erlang.3erl - Man Page

Returns an integer or float that is the arithmetical absolute value of Float or Int, for example:

> abs(-3.33).
3.33
> abs(-3).
3

Allowed in guard tests.

Types:

Data = iodata()

Computes and returns the adler32 checksum for Data.

Continues computing the adler32 checksum by combining the previous checksum, OldAdler, with the checksum of Data.

The following code:

X = erlang:adler32(Data1),
Y = erlang:adler32(X,Data2).

assigns the same value to Y as this:

Y = erlang:adler32([Data1,Data2]).

Combines two previously computed adler32 checksums. This computation requires the size of the data object for the second checksum to be known.

The following code:

Y = erlang:adler32(Data1),
Z = erlang:adler32(Y,Data2).

assigns the same value to Z as this:

X = erlang:adler32(Data1),
Y = erlang:adler32(Data2),
Z = erlang:adler32_combine(X,Y,iolist_size(Data2)).

Create an alias which can be used when sending messages to the process that created the alias. When the alias has been deactivated, messages sent using the alias will be dropped. An alias can be deactivated using unalias/1. Currently available options for alias/1:

explicit_unalias:

The alias can only be deactivated via a call to unalias/1. This is also the default behaviour if no options are passed or if alias/0 is called.

reply:

The alias will be automatically deactivated when a reply message sent via the alias is received. The alias can also still be deactivated via a call to unalias/1.

Example:

server() ->
    receive
        {request, AliasReqId, Request} ->
            Result = perform_request(Request),
            AliasReqId ! {reply, AliasReqId, Result}
    end,
    server().

client(ServerPid, Request) ->
    AliasReqId = alias([reply]),
    ServerPid ! {request, AliasReqId, Request},
    %% Alias will be automatically deactivated if we receive a reply
    %% since we used the 'reply' option...
    receive
        {reply, AliasReqId, Result} -> Result
    after 5000 ->
            unalias(AliasReqId),
            %% Flush message queue in case the reply arrived
            %% just before the alias was deactivated...
            receive {reply, AliasReqId, Result} -> Result
            after 0 -> exit(timeout)
            end
    end.

Note that both the server and the client in this example must be executing on at least OTP 24 systems in order for this to work.

For more information on process aliases see the Process Aliases  section of the Erlang Reference Manual .

Returns a new tuple that has one element more than Tuple1, and contains the elements in Tuple1 followed by Term as the last element. Semantically equivalent to list_to_tuple(tuple_to_list(Tuple1) ++ [Term]), but much faster. Example:

> erlang:append_element({one, two}, three).
{one,two,three}

Calls a fun, passing the elements in Args as arguments.

If the number of elements in the arguments are known at compile time, the call is better written as Fun(Arg1, Arg2, ... ArgN).

Earlier, Fun could also be specified as {Module, Function}, equivalent to apply(Module, Function, Args). This use is deprecated and will stop working in a future release.

Returns the result of applying Function in Module to Args. The applied function must be exported from Module. The arity of the function is the length of Args. Example:

> apply(lists, reverse, [[a, b, c]]).
[c,b,a]
> apply(erlang, atom_to_list, ['Erlang']).
"Erlang"

If the number of arguments are known at compile time, the call is better written as Module:Function(Arg1, Arg2, ..., ArgN).

Failure: error_handler:undefined_function/3 is called if the applied function is not exported. The error handler can be redefined (see process_flag/2). If error_handler is undefined, or if the user has redefined the default error_handler so the replacement module is undefined, an error with reason undef is generated.

Types:

Atom = atom()

The same as atom_to_binary(Atom, utf8).

Returns a binary corresponding to the text representation of Atom. If Encoding is latin1, one byte exists for each character in the text representation. If Encoding is utf8 or unicode, the characters are encoded using UTF-8 where characters may require multiple bytes.

As from Erlang/OTP 20, atoms can contain any Unicode character and atom_to_binary(Atom, latin1) may fail if the text representation for Atom contains a Unicode character > 255.

Example:

> atom_to_binary('Erlang', latin1).
<<"Erlang">>

Returns a list of unicode code points corresponding to the text representation of Atom, for example:

> atom_to_list('Erlang').
"Erlang"

> atom_to_list('你好').
[20320,22909]

See unicode(3) for how to convert the resulting list to different formats.

Extracts the part of the binary described by PosLen.

Negative length can be used to extract bytes at the end of a binary, for example:

1> Bin = <<1,2,3,4,5,6,7,8,9,10>>.
2> binary_part(Bin,{byte_size(Bin), -5}).
<<6,7,8,9,10>>

Failure: badarg if PosLen in any way references outside the binary.

Start is zero-based, that is:

1> Bin = <<1,2,3>>
2> binary_part(Bin,{0,2}).
<<1,2>>

For details about the PosLen semantics, see binary(3).

Allowed in guard tests.

The same as binary_part(Subject, {Start, Length}).

Allowed in guard tests.

Types:

Binary = binary()

The same as binary_to_atom(Binary, utf8).

Returns the atom whose text representation is Binary. If Encoding is utf8 or unicode, the binary must contain valid UTF-8 sequences.

As from Erlang/OTP 20, binary_to_atom(Binary, utf8) is capable of decoding any Unicode character. Earlier versions would fail if the binary contained Unicode characters > 255.

The number of characters that are permitted in an atom name is limited. The default limits can be found in the  efficiency guide (section Advanced).

There is configurable limit on how many atoms that can exist and atoms are not garbage collected. Therefore, it is recommended to consider whether binary_to_existing_atom/2 is a better option than binary_to_atom/2. The default limits can be found in efficiency guide (section Advanced).

Examples:

> binary_to_atom(<<"Erlang">>, latin1).

> binary_to_atom(<<1024/utf8>>, utf8).

Types:

Binary = binary()

The same as binary_to_existing_atom (Binary, utf8).

As binary_to_atom/2, but the atom must exist.

The Erlang system has a  configurable limit for the total number of atoms that can exist, and atoms are not garbage collected. Therefore, it is not safe to create many atoms from binaries that come from an untrusted source (for example, a file fetched from the Internet), for example, using binary_to_atom/2. This function is thus the appropriate option when the input binary comes from an untrusted source.

An atom exists in an Erlang system when included in a loaded Erlang module or when created programmatically (for example, by binary_to_atom/2). See the next note for an example of when an atom exists in the source code for an Erlang module but not in the compiled version of the same module.

Failure: badarg if the atom does not exist.

Note that the compiler may optimize away atoms. For example, the compiler will rewrite atom_to_list(some_atom) to "some_atom". If that expression is the only mention of the atom some_atom in the containing module, the atom will not be created when the module is loaded, and a subsequent call to binary_to_existing_atom(<<"some_atom">>, utf8) will fail.

The number of characters that are permitted in an atom name is limited. The default limits can be found in the  efficiency guide (section Advanced).

Returns the float whose text representation is Binary, for example:

> binary_to_float(<<"2.2017764e+0">>).
2.2017764

The float string format is the same as the format for Erlang float literals except for that underscores are not permitted.

Failure: badarg if Binary contains a bad representation of a float.

Returns an integer whose text representation is Binary, for example:

> binary_to_integer(<<"123">>).
123

binary_to_integer/1 accepts the same string formats as list_to_integer/1.

Failure: badarg if Binary contains a bad representation of an integer.

Returns an integer whose text representation in base Base is Binary, for example:

> binary_to_integer(<<"3FF">>, 16).
1023

binary_to_integer/2 accepts the same string formats as list_to_integer/2.

Failure: badarg if Binary contains a bad representation of an integer.

Types:

Binary = binary()

Returns a list of integers corresponding to the bytes of Binary.

As binary_to_list/1, but returns a list of integers corresponding to the bytes from position Start to position Stop in Binary. The positions in the binary are numbered starting from 1.

The one-based indexing for binaries used by this function is deprecated. New code is to use binary:bin_to_list/3 in STDLIB instead. All functions in module binary consistently use zero-based indexing.

Returns an Erlang term that is the result of decoding binary object Binary, which must be encoded according to the  Erlang external term format.

> Bin = term_to_binary(hello).
<<131,100,0,5,104,101,108,108,111>>
> hello = binary_to_term(Bin).
hello

When decoding binaries from untrusted sources, the untrusted source may submit data in a way to create resources, such as atoms and remote references, that cannot be garbage collected and lead to Denial of Service attack. In such cases, consider using binary_to_term/2 with the safe option.

See also term_to_binary/1 and binary_to_term/2.

As binary_to_term/1, but takes these options:

> binary_to_term(<<131,100,0,5,"hello">>, [safe]).
** exception error: bad argument
> hello.
hello
> binary_to_term(<<131,100,0,5,"hello">>, [safe]).
hello

> Input = <<131,100,0,5,"hello","world">>.
<<131,100,0,5,104,101,108,108,111,119,111,114,108,100>>
> {Term, Used} = binary_to_term(Input, [used]).
{hello, 9}
> split_binary(Input, Used).
{<<131,100,0,5,104,101,108,108,111>>, <<"world">>}

Failure: badarg if safe is specified and unsafe data is decoded.

See also term_to_binary/1, binary_to_term/1, and list_to_existing_atom/1.

Returns an integer that is the size in bits of Bitstring, for example:

> bit_size(<<433:16,3:3>>).
19
> bit_size(<<1,2,3>>).
24

Allowed in guard tests.

Returns a list of integers corresponding to the bytes of Bitstring. If the number of bits in the binary is not divisible by 8, the last element of the list is a bitstring containing the remaining 1-7 bits. Examples:

> bitstring_to_list(<<433:16>>).
[1,177]

> bitstring_to_list(<<433:16,3:3>>).
[1,177,<<3:3>>]

This implementation-dependent function increments the reduction counter for the calling process. In the Beam emulator, the reduction counter is normally incremented by one for each function and BIF call. A context switch is forced when the counter reaches the maximum number of reductions for a process (4000 reductions in Erlang/OTP 19.2 and later).

This BIF can be removed in a future version of the Beam machine without prior warning. It is unlikely to be implemented in other Erlang implementations.

Returns an integer that is the number of bytes needed to contain Bitstring. That is, if the number of bits in Bitstring is not divisible by 8, the resulting number of bytes is rounded up. Examples:

> byte_size(<<433:16,3:3>>).
3
> byte_size(<<1,2,3>>).
3

Allowed in guard tests.

Types:

TimerRef = reference()
Time = integer() >= 0
Result = Time | false

Cancels a timer. The same as calling erlang:cancel_timer(TimerRef, []).

Cancels a timer that has been created by erlang:start_timer or erlang:send_after. TimerRef identifies the timer, and was returned by the BIF that created the timer.

Options:

{async, Async}:

Asynchronous request for cancellation. Async defaults to false, which causes the cancellation to be performed synchronously. When Async is set to true, the cancel operation is performed asynchronously. That is, cancel_timer() sends an asynchronous request for cancellation to the timer service that manages the timer, and then returns ok.

{info, Info}:

Requests information about the Result of the cancellation. Info defaults to true, which means the Result is given. When Info is set to false, no information about the result of the cancellation is given.

• When Async is false: if Info is true, the Result is returned by erlang:cancel_timer(). otherwise ok is returned.
• When Async is true: if Info is true, a message on the form {cancel_timer, TimerRef, Result} is sent to the caller of erlang:cancel_timer() when the cancellation operation has been performed, otherwise no message is sent.

More Options may be added in the future.

If Result is an integer, it represents the time in milliseconds left until the canceled timer would have expired.

If Result is false, a timer corresponding to TimerRef could not be found. This can be either because the timer had expired, already had been canceled, or because TimerRef never corresponded to a timer. Even if the timer had expired, it does not tell you if the time-out message has arrived at its destination yet.

The timer service that manages the timer can be co-located with another scheduler than the scheduler that the calling process is executing on. If so, communication with the timer service takes much longer time than if it is located locally. If the calling process is in critical path, and can do other things while waiting for the result of this operation, or is not interested in the result of the operation, you want to use option {async, true}. If using option {async, false}, the calling process blocks until the operation has been performed.

See also erlang:send_after/4, erlang:start_timer/4, and erlang:read_timer/2.

Returns the smallest integer not less than Number. For example:

> ceil(5.5).
6

Allowed in guard tests.

Returns true if Module has old code, otherwise false.

See also code(3).

Types:

Pid = pid()
Module = module()
CheckResult = boolean()

The same as check_process_code(Pid, Module, []).

Checks if the node local process identified by Pid executes old code for Module.

Options:

{allow_gc, boolean()}:

Determines if garbage collection is allowed when performing the operation. If {allow_gc, false} is passed, and a garbage collection is needed to determine the result of the operation, the operation is aborted (see information on CheckResult below). The default is to allow garbage collection, that is, {allow_gc, true}.

{async, RequestId}:

The function check_process_code/3 returns the value async immediately after the request has been sent. When the request has been processed, the process that called this function is passed a message on the form {check_process_code, RequestId, CheckResult}.

If Pid equals self(), and no async option has been passed, the operation is performed at once. Otherwise a request for the operation is sent to the process identified by Pid, and is handled when appropriate. If no async option has been passed, the caller blocks until CheckResult is available and can be returned.

CheckResult informs about the result of the request as follows:

true:

The process identified by Pid executes old code for Module. That is, the current call of the process executes old code for this module, or the process has references to old code for this module, or the process contains funs that references old code for this module.

false:

The process identified by Pid does not execute old code for Module.

aborted:

The operation was aborted, as the process needed to be garbage collected to determine the operation result, and the operation was requested by passing option {allow_gc, false}.

Up until ERTS version 8.*, the check process code operation checks for all types of references to the old code. That is, direct references (e.g. return addresses on the process stack), indirect references (funs in process context), and references to literals in the code.

As of ERTS version 9.0, the check process code operation only checks for direct references to the code. Indirect references via funs will be ignored. If such funs exist and are used after a purge of the old code, an exception will be raised upon usage (same as the case when the fun is received by the process after the purge). Literals will be taken care of (copied) at a later stage. This behavior can as of ERTS version 8.1 be enabled when building OTP, and will automatically be enabled if dirty scheduler support is enabled.

See also code(3).

Failures:

badarg:

If Pid is not a node local process identifier.

badarg:

If Module is not an atom.

badarg:

If OptionList is an invalid list of options.

Converts the Time value of time unit FromUnit to the corresponding ConvertedTime value of time unit ToUnit. The result is rounded using the floor function.

You can lose accuracy and precision when converting between time units. To minimize such loss, collect all data at native time unit and do the conversion on the end result.

Types:

Data = iodata()

Computes and returns the crc32 (IEEE 802.3 style) checksum for Data.

Continues computing the crc32 checksum by combining the previous checksum, OldCrc, with the checksum of Data.

The following code:

X = erlang:crc32(Data1),
Y = erlang:crc32(X,Data2).

assigns the same value to Y as this:

Y = erlang:crc32([Data1,Data2]).

Combines two previously computed crc32 checksums. This computation requires the size of the data object for the second checksum to be known.

The following code:

Y = erlang:crc32(Data1),
Z = erlang:crc32(Y,Data2).

assigns the same value to Z as this:

X = erlang:crc32(Data1),
Y = erlang:crc32(Data2),
Z = erlang:crc32_combine(X,Y,iolist_size(Data2)).

Returns the current date as {Year, Month, Day}.

The time zone and Daylight Saving Time correction depend on the underlying OS. The return value is based on the OS System Time. Example:

> date().
{1995,2,19}

Decodes the binary Bin according to the packet protocol specified by Type. Similar to the packet handling done by sockets with option {packet,Type}.

If an entire packet is contained in Bin, it is returned together with the remainder of the binary as {ok,Packet,Rest}.

If Bin does not contain the entire packet, {more,Length} is returned. Length is either the expected total size of the packet, or undefined if the expected packet size is unknown. decode_packet can then be called again with more data added.

If the packet does not conform to the protocol format, {error,Reason} is returned.

Types:

raw | 0:

No packet handling is done. The entire binary is returned unless it is empty.

1 | 2 | 4:

Packets consist of a header specifying the number of bytes in the packet, followed by that number of bytes. The length of the header can be one, two, or four bytes; the order of the bytes is big-endian. The header is stripped off when the packet is returned.

line:

A packet is a line-terminated by a delimiter byte, default is the latin-1 newline character. The delimiter byte is included in the returned packet unless the line was truncated according to option line_length.

asn1 | cdr | sunrm | fcgi | tpkt:

The header is not stripped off.

The meanings of the packet types are as follows:

asn1 - ASN.1 BER:

sunrm - Sun's RPC encoding:

cdr - CORBA (GIOP 1.1):

fcgi - Fast CGI:

tpkt - TPKT format [RFC1006]:

http | httph | http_bin | httph_bin:

The Hypertext Transfer Protocol. The packets are returned with the format according to HttpPacket described earlier. A packet is either a request, a response, a header, or an end of header mark. Invalid lines are returned as HttpError.

Recognized request methods and header fields are returned as atoms. Others are returned as strings. Strings of unrecognized header fields are formatted with only capital letters first and after hyphen characters, for example, "Sec-Websocket-Key". Header field names are also returned in UnmodifiedField as strings, without any conversion or formatting.
The protocol type http is only to be used for the first line when an HttpRequest or an HttpResponse is expected. The following calls are to use httph to get HttpHeaders until http_eoh is returned, which marks the end of the headers and the beginning of any following message body.
The variants http_bin and httph_bin return strings (HttpString) as binaries instead of lists.
Since OTP 26.0, Host may be an IPv6 address enclosed in [], as defined in RFC2732 .

Options:

{packet_size, integer() >= 0}:

Sets the maximum allowed size of the packet body. If the packet header indicates that the length of the packet is longer than the maximum allowed length, the packet is considered invalid. Defaults to 0, which means no size limit.

{line_length, integer() >= 0}:

For packet type line, lines longer than the indicated length are truncated.

Option line_length also applies to http* packet types as an alias for option packet_size if packet_size itself is not set. This use is only intended for backward compatibility.

{line_delimiter, 0 =< byte() =< 255}:

For packet type line, sets the delimiting byte. Default is the latin-1 character $\n.

Examples:

> erlang:decode_packet(1,<<3,"abcd">>,[]).
{ok,<<"abc">>,<<"d">>}
> erlang:decode_packet(1,<<5,"abcd">>,[]).
{more,6}

Returns a new tuple with element at Index removed from tuple Tuple1, for example:

> erlang:delete_element(2, {one, two, three}).
{one,three}

Makes the current code for Module become old code and deletes all references for this module from the export table. Returns undefined if the module does not exist, otherwise true.

This BIF is intended for the code server (see code(3)) and is not to be used elsewhere.

Failure: badarg if there already is an old version of Module.

If MonitorRef is a reference that the calling process obtained by calling monitor/2, this monitoring is turned off. If the monitoring is already turned off, nothing happens.

Once demonitor(MonitorRef) has returned, it is guaranteed that no {'DOWN', MonitorRef, _, _, _} message, because of the monitor, will be placed in the caller message queue in the future. However, a {'DOWN', MonitorRef, _, _, _} message can have been placed in the caller message queue before the call. It is therefore usually advisable to remove such a 'DOWN' message from the message queue after monitoring has been stopped. demonitor(MonitorRef, [flush]) can be used instead of demonitor(MonitorRef) if this cleanup is wanted.

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

Before Erlang/OTP R11B (ERTS 5.5) demonitor/1 behaved completely asynchronously, that is, the monitor was active until the "demonitor signal" reached the monitored entity. This had one undesirable effect. You could never know when you were guaranteed not to receive a DOWN message because of the monitor.

The current behavior can be viewed as two combined operations: asynchronously send a "demonitor signal" to the monitored entity and ignore any future results of the monitor.

Failure: It is an error if MonitorRef refers to a monitoring started by another process. Not all such cases are cheap to check. If checking is cheap, the call fails with badarg, for example if MonitorRef is a remote reference.

The returned value is true unless info is part of OptionList.

demonitor(MonitorRef, []) is equivalent to demonitor(MonitorRef).

Options:

demonitor(MonitorRef),
receive
    {_, MonitorRef, _, _, _} ->
        true
after 0 ->
        true
end

More options can be added in a future release.

Failures:

badarg:

If OptionList is not a list.

badarg:

If Option is an invalid option.

badarg:

The same failure as for demonitor/1.

Forces the disconnection of a node. This appears to the node Node as if the local node has crashed. This BIF is mainly used in the Erlang network authentication protocols.

Returns true if disconnection succeeds, otherwise false. If the local node is not alive, ignored is returned.

This function may return before nodedown messages have been delivered.

Prints a text representation of Term on the standard output.

This BIF is intended for debugging only. The printed representation may contain internal details that do not match the high-level representation of the term in Erlang.

Get distribution channel data from the local node that is to be passed to the remote node. The distribution channel is identified by DHandle. If no data is available, the atom none is returned. One can request to be informed by a message when more data is available by calling erlang:dist_ctrl_get_data_notification(DHandle).

The returned value when there are data available depends on the value of the get_size option configured on the distribution channel identified by DHandle. For more information see the documentation of the get_size option for the erlang:dist_ctrl_set_opt/3 function.

Only the process registered as distribution controller for the distribution channel identified by DHandle is allowed to call this function.

This function is used when implementing an alternative distribution carrier using processes as distribution controllers. DHandle is retrieved via the callback f_handshake_complete. More information can be found in the documentation of ERTS User's Guide ➜ How to implement an Alternative Carrier for the Erlang Distribution ➜ Distribution Module.

Returns the value of the get_size option on the distribution channel identified by DHandle. For more information see the documentation of the get_size option for the erlang:dist_ctrl_set_opt/3 function.

Only the process registered as distribution controller for the distribution channel identified by DHandle is allowed to call this function.

This function is used when implementing an alternative distribution carrier using processes as distribution controllers. DHandle is retrieved via the callback f_handshake_complete. More information can be found in the documentation of ERTS User's Guide ➜ How to implement an Alternative Carrier for the Erlang Distribution ➜ Distribution Module.

Request notification when more data is available to fetch using erlang:dist_ctrl_get_data(DHandle) for the distribution channel identified by DHandle. When more data is present, the caller will be sent the message dist_data. Once a dist_data messages has been sent, no more dist_data messages will be sent until the dist_ctrl_get_data_notification/1 function has been called again.

Only the process registered as distribution controller for the distribution channel identified by DHandle is allowed to call this function.

This function is used when implementing an alternative distribution carrier using processes as distribution controllers. DHandle is retrieved via the callback f_handshake_complete. More information can be found in the documentation of ERTS User's Guide ➜ How to implement an Alternative Carrier for the Erlang Distribution ➜ Distribution Module.

Register an alternate input handler process for the distribution channel identified by DHandle. Once this function has been called, InputHandler is the only process allowed to call erlang:dist_ctrl_put_data(DHandle, Data) with the DHandle identifying this distribution channel.

Only the process registered as distribution controller for the distribution channel identified by DHandle is allowed to call this function.

This function is used when implementing an alternative distribution carrier using processes as distribution controllers. DHandle is retrieved via the callback f_handshake_complete. More information can be found in the documentation of ERTS User's Guide ➜ How to implement an Alternative Carrier for the Erlang Distribution ➜ Distribution Module.

Deliver distribution channel data from a remote node to the local node.

Only the process registered as distribution controller for the distribution channel identified by DHandle is allowed to call this function unless an alternate input handler process has been registered using erlang:dist_ctrl_input_handler(DHandle, InputHandler). If an alternate input handler has been registered, only the registered input handler process is allowed to call this function.

This function is used when implementing an alternative distribution carrier using processes as distribution controllers. DHandle is retrieved via the callback f_handshake_complete. More information can be found in the documentation of ERTS User's Guide ➜ How to implement an Alternative Carrier for the Erlang Distribution ➜ Distribution Module.

Sets the value of the get_size option on the distribution channel identified by DHandle. This option controls the return value of calls to erlang:dist_ctrl_get_data(DHandle) where DHandle equals DHandle used when setting this option. When the get_size option is:

false:

and there are distribution data available, a call to erlang:dist_ctrl_get_data(DHandle) will just return Data to pass over the channel. This is the default value of the get_size option.

true:

and there are distribution data available, a call to erlang:dist_ctrl_get_data(DHandle) will return Data to pass over the channel as well as the Size of Data in bytes. This is returned as a tuple on the form {Size, Data}.

All options are set to default when a channel is closed.

Only the process registered as distribution controller for the distribution channel identified by DHandle is allowed to call this function.

This function is used when implementing an alternative distribution carrier using processes as distribution controllers. DHandle is retrieved via the callback f_handshake_complete. More information can be found in the documentation of ERTS User's Guide ➜ How to implement an Alternative Carrier for the Erlang Distribution ➜ Distribution Module.

Returns the Nth element (numbering from 1) of Tuple, for example:

> element(2, {a, b, c}).
b

Allowed in guard tests.

Returns the process dictionary and deletes it, for example:

> put(key1, {1, 2, 3}),
put(key2, [a, b, c]),
erase().
[{key1,{1,2,3}},{key2,[a,b,c]}]

Returns the value Val associated with Key and deletes it from the process dictionary. Returns undefined if no value is associated with Key. The average time complexity for the current implementation of this function is O(1) and the worst case time complexity is O(N), where N is the number of items in the process dictionary. Example:

> put(key1, {merry, lambs, are, playing}),
X = erase(key1),
{X, erase(key1)}.
{{merry,lambs,are,playing},undefined}

Raises an exception of class error with the reason Reason. As evaluating this function causes an exception to be thrown, it has no return value.

The intent of the exception class error is to signal that an unexpected error has happened (for example, a function is called with a parameter that has an incorrect type). See the guide about errors and error handling for additional information. Example:

> catch error(foobar).
{'EXIT',{foobar,[{shell,apply_fun,3,
                        [{file,"shell.erl"},{line,906}]},
                 {erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,677}]},
                 {erl_eval,expr,5,[{file,"erl_eval.erl"},{line,430}]},
                 {shell,exprs,7,[{file,"shell.erl"},{line,687}]},
                 {shell,eval_exprs,7,[{file,"shell.erl"},{line,642}]},
                 {shell,eval_loop,3,[{file,"shell.erl"},{line,627}]}]}}

Raises an exception of class error with the reason Reason. Args is expected to be the list of arguments for the current function or the atom none. If it is a list, it is used to provide the arguments for the current function in the stack back-trace. If it is none, the arity of the calling function is used in the stacktrace. As evaluating this function causes an exception to be raised, it has no return value.

The intent of the exception class error is to signal that an unexpected error has happened (for example, a function is called with a parameter that has an incorrect type). See the guide about errors and error handling for additional information. Example:

test.erl:

-module(test).
-export([example_fun/2]).

example_fun(A1, A2) ->
    erlang:error(my_error, [A1, A2]).

Erlang shell:

6> c(test).
{ok,test}
7> test:example_fun(arg1,"this is the second argument").
** exception error: my_error
     in function  test:example_fun/2
         called as test:example_fun(arg1,"this is the second argument")

Raises an exception of class error with the reason Reason. Args is expected to be the list of arguments for the current function or the atom none. If it is a list, it is used to provide the arguments for the current function in the stack back-trace. If it is none, the arity of the calling function is used in the stacktrace. As evaluating this function causes an exception to be raised, it has no return value.

If the error_info option is given, the ErrorInfoMap will be inserted into the stacktrace. The information given in the ErrorInfoMap is to be used by error formatters such as erl_error to provide more context around an error.

The default module of the ErrorInfoMap is the module that the call to error/3 is made. The default function is format_error. See format_error/2 for more details on how this Module:Function/2 is to be used

The intent of the exception class error is to signal that an unexpected error has happened (for example, a function is called with a parameter that has an incorrect type). See the guide about errors and error handling for additional information.

Raises an exception of class exit with exit reason Reason. As evaluating this function causes an exception to be raised, it has no return value.

The intent of the exception class exit is that the current process should be stopped (for example when a message telling a process to stop is received).

This function differ from error/1,2,3 by causing an exception of a different class and by having a reason that does not include the list of functions from the call stack.

See the guide about errors and error handling for additional information.

Example:

> exit(foobar).
** exception exit: foobar
> catch exit(foobar).
{'EXIT',foobar}

If a process calls exit(kill) and does not catch the exception, it will terminate with exit reason kill and also emit exit signals with exit reason kill (not killed) to all linked processes. Such exit signals with exit reason kill can be trapped by the linked processes. Note that this means that signals with exit reason kill behave differently depending on how they are sent because the signal will be untrappable if a process sends such a signal to another process with erlang:exit/2.

Sends an exit signal with exit reason Reason to the process or port identified by Pid.

The following behavior applies if Reason is any term, except normal or kill, and P is the process or port identified by Pid:

• If P is not trapping exits, P exits with exit reason Reason.
• If P is trapping exits, the exit signal is transformed into a message {'EXIT', From, Reason}, where From is the process identifier of the process that sent the exit signal, and delivered to the message queue of P.

The following behavior applies if Reason is the term normal and Pid is the identifier of a process P which is not the same as the process that invoked erlang:exit(Pid, normal) (the behavior when a process sends a signal with the normal reason to itself is described in the warning):

• If P is trapping exits, the exit signal is transformed into a message {'EXIT', From, normal}, where From is the process identifier of the process that sent the exit signal, and delivered to P's message queue.
• The signal has no effect if P is not trapping exits.

If Reason is the atom kill, that is, if exit(Pid, kill) is called, an untrappable exit signal is sent to the process that is identified by Pid, which unconditionally exits with exit reason killed. The exit reason is changed from kill to killed to hint to linked processes that the killed process got killed by a call to exit(Pid, kill).

The functions erlang:exit/1 and erlang:exit/2 are named similarly but provide very different functionalities. The erlang:exit/1 function should be used when the intent is to stop the current process while erlang:exit/2 should be used when the intent is to send an exit signal to another process. Note also that erlang:exit/1 raises an exception that can be caught while erlang:exit/2 does not cause any exception to be raised.

The only scenario that has not been covered by the description above is when a process P sends an exit signal with reason normal to itself, that is erlang:exit(self(), normal). The behavior in this scenario is as follows:

• If P is trapping exits, the exit signal is transformed into a message {'EXIT', From, normal}, where From is P's process identifier, and delivered to P's message queue.
• P exits with reason normal if P is not trapping exits.

Note that the behavior described above is different from when a process sends an exit signal with reason normal to another process. This is arguably strange but this behavior is kept for backward compatibility reasons.

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

Calculates, without doing the encoding, the maximum byte size for a term encoded in the Erlang external term format. The following condition applies always:

> Size1 = byte_size(term_to_binary(Term)),
> Size2 = erlang:external_size(Term),
> true = Size1 =< Size2.
true

This is equivalent to a call to:

erlang:external_size(Term, [])

Calculates, without doing the encoding, the maximum byte size for a term encoded in the Erlang external term format. The following condition applies always:

> Size1 = byte_size(term_to_binary(Term, Options)),
> Size2 = erlang:external_size(Term, Options),
> true = Size1 =< Size2.
true

Option {minor_version, Version} specifies how floats are encoded. For a detailed description, see term_to_binary/2.

Returns a float by converting Number to a float, for example:

> float(55).
55.0

Allowed in guard tests.

If used on the top level in a guard, it tests whether the argument is a floating point number; for clarity, use is_float/1 instead.

When float/1 is used in an expression in a guard, such as 'float(A) == 4.0', it converts a number as described earlier.

Types:

Float = float()

The same as float_to_binary(Float,[{scientific,20}]).

Returns a binary corresponding to the text representation of Float using fixed decimal point formatting. Options behaves in the same way as float_to_list/2. Examples:

> float_to_binary(7.12, [{decimals, 4}]).
<<"7.1200">>
> float_to_binary(7.12, [{decimals, 4}, compact]).
<<"7.12">>
> float_to_binary(7.12, [{scientific, 3}]).
<<"7.120e+00">>
> float_to_binary(7.12, [short]).
<<"7.12">>
> float_to_binary(0.1+0.2, [short]).
<<"0.30000000000000004">>
> float_to_binary(0.1+0.2)
<<"3.00000000000000044409e-01">>

Types:

Float = float()

The same as float_to_list(Float,[{scientific,20}]).

Returns a string corresponding to the text representation of Float using fixed decimal point formatting.

Available options:

• If option decimals is specified, the returned value contains at most Decimals number of digits past the decimal point. If the number does not fit in the internal static buffer of 256 bytes, the function throws badarg.
• If option compact is specified, the trailing zeros at the end of the list are truncated. This option is only meaningful together with option decimals.
• If option scientific is specified, the float is formatted using scientific notation with Decimals digits of precision.
• If option short is specified, the float is formatted with the smallest number of digits that still guarantees that F =:= list_to_float(float_to_list(F, [short])). When the float is inside the range (-2⁵³, 2⁵³), the notation that yields the smallest number of characters is used (scientific notation or normal decimal notation). Floats outside the range (-2⁵³, 2⁵³) are always formatted using scientific notation to avoid confusing results when doing arithmetic operations.
• If Options is [], the function behaves as float_to_list/1.

Examples:

> float_to_list(7.12, [{decimals, 4}]).
"7.1200"
> float_to_list(7.12, [{decimals, 4}, compact]).
"7.12"
> float_to_list(7.12, [{scientific, 3}]).
"7.120e+00"
> float_to_list(7.12, [short]).
"7.12"
> float_to_list(0.1+0.2, [short]).
"0.30000000000000004"
> float_to_list(0.1+0.2)
"3.00000000000000044409e-01"

In the last example, float_to_list(0.1+0.2) evaluates to "3.00000000000000044409e-01". The reason for this is explained in Representation of Floating Point Numbers.

Returns the largest integer not greater than Number. For example:

> floor(-10.5).
-11

Allowed in guard tests.

Returns a list with information about the fun Fun. Each list element is a tuple. The order of the tuples is undefined, and more tuples can be added in a future release.

This BIF is mainly intended for debugging, but it can sometimes be useful in library functions that need to verify, for example, the arity of a fun.

Two types of funs have slightly different semantics:

• A fun created by fun M:F/A is called an external fun. Calling it will always call the function F with arity A in the latest code for module M. Notice that module M does not even need to be loaded when the fun fun M:F/A is created.
• All other funs are called local. When a local fun is called, the same version of the code that created the fun is called (even if a newer version of the module has been loaded).

The following elements are always present in the list for both local and external funs:

{type, Type}:

Type is local or external.

{module, Module}:

Module (an atom) is the module name.

If Fun is a local fun, Module is the module in which the fun is defined.
If Fun is an external fun, Module is the module that the fun refers to.

{name, Name}:

Name (an atom) is a function name.

If Fun is a local fun, Name is the name of the local function that implements the fun. (This name was generated by the compiler, and is only of informational use. As it is a local function, it cannot be called directly.) If no code is currently loaded for the fun, [] is returned instead of an atom.
If Fun is an external fun, Name is the name of the exported function that the fun refers to.

{arity, Arity}:

Arity is the number of arguments that the fun is to be called with.

{env, Env}:

Env (a list) is the environment or free variables for the fun. For external funs, the returned list is always empty.

The following elements are only present in the list if Fun is local:

Returns information about Fun as specified by Item, in the form {Item,Info}.

For any fun, Item can be any of the atoms module, name, arity, env, or type.

For a local fun, Item can also be any of the atoms index, new_index, new_uniq, uniq, and pid. For an external fun, the value of any of these items is always the atom undefined.

See erlang:fun_info/1.

Returns String that represents the code that created Fun.

String has the following form, if Fun was created by a fun expression of the form fun ModuleName:FuncName/Arity:

"fun ModuleName:FuncName/Arity"

The form of String when Fun is created from other types of fun expressions differs depending on if the fun expression was executed while executing compiled code or if the fun expression was executed while executing uncompiled code (uncompiled escripts, the Erlang shell, and other code executed by the erl_eval module):

compiled code:

"#Fun<M.I.U>", where M, I and U correspond to the values named module, index and uniq in the result of erlang:fun_info(Fun).

uncompiled code:

All funs created from fun expressions in uncompiled code with the same arity are mapped to the same list by fun_to_list/1.

Generally, one can not use fun_to_list/1 to check if two funs are equal as fun_to_list/1 does not take the fun's environment into account. See erlang:fun_info/1 for how to get the environment of a fun.

The output of fun_to_list/1 can differ between Erlang implementations and may change in future versions.

Examples:

-module(test).
-export([add/1, add2/0, fun_tuple/0]).
add(A) -> fun(B) -> A + B end.
add2() -> fun add/1.
fun_tuple() -> {fun() -> 1 end, fun() -> 1 end}.

> {fun test:add/1, test:add2()}.
{fun test:add/1,#Fun<test.1.107738983>}

Explanation: fun test:add/1 is upgradable but test:add2() is not upgradable.

> {test:add(1), test:add(42)}.
{#Fun<test.0.107738983>,#Fun<test.0.107738983>}

Explanation: test:add(1) and test:add(42) has the same string representation as the environment is not taken into account.

>test:fun_tuple().
{#Fun<test.2.107738983>,#Fun<test.3.107738983>}

Explanation: The string representations differ because the funs come from different fun expressions.

> {fun() -> 1 end, fun() -> 1 end}. > 
{#Fun<erl_eval.45.97283095>,#Fun<erl_eval.45.97283095>}

Explanation: All funs created from fun expressions of this form in uncompiled code with the same arity are mapped to the same list by fun_to_list/1.

Types:

Module = module()
Function = atom()
Arity = arity()

Returns true if the module Module is current and contains an exported function Function/Arity, or if there is a BIF (a built-in function implemented in C) with the specified name, otherwise returns false.

Forces an immediate garbage collection of the executing process. The function is not to be used unless it has been noticed (or there are good reasons to suspect) that the spontaneous garbage collection will occur too late or not at all.

Improper use can seriously degrade system performance.

Types:

Pid = pid()
GCResult = boolean()

The same as garbage_collect(Pid, []).

Garbage collects the node local process identified by Pid.

Option:

{async, RequestId}:

The function garbage_collect/2 returns the value async immediately after the request has been sent. When the request has been processed, the process that called this function is passed a message on the form {garbage_collect, RequestId, GCResult}.

{type, 'major' | 'minor'}:

Triggers garbage collection of requested type. Default value is 'major', which would trigger a fullsweep GC. The option 'minor' is considered a hint and may lead to either minor or major GC run.

If Pid equals self(), and no async option has been passed, the garbage collection is performed at once, that is, the same as calling garbage_collect/0. Otherwise a request for garbage collection is sent to the process identified by Pid, and will be handled when appropriate. If no async option has been passed, the caller blocks until GCResult is available and can be returned.

GCResult informs about the result of the garbage collection request as follows:

true:

The process identified by Pid has been garbage collected.

false:

No garbage collection was performed, as the process identified by Pid terminated before the request could be satisfied.

Notice that the same caveats apply as for garbage_collect/0.

Failures:

badarg:

If Pid is not a node local process identifier.

badarg:

If OptionList is an invalid list of options.

Returns the process dictionary as a list of {Key, Val} tuples. The items in the returned list can be in any order. Example:

> put(key1, merry),
put(key2, lambs),
put(key3, {are, playing}),
get().
[{key1,merry},{key2,lambs},{key3,{are,playing}}]

Returns the value Val associated with Key in the process dictionary, or undefined if Key does not exist. The expected time complexity for the current implementation of this function is O(1) and the worst case time complexity is O(N), where N is the number of items in the process dictionary. Example:

> put(key1, merry),
put(key2, lambs),
put({any, [valid, term]}, {are, playing}),
get({any, [valid, term]}).
{are,playing}

Types:

Cookie = atom()

Returns the magic cookie of the local node if the node is alive, otherwise the atom nocookie. This value is set by set_cookie/1.

Types:

Node = node()
Cookie = atom()

Returns the magic cookie for node Node if the local node is alive, otherwise the atom nocookie. This value is set by set_cookie/2.

Returns a list of all keys present in the process dictionary. The items in the returned list can be in any order. Example:

> put(dog, {animal,1}),
put(cow, {animal,2}),
put(lamb, {animal,3}),
get_keys().
[dog,cow,lamb]

Returns a list of keys that are associated with the value Val in the process dictionary. The items in the returned list can be in any order. Example:

> put(mary, {1, 2}),
put(had, {1, 2}),
put(a, {1, 2}),
put(little, {1, 2}),
put(dog, {1, 3}),
put(lamb, {1, 2}),
get_keys({1, 2}).
[mary,had,a,little,lamb]

Returns the process identifier of the group leader for the process evaluating the function.

Every process is a member of some process group and all groups have a group leader. All I/O from the group is channeled to the group leader. When a new process is spawned, it gets the same group leader as the spawning process.

Initially, at system startup, init is both its own group leader and the group leader of all processes. During the boot of a system the group leader for processes will be changed depending on the need of the system. Some examples where this is done are:

• When an application is started, the top supervisor of that application will have its group leader set to the application master. See application:start/2 for more details.
• When running tests, both common_test and eunit set the group leader in order to capture any I/O from the testcase.
• The interactive shell sets the group leader to intercept I/O.

Sets the group leader of Pid to GroupLeader. Typically, this is used when a process started from a certain shell is to have another group leader than init.

The group leader should be rarely changed in applications with a supervision tree, because OTP assumes the group leader of their processes is their application master.

Setting the group leader follows the signal ordering guarantees described in the  Processes Chapter in the Erlang Reference Manual .

See also group_leader/0 and OTP design principles related to starting and stopping applications.

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

The same as calling halt(0, []). Example:

> halt().
os_prompt%

The same as calling halt(Status, []). Example:

> halt(17).
os_prompt% echo $?
17
os_prompt%

The same as calling halt(abort, []).

The same as calling halt(CrashDumpSlogan, []).

Types:

halt_options() = [{flush, boolean()}]

Halt the runtime system with status code Status.

On many platforms, the OS supports only status codes 0-255. A too large status code is truncated by clearing the high bits.

Currently the following options are valid:

Types:

halt_options() = [{flush, boolean()}]

Halt the Erlang runtime system by aborting and produce a core dump if core dumping has been enabled in the environment that the runtime system is executing in.

The {flush, boolean()} option will be ignored, and flushing will be disabled.

Types:

halt_options() = [{flush, boolean()}]

Halt the Erlang runtime system and generate an Erlang crash dump. The string CrashDumpSlogan will be used as slogan in the Erlang crash dump created. The slogan will be trunkated if CrashDumpSlogan is longer than 1023 characters.

The {flush, boolean()} option will be ignored, and flushing will be disabled.

Behavior changes compared to earlier versions:

• Before OTP 24.2, the slogan was truncated if CrashDumpSlogan was longer than 200 characters. Now it will be truncated if longer than 1023 characters.
• Before OTP 20.1, only code points in the range 0-255 were accepted in the slogan. Now any Unicode string is valid.

Returns the head of List, that is, the first element.

It works with improper lists.

Examples:

> hd([1,2,3,4,5]).
1

> hd([first, second, third, so_on | improper_end]).
first

Allowed in guard tests.

Failure: badarg if List is an empty list [].

Puts the calling process into a wait state where its memory allocation has been reduced as much as possible. This is useful if the process does not expect to receive any messages soon.

The process is awaken when a message is sent to it, and control resumes in Module:Function with the arguments specified by Args with the call stack emptied, meaning that the process terminates when that function returns. Thus erlang:hibernate/3 never returns to its caller. The resume function Module:Function/Arity must be exported (Arity =:= length(Args)).

If the process has any message in its message queue, the process is awakened immediately in the same way as described earlier.

In more technical terms, erlang:hibernate/3 discards the call stack for the process, and then garbage collects the process. After this, all live data is in one continuous heap. The heap is then shrunken to the exact same size as the live data that it holds (even if that size is less than the minimum heap size for the process).

If the size of the live data in the process is less than the minimum heap size, the first garbage collection occurring after the process is awakened ensures that the heap size is changed to a size not smaller than the minimum heap size.

Notice that emptying the call stack means that any surrounding catch is removed and must be re-inserted after hibernation. One effect of this is that processes started using proc_lib (also indirectly, such as gen_server processes), are to use proc_lib:hibernate/3 instead, to ensure that the exception handler continues to work when the process wakes up.

Returns a new tuple with element Term inserted at position Index in tuple Tuple1. All elements from position Index and upwards are pushed one step higher in the new tuple Tuple2. Example:

> erlang:insert_element(2, {one, two, three}, new).
{one,new,two,three}

Returns a binary corresponding to the text representation of Integer, for example:

> integer_to_binary(77).
<<"77">>

Returns a binary corresponding to the text representation of Integer in base Base, for example:

> integer_to_binary(1023, 16).
<<"3FF">>

Returns a string corresponding to the text representation of Integer, for example:

> integer_to_list(77).
"77"

Returns a string corresponding to the text representation of Integer in base Base, for example:

> integer_to_list(1023, 16).
"3FF"

Returns an integer, that is the size in bytes, of the binary that would be the result of iolist_to_binary(Item), for example:

> iolist_size([1,2|<<3,4>>]).
4

Returns a binary that is made from the integers and binaries in IoListOrBinary, for example:

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6>>.
<<6>>
> iolist_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6>>

Returns an iovec that is made from the integers and binaries in IoListOrBinary. This function is useful when you want to flatten an iolist but you do not need a single binary. This can be useful for passing the data to nif functions such as enif_inspect_iovec or do more efficient message passing. The advantage of using this function over iolist_to_binary/1 is that it does not have to copy  off-heap binaries. Example:

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6>>.
<<6>>
%% If you pass small binaries and integers it works as iolist_to_binary
> erlang:iolist_to_iovec([Bin1,1,[2,3,Bin2],4|Bin3]).
[<<1,2,3,1,2,3,4,5,4,6>>]
%% If you pass larger binaries, they are split and returned in a form
%% optimized for calling the C function writev.
> erlang:iolist_to_iovec([<<1>>,<<2:8096>>,<<3:8096>>]).
[<<1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
   0,...>>,
 <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
   ...>>,
 <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,...>>]

Returns true if the local node is alive (that is, if the node can be part of a distributed system), otherwise false. A node is alive if it is started with:

• "erl -name LONGNAME" or,
• "erl -sname SHORTNAME".

A node can also be alive if it has got a name from a call to net_kernel:start/2 and has not been stopped by a call to net_kernel:stop/0.

Returns true if Term is an atom, otherwise false.

Allowed in guard tests.

Returns true if Term is a binary, otherwise false.

A binary always contains a complete number of bytes.

Allowed in guard tests.

Returns true if Term is a bitstring (including a binary), otherwise false.

Allowed in guard tests.

Returns true if Term is the atom true or the atom false (that is, a boolean). Otherwise returns false.

Allowed in guard tests.

This BIF is useful for builders of cross-reference tools.

Returns true if Module:Function/Arity is a BIF implemented in C, otherwise false.

Returns true if Term is a floating point number, otherwise false.

Allowed in guard tests.

Returns true if Term is a fun, otherwise false.

Allowed in guard tests.

Returns true if Term is a fun that can be applied with Arity number of arguments, otherwise false.

Allowed in guard tests.

Returns true if Term is an integer, otherwise false.

Allowed in guard tests.

Returns true if Term is a list with zero or more elements, otherwise false.

Allowed in guard tests.

Returns true if Term is a map, otherwise false.

Allowed in guard tests.

Returns true if map Map contains Key and returns false if it does not contain the Key.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> Map = #{"42" => value}.
#{"42" => value}
> is_map_key("42",Map).
true
> is_map_key(value,Map).
false

Allowed in guard tests.

Returns true if Term is an integer or a floating point number. Otherwise returns false.

Allowed in guard tests.

Returns true if Term is a process identifier, otherwise false.

Allowed in guard tests.

Returns true if Term is a port identifier, otherwise false.

Allowed in guard tests.

Pid must refer to a process at the local node.

Returns true if the process exists and is alive, that is, is not exiting and has not exited. Otherwise returns false.

If process P1 calls is_process_alive(P2Pid) it is guaranteed that all signals, sent from P1 to P2 (P2 is the process with identifier P2Pid) before the call, will be delivered to P2 before the aliveness of P2 is checked. This guarantee means that one can use is_process_alive/1 to let a process P1 wait until a process P2, which has got an exit signal with reason kill from P1, is killed. Example:

exit(P2Pid, kill),
% P2 might not be killed
is_process_alive(P2Pid),
% P2 is not alive (the call above always return false)

See the documentation about signals and erlang:exit/2 for more information about signals and exit signals.

Returns true if Term is a tuple and its first element is RecordTag. Otherwise returns false.

Normally the compiler treats calls to is_record/2 especially. It emits code to verify that Term is a tuple, that its first element is RecordTag, and that the size is correct. However, if RecordTag is not a literal atom, the BIF is_record/2 is called instead and the size of the tuple is not verified.

Allowed in guard tests, if RecordTag is a literal atom.

RecordTag must be an atom.

Returns true if Term is a tuple, its first element is RecordTag, and its size is Size. Otherwise returns false.

Allowed in guard tests if RecordTag is a literal atom and Size is a literal integer.

This BIF is documented for completeness. Usually is_record/2 is to be used.

Returns true if Term is a reference, otherwise false.

Allowed in guard tests.

Returns true if Term is a tuple, otherwise false.

Allowed in guard tests.

Returns the length of List, for example:

> length([1,2,3,4,5,6,7,8,9]).
9

Allowed in guard tests.

Sets up and activates a link between the calling process and another process or a port identified by PidOrPort. We will from here on call the identified process or port linkee. If the linkee is a port, it must reside on the same node as the caller.

If one of the participants of a link terminates, it will send an exit signal to the other participant. The exit signal will contain the exit reason of the terminated participant. Other cases when exit signals are triggered due to a link are when no linkee exist (noproc exit reason) and when the connection between linked processes on different nodes is lost or cannot be established (noconnection exit reason).

An existing link can be removed by calling unlink/1. For more information on links and exit signals due to links, see the Processes  chapter in the Erlang Reference Manual :

• Links
• Sending Exit Signals
• Receiving Exit Signals

For historical reasons, link/1 has a strange semi-synchronous behavior when it is "cheap" to check if the linkee exists or not, and the caller does not trap exits. If the above is true and the linkee does not exist, link/1 will raise a noproc error exception. The expected behavior would instead have been that link/1 returned true, and the caller later was sent an exit signal with noproc exit reason, but this is unfortunately not the case. The noproc  exception is not to be confused with an exit signal with exit reason noproc. Currently it is "cheap" to check if the linkee exists when it is supposed to reside on the same node as the calling process.

The link setup and activation is performed asynchronously. If the link already exists, or if the caller attempts to create a link to itself, nothing is done. A detailed description of the link protocol can be found in the Distribution Protocol  chapter of the ERTS User's Guide .

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

Failure:

• badarg if PidOrPort does not identify a process or a node local port.
• noproc linkee does not exist and it is "cheap" to check if it exists as described above.

Returns the atom whose text representation is String.

As from Erlang/OTP 20, String may contain any Unicode character. Earlier versions allowed only ISO-latin-1 characters as the implementation did not allow Unicode characters above 255.

The number of characters that are permitted in an atom name is limited. The default limits can be found in the  efficiency guide (section Advanced).

There is configurable limit on how many atoms that can exist and atoms are not garbage collected. Therefore, it is recommended to consider if list_to_existing_atom/1 is a better option than list_to_atom/1. The default limits can be found in the efficiency guide (section Advanced).

Example:

> list_to_atom("Erlang").

Returns a binary that is made from the integers and binaries in IoList, for example:

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6>>.
<<6>>
> list_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6>>

Returns a bitstring that is made from the integers and bitstrings in BitstringList. (The last tail in BitstringList is allowed to be a bitstring.) Example:

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6,7:4>>.
<<6,7:4>>
> list_to_bitstring([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6,7:4>>

Returns the atom whose text representation is String, but only if there already exists such atom. An atom exists if it has been created by the run-time system by either loading code or creating a term in which the atom is part.

Failure: badarg if there does not already exist an atom whose text representation is String.

Note that the compiler may optimize away atoms. For example, the compiler will rewrite atom_to_list(some_atom) to "some_atom". If that expression is the only mention of the atom some_atom in the containing module, the atom will not be created when the module is loaded, and a subsequent call to list_to_existing_atom("some_atom") will fail.

Returns the float whose text representation is String, for example:

> list_to_float("2.2017764e+0").
2.2017764

The float string format is the same as the format for Erlang float literals except for that underscores are not permitted.

Failure: badarg if String contains a bad representation of a float.

Returns an integer whose text representation is String, for example:

> list_to_integer("123").
123

> list_to_integer("-123").
-123

> list_to_integer("+123234982304982309482093833234234").
123234982304982309482093833234234

String must contain at least one digit character and can have an optional prefix consisting of a single "+" or "-" character (that is, String must match the regular expression "^[+-]?[0-9]+$").

Failure: badarg if String contains a bad representation of an integer.

Returns an integer whose text representation in base Base is String, for example:

> list_to_integer("3FF", 16).
1023

> list_to_integer("+3FF", 16).
1023

> list_to_integer("3ff", 16).
1023

> list_to_integer("3fF", 16).
1023

> list_to_integer("-3FF", 16).
-1023

For example, when Base is 16, String must match the regular expression "^[+-]?([0-9]|[A-F]|[a-f])+$".

Failure: badarg if String contains a bad representation of an integer.

Returns a process identifier whose text representation is a String, for example:

> list_to_pid("<0.4.1>").
<0.4.1>

Failure: badarg if String contains a bad representation of a process identifier.

This BIF is intended for debugging and is not to be used in application programs.

Returns a port identifier whose text representation is a String, for example:

> list_to_port("#Port<0.4>").
#Port<0.4>

Failure: badarg if String contains a bad representation of a port identifier.

This BIF is intended for debugging and is not to be used in application programs.

Returns a reference whose text representation is a String, for example:

> list_to_ref("#Ref<0.4192537678.4073193475.71181>").
#Ref<0.4192537678.4073193475.71181>

Failure: badarg if String contains a bad representation of a reference.

This BIF is intended for debugging and is not to be used in application programs.

Returns a tuple corresponding to List, for example

> list_to_tuple([share, ['Ericsson_B', 163]]).
{share, ['Ericsson_B', 163]}

List can contain any Erlang terms.

If Binary contains the object code for module Module, this BIF loads that object code. If the code for module Module already exists, all export references are replaced so they point to the newly loaded code. The previously loaded code is kept in the system as old code, as there can still be processes executing that code.

Returns either {module, Module}, or {error, Reason} if loading fails. Reason is one of the following:

badfile:

The object code in Binary has an incorrect format or the object code contains code for another module than Module.

not_purged:

Binary contains a module that cannot be loaded because old code for this module already exists.

on_load:

The code in Binary contains an on_load declaration that must be executed before Binary can become the current code. Any previous current code for Module will remain until the on_load call has finished.

not_allowed:

The code in Binary has been compiled with features that are currently not enabled in the runtime system.

This BIF is intended for the code server (see code(3)) and is not to be used elsewhere.

Loads and links a dynamic library containing native implemented functions (NIFs) for a module. Path is a file path to the shareable object/dynamic library file minus the OS-dependent file extension (.so for Unix and .dll for Windows). Notice that on most OSs the library has to have a different name on disc when an upgrade of the nif is done. If the name is the same, but the contents differ, the old library may be loaded instead. For information on how to implement a NIF library, see erl_nif(3).

LoadInfo can be any term. It is passed on to the library as part of the initialization. A good practice is to include a module version number to support future code upgrade scenarios.

The call to load_nif/2 must be made directly from the Erlang code of the module that the NIF library belongs to. It returns either ok, or {error,{Reason,Text}} if loading fails. Reason is one of the following atoms while Text is a human readable string that can give more information about the failure:

load_failed:

The OS failed to load the NIF library.

bad_lib:

The library did not fulfill the requirements as a NIF library of the calling module.

load | upgrade:

The corresponding library callback was unsuccessful.

reload:

A NIF library is already loaded for this module instance. The previously deprecated reload feature was removed in OTP 20.

old_code:

The call to load_nif/2 was made from the old code of a module that has been upgraded; this is not allowed.

If the -nifs() attribute is used (which is recommended), all NIFs in the dynamic library much be declared as such for load_nif/2 to succeed. On the other hand, all functions declared with the -nifs() attribute do not have to be implemented by the dynamic library. This allows a target independent Erlang file to contain fallback implementations for functions that may lack NIF support depending on target OS/hardware platform.

Returns a list of all loaded Erlang modules (current and old code), including preloaded modules.

See also code(3).

Returns the current local date and time, {{Year, Month, Day}, {Hour, Minute, Second}}, for example:

> erlang:localtime().
{{1996,11,6},{14,45,17}}

The time zone and Daylight Saving Time correction depend on the underlying OS. The return value is based on the OS System Time.

Converts local date and time to Universal Time Coordinated (UTC), if supported by the underlying OS. Otherwise no conversion is done and Localtime is returned. Example:

> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}).
{{1996,11,6},{13,45,17}}

Failure: badarg if Localtime denotes an invalid date and time.

Converts local date and time to Universal Time Coordinated (UTC) as erlang:localtime_to_universaltime/1, but the caller decides if Daylight Saving Time is active.

If IsDst == true, Localtime is during Daylight Saving Time, if IsDst == false it is not. If IsDst == undefined, the underlying OS can guess, which is the same as calling erlang:localtime_to_universaltime(Localtime).

Examples:

> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, true).
{{1996,11,6},{12,45,17}}
> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, false).
{{1996,11,6},{13,45,17}}
> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, undefined).
{{1996,11,6},{13,45,17}}

Failure: badarg if Localtime denotes an invalid date and time.

Returns a  unique reference. The reference is unique among connected nodes.

Before OTP-23 when a node is restarted multiple times with the same node name, references created on a newer node can be mistaken for a reference created on an older node with the same node name.

Creates a new tuple of the specified Arity, where all elements are InitialValue, for example:

> erlang:make_tuple(4, []).
{[],[],[],[]}

Creates a tuple of size Arity, where each element has value DefaultValue, and then fills in values from InitList. Each list element in InitList must be a two-tuple, where the first element is a position in the newly created tuple and the second element is any term. If a position occurs more than once in the list, the term corresponding to the last occurrence is used. Example:

> erlang:make_tuple(5, [], [{2,ignored},{5,zz},{2,aa}]).
{[],aa,[],[],zz}

Returns value Value associated with Key if Map contains Key.

The call fails with a {badmap,Map} exception if Map is not a map, or with a {badkey,Key} exception if no value is associated with Key.

Example:

> Key = 1337,
  Map = #{42 => value_two,1337 => "value one","a" => 1},
  map_get(Key,Map).
"value one"

Allowed in guard tests.

Returns an integer, which is the number of key-value pairs in Map, for example:

> map_size(#{a=>1, b=>2, c=>3}).
3

Allowed in guard tests.

Tests a match specification used in calls to ets:select/2 and erlang:trace_pattern/3. The function tests both a match specification for "syntactic" correctness and runs the match specification against the object. If the match specification contains errors, the tuple {error, Errors} is returned, where Errors is a list of natural language descriptions of what was wrong with the match specification.

If Type is table, the object to match against is to be a tuple. The function then returns {ok,Result,[],Warnings}, where Result is what would have been the result in a real ets:select/2 call, or false if the match specification does not match the object tuple.

If Type is trace, the object to match against is to be a list. The function returns {ok, Result, Flags, Warnings}, where Result is one of the following:

• true if a trace message is to be emitted
• false if a trace message is not to be emitted
• The message term to be appended to the trace message

Flags is a list containing all the trace flags to be enabled, currently this is only return_trace.

This is a useful debugging and test tool, especially when writing complicated match specifications.

See also ets:test_ms/2.

Returns the largest of Term1 and Term2. If the terms compare equal with the == operator, Term1 is returned.

The  Expressions section contains descriptions of the == operator and how terms are ordered.

Examples:

> max(1, 2).
2

> max(1.0, 1).
1.0

> max(1, 1.0).
1

> max("abc", "b").
"b"

Allowed in guard tests.

Allowed in guards tests from Erlang/OTP 26.

Computes an MD5 message digest from Data, where the length of the digest is 128 bits (16 bytes). Data is a binary or a list of small integers and binaries.

For more information about MD5, see  RFC 1321 - The MD5 Message-Digest Algorithm.

The MD5 Message-Digest Algorithm is not considered safe for code-signing or software-integrity purposes.

Types:

Context = Digest = binary()

Finishes the update of an MD5 Context and returns the computed MD5 message digest.

Types:

Context = binary()

Creates an MD5 context, to be used in the following calls to md5_update/2.

Types:

Context = binary()
Data = iodata()
NewContext = binary()

Update an MD5 Context with Data and returns a NewContext.

Returns a list with information about memory dynamically allocated by the Erlang emulator. Each list element is a tuple {Type, Size}. The first element Type is an atom describing memory type. The second element Size is the memory size in bytes.

Memory types:

total:

The total amount of memory currently allocated. This is the same as the sum of the memory size for processes and system.

processes:

The total amount of memory currently allocated for the Erlang processes.

processes_used:

The total amount of memory currently used by the Erlang processes. This is part of the memory presented as processes memory.

system:

The total amount of memory currently allocated for the emulator that is not directly related to any Erlang process. Memory presented as processes is not included in this memory. instrument(3) can be used to get a more detailed breakdown of what memory is part of this type.

atom:

The total amount of memory currently allocated for atoms. This memory is part of the memory presented as system memory.

atom_used:

The total amount of memory currently used for atoms. This memory is part of the memory presented as atom memory.

binary:

The total amount of memory currently allocated for binaries. This memory is part of the memory presented as system memory.

code:

The total amount of memory currently allocated for Erlang code. This memory is part of the memory presented as system memory.

ets:

The total amount of memory currently allocated for ETS tables. This memory is part of the memory presented as system memory.

maximum:

The maximum total amount of memory allocated since the emulator was started. This tuple is only present when the emulator is run with instrumentation.

For information on how to run the emulator with instrumentation, see instrument(3) and/or erl(1).

The system value is not complete. Some allocated memory that is to be part of this value is not.

When the emulator is run with instrumentation, the system value is more accurate, but memory directly allocated for malloc (and friends) is still not part of the system value. Direct calls to malloc are only done from OS-specific runtime libraries and perhaps from user-implemented Erlang drivers that do not use the memory allocation functions in the driver interface.

As the total value is the sum of processes and system, the error in system propagates to the total value.

The different amounts of memory that are summed are not gathered atomically, which introduces an error in the result.

The different values have the following relation to each other. Values beginning with an uppercase letter is not part of the result.

total      = processes + system
processes  = processes_used + ProcessesNotUsed
system     = atom + binary + code + ets + OtherSystem
atom       = atom_used + AtomNotUsed
RealTotal  = processes + RealSystem
RealSystem = system + MissedSystem

More tuples in the returned list can be added in a future release.

The total value is supposed to be the total amount of memory dynamically allocated by the emulator. Shared libraries, the code of the emulator itself, and the emulator stacks are not supposed to be included. That is, the total value is not supposed to be equal to the total size of all pages mapped to the emulator.

Also, because of fragmentation and prereservation of memory areas, the size of the memory segments containing the dynamically allocated memory blocks can be much larger than the total size of the dynamically allocated memory blocks.

As from ERTS 5.6.4, erlang:memory/0 requires that all erts_alloc(3) allocators are enabled (default behavior).

Failure: notsup if an erts_alloc(3) allocator has been disabled.

Types:

memory_type() = 
    total | processes | processes_used | system | atom |
    atom_used | binary | code | ets

Returns the memory size in bytes allocated for memory of type Type. The argument can also be specified as a list of memory_type() atoms, in which case a corresponding list of {memory_type(), Size :: integer >= 0} tuples is returned.

As from ERTS 5.6.4, erlang:memory/1 requires that all erts_alloc(3) allocators are enabled (default behavior).

Failures:

badarg:

If Type is not one of the memory types listed in the description of erlang:memory/0.

badarg:

If maximum is passed as Type and the emulator is not run in instrumented mode.

notsup:

If an erts_alloc(3) allocator has been disabled.

See also erlang:memory/0.

Returns the smallest of Term1 and Term2. If the terms compare equal with the == operator, Term1 is returned.

The  Expressions section contains descriptions of the == operator and how terms are ordered.

Examples:

> min(1, 2).
1

> min(1.0, 1).
1.0

> min(1, 1.0).
1

> min("abc", "b").
"abc"

Allowed in guard tests.

Allowed in guards tests from Erlang/OTP 26.

Types:

Module = module()

Returns true if the module Module is loaded as current code ; otherwise, false. It does not attempt to load the module.

Sends a monitor request of type Type to the entity identified by Item. If the monitored entity does not exist or it changes monitored state, the caller of monitor/2 is notified by a message on the following format:

{Tag, MonitorRef, Type, Object, Info}

The monitor request is an asynchronous signal. That is, it takes time before the signal reaches its destination.

Type can be one of the following atoms: process, port or time_offset.

A process or port monitor is triggered only once, after that it is removed from both monitoring process and the monitored entity. Monitors are fired when the monitored process or port terminates, does not exist at the moment of creation, or if the connection to it is lost. If the connection to it is lost, we do not know if it still exists. The monitoring is also turned off when demonitor/1 is called.

A process or port monitor by name resolves the RegisteredName to pid() or port() only once at the moment of monitor instantiation, later changes to the name registration will not affect the existing monitor.

When a process or port monitor is triggered, a 'DOWN' message is sent that has the following pattern:

{'DOWN', MonitorRef, Type, Object, Info}

In the monitor message MonitorRef and Type are the same as described earlier, and:

Object:

The monitored entity, which triggered the event. When monitoring a process or a local port, Object will be equal to the pid() or port() that was being monitored. When monitoring process or port by name, Object will have format {RegisteredName, Node} where RegisteredName is the name which has been used with monitor/2 call and Node is local or remote node name (for ports monitored by name, Node is always local node name).

Info:

Either the exit reason of the process, noproc (process or port did not exist at the time of monitor creation), or noconnection (no connection to the node where the monitored process resides).

{'CHANGE', MonitorRef, Type, Item, NewTimeOffset}

Making several calls to monitor/2 for the same Item and/or Type is not an error; it results in as many independent monitoring instances.

The monitor functionality is expected to be extended. That is, other Types and Items are expected to be supported in a future release.

If or when monitor/2 is extended, other possible values for Tag, Object, and Info in the monitor message will be introduced.

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

Provides an option list for modification of monitoring functionality provided by monitor/2. The Type and Item arguments have the same meaning as when passed to monitor/2. Currently available options:

server() ->
    receive
        {request, AliasReqId, Request} ->
            Result = perform_request(Request),
            AliasReqId ! {reply, AliasReqId, Result}
    end,
    server().

client(ServerPid, Request) ->
    AliasMonReqId = monitor(process, ServerPid, [{alias, reply_demonitor}]),
    ServerPid ! {request, AliasMonReqId, Request},
    %% Alias as well as monitor will be automatically deactivated if we
    %% receive a reply or a 'DOWN' message since we used 'reply_demonitor'
    %% as unalias option...
    receive
        {reply, AliasMonReqId, Result} ->
            Result;
        {'DOWN', AliasMonReqId, process, ServerPid, ExitReason} ->
            error(ExitReason)
    end.

server() ->
    receive
        {request, From, ReqId, Request} ->
            Result = perform_request(Request),
            From ! {reply, self(), ReqId, Result}
    end,
    server().

client(ServerPids, Request) when is_list(ServerPids) ->
    ReqId = make_ref(),
    lists:foreach(fun (ServerPid) ->
                          _ = monitor(process, ServerPid,
                                      [{tag, {'DOWN', ReqId}}]),
                          ServerPid ! {request, self(), ReqId, Request}
                  end,
                  ServerPids),
    receive_replies(ReqId, length(ServerPids), []).

receive_replies(_ReqId, 0, Acc) ->
    Acc;
receive_replies(ReqId, N, Acc) ->
    %% The compiler will detect that we match on the 'ReqId'
    %% reference in all clauses, and will enable the selective
    %% receive optimization which makes the receive able to
    %% skip past all messages present in the message queue at
    %% the time when the 'ReqId' reference was created...
    Res = receive
              {reply, ServerPid, ReqId, Result} ->
                  %% Here we typically would have deactivated the
                  %% monitor by a call to demonitor(Mon, [flush]) but
                  %% we ignore this in this example for simplicity...
                  {ok, ServerPid, Result};
              {{'DOWN', ReqId}, _Mon, process, ServerPid, ExitReason} ->
                  {error, ServerPid, ExitReason}
          end,
    receive_replies(ReqId, N-1, [Res | Acc]).

Monitor the status of the node Node. If Flag is true, monitoring is turned on. If Flag is false, monitoring is turned off.

Making several calls to monitor_node(Node, true) for the same Node is not an error; it results in as many independent monitoring instances.

If Node fails or does not exist, the message {nodedown, Node} is delivered to the process. If a process has made two calls to monitor_node(Node, true) and Node terminates, two nodedown messages are delivered to the process. If there is no connection to Node, an attempt is made to create one. If this fails, a nodedown message is delivered.

The delivery of the nodedown signal is not ordered with respect to other link or monitor signals from the node that goes down. If you need a guarantee that all signals from the remote node has been delivered before the nodedown signal is sent, you should use net_kernel:monitor_nodes/1.

Nodes connected through hidden connections can be monitored as any other nodes.

Failure: notalive if the local node is not alive.

Behaves as monitor_node/2 except that it allows an extra option to be specified, namely allow_passive_connect. This option allows the BIF to wait the normal network connection time-out for the monitored node to connect itself, even if it cannot be actively connected from this node (that is, it is blocked). The state where this can be useful can only be achieved by using the Kernel option dist_auto_connect once. If that option is not used, option allow_passive_connect has no effect.

Option allow_passive_connect is used internally and is seldom needed in applications where the network topology and the Kernel options in effect are known in advance.

Failure: badarg if the local node is not alive or the option list is malformed.

Returns the current Erlang monotonic time in native time unit. This is a monotonically increasing time since some unspecified point in time.

This is a  monotonically increasing time, but not a  strictly monotonically increasing time. That is, consecutive calls to erlang:monotonic_time/0 can produce the same result.

Different runtime system instances will use different unspecified points in time as base for their Erlang monotonic clocks. That is, it is pointless comparing monotonic times from different runtime system instances. Different runtime system instances can also place this unspecified point in time different relative runtime system start. It can be placed in the future (time at start is a negative value), the past (time at start is a positive value), or the runtime system start (time at start is zero). The monotonic time at runtime system start can be retrieved by calling erlang:system_info(start_time).

Returns the current Erlang monotonic time converted into the Unit passed as argument.

Same as calling erlang:convert_time_unit(erlang:monotonic_time(), native, Unit), however optimized for commonly used Units.

Types:

Reason = term()

Works exactly like error/1, but Dialyzer thinks that this BIF will return an arbitrary term. When used in a stub function for a NIF to generate an exception when the NIF library is not loaded, Dialyzer does not generate false warnings.

Types:

Reason = term()
Args = [term()]

Works exactly like error/2, but Dialyzer thinks that this BIF will return an arbitrary term. When used in a stub function for a NIF to generate an exception when the NIF library is not loaded, Dialyzer does not generate false warnings.

Returns the name of the local node. If the node is not alive, nonode@nohost is returned instead.

Allowed in guard tests.

Returns the node where Arg originates. Arg can be a process identifier, a reference, or a port. If Arg originates from the local node and the local node is not alive, nonode@nohost is returned.

Allowed in guard tests.

Types:

Nodes = [node()]

Returns a list of all nodes connected to this node through normal connections (that is, hidden nodes are not listed). Same as nodes(visible).

Returns a list of nodes according to the argument specified. The returned result, when the argument is a list, is the list of nodes satisfying the disjunction(s) of the list elements.

NodeTypes:

visible:

Nodes connected to this node through normal connections.

hidden:

Nodes connected to this node through hidden connections.

connected:

All nodes connected to this node.

this:

This node.

known:

Nodes that are known to this node. That is, connected nodes and nodes referred to by process identifiers, port identifiers, and references located on this node. The set of known nodes is garbage collected. Notice that this garbage collection can be delayed. For more information, see erlang:system_info(delayed_node_table_gc).

Some equalities: [node()] = nodes(this), nodes(connected) = nodes([visible, hidden]), and nodes() = nodes(visible).

Returns a list of NodeInfo tuples. The first element is the node name. Nodes to be included in the list are determined by the first argument Arg in the same way as for nodes(Arg). The second element of NodeInfo tuples is a map containing further information about the node identified by the first element. The information present in this map is determined by the InfoOpts map passed as the second argument. Currently the following associations are allowed in the InfoOpts map:

connection_id => boolean():

If the value of the association equals true, the Info map in the returned result will contain the key connection_id associated with the value ConnectionId. If ConnectionId equals undefined, the node is not connected to the node which the caller is executing on, or is the node which the caller is executing on. If ConnectionId is an integer, the node is currently connected to the node which the caller is executing on.

The integer connection identifier value together with a node name identifies a specific connection instance to the node with that node name. The connection identifier value is node local. That is, on the other node the connection identifier will not  be the same value. If a connection is taken down and then taken up again, the connection identifier value will change for the connection to that node. The amount of values for connection identifiers are limited, so it is possible to see the same value for different instances, but quite unlikely. It is undefined how the value change between two consecutive connection instances.

node_type => boolean():

If the value of the association equals true, the Info map in the returned result will contain the key node_type associated with the value NodeTypeInfo. Currently the following node types exist:

visible:

The node is connected to the node of the calling process through an ordinary visible connection. That is, the node name would appear in the result returned by nodes/0.

hidden:

The node is connected to the node of the calling process through a hidden connection. That is, the node name would not  appear in the result returned by nodes/0.

this:

This is the node of the calling process.

known:

The node is not connected but known to the node of the calling process.

Example:

(a@localhost)1> nodes([this, connected], #{connection_id=>true, node_type=>true}). 
[{c@localhost,#{connection_id => 13892108,node_type => hidden}},
 {b@localhost,#{connection_id => 3067553,node_type => visible}},
 {a@localhost,#{connection_id => undefined,node_type => this}}]
(a@localhost)2>

This function is deprecated. Do not use it.

For more information, see section Time and Time Correction in the User's Guide. Specifically, section  Dos and Dont's describes what to use instead of erlang:now/0.

Returns the tuple {MegaSecs, Secs, MicroSecs}, which is the elapsed time since 00:00 GMT, January 1, 1970 (zero hour), if provided by the underlying OS. Otherwise some other point in time is chosen. It is also guaranteed that the following calls to this BIF return continuously increasing values. Hence, the return value from erlang:now/0 can be used to generate unique time stamps. If it is called in a tight loop on a fast machine, the time of the node can become skewed.

Can only be used to check the local time of day if the time-zone information of the underlying OS is properly configured.

Returns a port identifier as the result of opening a new Erlang port. A port can be seen as an external Erlang process.

The name of the executable as well as the arguments specified in cd, env, args, and arg0 are subject to Unicode filename translation if the system is running in Unicode filename mode. To avoid translation or to force, for example UTF-8, supply the executable and/or arguments as a binary in the correct encoding. For details, see the module file(3), the function file:native_name_encoding/0 in Kernel, and the Using Unicode in Erlang User's Guide.

The characters in the name (if specified as a list) can only be > 255 if the Erlang virtual machine is started in Unicode filename translation mode. Otherwise the name of the executable is limited to the ISO Latin-1 character set.

PortNames:

PortSettings is a list of settings for the port. The valid settings are as follows:

{packet, N}:

Messages are preceded by their length, sent in N bytes, with the most significant byte first. The valid values for N are 1, 2, and 4.

stream:

Output messages are sent without packet lengths. A user-defined protocol must be used between the Erlang process and the external object.

{line, L}:

Messages are delivered on a per line basis. Each line (delimited by the OS-dependent newline sequence) is delivered in a single message. The message data format is {Flag, Line}, where Flag is eol or noeol, and Line is the data delivered (without the newline sequence).

L specifies the maximum line length in bytes. Lines longer than this are delivered in more than one message, with Flag set to noeol for all but the last message. If end of file is encountered anywhere else than immediately following a newline sequence, the last line is also delivered with Flag set to noeol. Otherwise lines are delivered with Flag set to eol.
The {packet, N} and {line, L} settings are mutually exclusive.

{cd, Dir}:

Only valid for {spawn, Command} and {spawn_executable, FileName}. The external program starts using Dir as its working directory. Dir must be a string.

{env, Env}:

Types:
Name = os:env_var_name()
Val = os:env_var_value() | false
Env = [{Name, Val}]

Only valid for {spawn, Command}, and {spawn_executable, FileName}. The environment of the started process is extended using the environment specifications in Env.
Env is to be a list of tuples {Name, Val}, where Name is the name of an environment variable, and Val is the value it is to have in the spawned port process. Both Name and Val must be strings. The one exception is Val being the atom false (in analogy with os:getenv/1), which removes the environment variable.
For information about encoding requirements, see documentation of the types for Name and Val.

{args, [ string() | binary() ]}:

Only valid for {spawn_executable, FileName} and specifies arguments to the executable. Each argument is specified as a separate string and (on Unix) eventually ends up as one element each in the argument vector. On other platforms, a similar behavior is mimicked.

The arguments are not expanded by the shell before they are supplied to the executable. Most notably this means that file wildcard expansion does not occur. To expand wildcards for the arguments, use filelib:wildcard/1. Notice that even if the program is a Unix shell script, meaning that the shell ultimately is invoked, wildcard expansion does not occur, and the script is provided with the untouched arguments. On Windows, wildcard expansion is always up to the program itself, therefore this is not an issue.
The executable name (also known as argv[0]) is not to be specified in this list. The proper executable name is automatically used as argv[0], where applicable.
If you explicitly want to set the program name in the argument vector, option arg0 can be used.

{arg0, string() | binary()}:

Only valid for {spawn_executable, FileName} and explicitly specifies the program name argument when running an executable. This can in some circumstances, on some OSs, be desirable. How the program responds to this is highly system-dependent and no specific effect is guaranteed.

exit_status:

Only valid for {spawn, Command}, where Command refers to an external program, and for {spawn_executable, FileName}.

When the external process connected to the port exits, a message of the form {Port,{exit_status,Status}} is sent to the connected process, where Status is the exit status of the external process. If the program aborts on Unix, the same convention is used as the shells do (that is, 128+signal).
If option eof is specified also, the messages eof and exit_status appear in an unspecified order.
If the port program closes its stdout without exiting, option exit_status does not work.

use_stdio:

Only valid for {spawn, Command} and {spawn_executable, FileName}. It allows the standard input and output (file descriptors 0 and 1) of the spawned (Unix) process for communication with Erlang.

nouse_stdio:

The opposite of use_stdio. It uses file descriptors 3 and 4 for communication with Erlang.

stderr_to_stdout:

Affects ports to external programs. The executed program gets its standard error file redirected to its standard output file. stderr_to_stdout and nouse_stdio are mutually exclusive.

overlapped_io:

Affects ports to external programs on Windows only. The standard input and standard output handles of the port program are, if this option is supplied, opened with flag FILE_FLAG_OVERLAPPED, so that the port program can (and must) do overlapped I/O on its standard handles. This is not normally the case for simple port programs, but an option of value for the experienced Windows programmer. On all other platforms, this option is silently discarded.

in:

The port can only be used for input.

out:

The port can only be used for output.

binary:

All I/O from the port is binary data objects as opposed to lists of bytes.

eof:

The port is not closed at the end of the file and does not produce an exit signal. Instead, it remains open and a {Port, eof} message is sent to the process holding the port.

hide:

When running on Windows, suppresses creation of a new console window when spawning the port program. (This option has no effect on other platforms.)

{parallelism, Boolean}:

Sets scheduler hint for port parallelism. If set to true, the virtual machine schedules port tasks; when doing so, it improves parallelism in the system. If set to false, the virtual machine tries to perform port tasks immediately, improving latency at the expense of parallelism. The default can be set at system startup by passing command-line argument +spp to erl(1).

{busy_limits_port, {Low, High} | disabled}:

Sets limits that will be used for controlling the busy state of the port.

When the ports internal output queue size becomes larger than or equal to High bytes, it enters the busy state. When it becomes less than Low bytes it leaves the busy state. When the port is in the busy state, processes sending commands to it will be suspended until the port leaves the busy state. Commands are in this context either Port ! {Owner, {command, Data}} or port_command/[2,3].
The Low limit is automatically adjusted to the same as High if it is set larger then High. Valid range of values for Low and High is [1, (1 bsl (8*erlang:system_info(wordsize)))-2]. If the atom disabled is passed, the port will never enter the busy state.
The defaults are Low = 4096 and High = 8192.
Note that this option is only valid when spawning an executable (port program) by opening the spawn driver and when opening the fd driver. This option will cause a failure with a badarg exception when opening other drivers.

{busy_limits_msgq, {Low, High} | disabled}:

Sets limits that will be used for controlling the busy state of the port message queue.

When the ports message queue size becomes larger than or equal to High bytes it enters the busy state. When it becomes less than Low bytes it leaves the busy state. When the port message queue is in the busy state, processes sending commands to it will be suspended until the port message queue leaves the busy state. Commands are in this context either Port ! {Owner, {command, Data}} or port_command/[2,3].
The Low limit is automatically adjusted to the same as High if it is set larger then High. Valid range of values for Low and High is [1, (1 bsl (8*erlang:system_info(wordsize)))-2]. If the atom disabled is passed, the port message queue will never enter the busy state.
Note that if the driver statically has disabled the use of this feature, a failure with a badarg exception will be raised unless this option also is set to disable or not passed at all.
The defaults are Low = 4096 and High = 8192 unless the driver itself does modifications of these values.
Note that the driver might fail if it also adjust these limits by itself and you have disabled this feature.
The spawn driver (used when spawning an executable) and the fd driver do not disable this feature and do not adjust these limits by themselves.
For more information see the documentation erl_drv_busy_msgq_limits().

Default is stream for all port types and use_stdio for spawned ports.

Failure: if the port cannot be opened, the exit reason is badarg, system_limit, or the POSIX error code that most closely describes the error, or einval if no POSIX code is appropriate:

badarg:

Bad input arguments to open_port.

system_limit:

All available ports in the Erlang emulator are in use.

enomem:

Not enough memory to create the port.

eagain:

No more available OS processes.

enametoolong:

Too long external command.

emfile:

No more available file descriptors (for the OS process that the Erlang emulator runs in).

enfile:

Full file table (for the entire OS).

eacces:

Command specified in {spawn_executable, Command} does not point out an executable file.

enoent:

FileName specified in {spawn_executable, FileName} does not point out an existing file.

During use of a port opened using {spawn, Name}, {spawn_driver, Name}, or {spawn_executable, Name}, errors arising when sending messages to it are reported to the owning process using signals of the form {'EXIT', Port, PosixCode}. For the possible values of PosixCode, see file(3).

The maximum number of ports that can be open at the same time can be configured by passing command-line flag +Q to erl(1).

This function is deprecated as erlang:phash2/2 should be used for new code. Note that erlang:phash(X,N) is not necessary equal to erlang:phash2(X,N)

Portable hash function that gives the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 4.9.1.1). The function returns a hash value for Term within the range 1..Range. The maximum value for Range is 2^32.

Portable hash function that gives the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 5.2). The function returns a hash value for Term within the range 0..Range-1. The maximum value for Range is 2^32. When without argument Range, a value in the range 0..2^27-1 is returned.

This BIF is always to be used for hashing terms. It distributes small integers better than phash/2, and it is faster for bignums and binaries.

Notice that the range 0..Range-1 is different from the range of phash/2, which is 1..Range.

Returns a string corresponding to the text representation of Pid. Example:

> erlang:pid_to_list(self()).
"<0.85.0>"

The creation for the node is not included in the list representation of Pid. This means that processes in different incarnations of a node with a specific name can get the same list representation.

Performs a synchronous call to a port. The meaning of Operation and Data depends on the port, that is, on the port driver. Not all port drivers support this feature.

Port is a port identifier, referring to a driver.

Operation is an integer, which is passed on to the driver.

Data is any Erlang term. This data is converted to binary term format and sent to the port.

Returns a term from the driver. The meaning of the returned data also depends on the port driver.

Failures:

Closes an open port. Roughly the same as Port ! {self(), close} except for the error behavior (see below), being synchronous, and that the port does not reply with {Port, closed}. Any process can close a port with port_close/1, not only the port owner (the connected process). If the calling process is linked to the port identified by Port, the exit signal from the port is guaranteed to be delivered before port_close/1 returns.

For comparison: Port ! {self(), close} only fails with badarg if Port does not refer to a port or a process. If Port is a closed port, nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, closed} when all buffers have been flushed and the port really closes. If the calling process is not the port owner, the port owner fails with badsig.

Notice that any process can close a port using Port ! {PortOwner, close} as if it itself was the port owner, but the reply always goes to the port owner.

As from Erlang/OTP R16, Port ! {PortOwner, close} is truly asynchronous. Notice that this operation has always been documented as an asynchronous operation, while the underlying implementation has been synchronous. port_close/1 is however still fully synchronous because of its error behavior.

Failure: badarg if Port is not an identifier of an open port, or the registered name of an open port. If the calling process was previously linked to the closed port, identified by Port, the exit signal from the port is guaranteed to be delivered before this badarg exception occurs.

Sends data to a port. Same as Port ! {PortOwner, {command, Data}} except for the error behavior and being synchronous (see below). Any process can send data to a port with port_command/2, not only the port owner (the connected process).

For comparison: Port ! {PortOwner, {command, Data}} only fails with badarg if Port does not refer to a port or a process. If Port is a closed port, the data message disappears without a sound. If Port is open and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Data is an invalid I/O list.

Notice that any process can send to a port using Port ! {PortOwner, {command, Data}} as if it itself was the port owner.

If the port is busy, the calling process is suspended until the port is not busy any more.

As from Erlang/OTP R16, Port ! {PortOwner, {command, Data}} is truly asynchronous. Notice that this operation has always been documented as an asynchronous operation, while the underlying implementation has been synchronous. port_command/2 is however still fully synchronous because of its error behavior.

Failures:

badarg:

If Port is not an identifier of an open port, or the registered name of an open port. If the calling process was previously linked to the closed port, identified by Port, the exit signal from the port is guaranteed to be delivered before this badarg exception occurs.

badarg:

If Data is an invalid I/O list.

Do not send data to an unknown port. Any undefined behavior is possible (including node crash) depending on how the port driver interprets the data.

Sends data to a port. port_command(Port, Data, []) equals port_command(Port, Data).

If the port command is aborted, false is returned, otherwise true.

If the port is busy, the calling process is suspended until the port is not busy anymore.

Options:

force:

The calling process is not suspended if the port is busy, instead the port command is forced through. The call fails with a notsup exception if the driver of the port does not support this. For more information, see driver flag ERL_DRV_FLAG_SOFT_BUSY.

nosuspend:

The calling process is not suspended if the port is busy, instead the port command is aborted and false is returned.

More options can be added in a future release.

Failures:

badarg:

If Port is not an identifier of an open port, or the registered name of an open port. If the calling process was previously linked to the closed port, identified by Port, the exit signal from the port is guaranteed to be delivered before this badarg exception occurs.

badarg:

If Data is an invalid I/O list.

badarg:

If OptionList is an invalid option list.

notsup:

If option force has been passed, but the driver of the port does not allow forcing through a busy port.

Do not send data to an unknown port. Any undefined behavior is possible (including node crash) depending on how the port driver interprets the data.

Sets the port owner (the connected port) to Pid. Roughly the same as Port ! {Owner, {connect, Pid}} except for the following:

• The error behavior differs, see below.
• The port does not reply with {Port,connected}.
• port_connect/1 is synchronous, see below.
• The new port owner gets linked to the port.

The old port owner stays linked to the port and must call unlink(Port) if this is not desired. Any process can set the port owner to be any process with port_connect/2.

For comparison: Port ! {self(), {connect, Pid}} only fails with badarg if Port does not refer to a port or a process. If Port is a closed port, nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, connected} to the old port owner. Notice that the old port owner is still linked to the port, while the new is not. If Port is an open port and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Pid is not an existing local process identifier.

Notice that any process can set the port owner using Port ! {PortOwner, {connect, Pid}} as if it itself was the port owner, but the reply always goes to the port owner.

As from Erlang/OTP R16, Port ! {PortOwner, {connect, Pid}} is truly asynchronous. Notice that this operation has always been documented as an asynchronous operation, while the underlying implementation has been synchronous. port_connect/2 is however still fully synchronous because of its error behavior.

Failures:

badarg:

If Port is not an identifier of an open port, or the registered name of an open port. If the calling process was previously linked to the closed port, identified by Port, the exit signal from the port is guaranteed to be delivered before this badarg exception occurs.

badarg:

If the process identified by Pid is not an existing local process.

Performs a synchronous control operation on a port. The meaning of Operation and Data depends on the port, that is, on the port driver. Not all port drivers support this control feature.

Returns a list of integers in the range 0..255, or a binary, depending on the port driver. The meaning of the returned data also depends on the port driver.

Failures:

Returns a list containing tuples with information about Port, or undefined if the port is not open. The order of the tuples is undefined, and all the tuples are not mandatory. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/1 returns undefined.

The result contains information about the following Items:

• registered_name (if the port has a registered name)
• id
• connected
• links
• name
• input
• output

For more information about the different Items, see port_info/2.

Failure: badarg if Port is not a local port identifier, or an atom.

Pid is the process identifier of the process connected to the port.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Index is the internal index of the port. This index can be used to separate ports.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Bytes is the total number of bytes read from the port.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Pids is a list of the process identifiers of the processes that the port is linked to.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Locking is one of the following:

• port_level (port-specific locking)
• driver_level (driver-specific locking)

Notice that these results are highly implementation-specific and can change in a future release.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Bytes is the total number of bytes allocated for this port by the runtime system. The port itself can have allocated memory that is not included in Bytes.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Monitors represent processes monitored by this port.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Returns list of pids that are monitoring given port at the moment.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Name is the command name set by open_port/2.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

OsPid is the process identifier (or equivalent) of an OS process created with open_port({spawn | spawn_executable, Command}, Options). If the port is not the result of spawning an OS process, the value is undefined.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Bytes is the total number of bytes written to the port from Erlang processes using port_command/2, port_command/3, or Port ! {Owner, {command, Data}.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Types:

Port = port() | atom()
Boolean = boolean()

Boolean corresponds to the port parallelism hint used by this port. For more information, see option parallelism of open_port/2.

Bytes is the total number of bytes queued by the port using the ERTS driver queue implementation.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

RegisteredName is the registered name of the port. If the port has no registered name, [] is returned.

If the port identified by Port is not open, undefined is returned. If the port is closed and the calling process was previously linked to the port, the exit signal from the port is guaranteed to be delivered before port_info/2 returns undefined.

Failure: badarg if Port is not a local port identifier, or an atom.

Types:

Port = port()

Returns a string corresponding to the text representation of the port identifier Port.

Returns a list of port identifiers corresponding to all the ports existing on the local node.

Notice that an exiting port exists, but is not open.

Returns a list of Erlang modules that are preloaded in the run-time system. Pre-loaded modules are Erlang modules that are needed to bootstrap the system to load the first Erlang modules from either disk or by using erl_boot_server.

Types:

Pid = pid()
Type = backtrace

Writes information about the local process Pid on standard error. The only allowed value for the atom Type is backtrace, which shows the contents of the call stack, including information about the call chain, with the current function printed first. The format of the output is not further defined.

Enable or disable fully asynchronous distributed signaling  for the calling process. When disabled, which is the default, the process sending a distributed signal will block in the send operation if the buffer for the distribution channel reach the distribution buffer busy limit. The process will remain blocked until the buffer shrinks enough. This might in some cases take a substantial amount of time. When async_dist is enabled, send operations of distributed signals will always buffer the signal on the outgoing distribution channel and then immediately return. That is, these send operations will never block the sending process.

Since no flow control is enforced by the runtime system when async_dist process flag is enabled, you need to make sure that flow control for such data is implemented, or that the amount of such data is known to always be limited. Unlimited signaling with async_dist enabled in the absence of flow control will typically cause the sending runtime system to crash on an out of memory condition.

Blocking due to disabled async_dist can be monitored by erlang:system_monitor() using the busy_dist_port option. Only data buffered by processes which (at the time of sending a signal) have disabled async_dist will be counted when determining whether or not an operation should block the caller.

The async_dist flag can also be set on a new process when spawning it using the spawn_opt() BIF with the option {async_dist, Enable}. The default async_dist flag to use on newly spawned processes can be set by passing the command line argument +pad <boolean> when starting the runtime system. If the +pad <boolean> command line argument is not passed, the default value of the async_dist flag will be false.

You can inspect the state of the async_dist process flag of a process by calling process_info(Pid, async_dist).

Returns the old value of the async_dist flag.

When trap_exit is set to true, exit signals arriving to a process are converted to {'EXIT', From, Reason} messages, which can be received as ordinary messages. If trap_exit is set to false, the process exits if it receives an exit signal other than normal and the exit signal is propagated to its linked processes. Application processes are normally not to trap exits.

Returns the old value of the flag.

See also exit/2.

Used by a process to redefine the error handler for undefined function calls and undefined registered processes. Use this flag with substantial caution, as code auto-loading depends on the correct operation of the error handling module.

Returns the old value of the flag.

Changes the maximum number of generational collections before forcing a fullsweep for the calling process.

Returns the old value of the flag.

Changes the minimum heap size for the calling process.

Returns the old value of the flag.

Changes the minimum binary virtual heap size for the calling process.

Returns the old value of the flag.

This flag sets the maximum heap size for the calling process. If MaxHeapSize is an integer, the system default values for kill and error_logger are used.

For details on how the heap grows, see Sizing the heap in the ERTS internal documentation.

size:

The maximum size in words of the process. If set to zero, the heap size limit is disabled. badarg is be thrown if the value is smaller than min_heap_size. The size check is only done when a garbage collection is triggered.

size is the entire heap of the process when garbage collection is triggered. This includes all generational heaps, the process stack, any  messages that are considered to be part of the heap, and any extra memory that the garbage collector needs during collection.
size is the same as can be retrieved using erlang:process_info(Pid, total_heap_size), or by adding heap_block_size, old_heap_block_size and mbuf_size from erlang:process_info(Pid, garbage_collection_info).

kill:

When set to true, the runtime system sends an untrappable exit signal with reason kill to the process if the maximum heap size is reached. The garbage collection that triggered the kill is not completed, instead the process exits as soon as possible. When set to false, no exit signal is sent to the process, instead it continues executing.

If kill is not defined in the map, the system default will be used. The default system default is true. It can be changed by either option +hmaxk in erl(1), or erlang:system_flag(max_heap_size, MaxHeapSize).

error_logger:

When set to true, the runtime system logs an error event via logger, containing details about the process when the maximum heap size is reached. One log event is sent each time the limit is reached.

If error_logger is not defined in the map, the system default is used. The default system default is true. It can be changed by either the option +hmaxel int erl(1), or erlang:system_flag(max_heap_size, MaxHeapSize).

include_shared_binaries:

When set to true, off-heap binaries are included in the total sum compared against the size limit. Off-heap binaries are typically larger binaries that may be shared between processes. The size of a shared binary is included by all processes that are referring it. Also, the entire size of a large binary may be included even if only a smaller part of it is referred by the process.

If include_shared_binaries is not defined in the map, the system default is used. The default system default is false. It can be changed by either the option +hmaxib in erl(1), or erlang:system_flag(max_heap_size, MaxHeapSize).

The heap size of a process is quite hard to predict, especially the amount of memory that is used during the garbage collection. When contemplating using this option, it is recommended to first run it in production with kill set to false and inspect the log events to see what the normal peak sizes of the processes in the system is and then tune the value accordingly.

Determines how messages in the message queue are stored, as follows:

off_heap:

All messages in the message queue will be stored outside the process heap. This implies that no messages in the message queue will be part of a garbage collection of the process.

on_heap:

All messages in the message queue will eventually be placed on the process heap. They can, however, be temporarily stored off the heap. This is how messages have always been stored up until ERTS 8.0.

The default value of the message_queue_data process flag is determined by the command-line argument +hmqd in erl(1).

If the process may potentially accumulate a large number of messages in its queue it is recommended to set the flag value to off_heap. This is due to the fact that the garbage collection of a process that has a large number of messages stored on the heap can become extremely expensive and the process can consume large amounts of memory. The performance of the actual message passing is, however, generally better when the flag value is on_heap.

Changing the flag value causes any existing messages to be moved. The move operation is initiated, but not necessarily completed, by the time the function returns.

Returns the old value of the flag.

Sets the process priority. Level is an atom. Four priority levels exist: low, normal, high, and max. Default is normal.

Priority level max is reserved for internal use in the Erlang runtime system, and is not to be used by others.

Internally in each priority level, processes are scheduled in a round robin fashion.

Execution of processes on priority normal and low are interleaved. Processes on priority low are selected for execution less frequently than processes on priority normal.

When runnable processes on priority high exist, no processes on priority low or normal are selected for execution. Notice however that this does not mean that no processes on priority low or normal can run when processes are running on priority high. When using multiple schedulers, more processes can be running in parallel than processes on priority high. That is, a low and a high priority process can execute at the same time.

When runnable processes on priority max exist, no processes on priority low, normal, or high are selected for execution. As with priority high, processes on lower priorities can execute in parallel with processes on priority max.

Scheduling is pre-emptive. Regardless of priority, a process is pre-empted when it has consumed more than a certain number of reductions since the last time it was selected for execution.

Do not depend on the scheduling to remain exactly as it is today. Scheduling is likely to be changed in a future release to use available processor cores better.

There is no automatic mechanism for avoiding priority inversion, such as priority inheritance or priority ceilings. When using priorities, take this into account and handle such scenarios by yourself.

Making calls from a high priority process into code that you has no control over can cause the high priority process to wait for a process with lower priority. That is, effectively decreasing the priority of the high priority process during the call. Even if this is not the case with one version of the code that you have no control over, it can be the case in a future version of it. This can, for example, occur if a high priority process triggers code loading, as the code server runs on priority normal.

Other priorities than normal are normally not needed. When other priorities are used, use them with care, especially priority high. A process on priority high is only to perform work for short periods. Busy looping for long periods in a high priority process causes most likely problems, as important OTP servers run on priority normal.

Returns the old value of the flag.

N must be an integer in the interval 0..10000. If N > 0, call saving is made active for the process. This means that information about the N most recent global function calls, BIF calls, sends, and receives made by the process are saved in a list, which can be retrieved with process_info(Pid, last_calls). A global function call is one in which the module of the function is explicitly mentioned. Only a fixed amount of information is saved, as follows:

• A tuple {Module, Function, Arity} for function calls
• The atoms send, 'receive', and timeout for sends and receives ('receive' when a message is received and timeout when a receive times out)

If N = 0, call saving is disabled for the process, which is the default. Whenever the size of the call saving list is set, its contents are reset.

Returns the old value of the flag.

Sets or clears flag sensitive for the current process. When a process has been marked as sensitive by calling process_flag(sensitive, true), features in the runtime system that can be used for examining the data or inner working of the process are silently disabled.

Features that are disabled include (but are not limited to) the following:

• Tracing. Trace flags can still be set for the process, but no trace messages of any kind are generated. (If flag sensitive is turned off, trace messages are again generated if any trace flags are set.)
• Sequential tracing. The sequential trace token is propagated as usual, but no sequential trace messages are generated.

process_info/1,2 cannot be used to read out the message queue or the process dictionary (both are returned as empty lists).

Stack back-traces cannot be displayed for the process.

In crash dumps, the stack, messages, and the process dictionary are omitted.

If {save_calls,N} has been set for the process, no function calls are saved to the call saving list. (The call saving list is not cleared. Also, send, receive, and time-out events are still added to the list.)

Returns the old value of the flag.

Sets certain flags for the process Pid, in the same manner as process_flag/2. Returns the old value of the flag. The valid values for Flag are only a subset of those allowed in process_flag/2, namely save_calls.

Failure: badarg if Pid is not a local process.

Returns a list containing InfoTuples with miscellaneous information about the process identified by Pid, or undefined if the process is not alive.

The order of the InfoTuples is undefined and all InfoTuples are not mandatory. The InfoTuples part of the result can be changed without prior notice.

The InfoTuples with the following items are part of the result:

• current_function
• initial_call
• status
• message_queue_len
• links
• dictionary
• trap_exit
• error_handler
• priority
• group_leader
• total_heap_size
• heap_size
• stack_size
• reductions
• garbage_collection

If the process identified by Pid has a registered name, also an InfoTuple with item registered_name is included.

For information about specific InfoTuples, see process_info/2.

This BIF is intended for debugging only. For all other purposes, use process_info/2.

Failure: badarg if Pid is not a local process.

Returns information about the process identified by Pid, as specified by Item or ItemList. Returns undefined if the process is not alive.

If the process is alive and a single Item is specified, the returned value is the corresponding InfoTuple, unless Item =:= registered_name and the process has no registered name. In this case, [] is returned. This strange behavior is because of historical reasons, and is kept for backward compatibility.

If ItemList is specified, the result is InfoTupleList. The InfoTuples in InfoTupleList are included with the corresponding Items in the same order as the Items were included in ItemList. Valid Items can be included multiple times in ItemList.

Getting process information follows the signal ordering guarantees described in the  Processes Chapter in the Erlang Reference Manual .

If registered_name is part of ItemList and the process has no name registered, a {registered_name, []}, InfoTuple will be included in the resulting InfoTupleList. This behavior is different when a single Item =:= registered_name is specified, and when process_info/1 is used.

Valid InfoTuples with corresponding Items:

{async_dist, Enabled}:

Since: OTP 25.3

Current value of the async_dist process flag.

{backtrace, Bin}:

Binary Bin contains the same information as the output from erlang:process_display(Pid, backtrace). Use binary_to_list/1 to obtain the string of characters from the binary.

{binary, BinInfo}:

BinInfo is a list containing miscellaneous information about binaries on the heap of this process. This InfoTuple can be changed or removed without prior notice. In the current implementation BinInfo is a list of tuples. The tuples contain; BinaryId, BinarySize, BinaryRefcCount.

Depending on the value of the message_queue_data process flag the message queue may be stored on the heap.

{catchlevel, CatchLevel}:

CatchLevel is the number of currently active catches in this process. This InfoTuple can be changed or removed without prior notice.

{current_function, {Module, Function, Arity} | undefined}:

Module, Function, Arity is the current function call of the process. The value undefined can be returned if the process is currently executing native compiled code.

{current_location, {Module, Function, Arity, Location}}:

Module, Function, Arity is the current function call of the process. Location is a list of two-tuples describing the location in the source code.

{current_stacktrace, Stack}:

Returns the current call stack back-trace (stacktrace) of the process. The stack has the same format as in the catch part of a try. See The call-stack back trace (stacktrace). The depth of the stacktrace is truncated according to the backtrace_depth system flag setting.

{dictionary, Dictionary}:

Dictionary is the process dictionary.

{{dictionary, Key}, Value}:

Value associated with Key in the process dictionary.

{error_handler, Module}:

Module is the error handler module used by the process (for undefined function calls, for example).

{garbage_collection, GCInfo}:

GCInfo is a list containing miscellaneous information about garbage collection for this process. The content of GCInfo can be changed without prior notice.

{garbage_collection_info, GCInfo}:

GCInfo is a list containing miscellaneous detailed information about garbage collection for this process. The content of GCInfo can be changed without prior notice. For details about the meaning of each item, see gc_minor_start in erlang:trace/3.

{group_leader, GroupLeader}:

GroupLeader is the group leader for the I/O of the process.

{heap_size, Size}:

Size is the size in words of the youngest heap generation of the process. This generation includes the process stack. This information is highly implementation-dependent, and can change if the implementation changes.

{initial_call, {Module, Function, Arity}}:

Module, Function, Arity is the initial function call with which the process was spawned.

{links, PidsAndPorts}:

PidsAndPorts is a list of process identifiers and port identifiers, with processes or ports to which the process has a link.

{last_calls, false|Calls}:

The value is false if call saving is not active for the process (see process_flag/3). If call saving is active, a list is returned, in which the last element is the most recent called.

{memory, Size}:

Size is the size in bytes of the process. This includes call stack, heap, and internal structures.

{message_queue_len, MessageQueueLen}:

MessageQueueLen is the number of messages currently in the message queue of the process. This is the length of the list MessageQueue returned as the information item messages (see below).

{messages, MessageQueue}:

MessageQueue is a list of the messages to the process, which have not yet been processed.

{min_heap_size, MinHeapSize}:

MinHeapSize is the minimum heap size for the process.

{min_bin_vheap_size, MinBinVHeapSize}:

MinBinVHeapSize is the minimum binary virtual heap size for the process.

{monitored_by, MonitoredBy}:

A list of identifiers for all the processes, ports and NIF resources, that are monitoring the process.

{monitors, Monitors}:

A list of monitors (started by monitor/2) that are active for the process. For a local process monitor or a remote process monitor by a process identifier, the list consists of:

{process, Pid}:

Process is monitored by pid.

{process, {RegName, Node}}:

Local or remote process is monitored by name.

{port, PortId}:

Local port is monitored by port id.

{port, {RegName, Node}}:

Local port is monitored by name. Please note, that remote port monitors are not supported, so Node will always be the local node name.

{message_queue_data, MQD}:

MQD is the current value of the message_queue_data process flag, which can be either off_heap or on_heap. For more information, see the documentation of process_flag(message_queue_data, MQD).

{parent, Pid}:

Pid is the identifier of the parent process, the one that spawned current process. When the process does not have a parent undefined is returned. Only the initial process (init) on a node lacks a parent, though.

{priority, Level}:

Level is the current priority level for the process. For more information on priorities, see process_flag(priority, Level).

{reductions, Number}:

Number is the number of reductions executed by the process.

{registered_name, Atom}:

Atom is the registered process name. If the process has no registered name, this tuple is not present in the list.

{sequential_trace_token, [] | SequentialTraceToken}:

SequentialTraceToken is the sequential trace token for the process. This InfoTuple can be changed or removed without prior notice.

{stack_size, Size}:

Size is the stack size, in words, of the process.

{status, Status}:

Status is the status of the process and is one of the following:

• exiting
• garbage_collecting
• waiting (for a message)
• running
• runnable (ready to run, but another process is running)
• suspended (suspended on a "busy" port or by the BIF erlang:suspend_process/1,2)

{suspending, SuspendeeList}:

SuspendeeList is a list of {Suspendee, ActiveSuspendCount, OutstandingSuspendCount} tuples. Suspendee is the process identifier of a process that has been, or is to be, suspended by the process identified by Pid through the BIF erlang:suspend_process/2 or erlang:suspend_process/1.

ActiveSuspendCount is the number of times Suspendee has been suspended by Pid. OutstandingSuspendCount is the number of not yet completed suspend requests sent by Pid, that is:

• If ActiveSuspendCount =/= 0, Suspendee is currently in the suspended state.
• If OutstandingSuspendCount =/= 0, option asynchronous of erlang:suspend_process/2 has been used and the suspendee has not yet been suspended by Pid.

Notice that ActiveSuspendCount and OutstandingSuspendCount are not the total suspend count on Suspendee, only the parts contributed by Pid.

{total_heap_size, Size}:

Size is the total size, in words, of all heap fragments of the process. This includes the process stack and any unreceived messages that are considered to be part of the heap.

{trace, InternalTraceFlags}:

InternalTraceFlags is an integer representing the internal trace flag for this process. This InfoTuple can be changed or removed without prior notice.

{trap_exit, Boolean}:

Boolean is true if the process is trapping exits, otherwise false.

Notice that not all implementations support all these Items.

Failures:

badarg:

If Pid is not a local process.

badarg:

If Item is an invalid item.

Returns a list of process identifiers corresponding to all the processes currently existing on the local node.

Notice that an exiting process exists, but is not alive. That is, is_process_alive/1 returns false for an exiting process, but its process identifier is part of the result returned from processes/0.

Example:

> processes().
[<0.0.0>,<0.2.0>,<0.4.0>,<0.5.0>,<0.7.0>,<0.8.0>]

Removes old code for Module. Before this BIF is used, check_process_code/2 is to be called to check that no processes execute old code in the module.

This BIF is intended for the code server (see code(3)) and is not to be used elsewhere.

As from ERTS 8.0 (Erlang/OTP 19), any lingering processes that still execute the old code is killed by this function. In earlier versions, such incorrect use could cause much more fatal failures, like emulator crash.

Failure: badarg if there is no old code for Module.

Adds a new Key to the process dictionary, associated with the value Val, and returns undefined. If Key exists, the old value is deleted and replaced by Val, and the function returns the old value. The average time complexity for the current implementation of this function is O(1) and the worst case time complexity is O(N), where N is the number of items in the process dictionary. Example:

> X = put(name, walrus), Y = put(name, carpenter),
Z = get(name),
{X, Y, Z}.
{undefined,walrus,carpenter}

The values stored when put is evaluated within the scope of a catch are not retracted if a throw is evaluated, or if an error occurs.

Raises an exception of the specified class, reason, and call stack backtrace (stacktrace).

Class is error, exit, or throw. So, if it were not for the stacktrace, erlang:raise(Class, Reason, Stacktrace) is equivalent to erlang:Class(Reason) (given that Class is a valid class).

Reason can be any term.

Stacktrace is a list as provided in a try-catch clause.

try
    ...
catch Class:Reason:Stacktrace ->
    ...
end

That is, a list of four-tuples {Module, Function, Arity | Args, ExtraInfo}, where Module and Function are atoms, and the third element is an integer arity or an argument list. The stacktrace can also contain {Fun, Args, ExtraInfo} tuples, where Fun is a local fun and Args is an argument list.

Element ExtraInfo at the end is optional. Omitting it is equivalent to specifying an empty list.

The stacktrace is used as the exception stacktrace for the calling process; it is truncated to the current maximum stacktrace depth.

As evaluating this function causes the process to terminate, it has no return value unless the arguments are invalid, in which case the function returns the error reason badarg. If you want to be sure not to return, you can call error(erlang:raise(Class, Reason, Stacktrace)) and hope to distinguish exceptions later.

See the reference manual about errors and error handling for more information about exception classes and how to catch exceptions.

Types:

TimerRef = reference()
Time = integer() >= 0
Result = Time | false

Reads the state of a timer. The same as calling erlang:read_timer(TimerRef, []).

Reads the state of a timer that has been created by either erlang:start_timer or erlang:send_after. TimerRef identifies the timer, and was returned by the BIF that created the timer.

Options:

{async, Async}:

Asynchronous request for state information. Async defaults to false, which causes the operation to be performed synchronously. In this case, the Result is returned by erlang:read_timer. When Async is true, erlang:read_timer sends an asynchronous request for the state information to the timer service that manages the timer, and then returns ok. A message on the format {read_timer, TimerRef, Result} is sent to the caller of erlang:read_timer when the operation has been processed.

More Options can be added in the future.

If Result is an integer, it represents the time in milliseconds left until the timer expires.

If Result is false, a timer corresponding to TimerRef could not be found. This because the timer had expired, or been canceled, or because TimerRef never has corresponded to a timer. Even if the timer has expired, it does not tell you whether or not the time-out message has arrived at its destination yet.

The timer service that manages the timer can be co-located with another scheduler than the scheduler that the calling process is executing on. If so, communication with the timer service takes much longer time than if it is located locally. If the calling process is in a critical path, and can do other things while waiting for the result of this operation, you want to use option {async, true}. If using option {async, false}, the calling process is blocked until the operation has been performed.

See also erlang:send_after/4, erlang:start_timer/4, and erlang:cancel_timer/2.

Returns a string corresponding to the text representation of Ref.

This BIF is intended for debugging and is not to be used in application programs.

Registers the name RegName with a process identifier (pid) or a port identifier in the name registry. RegName, which must be an atom, can be used instead of the pid or port identifier in send operator (RegName ! Message) and most other BIFs that take a pid or port identifies as an argument. Example:

> register(db, Pid).
true

The registered name is considered a  Directly Visible Erlang Resource and is automatically unregistered when the process terminates.

Failures:

badarg:

If PidOrPort is not an existing local process or port.

badarg:

If RegName is already in use.

badarg:

If the process or port is already registered (already has a name).

badarg:

If RegName is the atom undefined.

Returns a list of names that have been registered using register/2, for example:

> registered().
[code_server, file_server, init, user, my_db]

Decreases the suspend count on the process identified by Suspendee. Suspendee is previously to have been suspended through erlang:suspend_process/2 or erlang:suspend_process/1 by the process calling erlang:resume_process(Suspendee). When the suspend count on Suspendee reaches zero, Suspendee is resumed, that is, its state is changed from suspended into the state it had before it was suspended.

This BIF is intended for debugging only.

Failures:

badarg:

If Suspendee is not a process identifier.

badarg:

If the process calling erlang:resume_process/1 had not previously increased the suspend count on the process identified by Suspendee.

badarg:

If the process identified by Suspendee is not alive.

Returns an integer by rounding Number, for example:

round(42.1).
42

round(5.5).
6

round(-5.5).
-6

round(36028797018963969.0).
36028797018963968

In the last example, round(36028797018963969.0) evaluates to 36028797018963968. The reason for this is that the number 36028797018963969.0 cannot be represented exactly as a float value. Instead, the float literal is represented as 36028797018963968.0, which is the closest number that can be represented exactly as a float value. See Representation of Floating Point Numbers for additional information.

Allowed in guard tests.

Returns the process identifier of the calling process, for example:

> self().
<0.26.0>

Allowed in guard tests.

Sends a message and returns Msg. This is the same as using the  send operator: Dest ! Msg.

Dest can be a remote or local process identifier, an alias, a (local) port, a locally registered name, or a tuple {RegName, Node} for a registered name at another node.

The function fails with a badarg run-time error if Dest is an atom name, but this name is not registered. This is the only case when send fails for an unreachable destination Dest (of correct type).

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

Either sends a message and returns ok, or does not send the message but returns something else (see below). Otherwise the same as erlang:send/2. For more detailed explanation and warnings, see erlang:send_nosuspend/2,3.

Options:

nosuspend:

If the sender would have to be suspended to do the send, nosuspend is returned instead.

noconnect:

If the destination node would have to be auto-connected to do the send, noconnect is returned instead.

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

As with erlang:send_nosuspend/2,3: use with extreme care.

Types:

Time = integer() >= 0
Dest = pid() | atom()
Msg = term()
TimerRef = reference()

Starts a timer. The same as calling erlang:send_after(Time, Dest, Msg, []).

Types:

Time = integer()
Dest = pid() | atom()
Msg = term()
Options = [Option]
Abs = boolean()
Option = {abs, Abs}
TimerRef = reference()

Starts a timer. When the timer expires, the message Msg is sent to the process identified by Dest. Apart from the format of the time-out message, this function works exactly as erlang:start_timer/4.

The same as erlang:send(Dest, Msg, [nosuspend]), but returns true if the message was sent and false if the message was not sent because the sender would have had to be suspended.

This function is intended for send operations to an unreliable remote node without ever blocking the sending (Erlang) process. If the connection to the remote node (usually not a real Erlang node, but a node written in C or Java) is overloaded, this function does not send the message and returns false.

The same occurs if Dest refers to a local port that is busy. For all other destinations (allowed for the ordinary send operator '!'), this function sends the message and returns true.

This function is only to be used in rare circumstances where a process communicates with Erlang nodes that can disappear without any trace, causing the TCP buffers and the drivers queue to be over-full before the node is shut down (because of tick time-outs) by net_kernel. The normal reaction to take when this occurs is some kind of premature shutdown of the other node.

Notice that ignoring the return value from this function would result in an unreliable message passing, which is contradictory to the Erlang programming model. The message is not sent if this function returns false.

In many systems, transient states of overloaded queues are normal. Although this function returns false does not mean that the other node is guaranteed to be non-responsive, it could be a temporary overload. Also, a return value of true does only mean that the message can be sent on the (TCP) channel without blocking; the message is not guaranteed to arrive at the remote node. For a disconnected non-responsive node, the return value is true (mimics the behavior of operator !). The expected behavior and the actions to take when the function returns false are application- and hardware-specific.

Use with extreme care.

The same as erlang:send(Dest, Msg, [nosuspend | Options]), but with a Boolean return value.

This function behaves like erlang:send_nosuspend/2, but takes a third parameter, a list of options. The only option is noconnect, which makes the function return false if the remote node is not currently reachable by the local node. The normal behavior is to try to connect to the node, which can stall the process during a short period. The use of option noconnect makes it possible to be sure not to get the slightest delay when sending to a remote process. This is especially useful when communicating with nodes that expect to always be the connecting part (that is, nodes written in C or Java).

Whenever the function returns false (either when a suspend would occur or when noconnect was specified and the node was not already connected), the message is guaranteed not to have been sent.

Use with extreme care.

Sets the magic cookie of the local node to the atom Cookie, which is also the cookie for all nodes that have no explicit cookie set with set_cookie/2 Cookie (see section  Distributed Erlang in the Erlang Reference Manual in System Documentation).

You can get this value using get_cookie/0.

Failure: function_clause if the local node is not alive.

Sets the magic cookie for Node to the atom Cookie. If Node is the local node, the function sets the cookie of all other nodes (that have no explicit cookie set with this function) to Cookie (see section  Distributed Erlang in the Erlang Reference Manual in System Documentation).

You can get this value using get_cookie/1.

Failure: function_clause if the local node is not alive.

Returns a tuple that is a copy of argument Tuple1 with the element specified by integer argument Index (the first element is the element with index 1) replaced by argument Value, for example:

> setelement(2, {10, green, bottles}, red).
{10,red,bottles}

Returns the number of elements in a tuple or the number of bytes in a binary or bitstring, for example:

> size({morni, mulle, bwange}).
3
> size(<<11, 22, 33>>).
3

For bitstrings, the number of whole bytes is returned. That is, if the number of bits in the bitstring is not divisible by 8, the resulting number of bytes is rounded down.

Allowed in guard tests.

See also tuple_size/1, byte_size/1, and bit_size/1.

Types:

Fun = function()

Returns the process identifier of a new process started by the application of Fun to the empty list []. Otherwise works like spawn/3.

Types:

Node = node()
Fun = function()

Returns the process identifier of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn/3.

Returns the process identifier of a new process started by the application of Module:Function to Args.

error_handler:undefined_function(Module, Function, Args) is evaluated by the new process if Module:Function/Arity does not exist (where Arity is the length of Args). The error handler can be redefined (see process_flag/2). If error_handler is undefined, or the user has redefined the default error_handler and its replacement is undefined, a failure with reason undef occurs.

Example:

> spawn(speed, regulator, [high_speed, thin_cut]).
<0.13.1>

Types:

Node = node()
Module = module()
Function = atom()
Args = [term()]

Returns the process identifier (pid) of a new process started by the application of Module:Function to Args on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn/3.

Types:

Fun = function()

Returns the process identifier of a new process started by the application of Fun to the empty list []. A link is created between the calling process and the new process, atomically. Otherwise works like spawn/3.

Types:

Node = node()
Fun = function()

Returns the process identifier (pid) of a new process started by the application of Fun to the empty list [] on Node. A link is created between the calling process and the new process, atomically. If Node does not exist, a useless pid is returned and an exit signal with reason noconnection is sent to the calling process. Otherwise works like spawn/3.

Types:

Module = module()
Function = atom()
Args = [term()]

Returns the process identifier of a new process started by the application of Module:Function to Args. A link is created between the calling process and the new process, atomically. Otherwise works like spawn/3.

Types:

Node = node()
Module = module()
Function = atom()
Args = [term()]

Returns the process identifier (pid) of a new process started by the application of Module:Function to Args on Node. A link is created between the calling process and the new process, atomically. If Node does not exist, a useless pid is returned and an exit signal with reason noconnection is sent to the calling process. Otherwise works like spawn/3.

Types:

Fun = function()

Returns the process identifier of a new process, started by the application of Fun to the empty list [], and a reference for a monitor created to the new process. Otherwise works like spawn/3.

Returns the process identifier of a new process, started by the application of Fun to the empty list [] on the node Node, and a reference for a monitor created to the new process. Otherwise works like spawn/3.

If the node identified by Node does not support distributed spawn_monitor(), the call will fail with a notsup exception.

Types:

Module = module()
Function = atom()
Args = [term()]

A new process is started by the application of Module:Function to Args. The process is monitored at the same time. Returns the process identifier and a reference for the monitor. Otherwise works like spawn/3.

A new process is started by the application of Module:Function to Args on the node Node. The process is monitored at the same time. Returns the process identifier and a reference for the monitor. Otherwise works like spawn/3.

If the node identified by Node does not support distributed spawn_monitor(), the call will fail with a notsup exception.

Returns the process identifier (pid) of a new process started by the application of Fun to the empty list []. Otherwise works like spawn_opt/4.

If option monitor is specified, the newly created process is monitored, and both the pid and reference for the monitor are returned.

Returns the process identifier (pid) of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

Valid options depends on what options are supported by the node identified by Node. A description of valid Options for the local node of current OTP version can be found in the documentation of spawn_opt/4.

Works as spawn/3, except that an extra option list is specified when creating the process.

If option monitor is specified, the newly created process is monitored, and both the pid and reference for the monitor are returned.

Options:

link:

Sets a link to the parent process (like spawn_link/3 does).

monitor:

Monitors the new process (like monitor(process, Pid) does). A {Pid, MonitorRef} tuple will be returned instead of just a Pid.

{monitor, MonitorOpts}:

Monitors the new process with options (like monitor(process, Pid, MonitorOpts) does). A {Pid, MonitorRef} tuple will be returned instead of just a Pid.

{priority, Level}:

Sets the priority of the new process. Equivalent to executing process_flag(priority, Level) in the start function of the new process, except that the priority is set before the process is selected for execution for the first time. For more information on priorities, see process_flag(priority, Level).

{fullsweep_after, Number}:

Useful only for performance tuning. Do not use this option unless you know that there is problem with execution times or memory consumption, and ensure that the option improves matters.

The Erlang runtime system uses a generational garbage collection scheme, using an "old heap" for data that has survived at least one garbage collection. When there is no more room on the old heap, a fullsweep garbage collection is done.
Option fullsweep_after makes it possible to specify the maximum number of generational collections before forcing a fullsweep, even if there is room on the old heap. Setting the number to zero disables the general collection algorithm, that is, all live data is copied at every garbage collection.
A few cases when it can be useful to change fullsweep_after:

• If binaries that are no longer used are to be thrown away as soon as possible. (Set Number to zero.)
• A process that mostly have short-lived data is fullsweeped seldom or never, that is, the old heap contains mostly garbage. To ensure a fullsweep occasionally, set Number to a suitable value, such as 10 or 20.
• In embedded systems with a limited amount of RAM and no virtual memory, you might want to preserve memory by setting Number to zero. (The value can be set globally, see erlang:system_flag/2.)

{min_heap_size, Size}:

Useful only for performance tuning. Do not use this option unless you know that there is problem with execution times or memory consumption, and ensure that the option improves matters.

Gives a minimum heap size, in words. Setting this value higher than the system default can speed up some processes because less garbage collection is done. However, setting a too high value can waste memory and slow down the system because of worse data locality. Therefore, use this option only for fine-tuning an application and to measure the execution time with various Size values.

{min_bin_vheap_size, VSize}:

Useful only for performance tuning. Do not use this option unless you know that there is problem with execution times or memory consumption, and ensure that the option improves matters.

Gives a minimum binary virtual heap size, in words. Setting this value higher than the system default can speed up some processes because less garbage collection is done. However, setting a too high value can waste memory. Therefore, use this option only for fine-tuning an application and to measure the execution time with various VSize values.

{max_heap_size, Size}:

Sets the max_heap_size process flag. The default max_heap_size is determined by command-line argument +hmax in erl(1). For more information, see the documentation of process_flag(max_heap_size, Size).

{message_queue_data, MQD}:

Sets the value of the message_queue_data process flag. MQD can be either off_heap or on_heap. The default value of the message_queue_data process flag is determined by the command-line argument +hmqd in erl(1). For more information, see the documentation of process_flag(message_queue_data, MQD).

{async_dist, Enabled}:

Since: OTP 25.3

Set the async_dist process flag of the spawned process. This option will override the default value set by the command line argument +pad <boolean>.

Returns the process identifier (pid) of a new process started by the application of Module:Function to Args on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

Valid options depends on what options are supported by the node identified by Node. A description of valid Options for the local node of current OTP version can be found in the documentation of spawn_opt/4.

Types:

Fun = function()
ReqId = reference()

The same as the call spawn_request(node(),Fun,[]). That is, a spawn request on the local node with no options.

Types:

Fun = function()
Option =
   {reply_tag, ReplyTag} | {reply, Reply} | spawn_opt_option()
ReplyTag = term()
Reply = yes | no | error_only | success_only
Options = [Option]
ReqId = reference()

The same as the call spawn_request(node(),Fun,Options). That is, a spawn request on the local node.

Types:

Node = node()
Fun = function()
ReqId = reference()

The same as the call spawn_request(Node,Fun,[]). That is, a spawn request with no options.

The same as spawn_request(Node,erlang,apply,[Fun,[]],Options). That is, a spawn request using the fun Fun of arity zero as entry point.

This function will fail with a badarg exception if:

• Node is not an atom.
• Fun is not a fun of arity zero.
• Options is not a proper list of terms.

Types:

Module = module()
Function = atom()
Args = [term()]
ReqId = reference()

The same as the call spawn_request(node(),Module,Function,Args,[]). That is, a spawn request on the local node with no options.

Types:

Node = node()
Module = module()
Function = atom()
Args = [term()]
ReqId = reference()

The same as the call spawn_request(Node,Module,Function,Args,[]). That is, a spawn request with no options.

Types:

Module = module()
Function = atom()
Args = [term()]
Option =
   {reply_tag, ReplyTag} | {reply, Reply} | spawn_opt_option()
ReplyTag = term()
Reply = yes | no | error_only | success_only
Options = [Option]
ReqId = reference()

The same as the call spawn_request(node(),Module,Function,Args,Options). That is, a spawn request on the local node.

Asynchronously send a spawn request. Returns a request identifier ReqId.

If the spawn operation succeeds, a new process is created on the node identified by Node. When a spawn operation succeeds, the caller will by default be sent a message on the form {ReplyTag, ReqId, ok, Pid} where Pid is the process identifier of the newly created process. Such a message is referred to as a success message  below in the text. ReplyTag is by default the atom spawn_reply unless modified by the {reply_tag, ReplyTag} option. The new process is started by the application of Module:Function to Args.

The spawn operation fails either if creation of a new process failed or if the spawn operation was interrupted by a connection failure. When a spawn operation fails, the caller will by default be sent a message on the form {ReplyTag, ReqId, error, Reason} where Reason is the error reason. Such a message is referred to as an error message  below in the text. Currently the following spawn error Reasons are defined, but other reasons can appear at any time without prior notice:

badopt:

An invalid Option was passed as argument. Note that different runtime systems may support different options.

notsup:

The node identified by Node does not support spawn operations issued by spawn_request().

noconnection:

Failure to set up a connection to the node identified by Node or the connection to that node was lost during the spawn operation. In the case the connection was lost, a process may or may not have been created.

system_limit:

Could not create a new process due to that some system limit was reached. Typically the process table was full.

Valid Options:

monitor:

In the absence of spawn operation failures, atomically sets up a monitor to the newly created process. That is, as if the calling process had called monitor(process, Pid) where Pid is the process identifier of the newly created process. The ReqId returned by spawn_request() is also used as monitor reference as if it was returned from monitor(process, Pid).

The monitor will not be activated for the calling process until the spawn operation has succeeded. The monitor can not be demonitored before the operation has succeeded. A 'DOWN' message for the corresponding monitor is guaranteed not to be delivered before a success message  that corresponds to the spawn operation. If the spawn operation fails, no 'DOWN' message will be delivered.
If the connection between the nodes involved in the spawn operation is lost during the spawn operation, the spawn operation will fail with an error reason of noconnection. A new process may or may not have been created.

{monitor, MonitorOpts}:

In the absence of spawn operation failures, atomically sets up a monitor to the newly created process. That is, as if the calling process had called monitor(process, Pid, MonitorOpts) where Pid is the process identifier of the newly created process. See the monitor option above for more information.

Note that the monitor will not be activated for the calling process until the spawn operation has succeeded. For example, in the case that an alias is created using the monitor option, the alias will not be active until the monitor is activated.

link:

In absence of spawn operation failures, atomically sets up a link between the calling process and the newly created process. That is, as if the calling process had called link(Pid) where Pid is the process identifier of the newly created process.

The link will not be activated for the calling process until the spawn operation has succeeded. The link can not be removed before the operation has succeeded. An exit signal due to the link is guaranteed not to be delivered before a success message  that corresponds to the spawn operation. If the spawn operation fails, no exit signal due to the link will be delivered to the caller of spawn_request().
If the connection between the nodes involved in the spawn operation is lost during the spawn operation, the spawn operation will fail with an error reason of noconnection. A new process may or may not have been created. If it has been created, it will be delivered an exit signal with an exit reason of noconnection.

{reply, Reply}:

Valid Reply values:

yes:

A spawn reply message will be sent to the caller regardless of whether the operation succeeds or not. If the call to spawn_request() returns without raising an exception and the reply option is set to yes, the caller is guaranteed to be delivered either a success message  or an error message . The reply option is by default set to yes.

no:

No spawn reply message will be sent to the caller when the spawn operation completes. This regardless of whether the operation succeeds or not.

error_only:

No spawn reply message will be sent to the caller if the spawn operation succeeds, but an error message  will be sent to the caller if the operation fails.

success_only:

No spawn reply message will be sent to the caller if the spawn operation fails, but a success message  will be sent to the caller if the operation succeeds.

{reply_tag, ReplyTag}:

Sets the reply tag to ReplyTag in the reply message. That is, in the success  or error  message that is sent to the caller due to the spawn operation. The default reply tag is the atom spawn_reply.

OtherOption:

Other valid options depends on what options are supported by the node identified by Node. A description of other valid Options for the local node of current OTP version can be found in the documentation of spawn_opt/4.

If a spawn reply message is delivered, it is guaranteed to be delivered before any other signals from the newly spawned process are delivered to the process issuing the spawn request.

This function will fail with a badarg exception if:

• Node is not an atom.
• Module is not an atom.
• Function is not an atom.
• Args is not a proper list of terms.
• Options is not a proper list of terms.

Note that not all individual Options are checked when the spawn request is sent. Some Options can only be checked on reception of the request. Therefore an invalid option does not cause a badarg exception, but will cause the spawn operation to fail with an error reason of badopt.

A spawn request can be abandoned by calling spawn_request_abandon/1.

For some important information about distributed signals, see the Blocking Signaling Over Distribution  section in the Processes  chapter of the Erlang Reference Manual .

Abandon a previously issued spawn request. ReqId corresponds to a request identifier previously returned by spawn_request() in a call from current process. That is, only the process that has made the request can abandon the request.

A spawn request can only be successfully abandoned until the spawn request has completed. When a spawn request has been successfully abandoned, the caller will not be effected by future direct effects of the spawn request itself. For example, it will not receive a spawn reply message. The request is however not withdrawn, so a new process may or may not be created due to the request. If a new process is created after the spawn request was abandoned, no monitors nor links will be set up to the caller of spawn_request_abandon/1 due to the spawn request. If the spawn request included the link option, the process created due to this request will be sent an exit signal from its parent with the exit reason abandoned when it is detected that the spawn operation has succeeded.

A process created due to a spawn request that has been abandoned may communicate with its parent as any other process. It is only the direct effects on the parent of the actual spawn request, that will be canceled by abandoning a spawn request.

Return values:

true:

The spawn request was successfully abandoned.

false:

No spawn request was abandoned. The ReqId request identifier did not correspond to an outstanding spawn request issued by the calling process. The reason for this is either:

• ReqId corresponds to a spawn request previoulsy made by the calling process. The spawn operation has completed and a spawn reply has already been delivered to the calling process unless the spawn reply was disabled in the request.
• ReqId does not correspond to a spawn request that has been made by the calling process.

This function fail with a badarg exception if ReqId is not a reference.

Returns a tuple containing the binaries that are the result of splitting Bin into two parts at position Pos. This is not a destructive operation. After the operation, there are three binaries altogether. Example:

> B = list_to_binary("0123456789").
<<"0123456789">>
> byte_size(B).
10
> {B1, B2} = split_binary(B,3).
{<<"012">>,<<"3456789">>}
> byte_size(B1).
3
> byte_size(B2).
7

Types:

Time = integer() >= 0
Dest = pid() | atom()
Msg = term()
TimerRef = reference()

Starts a timer. The same as calling erlang:start_timer(Time, Dest, Msg, []).

Starts a timer. When the timer expires, the message {timeout, TimerRef, Msg} is sent to the process identified by Dest.

Options:

{abs, false}:

This is the default. It means the Time value is interpreted as a time in milliseconds relative current Erlang monotonic time.

{abs, true}:

Absolute Time value. The Time value is interpreted as an absolute Erlang monotonic time in milliseconds.

More Options can be added in the future.

The absolute point in time, the timer is set to expire on, must be in the interval [ erlang:convert_time_unit(erlang:system_info(start_time), native, millisecond), erlang:convert_time_unit(erlang:system_info(end_time), native, millisecond) ]. If a relative time is specified, the Time value is not allowed to be negative.

If Dest is a pid(), it must be a pid() of a process created on the current runtime system instance. This process has either terminated or not. If Dest is an atom(), it is interpreted as the name of a locally registered process. The process referred to by the name is looked up at the time of timer expiration. No error is returned if the name does not refer to a process.

If Dest is a pid(), the timer is automatically canceled if the process referred to by the pid() is not alive, or if the process exits. This feature was introduced in ERTS 5.4.11. Notice that timers are not automatically canceled when Dest is an atom().

See also erlang:send_after/4, erlang:cancel_timer/2, and erlang:read_timer/2.

Failure: badarg if the arguments do not satisfy the requirements specified here.

Types:

ActiveTasks = integer() >= 0

Returns the same as statistics(active_tasks_all) with the exception that no information about the dirty IO run queue and its associated schedulers is part of the result. That is, only tasks that are expected to be CPU bound are part of the result.

Returns a list where each element represents the amount of active processes and ports on each run queue and its associated schedulers. That is, the number of processes and ports that are ready to run, or are currently running. Values for normal run queues and their associated schedulers are located first in the resulting list. The first element corresponds to scheduler number 1 and so on. If support for dirty schedulers exist, an element with the value for the dirty CPU run queue and its associated dirty CPU schedulers follow and then as last element the value for the dirty IO run queue and its associated dirty IO schedulers follow. The information is not gathered atomically. That is, the result is not necessarily a consistent snapshot of the state, but instead quite efficiently gathered.

Each normal scheduler has one run queue that it manages. If dirty schedulers are supported, all dirty CPU schedulers share one run queue, and all dirty IO schedulers share one run queue. That is, we have multiple normal run queues, one dirty CPU run queue and one dirty IO run queue. Work can not migrate between the different types of run queues. Only work in normal run queues can migrate to other normal run queues. This has to be taken into account when evaluating the result.

See also statistics(total_active_tasks), statistics(run_queue_lengths), statistics(run_queue_lengths_all), statistics(total_run_queue_lengths), and statistics(total_run_queue_lengths_all).

Types:

ContextSwitches = integer() >= 0

Returns the total number of context switches since the system started.

Returns the number of exact reductions.

statistics(exact_reductions) is a more expensive operation than  statistics(reductions).

Returns information about garbage collection, for example:

> statistics(garbage_collection).
{85,23961,0}

This information can be invalid for some implementations.

Types:

Input = Output = integer() >= 0

Returns Input, which is the total number of bytes received through ports, and Output, which is the total number of bytes output to ports.

Microstate accounting can be used to measure how much time the Erlang runtime system spends doing various tasks. It is designed to be as lightweight as possible, but some overhead exists when this is enabled. Microstate accounting is meant to be a profiling tool to help finding performance bottlenecks. To start/stop/reset microstate accounting, use system flag microstate_accounting.

statistics(microstate_accounting) returns a list of maps representing some of the OS threads within ERTS. Each map contains type and id fields that can be used to identify what thread it is, and also a counters field that contains data about how much time has been spent in the various states.

Example:

> erlang:statistics(microstate_accounting).
[#{counters => #{aux => 1899182914,
                 check_io => 2605863602,
                 emulator => 45731880463,
                 gc => 1512206910,
                 other => 5421338456,
                 port => 221631,
                 sleep => 5150294100},
   id => 1,
   type => scheduler}|...]

The time unit is the same as returned by os:perf_counter/0. So, to convert it to milliseconds, you can do something like this:

lists:map(
  fun(#{ counters := Cnt } = M) ->
          MsCnt = maps:map(fun(_K, PerfCount) ->
                                   erlang:convert_time_unit(PerfCount, perf_counter, 1000)
                           end, Cnt),
         M#{ counters := MsCnt }
  end, erlang:statistics(microstate_accounting)).

Notice that these values are not guaranteed to be the exact time spent in each state. This is because of various optimisation done to keep the overhead as small as possible.

MSAcc_Thread_Types:

scheduler:

The main execution threads that do most of the work. See erl +S for more details.

dirty_cpu_scheduler:

The threads for long running cpu intensive work. See erl +SDcpu for more details.

dirty_io_scheduler:

The threads for long running I/O work. See erl +SDio for more details.

async:

Async threads are used by various linked-in drivers (mainly the file drivers) do offload non-CPU intensive work. See erl +A for more details.

aux:

Takes care of any work that is not specifically assigned to a scheduler.

poll:

Does the IO polling for the emulator. See erl +IOt for more details.

The following MSAcc_Thread_States are available. All states are exclusive, meaning that a thread cannot be in two states at once. So, if you add the numbers of all counters in a thread, you get the total runtime for that thread.

aux:

Time spent handling auxiliary jobs.

check_io:

Time spent checking for new I/O events.

emulator:

Time spent executing Erlang processes.

gc:

Time spent doing garbage collection. When extra states are enabled this is the time spent doing non-fullsweep garbage collections.

other:

Time spent doing unaccounted things.

port:

Time spent executing ports.

sleep:

Time spent sleeping.

More fine-grained MSAcc_Thread_States can be added through configure (such as ./configure --with-microstate-accounting=extra). Enabling these states causes performance degradation when microstate accounting is turned off and increases the overhead when it is turned on.

alloc:

Time spent managing memory. Without extra states this time is spread out over all other states.

bif:

Time spent in BIFs. Without extra states this time is part of the emulator state.

busy_wait:

Time spent busy waiting. This is also the state where a scheduler no longer reports that it is active when using statistics(scheduler_wall_time). So, if you add all other states but this and sleep, and then divide that by all time in the thread, you should get something very similar to the scheduler_wall_time fraction. Without extra states this time is part of the other state.

ets:

Time spent executing ETS BIFs. Without extra states this time is part of the emulator state.

gc_full:

Time spent doing fullsweep garbage collection. Without extra states this time is part of the gc state.

nif:

Time spent in NIFs. Without extra states this time is part of the emulator state.

send:

Time spent sending messages (processes only). Without extra states this time is part of the emulator state.

timers:

Time spent managing timers. Without extra states this time is part of the other state.

The utility module msacc(3) can be used to more easily analyse these statistics.

Returns undefined if system flag microstate_accounting is turned off.

The list of thread information is unsorted and can appear in different order between calls.

The threads and states are subject to change without any prior notice.

Returns information about reductions, for example:

> statistics(reductions).
{2046,11}

As from ERTS 5.5 (Erlang/OTP R11B), this value does not include reductions performed in current time slices of currently scheduled processes. If an exact value is wanted, use statistics(exact_reductions).

Returns the total length of all normal and dirty CPU run queues. That is, queued work that is expected to be CPU bound. The information is gathered atomically. That is, the result is a consistent snapshot of the state, but this operation is much more expensive compared to statistics(total_run_queue_lengths), especially when a large amount of schedulers is used.

Types:

RunQueueLength = integer() >= 0

Returns the same as statistics(run_queue_lengths_all) with the exception that no information about the dirty IO run queue is part of the result. That is, only run queues with work that is expected to be CPU bound is part of the result.

Returns a list where each element represents the amount of processes and ports ready to run for each run queue. Values for normal run queues are located first in the resulting list. The first element corresponds to the normal run queue of scheduler number 1 and so on. If support for dirty schedulers exist, values for the dirty CPU run queue and the dirty IO run queue follow (in that order) at the end. The information is not gathered atomically. That is, the result is not necessarily a consistent snapshot of the state, but instead quite efficiently gathered.

Each normal scheduler has one run queue that it manages. If dirty schedulers are supported, all dirty CPU schedulers share one run queue, and all dirty IO schedulers share one run queue. That is, we have multiple normal run queues, one dirty CPU run queue and one dirty IO run queue. Work can not migrate between the different types of run queues. Only work in normal run queues can migrate to other normal run queues. This has to be taken into account when evaluating the result.

See also statistics(run_queue_lengths), statistics(total_run_queue_lengths_all), statistics(total_run_queue_lengths), statistics(active_tasks), statistics(active_tasks_all), and statistics(total_active_tasks), statistics(total_active_tasks_all).

Returns information about runtime, in milliseconds.

This is the sum of the runtime for all threads in the Erlang runtime system and can therefore be greater than the wall clock time.

This value might wrap due to limitations in the underlying functionality provided by the operating system that is used.

Example:

> statistics(runtime).
{1690,1620}

Returns information describing how much time normal and dirty CPU schedulers in the system have been busy. This value is normally a better indicator of how much load an Erlang node is under instead of looking at the CPU utilization provided by tools such as top or sysstat. This is because scheduler_wall_time also includes time where the scheduler is waiting for some other reasource (such as an internal mutex) to be available but does not use the CPU. In order to better understand what a scheduler is busy doing you can use microstate accounting.

The definition of a busy scheduler is when it is not idle and not busy waiting for new work, that is:

• Executing process code
• Executing linked-in driver or NIF code
• Executing BIFs, or any other runtime handling
• Garbage collecting
• Handling any other memory management

Notice that a scheduler can also be busy even if the OS has scheduled out the scheduler thread.

It is recommended to use the module scheduler instead of this function directly as it provides an easier way to get the information that you usually want.

If enabled this function returns a list of tuples with {SchedulerId, ActiveTime, TotalTime}, where SchedulerId is an integer ID of the scheduler, ActiveTime is the duration the scheduler has been busy, and TotalTime is the total time duration since scheduler_wall_time activation for the specific scheduler. The time unit returned is undefined and can be subject to change between releases, OSs, and system restarts. scheduler_wall_time is only to be used to calculate relative values for scheduler utilization. The ActiveTime can never exceed TotalTime. The list of scheduler information is unsorted and can appear in different order between calls.

The disabled this function returns undefined.

The activation time can differ significantly between schedulers. Currently dirty schedulers are activated at system start while normal schedulers are activated some time after the scheduler_wall_time functionality is enabled.

Only information about schedulers that are expected to handle CPU bound work is included in the return values from this function. If you also want information about dirty I/O schedulers, use statistics(scheduler_wall_time_all) instead.

Normal schedulers will have scheduler identifiers in the range 1 =< SchedulerId =< erlang:system_info(schedulers). Dirty CPU schedulers will have scheduler identifiers in the range erlang:system_info(schedulers) < SchedulerId =< erlang:system_info(schedulers) + erlang:system_info(dirty_cpu_schedulers).

The different types of schedulers handle specific types of jobs. Every job is assigned to a specific scheduler type. Jobs can migrate between different schedulers of the same type, but never between schedulers of different types. This fact has to be taken under consideration when evaluating the result returned.

You can use scheduler_wall_time to calculate scheduler utilization. First you take a sample of the values returned by erlang:statistics(scheduler_wall_time).

> erlang:system_flag(scheduler_wall_time, true).
false
> Ts0 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.
ok

Some time later the user takes another snapshot and calculates scheduler utilization per scheduler, for example:

> Ts1 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.
ok
> lists:map(fun({{I, A0, T0}, {I, A1, T1}}) -> {I, (A1 - A0)/(T1 - T0)} end, lists:zip(Ts0,Ts1)).
[{1,0.9743474730177548},
 {2,0.9744843782751444},
 {3,0.9995902361669045},
 {4,0.9738012596572161},
 {5,0.9717956667018103},
 {6,0.9739235846420741},
 {7,0.973237033077876},
 {8,0.9741297293248656}]

Using the same snapshots to calculate a total scheduler utilization:

> {A, T} = lists:foldl(fun({{_, A0, T0}, {_, A1, T1}}, {Ai,Ti}) -> {Ai + (A1 - A0), Ti + (T1 - T0)} end, {0, 0}, lists:zip(Ts0,Ts1)), TotalSchedulerUtilization = A/T.
0.9769136803764825

Total scheduler utilization will equal 1.0 when all schedulers have been active all the time between the two measurements.

Another (probably more) useful value is to calculate total scheduler utilization weighted against maximum amount of available CPU time:

> WeightedSchedulerUtilization = (TotalSchedulerUtilization * (erlang:system_info(schedulers) + erlang:system_info(dirty_cpu_schedulers))) / erlang:system_info(logical_processors_available).
0.9769136803764825

This weighted scheduler utilization will reach 1.0 when schedulers are active the same amount of time as maximum available CPU time. If more schedulers exist than available logical processors, this value may be greater than 1.0.

As of ERTS version 9.0, the Erlang runtime system will as default have more schedulers than logical processors. This due to the dirty schedulers.

scheduler_wall_time is by default disabled. To enable it, use erlang:system_flag(scheduler_wall_time, true).

The same as statistics(scheduler_wall_time), except that it also include information about all dirty I/O schedulers.

Dirty IO schedulers will have scheduler identifiers in the range erlang:system_info(schedulers) + erlang:system_info(dirty_cpu_schedulers) < SchedulerId =< erlang:system_info(schedulers) + erlang:system_info(dirty_cpu_schedulers) + erlang:system_info(dirty_io_schedulers).

Note that work executing on dirty I/O schedulers are expected to mainly wait for I/O. That is, when you get high scheduler utilization on dirty I/O schedulers, CPU utilization is not expected to be high due to this work.

Types:

ActiveTasks = integer() >= 0

The same as calling lists:sum(statistics(active_tasks)), but more efficient.

Types:

ActiveTasks = integer() >= 0

The same as calling lists:sum(statistics(active_tasks_all)), but more efficient.

Types:

TotalRunQueueLengths = integer() >= 0

The same as calling lists:sum(statistics(run_queue_lengths)), but more efficient.

Types:

TotalRunQueueLengths = integer() >= 0

The same as calling lists:sum(statistics(run_queue_lengths_all)), but more efficient.

Types:

Total_Wallclock_Time = Wallclock_Time_Since_Last_Call = integer() >= 0

Returns information about wall clock. wall_clock can be used in the same manner as runtime, except that real time is measured as opposed to runtime or CPU time.

Suspends the process identified by Suspendee. The same as calling erlang:suspend_process(Suspendee, []).

This BIF is intended for debugging only.

Increases the suspend count on the process identified by Suspendee and puts it in the suspended state if it is not already in that state. A suspended process is not scheduled for execution until the process has been resumed.

A process can be suspended by multiple processes and can be suspended multiple times by a single process. A suspended process does not leave the suspended state until its suspend count reaches zero. The suspend count of Suspendee is decreased when erlang:resume_process(Suspendee) is called by the same process that called erlang:suspend_process(Suspendee). All increased suspend counts on other processes acquired by a process are automatically decreased when the process terminates.

Options (Opts):

asynchronous:

A suspend request is sent to the process identified by Suspendee. Suspendee eventually suspends unless it is resumed before it could suspend. The caller of erlang:suspend_process/2 returns immediately, regardless of whether Suspendee has suspended yet or not. The point in time when Suspendee suspends cannot be deduced from other events in the system. It is only guaranteed that Suspendee eventually suspends (unless it is resumed). If no asynchronous options has been passed, the caller of erlang:suspend_process/2 is blocked until Suspendee has suspended.

{asynchronous, ReplyTag}:

A suspend request is sent to the process identified by Suspendee. When the suspend request has been processed, a reply message is sent to the caller of this function. The reply is on the form {ReplyTag, State} where State is either:

exited:

Suspendee has exited.

suspended:

Suspendee is now suspended.

not_suspended:

Suspendee is not suspended. This can only happen when the process that issued this request, have called resume_process(Suspendee) before getting the reply.

Apart from the reply message, the {asynchronous, ReplyTag} option behaves exactly the same as the asynchronous option without reply tag.

unless_suspending:

The process identified by Suspendee is suspended unless the calling process already is suspending Suspendee. If unless_suspending is combined with option asynchronous, a suspend request is sent unless the calling process already is suspending Suspendee or if a suspend request already has been sent and is in transit. If the calling process already is suspending Suspendee, or if combined with option asynchronous and a send request already is in transit, false is returned and the suspend count on Suspendee remains unchanged.

If the suspend count on the process identified by Suspendee is increased, true is returned, otherwise false.

This BIF is intended for debugging only.

You can easily create deadlocks if processes suspends each other (directly or in circles). In ERTS versions prior to ERTS version 10.0, the runtime system prevented such deadlocks, but this prevention has now been removed due to performance reasons.

Failures:

badarg:

If Suspendee is not a process identifier.

badarg:

If the process identified by Suspendee is the same process as the process calling erlang:suspend_process/2.

badarg:

If the process identified by Suspendee is not alive.

badarg:

If the process identified by Suspendee resides on another node.

badarg:

If OptList is not a proper list of valid Opts.

system_limit:

If the process identified by Suspendee has been suspended more times by the calling process than can be represented by the currently used internal data structures. The system limit is greater than 2,000,000,000 suspends and will never be lower.

Sets the maximum depth of call stack back-traces in the exit reason element of 'EXIT' tuples. The flag also limits the stacktrace depth returned by process_info item current_stacktrace.

Returns the old value of the flag.

This argument is deprecated. Instead of using this argument, use command-line argument +sct in erl(1).

When this argument is removed, a final CPU topology to use is determined at emulator boot time.

Sets the user-defined CpuTopology. The user-defined CPU topology overrides any automatically detected CPU topology. By passing undefined as CpuTopology, the system reverts to the CPU topology automatically detected. The returned value equals the value returned from erlang:system_info(cpu_topology) before the change was made.

Returns the old value of the flag.

The CPU topology is used when binding schedulers to logical processors. If schedulers are already bound when the CPU topology is changed, the schedulers are sent a request to rebind according to the new CPU topology.

The user-defined CPU topology can also be set by passing command-line argument +sct to erl(1).

For information on type CpuTopology and more, see erlang:system_info(cpu_topology) as well as command-line flags +sct and +sbt in erl(1).

Sets the number of dirty CPU schedulers online. Range is 1 <= DirtyCPUSchedulersOnline <= N, where N is the smallest of the return values of erlang:system_info(dirty_cpu_schedulers) and erlang:system_info(schedulers_online).

Returns the old value of the flag.

The number of dirty CPU schedulers online can change if the number of schedulers online changes. For example, if 12 schedulers and 6 dirty CPU schedulers are online, and system_flag/2 is used to set the number of schedulers online to 6, then the number of dirty CPU schedulers online is automatically decreased by half as well, down to 3. Similarly, the number of dirty CPU schedulers online increases proportionally to increases in the number of schedulers online.

For more information, see erlang:system_info(dirty_cpu_schedulers) and erlang:system_info(dirty_cpu_schedulers_online).

Sets system flags for erts_alloc(3). Alloc is the allocator to affect, for example binary_alloc. F is the flag to change and V is the new value.

Only a subset of all erts_alloc flags can be changed at run time. This subset is currently only the flag sbct.

Returns ok if the flag was set or notsup if not supported by erts_alloc.

Sets system flag fullsweep_after. Number is a non-negative integer indicating how many times generational garbage collections can be done without forcing a fullsweep collection. The value applies to new processes, while processes already running are not affected.

Returns the old value of the flag.

In low-memory systems (especially without virtual memory), setting the value to 0 can help to conserve memory.

This value can also be set through (OS) environment variable ERL_FULLSWEEP_AFTER.

Turns on/off microstate accounting measurements. When passing reset, all counters are reset to 0.

For more information see statistics(microstate_accounting).

Sets the default minimum heap size for processes. The size is specified in words. The new min_heap_size effects only processes spawned after the change of min_heap_size has been made. min_heap_size can be set for individual processes by using spawn_opt/4 or process_flag/2.

Returns the old value of the flag.

Sets the default minimum binary virtual heap size for processes. The size is specified in words. The new min_bin_vhheap_size effects only processes spawned after the change of min_bin_vheap_size has been made. min_bin_vheap_size can be set for individual processes by using spawn_opt/2,3,4 or process_flag/2.

Returns the old value of the flag.

Sets the default maximum heap size settings for processes. The size is specified in words. The new max_heap_size effects only processes spawned after the change has been made. max_heap_size can be set for individual processes using spawn_opt/2,3,4 or process_flag/2.

Returns the old value of the flag.

For details on how the heap grows, see Sizing the heap in the ERTS internal documentation.

If multi-scheduling is enabled, more than one scheduler thread is used by the emulator. Multi-scheduling can be blocked in two different ways. Either all schedulers but one is blocked, or all normal schedulers but one is blocked. When only normal schedulers are blocked, dirty schedulers are free to continue to schedule processes.

If BlockState =:= block, multi-scheduling is blocked. That is, one and only one scheduler thread will execute. If BlockState =:= unblock and no one else blocks multi-scheduling, and this process has blocked only once, multi-scheduling is unblocked.

If BlockState =:= block_normal, normal multi-scheduling is blocked. That is, only one normal scheduler thread will execute, but multiple dirty schedulers can execute. If BlockState =:= unblock_normal and no one else blocks normal multi-scheduling, and this process has blocked only once, normal multi-scheduling is unblocked.

One process can block multi-scheduling and normal multi-scheduling multiple times. If a process has blocked multiple times, it must unblock exactly as many times as it has blocked before it has released its multi-scheduling block. If a process that has blocked multi-scheduling or normal multi-scheduling exits, it automatically releases its blocking of multi-scheduling and normal multi-scheduling.

The return values are disabled, blocked, blocked_normal, or enabled. The returned value describes the state just after the call to erlang:system_flag(multi_scheduling, BlockState) has been made. For information about the return values, see erlang:system_info(multi_scheduling).

Blocking of multi-scheduling and normal multi-scheduling is normally not needed. If you feel that you need to use these features, consider it a few more times again. Blocking multi-scheduling is only to be used as a last resort, as it is most likely a very inefficient way to solve the problem.

See also erlang:system_info(multi_scheduling), erlang:system_info(normal_multi_scheduling_blockers), erlang:system_info(multi_scheduling_blockers), and erlang:system_info(schedulers).

Sets a limit on the amount of outstanding requests made by a system process orchestrating system wide changes. Currently there are two such processes:

The Code Purger:

The code purger orchestrates checking of references to old code before old code is removed from the system.

The Literal Area Collector:

The literal area collector orchestrates copying of references from old literal areas before removal of such areas from the system.

Each of these processes are allowed to have as many outstanding requests as this limit is set to. By default this limit is set to twice the amount of schedulers on the system. This will ensure that schedulers will have enough work scheduled to perform these operations as quickly as possible at the same time as other work will be interleaved with this work. Currently used limit can be checked by calling erlang:system_info(outstanding_system_requests_limit).

This limit can also be set by passing the command line argument +zosrl <Limit> to erl.