sys_socket.h - Man Page

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

#include <sys/socket.h>

The <sys/socket.h> header shall define the socklen_t type, which is an integer type of width of at least 32 bits; see Application Usage.

The <sys/socket.h> header shall define the sa_family_t unsigned integer type.

The <sys/socket.h> header shall define the sockaddr structure, which shall include at least the following members:

sa_family_t  sa_family  Address family.
char         sa_data[]  Socket address (variable-length data).

The sockaddr structure is used to define a socket address which is used in the bind(), connect(), getpeername(), getsockname(), recvfrom(), and sendto() functions.

The <sys/socket.h> header shall define the sockaddr_storage structure, which shall be:

*

Large enough to accommodate all supported protocol-specific address structures

*

Aligned at an appropriate boundary so that pointers to it can be cast as pointers to protocol-specific address structures and used to access the fields of those structures without alignment problems

The sockaddr_storage structure shall include at least the following members:

sa_family_t   ss_family

When a pointer to a sockaddr_storage structure is cast as a pointer to a sockaddr structure, the ss_family field of the sockaddr_storage structure shall map onto the sa_family field of the sockaddr structure. When a pointer to a sockaddr_storage structure is cast as a pointer to a protocol-specific address structure, the ss_family field shall map onto a field of that structure that is of type sa_family_t and that identifies the protocol's address family.

The <sys/socket.h> header shall define the msghdr structure, which shall include at least the following members:

void          *msg_name        Optional address.
socklen_t      msg_namelen     Size of address.
struct iovec  *msg_iov         Scatter/gather array.
int            msg_iovlen      Members in msg_iov.
void          *msg_control     Ancillary data; see below.
socklen_t      msg_controllen  Ancillary data buffer len.
int            msg_flags       Flags on received message.

The msghdr structure is used to minimize the number of directly supplied parameters to the recvmsg() and sendmsg() functions. This structure is used as a value-result parameter in the recvmsg() function and value only for the sendmsg() function.

The <sys/socket.h> header shall define the iovec structure as described in <sys/uio.h>.

The <sys/socket.h> header shall define the cmsghdr structure, which shall include at least the following members:

socklen_t  cmsg_len    Data byte count, including the cmsghdr.
int        cmsg_level  Originating protocol.
int        cmsg_type   Protocol-specific type.

The cmsghdr structure is used for storage of ancillary data object information.

Ancillary data consists of a sequence of pairs, each consisting of a cmsghdr structure followed by a data array. The data array contains the ancillary data message, and the cmsghdr structure contains descriptive information that allows an application to correctly parse the data.

The values for cmsg_level shall be legal values for the level argument to the getsockopt() and setsockopt() functions. The system documentation shall specify the cmsg_type definitions for the supported protocols.

Ancillary data is also possible at the socket level. The <sys/socket.h> header shall define the following symbolic constant for use as the cmsg_type value when cmsg_level is SOL_SOCKET:

SCM_RIGHTS

Indicates that the data array contains the access rights to be sent or received.

The <sys/socket.h> header shall define the following macros to gain access to the data arrays in the ancillary data associated with a message header:

CMSG_DATA(cmsg)

If the argument is a pointer to a cmsghdr structure, this macro shall return an unsigned character pointer to the data array associated with the cmsghdr structure.

CMSG_NXTHDR(mhdr,cmsg)

If the first argument is a pointer to a msghdr structure and the second argument is a pointer to a cmsghdr structure in the ancillary data pointed to by the msg_control field of that msghdr structure, this macro shall return a pointer to the next cmsghdr structure, or a null pointer if this structure is the last cmsghdr in the ancillary data.

CMSG_FIRSTHDR(mhdr)

If the argument is a pointer to a msghdr structure, this macro shall return a pointer to the first cmsghdr structure in the ancillary data associated with this msghdr structure, or a null pointer if there is no ancillary data associated with the msghdr structure.

The <sys/socket.h> header shall define the linger structure, which shall include at least the following members:

int  l_onoff   Indicates whether linger option is enabled.
int  l_linger  Linger time, in seconds.

The <sys/socket.h> header shall define the following symbolic constants with distinct values:

SOCK_DGRAM

Datagram socket.

SOCK_RAW

Raw Protocol Interface.

SOCK_SEQPACKET

Sequenced-packet socket.

SOCK_STREAM

Byte-stream socket.

The <sys/socket.h> header shall define the following symbolic constant for use as the level argument of setsockopt() and getsockopt().

SOL_SOCKET

Options to be accessed at socket level, not protocol level.

The <sys/socket.h> header shall define the following symbolic constants with distinct values for use as the option_name argument in getsockopt() or setsockopt() calls (see the System Interfaces volume of POSIX.1-2017, Section 2.10.16, Use of Options):

SO_ACCEPTCONN

Socket is accepting connections.

SO_BROADCAST

Transmission of broadcast messages is supported.

SO_DEBUG

Debugging information is being recorded.

SO_DONTROUTE

Bypass normal routing.

SO_ERROR

Socket error status.

SO_KEEPALIVE

Connections are kept alive with periodic messages.

SO_LINGER

Socket lingers on close.

SO_OOBINLINE

Out-of-band data is transmitted in line.

SO_RCVBUF

Receive buffer size.

SO_RCVLOWAT

Receive “low water mark”.

SO_RCVTIMEO

Receive timeout.

SO_REUSEADDR

Reuse of local addresses is supported.

SO_SNDBUF

Send buffer size.

SO_SNDLOWAT

Send “low water mark”.

SO_SNDTIMEO

Send timeout.

SO_TYPE

Socket type.

The <sys/socket.h> header shall define the following symbolic constant for use as the maximum backlog queue length which may be specified by the backlog field of the listen() function:

SOMAXCONN

The maximum backlog queue length.

The <sys/socket.h> header shall define the following symbolic constants with distinct values for use as the valid values for the msg_flags field in the msghdr structure, or the flags parameter in recv(), recvfrom(), recvmsg(), send(), sendmsg(), or sendto() calls:

MSG_CTRUNC

Control data truncated.

MSG_DONTROUTE

Send without using routing tables.

MSG_EOR

Terminates a record (if supported by the protocol).

MSG_OOB

Out-of-band data.

MSG_NOSIGNAL

No SIGPIPE generated when an attempt to send is made on a stream-oriented socket that is no longer connected.

MSG_PEEK

Leave received data in queue.

MSG_TRUNC

Normal data truncated.

MSG_WAITALL

Attempt to fill the read buffer.

The <sys/socket.h> header shall define the following symbolic constants with distinct values:

AF_INET

Internet domain sockets for use with IPv4 addresses.

AF_INET6

Internet domain sockets for use with IPv6 addresses.

AF_UNIX

UNIX domain sockets.

AF_UNSPEC

Unspecified.

The value of AF_UNSPEC shall be 0.

The <sys/socket.h> header shall define the following symbolic constants with distinct values:

SHUT_RD

Disables further receive operations.

SHUT_RDWR

Disables further send and receive operations.

SHUT_WR

Disables further send operations.

The <sys/socket.h> header shall define the size_t and ssize_t types as described in <sys/types.h>.

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

int     accept(int, struct sockaddr *restrict, socklen_t *restrict);
int     bind(int, const struct sockaddr *, socklen_t);
int     connect(int, const struct sockaddr *, socklen_t);
int     getpeername(int, struct sockaddr *restrict, socklen_t *restrict);
int     getsockname(int, struct sockaddr *restrict, socklen_t *restrict);
int     getsockopt(int, int, int, void *restrict, socklen_t *restrict);
int     listen(int, int);
ssize_t recv(int, void *, size_t, int);
ssize_t recvfrom(int, void *restrict, size_t, int,
        struct sockaddr *restrict, socklen_t *restrict);
ssize_t recvmsg(int, struct msghdr *, int);
ssize_t send(int, const void *, size_t, int);
ssize_t sendmsg(int, const struct msghdr *, int);
ssize_t sendto(int, const void *, size_t, int, const struct sockaddr *,
        socklen_t);
int     setsockopt(int, int, int, const void *, socklen_t);
int     shutdown(int, int);
int     sockatmark(int);
int     socket(int, int, int);
int     socketpair(int, int, int, int [2]);

Inclusion of <sys/socket.h> may also make visible all symbols from <sys/uio.h>.

The following sections are informative.

To forestall portability problems, it is recommended that applications not use values larger than 2³¹ -1 for the socklen_t type.

The sockaddr_storage structure solves the problem of declaring storage for automatic variables which is both large enough and aligned enough for storing the socket address data structure of any family. For example, code with a file descriptor and without the context of the address family can pass a pointer to a variable of this type, where a pointer to a socket address structure is expected in calls such as getpeername(), and determine the address family by accessing the received content after the call.

The example below illustrates a data structure which aligns on a 64-bit boundary. An implementation-defined field _ss_align following _ss_pad1 is used to force a 64-bit alignment which covers proper alignment good enough for needs of at least sockaddr_in6 (IPv6) and sockaddr_in (IPv4) address data structures. The size of padding field _ss_pad1 depends on the chosen alignment boundary. The size of padding field _ss_pad2 depends on the value of overall size chosen for the total size of the structure. This size and alignment are represented in the above example by implementation-defined (not required) constants _SS_MAXSIZE (chosen value 128) and _SS_ALIGNMENT (with chosen value 8). Constants _SS_PAD1SIZE (derived value 6) and _SS_PAD2SIZE (derived value 112) are also for illustration and not required. The implementation-defined definitions and structure field names above start with an <underscore> to denote implementation private name space. Portable code is not expected to access or reference those fields or constants.

/*
 *  Desired design of maximum size and alignment.
 */
#define _SS_MAXSIZE 128
    /* Implementation-defined maximum size. */
#define _SS_ALIGNSIZE (sizeof(int64_t))
    /* Implementation-defined desired alignment. */

/*
 *  Definitions used for sockaddr_storage structure paddings design.
 */
#define _SS_PAD1SIZE (_SS_ALIGNSIZE - sizeof(sa_family_t))
#define _SS_PAD2SIZE (_SS_MAXSIZE - (sizeof(sa_family_t)+ \
                      _SS_PAD1SIZE + _SS_ALIGNSIZE))
struct sockaddr_storage {
    sa_family_t  ss_family;  /* Address family. */
/*
 *  Following fields are implementation-defined.
 */
    char _ss_pad1[_SS_PAD1SIZE];
        /* 6-byte pad; this is to make implementation-defined
           pad up to alignment field that follows explicit in
           the data structure. */
    int64_t _ss_align;  /* Field to force desired structure
                           storage alignment. */
    char _ss_pad2[_SS_PAD2SIZE];
        /* 112-byte pad to achieve desired size,
           _SS_MAXSIZE value minus size of ss_family
           __ss_pad1, __ss_align fields is 112. */
};

None.

None.

<sys_types.h>, <sys_uio.h>

The System Interfaces volume of POSIX.1-2017, accept(), bind(), connect(), getpeername(), getsockname(), getsockopt(), listen(), recv(), recvfrom(), recvmsg(), send(), sendmsg(), sendto(), setsockopt(), shutdown(), sockatmark(), socket(), socketpair()

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

accept(3p), bind(3p), connect(3p), freeaddrinfo(3p), getnameinfo(3p), getpeername(3p), getsockname(3p), getsockopt(3p), listen(3p), netdb.h(0p), netinet_in.h(0p), netinet_tcp.h(0p), recv(3p), recvfrom(3p), recvmsg(3p), send(3p), sendmsg(3p), sendto(3p), setsockopt(3p), shutdown(3p), sockatmark(3p), socket(3p), socketpair(3p), sys_un.h(0p), unistd.h(0p).