fblocked - Man Page

fblocked channelId

The fblocked command returns 1 if the most recent input operation on channelId returned less information than requested because all available input was exhausted. For example, if gets is invoked when there are only three characters available for input and no end-of-line sequence, gets returns an empty string and a subsequent call to fblocked will return 1.

ChannelId must be an identifier for an open channel such as a Tcl standard channel (stdin, stdout, or stderr), the return value from an invocation of open or socket, or the result of a channel creation command provided by a Tcl extension.

The fblocked command is particularly useful when writing network servers, as it allows you to write your code in a line-by-line style without preventing the servicing of other connections.  This can be seen in this simple echo-service:

# This is called whenever a new client connects to the server proc connect {chan host port} {
   set clientName [format <%s:%d> $host $port]
   puts "connection from $clientName"
   fconfigure $chan -blocking 0 -buffering line
   fileevent $chan readable [list echoLine $chan $clientName] }

# This is called whenever either at least one byte of input # data is available, or the channel was closed by the client. proc echoLine {chan clientName} {
   gets $chan line
   if {[eof $chan]} {
       puts "finishing connection from $clientName"
       close $chan
   } elseif {![fblocked $chan]} {
       # Didn't block waiting for end-of-line
       puts "$clientName - $line"
       puts $chan $line
   } }

# Create the server socket and enter the event-loop to wait # for incoming connections... socket -server connect 12345 vwait forever

gets(n), open(n), read(n), socket(n), Tcl_StandardChannels(3)

blocking, nonblocking

chan(n), eof(n), fcopy(n), file(n), gets(n), read(n).