Tcl8.6.14/Tk8.6.14 Documentation > Tcl Commands, version 8.6.14 > fblocked

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fblocked — Test whether the last input operation exhausted all available input


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, open, read, socket, Tcl_StandardChannels


blocking, nonblocking
Copyright © 1996 Sun Microsystems, Inc.