fblocked - Test whether the last input operation exhausted all available input
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.
http://purl.org/tcl/home/man/tcl8.4/TclCmd/fblocked.htm