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SuSE 11.3 - man page for tell (suse section n)

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tell(n) 			      Tcl Built-In Commands				  tell(n)


       tell - Return current access position for an open channel

       tell channelId

       Returns	an  integer  string  giving the current access position in channelId.  This value
       returned is a byte offset that can be passed to seek in order to set the channel to a par-
       ticular	position.   Note  that this value is in terms of bytes, not characters like read.
       The value returned is -1 for channels that do not support seeking.

       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.

       Read a line from a file channel only if it starts with foobar:
	      # Save the offset in case we need to undo the read...
	      set offset [tell $chan]
	      if {[read $chan 6] eq "foobar"} {
		  gets $chan line
	      } else {
		  set line {}
		  # Undo the read...
		  seek $chan $offset

       file(n), open(n), close(n), gets(n), seek(n), Tcl_StandardChannels(3)

       access position, channel, seeking

Tcl					       8.1					  tell(n)
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