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 particular 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







  

    8.1
    Tcl