'\"
'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
'\" RCS: @(#) $Id: tell.n 29250 2007-11-06 21:35:32Z erikgreenwald $
'\" 
.so man.macros
.TH tell n 8.1 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
tell \- Return current access position for an open channel
.SH SYNOPSIS
\fBtell \fIchannelId\fR
.BE

.SH DESCRIPTION
.PP
Returns an integer string giving the current access position in
\fIchannelId\fR.  This value returned is a byte offset that can be passed to
\fBseek\fR in order to set the channel to a particular position.  Note
that this value is in terms of bytes, not characters like \fBread\fR.
The value returned is -1 for channels that do not support
seeking.
.PP
\fIChannelId\fR must be an identifier for an open channel such as a
Tcl standard channel (\fBstdin\fR, \fBstdout\fR, or \fBstderr\fR),
the return value from an invocation of \fBopen\fR or \fBsocket\fR, or
the result of a channel creation command provided by a Tcl extension.
.SH EXAMPLE
Read a line from a file channel only if it starts with \fBfoobar\fR:
.CS
# Save the offset in case we need to undo the read...
set offset [\fBtell\fR $chan]
if {[read $chan 6] eq "foobar"} {
    gets $chan line
} else {
    set line {}
    # Undo the read...
    seek $chan $offset
}
.CE

.SH "SEE ALSO"
file(n), open(n), close(n), gets(n), seek(n), Tcl_StandardChannels(3)

.SH KEYWORDS
access position, channel, seeking
