tell — Return current access position for an open channel
tell channelId
Returns an integer 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, open, close, gets, seek, Tcl_StandardChannels
access position, channel, seeking
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.