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Man Pages
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

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