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tell(3tcl) Tcl Built-In Commands tell(3tcl)


NAME

tell - Return current access position for an open channel

SYNOPSIS

tell channelId

DESCRIPTION

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.

EXAMPLE

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
}

SEE ALSO

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

KEYWORDS

access position, channel, seeking
8.1 Tcl