seek(n) Tcl Built-In Commands seek(n)
NAME
seek - Change the access position for an open channel
SYNOPSIS
seek channelId offset ?origin?
DESCRIPTION
Changes the current access position for channelId.
ChannelId must be an identifier for an open channel such as a Tcl stan-
dard channel (stdin, stdout, or stderr), the return value from an invo-
cation of open or socket, or the result of a channel creation command
provided by a Tcl extension.
The offset and origin arguments specify the position at which the next
read or write will occur for channelId. Offset must be an integer
(which may be negative) and origin must be one of the following:
start The new access position will be offset bytes from the start
of the underlying file or device.
current The new access position will be offset bytes from the current
access position; a negative offset moves the access position
backwards in the underlying file or device.
end The new access position will be offset bytes from the end of
the file or device. A negative offset places the access
position before the end of file, and a positive offset places
the access position after the end of file.
The origin argument defaults to start.
The command flushes all buffered output for the channel before the com-
mand returns, even if the channel is in non-blocking mode. It also
discards any buffered and unread input. This command returns an empty
string. An error occurs if this command is applied to channels whose
underlying file or device does not support seeking.
Note that offset values are byte offsets, not character offsets. Both
seek and tell operate in terms of bytes, not characters, unlike read.
EXAMPLES
Read a file twice:
set f [open file.txt] set data1 [read $f] seek $f 0 set data2 [read $f]
close $f # $data1 eq $data2 if the file wasn't updated
Read the last 10 bytes from a file:
set f [open file.data] # This is guaranteed to work with binary data
but # may fail with other encodings... fconfigure $f -translation
binary seek $f -10 end set data [read $f 10] close $f
SEE ALSO
file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)
KEYWORDS
access position, file, seek
Tcl 8.1 seek(n)
tcl 8.6.0 - Generated Fri Jan 11 15:10:42 CST 2013