seek - Change the access position for an open channel
seek channelId offset
Changes the current access position for channelId
must be an identifier for an open channel such as a Tcl
standard channel ( stdin
, or stderr
), the return
value from an invocation of open
, or the result of a
channel creation command provided by a Tcl extension.
arguments specify the position at which the
next read or write will occur for channelId
must be an
integer (which may be negative) and origin
must be one of the
- The new access position will be offset bytes from the start of the
underlying file or device.
- 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.
- 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.
argument defaults to start
The command flushes all buffered output for the channel before the command
returns, even if the channel is in nonblocking 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
operate in terms of bytes, not characters, unlike
Read a file twice:
set f [open file.txt]
set data1 [read $f]
seek $f 0
set data2 [read $f]
# $data1 == $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]
file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)
access position, file, seek