seek - Change the access position for an open channel
seek channelId offset ?
origin?
Changes the current access position for
channelId.
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.
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 command
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.
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
file(3tcl),
open(3tcl),
close(3tcl),
gets(3tcl),
tell(3tcl),
Tcl_StandardChannels(3tcl)
access position, file, seek