seek

seek(n)                      Tcl Built-In Commands                     seek(n)



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NAME
       seek - Change the access position for an open channel

SYNOPSIS
       seek channelId offset ?origin?
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DESCRIPTION
       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.

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)