Provided by: tcl8.6-doc_8.6.10+dfsg-1_all bug


       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

       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]
              # 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