seek(3tcl)

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 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
   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 == $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(3tcl),    open(3tcl),    close(3tcl),    gets(3tcl),   tell(3tcl),
   Tcl_StandardChannels(3tcl)

KEYWORDS

   access position, file, seek

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