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man pages section 1: User Commands

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Updated: Thursday, June 13, 2019

seek (1t)


seek - Change the access position for an open channel


seek channelId offset ?origin?


seek(1t)                     Tcl Built-In Commands                    seek(1t)


       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 stan-
       dard channel (stdin, stdout, or stderr), the return value from an invo-
       cation  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 com-
       mand  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

       See attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |
       file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)

       access position, file, seek

       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   8.1                             seek(1t)