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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 nega-tive negative
                 tive 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 chan-nel channel
       nel 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(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)


KEYWORDS
       access position, file, seek



Tcl                                                  8.1                                             seek(n)