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# seek

Perl 5 version 26.0 documentation
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# seek

Perl functions A-Z | Perl functions by category | The 'perlfunc' manpage

- seek FILEHANDLE,POSITION,WHENCE

  Sets FILEHANDLE's position, just like the fseek(3) call of C `stdio` . FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are `0` to set the new position in bytes to POSITION; `1` to set it to the current position plus POSITION; and `2` to set it to EOF plus POSITION, typically negative. For WHENCE you may use the constants `SEEK_SET` , `SEEK_CUR` , and `SEEK_END` (start of the file, current position, end of the file) from the Fcntl module. Returns `1` on success, false otherwise.

  Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the `:encoding(UTF-8)` I/O layer), the seek, tell, and sysseek family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file.

  If you want to position the file for sysread or syswrite, don't use seek, because buffering makes its effect on the file's read-write position unpredictable and non-portable. Use sysseek instead.

  Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of `1` (`SEEK_CUR` ) is useful for not moving the file position:

          seek($fh, 0, 1);

  This is also useful for applications emulating `tail -f` . Once you hit EOF on your read and then sleep for a while, you (probably) have to stick in a dummy seek to reset things. The seek doesn't change the position, but it does clear the end-of-file condition on the handle, so that the next `readline FILE` makes Perl try again to read something. (We hope.)

  If that doesn't work (some I/O implementations are particularly cantankerous), you might need something like this:

          for (;;) {        for ($curpos = tell($fh); $_ = readline($fh);             $curpos = tell($fh)) {            # search for some stuff and put it into files        }        sleep($for_a_while);        seek($fh, $curpos, 0);    }