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seek

Perl 5 version 8.9 documentation
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seek

  • seek FILEHANDLE,POSITION,WHENCE

    Sets FILEHANDLE's position, just like the fseek call of 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 upon success, 0 otherwise.

    Note the in bytes: even if the filehandle has been set to operate on characters (for example by using the :encoding(utf8) open layer), tell() will return byte offsets, not character offsets (because implementing that would render seek() and tell() rather slow).

    If you want to position file for sysread or syswrite, don't use seek--buffering makes its effect on the file's system 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:

    1. seek(TEST,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 might have to stick in a seek() to reset things. The seek doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <FILE> makes Perl try again to read something. We hope.

    If that doesn't work (some IO implementations are particularly cantankerous), then you may need something more like this:

    1. for (;;) {
    2. for ($curpos = tell(FILE); $_ = <FILE>;
    3. $curpos = tell(FILE)) {
    4. # search for some stuff and put it into files
    5. }
    6. sleep($for_a_while);
    7. seek(FILE, $curpos, 0);
    8. }