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perlfunc:seek

by gods
on Aug 24, 1999 at 22:43 UTC ( #300=perlfunc: print w/ replies, xml ) Need Help??

seek

See the current Perl documentation for seek.

Here is our local, out-dated (pre-5.6) version:


seek - reposition file pointer for random-access I/O



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 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 from either the IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.

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.

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(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 stdios are particularly cantankerous), then you may need something more like this:

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

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