in reply to Re: (OT?) File Locking
in thread (OT?) File Locking

Eeek! No! That code snippet is incomplete, and will eventually get people into trouble if they're trying to run on multiple platforms.

If you intent to append to a file, you need to handle the small, but very real timing window between opening the file and acquiring the lock. Within that window, a different process can write the the file, leaving you with a bogus seek pointer. Best case, you'll lose the last record(s) written. Worse case, you'll write into the midst of them, leaving corrupted records.

To mitigate the timing window, add the following line 

    use Fcntl ':flock';
    ...
    open FH, ">>$file" or die "Can't open $file: $!";
    flock FH, LOCK_EX; # lock it
+++ seek FH, 0, 2; # in case eof moved

    # ... print() to file here

    flock FH, LOCK_UN;
    close FH;

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Re: Re: Re: (OT?) File Locking
by no_slogan (Deacon) on Apr 14, 2002 at 18:37 UTC
    Under UNIX, a file opened for appending automatically seeks to the end on every write(). No locking should be required, even with multiple processes appending, as long as every write is accomplished with a single system call (you can check that with strace/truss). If you take a look at the apache source, it doesn't lock the log file when it writes a message, and that's certainly a case where many processes are appending to the same file at once.

    In perl, you'll want to set $| on the log file handle, or the output buffers from the different processes will interleave. Or use syswrite.

    I don't know if this is true in Windows.

[reply]
      I don't know if this [automatic seek to eof when writing a file opened for append] is true in Windows.

      It isn't. This is a frequent source of hard-to-track file corruption.

[reply]

        The automatic seek is there, it is just that the seek and subsequent write are not done as an atomic operation so there is a race condition. Looking at the source code, I see that the translation of "\n" to "\r\n" is done between the seek and the write, so the race condition will probably be noticeably worse if you didn't use binmode.

                - tye (but my friends call me "Tye")
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