in reply to File Locking Problem

So don't clobber it first. See open for the scoop on the +< mode.

open HANDLE, "<", $file or die "Can't open $file for reading: $!";
flock HANDLE, LOCK_SH or die "Can't get a shared lock to $file: $!";

# Open the file without clobbering it.
open WRITER, "+<", $file or die "Can't open $file for writing: $!";
flock WRITER, LOCK_EX or die "Can't get an exclusive lock to $file: $!
+";
truncate WRITER, 0 or die "Can't truncate $file: $!";

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Re^2: File Locking Problem
by Anonymous Monk on Jul 28, 2004 at 21:07 UTC
    Just be careful not to assume that $file exists, because if it doesn't you won't be able to open it with +<. You may want to do something like:
    if (-e "$file") {
     open WRITER, "+<$file", $file or die "Can't open: $!";
     flock WRITER, LOCK_EX or die "Can't lock $file: $!";
     truncate WRITER, 0 or die "Can't truncate $file: $!";
} else {
     open WRITER, ">$file" or die "Can't open: $!";
     flock(WRITER, LOCK_EX or die "Can't lock $file: $!";
}

#do whatever....

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[d/l]
      How does the overwriting open prevent stepping on a file if the file is created after the -e test and before the overwrite?
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        It doesn't, but you will at least be able to open a filehandle (you just have to hope that the interval between the two is small enough that you won't run into problems). It's not an ideal solution, certainly, but is there a way to open a read/write handle on a file without clobbering it if it does exist and that won't fail if the file doesn't exist?
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Re^2: File Locking Problem
by Anonymous Monk on Jul 28, 2004 at 20:10 UTC
    *thwack*

    Good call. Should have seen that one. =)
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