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Locking...

  For more locking info please see C/locking.hints

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Per File Locking...

WARNING: the file must be opened for write or append for write locks to
work correctly!

This is especially the case for Solaris 9 where shared locks are the same as
exclusive locks, AND the file MUST be writable even if you never write to
it! If it isn't writable you get a "Bad file number" Error.

Example code....

#!/usr/bin/perl
#
# Using flock(), this uses the systems broadest locking mechnism available
use Fcntl qw{:DEFAULT :flock};

sub lock {
  local(*FILE) = @_;
  flock(*FILE, LOCK_EX | LOCK_NB ) or do {
    warn "$prog: File is locked -- continuing lock attempt...\n";
    flock(*FILE, $lock_types{$type} ) or
      die ("$prog: Flock failure: $!\n");
  };
}
sub unlock {  # not really needed if file closes
  flock(*FILE, LOCK_UN);
}

# ---- Example Use ----
(my $prog = $0) =~ s/^.*\///;
system("touch /tmp/t");

print "opening...\n";
open(TEST, ">>/tmp/t") || die "cant open file \"/tmp/t\" for write";
print "locking...\n";
&lock(TEST);                 # lock file for writing


print "locked\n";
sleep(10);                        # sleep for 10 seconds


print "unlocking...\n";
&unlock(TEST);                 # unlock file
print "closing...\n";
close(TEST);

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Lock File Locking (via file creation)

Using a standard lock file (non-root)
See also file locking in shell/file.hints and C/C.hints
Particularly for the more standard lock file methods
NOTE: this is not atomic in NFS v2 but is in NFS v3

Perl 5

  use Fcntl;  # get the open flags

  sub lock($) {  # create a file lock
    my ($file) = @_;

    until ( sysopen( LOCK, $file, O_RDONLY|O_CREAT|O_EXCL, 0444 ) ) {
      sleep 1;
    }
  }

See the Perl4 sysopen() subroutine replacement in "perl_file.hints"


Alturnativally you can use the simple procmail lockfile program to do all
the hard work...

  system('/opt/bin/lockfile' $lockfile);

  ...

  unlink $lockfile;


What is atomic is rename or linking a file!
That is create a unique file for your process and try to 
rename()  or  link() it to the lockfile name.

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