Programming Perl, 4th Edition

Handling Timing Glitches

Sometimes your program’s behavior is exquisitely sensitive to the timing of external events beyond your control. This is always a concern when other programs, particularly inimical ones, might be vying with your program for the same resources (such as files or devices). In a multitasking environment, you cannot predict the order in which processes waiting to run will be granted access to the processor. Instruction streams among all eligible processes are interleaved, so first one process gets some CPU, and then another process, and so on. Whose turn it is to run, and how long they’re allowed to run, appears to be random. With just one program that’s not a problem. However, it can be a problem when several programs share common resources.

Thread programmers are especially sensitive to these issues. They quickly learn not to say:

$var++ if $var == 0;

when they should say:

{
    lock($var);
    $var++ if $var == 0;
}

The former produces unpredictable results when multiple execution threads attempt to run this code at the same time. If you think of files as shared objects, and processes as threads contending for access to those shared objects, you can see how the same issues arise. A process, after all, is really just a thread with an attitude. Or vice versa.

Timing unpredictabilities affect both privileged and nonprivileged situations. We’ll first describe how to cope with a long-standing bug in old Unix kernels that affects any set-id program. Then we’ll move on to discuss ...