How to avoid shared memory leaks

Question

I'm using shared memory between 2 processes on Suse Linux and I'm wondering how can I avoid the shared memory leaks in case one process crashes or both. Does a leak occur in this case? If yes, how can I avoid it?

Answer 1

You could allocate space for two counters in the shared memory region: one for each process. Every few seconds, each process increments its counter, and checks that the other counter has been incremented as well. That makes it easy for these two processes, or an external watchdog, to tear down the shared memory if somebody crashes or exits.

Answer 2

• If the subprocess is a simple fork() from the parent process, then mmap() with MAP_SHARED should work.

• If the subprocess does an exec() to start a different executable, you can often pass file descriptors from shm_open() or a similar non-portable system call (see Is there anything like shm_open() without filename?) On many operating systems, including Linux, you can shm_unlink() the file descriptor from shm_open() so it doesn't leak memory when your processes die, and use fcntl() to clear the close-on-exec flag on the shm file descriptor so that your child process can inherit it across exec. This is not well defined in the POSIX standard but it appears to be very portable in practice.

• If you need to use a filename instead of just a file descriptor number to pass the shared memory object to an unrelated process, then you have to figure out some way to shm_unlink() the file yourself when it's no longer needed; see John Zwinck's answer for one method.