Question

I’ve done a bit of digging, and the closest I’ve come to direct answer to my question is here.

I’ve created a mutex then spin into a loop where I listen for activity on a TCP/IP port, fork, respond, then exit from the child. Part of the ‘respond’ requires that the child process access hardware autonomously, that is protected by a mutex.

If I initialize a mutex before creating child processes, will all the potentially concurrently running children honor the mutex?

Answer 1

Not by default.

You have to store the mutex in a shared memory block. You can get one with SysV IPC or with the MAP_SHARED flag to mmap().

You also have to set the attributes on the mutex properly. You need to make a mutex attribute with pthread_mutexattr_init() and call pthread_mutexattr_setpshared(), then pass that attribute to pthread_mutex_init().

If you do not use shared memory, the child processes will not be able to see the mutex state because each will be using a private copy. If you do not set the pshared attribute the mutex will probably crash the program because it will not expect other processes to be changing the data.