Question

people. For a academic exercise i have to implement a program in c for nix platform which is to synchronize multiple processes through signal handling, using only signal,pause,kill and fork basic functions.
I searched google and have not found any clear example: I hope the wisdom of one of you will light my way.
Thanks!

Answer 1

pause doesn’t return until a signal is received. The basic design is thus:

• fork to create the necessary workers
• catch SIGINT in each worker. The handler sets a flag meaning the process should exit after finishing it’s current job.
• each process does work while it can, then pauses. Repeat unless SIGINT is received (test before & after pause).
• when one process has work available for another process, it signals the other process with SIGCONT
• when there is no more work for a process, signal it with SIGINT.

This doesn’t quite include synchronized access to shared data. To do that, you could add an additional rule:

• when a process signals another that work is available, it should pause

Of course, this rather defeats the purpose of concurrent programming.

Since most system calls are interrupted by signals (causing them to return -1, with errno set to EINTR), you’ll have to handle this contingency, repeating each affected system call until it’s successful. For example:

while ((readCount = read(...)) < 0 && errno == EINTR) {}