Sending data in a signal
Craig said on sending additional data or context in a UNIX signal:
"[...] One thing that isn’t as well known is besides sending a signal to a process, you can send some data to it. This can either be an integer or a pointer and uses similar semantics to the known kill and signal handler. [...]"
The key is using sigqueue() instead of signal() and a signal handler like:
void signal_handler(int signum, siginfo_t *siginfo, void *ucontext)
{
if (signum != SIGUSR1) return;
if (siginfo->si_code != SI_QUEUE) return;
printf("receiver: Got value %d\n", siginfo->si_int);
}