I want to send a copy of some data to a signal handler function, take the simplified example:
signal(SIGUSR1, handler);
void handler(int sig){
//...
_exit(some_variable);
}
The goal is to stop the process and make it return some_variable value to the parent without making it a global variable. Mind that some_variable is local to the project that catches the signal, not the process that raises the signal.
I woud like to be able to both raise the signal in another process (kill) (not the parent btw) and to be able to do it from a console (kill) or using a script, and that's the main reason for using signal(), as it's good for both.
I understand that there are other ways of doing this but would really like to know if this can be done using signal() or a related method.
Other threads on the site suggest that this can't be done like this but they are quite old, perhaps some newer solution exists.
If the signal is sent from the same process, just use a global variable. There is no basis/justification for objecting to using a global variable here, because signal dispositions are already global variables. That is, by committing yourself to installing a handler for SIGUSR1 and choosing a purpose for it, you've already chosen to have a singleton here; adding a variable as part of that singleton is no further sin. However in this case (same process) there's probably no reason to be using a signal handler anyway. There are all sorts of far-less-dangerous ways to communicate between parts of your program.
If the signal is being sent from a different process, you can pass small amounts of data (one integer) via union sigval queued with sigqueue, but that only works for sigqueue; it doesn't work if you want to be able to just use the kill command. And once you choose to require a special tool to interact, it makes a lot more sense to drop signals and use some better IPC mechanism like unix sockets, named pipes, shared memory with some synchronization primitives (semaphore or mutex+condvar) living inside it, etc.