Using signals between unrelated processes (C)

[sauronnikko]

Hi. I'm having quite a headache trying to communicate two processes which have no relation (one is not the child of the other). I use SIGUSR1 for this purpose. i just want to send one signal from process 1 to process 2 so that this last process prints something in the console. Instead, it prints "User defined signal 1" and exits. I know this is the default behaviour but i'm installing a signal handler which supposedly just prints something. Any help'll be appreciated. Thanks

[soltanis]

Show us the code for the signal handler you're trying to execute, and the relevant section of code where you assign the handler with signal (2).

[sauronnikko]

This is the signal handler function:

void signalHandler (int sig) {
printf ("PRINT\n");
}

This is where I install the signal handler (I do this in main):

signal (SIGUSR1, signalHandler);

And now this is where I try to send a signal to the other process (id is that process id):

kill (id, SIGUSR1);

All of this is written in the process which sends the signal. The other process has nothing related to signals. Of this I am doubting, perhaps it should have something. Anyway, my objective is to have the program which receives the signal print "PRINT" as shown in signalHandler.

[stroyan]

A signal handler only affects the reaction when a signal is received by the process that set up the signal handler. It won't affect the behavior of another process that you are sending a signal to.

Signals have other drawbacks as a means of interprocess communication:

A signal can be deadly to a process that is not ready for it.
Multiple signals will not be queued and delivered if they are received when the process is handling one signal. (Additional signals will either be dropped or may cause a process to exit if you are using "signal" instead of "sigaction" to set up the handler.)
Very few library and system calls are safe to use within a signal handler. Event printf is not safe to call in a handler.


You may be much happier in the long run using a different mechanism such as sockets or message queues. Have a look at http://en.wikipedia.org/wiki/Inter-process_communication for some options.

[sauronnikko]

Thank you. You were right, I needed to set up a signal handler for the process that was receiving the signal. Also, I know that other ways of communications such as pipes are better, but I was curious about how to deal this kind of thing with signals