Question

I currently have a program I have written in C on a server that has an infinite loop that processes information, each loop takes about 5 minutes to complete. I would like to have the following functionality in a shell script:

• Terminate C program
• Make source
• Run program

The problem is, I don’t know how to tell my C program to exit without doing something like ctrl+c, I would rather it finished processing the information it is currently working on before terminating itself.

Answer 1

The POSIX standard way to tell a process to finish its business and exit cleanly is to send it a SIGTERM signal. Depending on your application it may or may not be appropriate to exit on SIGINT, which is meant to interrupt a process, not terminate it. (Control-c sends SIGINT.)

Try putting a flag in your tight loop; check the flag at a time when it is easy to exit, but still frequently enough to exit promptly. In your case, receipt of a SIGTERM might put a message on the system log right away, then promise to exit within the next 5 minutes.

Your signal handler will look like this:

static int signalled;  // if nonzero, what signal have we been sent?

static void SignalHandler(int signum) {
  signalled = signum;
}

I check the global static variable signalled after every I/O operation, which means many times per second.

Here’s my code to catch and restore signals:

static __sighandler_t sh, si, st;

static void catch_signals(void) {
  if ((sh = signal(SIGHUP, SignalHandler)) == SIG_IGN) signal(SIGHUP, SIG_IGN);
  if ((si = signal(SIGINT, SignalHandler)) == SIG_IGN) signal(SIGINT, SIG_IGN);
  if ((st = signal(SIGTERM, SignalHandler)) == SIG_IGN) signal(SIGTERM, SIG_IGN);
  signalled = 0;
}

static void restore_signals(void) {
  signal(SIGHUP, sh);
  signal(SIGINT, si);
  signal(SIGTERM, st);
}

(This code is from a library, so I’m being extra careful to leave things the way I found them.)

Bonus trick: when time expires (this is a TV recording library), the timer just sets signalled = SIGTERM, and the same logic is used to exit the recorder normally.