Question

So I’m trying to create a signal handler using pthreads which works on both OS X and Linux. The code below works on OS X but doesn’t work on Fedora 13.

The application is fairly simple. It spawns a pthread, registers SIGHUP and waits for a signal. After spawning the signal handler I block SIGHUP in the main thread so the signal should only be sent to the signal_handler thread.

On OS X this works fine, if I compile, run and send SIGHUP to the process it prints “Got SIGHUP”. On Linux it just kills the process (and prints Hangup). If I comment out the signal_handler pthread_create the application doesn’t die.

I know the application gets to the sigwait and blocks but instead of return the signal code it just kills the application.

I ran the test using the following commands:

g++ test.cc -lpthread -o test
./test &
PID="$!"
sleep 1
kill -1 "$PID"

test.cc

#include <pthread.h>
#include <signal.h>
#include <iostream>

using namespace std;

void *signal_handler(void *arg) {
  int sig;
  sigset_t set;

  sigemptyset(&set);
  sigaddset(&set, SIGHUP);

  while (true) {
    cout << "Wait for signal" << endl;
    sigwait(&set, &sig);
    if (sig == SIGHUP) {
      cout << "Got SIGHUP" << endl;
    }
  }
}

int main() {
  pthread_t handler;
  sigset_t set;

  // Create signal handler
  pthread_create(&handler, NULL, signal_handler, NULL);

  // Ignore SIGHUP in main thread
  sigfillset(&set);
  sigaddset(&set, SIGHUP);
  pthread_sigmask(SIG_BLOCK, &set, NULL);

  for (int i = 1; i < 5; i++) {
    cout << "Sleeping..." << endl;
    sleep(1);
  }

  pthread_join(handler, NULL);
  return 0;
}

Answer 1

The POSIX spec for sigwait() says:

The signals defined by set shall have
been blocked at the time of the call
to sigwait(); otherwise, the behavior
is undefined.

You’re not doing this. If you add:

pthread_sigmask(SIG_BLOCK, &set, NULL);

to your signal_handler() function immediately after the sigaddset(), then it works correctly.