cond.c

/*

In one terminal:

$ ./cond
pid is 20274
Running busy loop thread
Waiting on condition!
pthread_cond_wait returned 0
Got past condition, handled signal: 1

In another terminal:

$ kill -USR1 20274

*/

#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>

static int handledSignal = 0;
static pthread_t busyLoopThread;
static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

static void *busyLoop(void *ctx) {
	static pthread_mutex_t sleepLock = PTHREAD_MUTEX_INITIALIZER;

	printf("Running busy loop thread\n");
	
	// deadlock myself
	pthread_mutex_lock(&sleepLock);
	pthread_mutex_lock(&sleepLock);

	return NULL;
}

static void handleSignal(int signal) {
	handledSignal = 1;
}

int main() {
	printf("pid is %d\n", getpid());

	struct sigaction sa = {};
	sa.sa_handler = &handleSignal;
	sa.sa_flags = SA_RESTART;
	sigfillset(&sa.sa_mask);
	if (sigaction(SIGUSR1, &sa, NULL) == -1) {
		perror("sigaction");
	}

	// copied from StartWebThread, unlikely to be relevant
	pthread_attr_t tattr;
	pthread_attr_init(&tattr);
	pthread_attr_setscope(&tattr, PTHREAD_SCOPE_SYSTEM);
	pthread_attr_setstacksize(&tattr, 200 * 4096);

	pthread_create(&busyLoopThread, &tattr, busyLoop, NULL);
	pthread_attr_destroy(&tattr);

	pthread_mutex_lock(&lock);
	printf("Waiting on condition!\n");
	int result = pthread_cond_wait(&cond, &lock);
	if (result != 0) {
		perror("pthread_cond_wait");
	} else {
		printf("pthread_cond_wait returned 0\n");
	}
	pthread_mutex_unlock(&lock);

	printf("Got past condition, handled signal: %d\n", handledSignal);

	return 0;
}