[ltt-dev] UST use case: Tracing QEMU/KVM

[Stefan Hajnoczi]

/*
 * This example demonstrates the UST listener thread receiving signals and
 * returning from poll().
 *
 * $ gcc -o poll_eintr poll_eintr.c -lust -lpthread
 * $ ./poll_eintr
 * main thread id=0x7f9c31a766a0
 * ^CSIGINT handled in thread id=0x7f9c30ed7910
 * libust[2615/2616]: Error: poll: Interrupted system call (in multipoll_poll() at ../libustcomm/multipoll.c:84)
 * libust[2615/2616]: Error: error in multipoll_poll (in listener_main() at tracectl.c:1015)
 *
 * This happens because all other threads have SIGINT blocked, whereas libust
 * does not block signals in the client thread.  Therefore the signal is
 * handled in the client thread, poll() is interrupted, and an error is
 * printed.
 *
 * I think the correct behavior is to block signals in the client thread so it
 * will not interfere with signal handling of the main program.
 */

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

static void sigint_handler(int signo)
{
	fprintf(stderr, "SIGINT handled in thread id=0x%lx\n", pthread_self());
}

int main(int argc, char **argv)
{
	sigset_t newmask;

	fprintf(stderr, "main thread id=0x%lx\n", pthread_self());

	/* Block SIGINT in this thread */
	sigemptyset(&newmask);
	sigaddset(&newmask, SIGINT);
	pthread_sigmask(SIG_SETMASK, &newmask, NULL);

	/* Set up a handler for SIGINT */
	signal(SIGINT, sigint_handler);

	/* Now wait for signals, the UST listener thread will receive it */
	for (;;) {
		pause();
		fprintf(stderr, "pause returned in main thread\n");
	}
	return 0;
}