pthread_sigmask() not work in multithreaded program

Question

I'm a newbie in c development. Recently, I noticed a problem when I was learning multi-threaded development, when I set a signal in the main thread of Action and when I try to block the signal action set by the main thread in the child thread, I find that it does not work.

Here is a brief description of the code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <pthread.h>
#include <unistd.h>
#include <signal.h>

void *thread_start(void *_arg) {
  sleep(2);

  sigset_t mask;
  sigemptyset(&mask);
  sigaddset(&mask, SIGUSR2);
  pthread_sigmask(SIG_BLOCK, &mask, NULL);

  printf("child-thread executed\n");

  while (true) {
    sleep(1);
  }

  return NULL;
}

void sig_handler(int _sig) {
  printf("executed\n");
}

int main(int argc, char *argv[]) {
  pthread_t t_id;
  int s = pthread_create(&t_id, NULL, thread_start, NULL);
  if (s != 0) {
    char *msg = strerror(s);
    printf("%s\n", msg);
  }

  printf("main-thread executed, create [%lu]\n", t_id);

  signal(SIGUSR2, sig_handler);

  while (true) {
    sleep(1);
  }

  return EXIT_SUCCESS;
}

Answer 1

As I wrote in a comment, any USR1 signal sent to the process will be delivered using the main thread. It's output will not tell you exactly what happened, so it is not really a good way to test threads and signal masks. Additionally, it uses printf() in a signal handler, which may or may not work: printf() is not an async-signal safe function, so it must not be used in a signal handler.

Here is a better example:

#define _POSIX_C_SOURCE 200809L
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <pthread.h>
#include <limits.h>
#include <string.h>
#include <errno.h>
#include <stdio.h>

/* This function writes a message directly to standard error,
   without using the stderr stream.  This is async-signal safe.
   Returns 0 if success, errno error code if an error occurs.
   errno is kept unchanged. */
static int write_stderr(const char *msg)
{
    const char *end = msg;
    const int   saved_errno = errno;
    int         retval = 0;
    ssize_t     n;

    /* If msg is non-NULL, find the string-terminating '\0'. */
    if (msg)
        while (*end)
            end++;

    /* Write the message to standard error. */
    while (msg < end) {
        n = write(STDERR_FILENO, msg, (size_t)(end - msg));
        if (n > 0) {
            msg += n;
        } else
        if (n != 0) {
            /* Bug, should not occur */
            retval = EIO;
            break;
        } else
        if (errno != EINTR) {
            retval = errno;
            break;
        }
    }

    /* Paranoid check that exactly the message was written */
    if (!retval)
        if (msg != end)
            retval = EIO;

    errno = saved_errno;
    return retval;
}

static volatile sig_atomic_t  done = 0;
pthread_t                     main_thread;
pthread_t                     other_thread;

static void signal_handler(int signum)
{
    const pthread_t  id = pthread_self();
    const char *thread = (id == main_thread) ? "Main thread" :
                         (id == other_thread) ? "Other thread" : "Unknown thread";
    const char *event = (signum == SIGHUP) ? "HUP" :
                        (signum == SIGUSR1) ? "USR1" :
                        (signum == SIGINT) ? "INT" :
                        (signum == SIGTERM) ? "TERM" : "Unknown signal";

    if (signum == SIGTERM || signum == SIGINT)
        done = 1;

    write_stderr(thread);
    write_stderr(": ");
    write_stderr(event);
    write_stderr(".\n");
}

static int install_handler(int signum)
{
    struct sigaction  act;
    memset(&act, 0, sizeof act);
    sigemptyset(&act.sa_mask);
    act.sa_handler = signal_handler;
    act.sa_flags = 0;
    if (sigaction(signum, &act, NULL) == -1)
        return -1;
    return 0;
}

void *other(void *unused __attribute__((unused)))
{
    sigset_t  mask;

    sigemptyset(&mask);
    sigaddset(&mask, SIGTERM);
    sigaddset(&mask, SIGHUP);
    pthread_sigmask(SIG_BLOCK, &mask, NULL);

    while (!done)
        sleep(1);

    return NULL;
}

int main(void)
{
    pthread_attr_t  attrs;
    sigset_t        mask;
    int             result;

    main_thread = pthread_self();
    other_thread = pthread_self(); /* Just to initialize it to a sane value */

    /* Install HUP, USR1, INT, and TERM signal handlers. */
    if (install_handler(SIGHUP) ||
        install_handler(SIGUSR1) ||
        install_handler(SIGINT) ||
        install_handler(SIGTERM)) {
        fprintf(stderr, "Cannot install signal handlers: %s.\n", strerror(errno));
        return EXIT_FAILURE;
    }

    /* Create the other thread. */
    pthread_attr_init(&attrs);
    pthread_attr_setstacksize(&attrs, 2*PTHREAD_STACK_MIN);
    result = pthread_create(&other_thread, &attrs, other, NULL);
    pthread_attr_destroy(&attrs);
    if (result) {
        fprintf(stderr, "Cannot create a thread: %s.\n", strerror(result));
        return EXIT_FAILURE;
    }

    /* This thread blocks SIGUSR1. */
    sigemptyset(&mask);
    sigaddset(&mask, SIGUSR1);
    pthread_sigmask(SIG_BLOCK, &mask, NULL);

    /* Ready to handle signals. */
    printf("Send a HUP, USR1, or TERM signal to process %d.\n", (int)getpid());
    fflush(stdout);

    while (!done)
        sleep(1);

    pthread_join(other_thread, NULL);

    return EXIT_SUCCESS;
}

Save it as e.g. example.c, and compile and run using

gcc -Wall -O2 example.c -pthread -o exprog
./exprog

It will block the USR1 signal in the main thread, and HUP and TERM in the other thread. It will also catch the INT signal (Ctrl+C), which is not blocked in either thread. When you send it the INT or TERM signal, the program will exit.

If you send the program the USR1 signal, you'll see that it will always be delivered using the other thread.

If you send the program a HUP signal, you'll see that it will always be delivered using the main thread.

If you send the program a TERM signal, it too will be delivered using the main thread, but it will also cause the program to exit (nicely).

If you send the program an INT signal, it will be delivered using one of the threads. It depends on several factors whether you'll always see it being delivered using the same thread or not, but at least in theory, it can be delivered using either thread. This signal too will cause the program to exit (nicely).

Answer 2

The signal mask is a per-thread property, a thread will inherit whatever the parent has at time of thread creation but, after that, it controls its own copy.

In other words, blocking a signal in a thread only affects the delivery of signals for that thread, not for any other.

In any case, even if it were shared (it's not), you would have a potential race condition since you start the child thread before setting up the signal in the main thread. Hence it would be indeterminate as to whether the order was "parent sets up signal, then child blocks" or vice versa. But, as stated, that's irrelevant due to the thread-specific nature of the signal mask.

If you want a thread to control the signal mask of another thread, you will need to use some form of inter-thread communication to let the other thread do it itself.