sigwait() and signal handler

Question

If I setup and signal handler for SIGABRT and meanwhile I have a thread that waits on sigwait() for SIGABRT to come (I have a blocked SIGABRT in other threads by pthread_sigmask).

So which one will be processed first ? Signal handler or sigwait() ?

[I am facing some issues that sigwait() is get blocked for ever. I am debugging it currently]

main()
{
    sigset_t                    signal_set;

    sigemptyset(&signal_set);
    sigaddset(&signal_set, SIGABRT); 
    sigprocmask(SIG_BLOCK, &signal_set, NULL); 

    // Dont deliver SIGABORT while running this thread and it's kids.
    pthread_sigmask(SIG_BLOCK, &signal_set, NULL);

    pthread_create(&tAbortWaitThread, NULL, WaitForAbortThread, NULL);
    ..
    Create all other threads
    ...
}   

static void*    WaitForAbortThread(void* v)
{
    sigset_t signal_set;
    int stat;
    int sig;

    sigfillset( &signal_set);
    pthread_sigmask( SIG_BLOCK, &signal_set, NULL ); // Dont want any signals


    sigemptyset(&signal_set);
    sigaddset(&signal_set, SIGABRT);     // Add only SIGABRT

    // This thread while executing , will handle the SIGABORT signal via signal handler.
    pthread_sigmask(SIG_UNBLOCK, &signal_set, NULL); 
    stat= sigwait( &signal_set, &sig  ); // lets wait for signal handled in CatchAbort().
    while (stat == -1)
    {
        stat= sigwait( &signal_set, &sig  );
    }

    TellAllThreadsWeAreGoingDown();

    sleep(10);

    return null;
}

// Abort signal handler executed via sigaction().
static void CatchAbort(int i, siginfo_t* info, void* v)
{
    sleep(20); // Dont return , hold on till the other threads are down.
}

Here at sigwait(), i will come to know that SIGABRT is received. I will tell other threads about it. Then will hold abort signal handler so that process is not terminated.

I wanted to know the interaction of sigwait() and the signal handler.

Answer 1

I run serveral tests and the conbinations and results are:

For all test cases, I register a signal handler by calling sigaction in the main thread.

1. main thread block target signal, thread A unblock target signal by calling pthread_sigmask, thread A sleep, send target signal.

   result: signal handler is executed in thread A.

2. main thread block target signal, thread A unblock target signal by calling pthread_sigmask, thread A calls sigwait, send target signal.

   result: sigwait is executed.

3. main thread does not block target signal, thread A does not block target signal, thread A calls sigwait, send target signal.

   result: main thread is chosen and the registered signal handler is executed in the main thread.

---

As you can see, conbination 1 and 2 are easy to understand and conclude.

It is:

If a signal is blocked by a thread, then the process-wide signal handler registered by sigaction just can't catch or even know it.

If a signal is not blocked, and it's sent before calling sigwait, the process-wide signal handler wins. And that's why APUE the books require us to block the target signal before calling sigwait. Here I use sleep in thread A to simulate a long "window time".

If a signal is not blocked, and it's sent when sigwait has already been waiting, sigwait wins.

But you should notice that for test case 1 and 2, main thread is designed to block the target signal.

At last for test case 3, when main thread is not blocked the target signal, and sigwait in thread A is also waiting, the signal handler is executed in the main thread.

I believe the behaviour of test case 3 is what APUE talks about:

From APUE §12.8:

If a signal is being caught (the process has established a signal handler by using sigaction, for example) and a thread is waiting for the same signal in a call to sigwait, it is left up to the implementation to decide which way to deliver the signal. The implementation could either allow sigwait to return or invoke the signal handler, but not both.

---

Above all, if you want to accomplish one thread <-> one signal model, you should:

1. block all signals in the main thread with pthread_sigmask (subsequent thread created in main thread inheris the signal mask)
2. create threads and call sigwait(target_signal) with target signal.

---

test code

#define _POSIX_C_SOURCE 200809L

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

FILE* file;


void* threadA(void* argv){
    fprintf(file, "%ld\n", pthread_self());

    sigset_t m;
    sigemptyset(&m);
    sigaddset(&m, SIGUSR1);
    int signo;
    int err;


    // sigset_t q;
    // sigemptyset(&q);
    // pthread_sigmask(SIG_SETMASK, &q, NULL);
    // sleep(50);

    fprintf(file, "1\n");


    err = sigwait(&m, &signo);
    if (err != 0){
        fprintf(file, "sigwait error\n");
        exit(1);
    }

    switch (signo)
    {
    case SIGUSR1:
        fprintf(file, "SIGUSR1 received\n");
        break;

    default:
        fprintf(file, "?\n");
        break;
    }

    fprintf(file, "2\n");
}

void hello(int signo){
    fprintf(file, "%ld\n", pthread_self());
    fprintf(file, "hello\n");
}


int main(){
    file = fopen("daemon", "wb");

    setbuf(file, NULL);


    struct sigaction sa;
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = hello;
    sigaction(SIGUSR1, &sa, NULL);

    sigset_t n;
    sigemptyset(&n);
    sigaddset(&n, SIGUSR1);

    // pthread_sigmask(SIG_BLOCK, &n, NULL);

    pthread_t pid;
    int err;
    err = pthread_create(&pid, NULL, threadA, NULL);

    if(err != 0){
        fprintf(file, "create thread error\n");
        exit(1);
    }

    pause();

    fprintf(file, "after pause\n");
    fclose(file);
    return 0;

}

run with ./a.out & (run in the background), and use kill -SIGUSR1 pid to test. Do not use raise. raise, sleep, pause are thread-wide.

Answer 2

I know this question is about a year old, but I often use a pattern, which solves exactly this issue using pthreads and signals. It is a little length but takes care of any issues I am aware of.

I recently used in combination with a library wrapped with SWIG and called from within Python. An annoying issue was that my IRQ thread waiting for SIGINT using sigwait never received the SIGINT signal. The same library worked perfectly when called from Matlab, which didn't capture the SIGINT signal.

The solution was to install a signal handler

#define _NTHREADS 8

#include <signal.h>
#include <pthread.h>
#include <unistd.h>
#include <sched.h>
#include <linux/unistd.h>
#include <sys/signal.h>
#include <sys/syscall.h>
#include <setjmp.h>

#include <stdio.h>
#include <stdlib.h>

#include <errno.h>
#include <string.h> // strerror

#define CallErr(fun, arg)  { if ((fun arg)<0)          \
      FailErr(#fun) }

#define CallErrExit(fun, arg, ret)  { if ((fun arg)<0) \
      FailErrExit(#fun,ret) }

#define FailErrExit(msg,ret) {                    \
  (void)fprintf(stderr, "FAILED: %s(errno=%d strerror=%s)\n", \
        msg, errno, strerror(errno));             \
  (void)fflush(stderr);                       \
  return ret; }

#define FailErr(msg) {                                        \
  (void)fprintf(stderr, "FAILED: %s(errno=%d strerror=%s)\n", \
        msg, errno, strerror(errno));             \
  (void)fflush(stderr);}

typedef struct thread_arg {
  int cpu_id;
  int thread_id;
} thread_arg_t;

static jmp_buf jmp_env;

static struct sigaction act;
static struct sigaction oact;

size_t exitnow = 0;
pthread_mutex_t exit_mutex;

pthread_attr_t attr;

pthread_t pids[_NTHREADS];
pid_t     tids[_NTHREADS+1];

static volatile int status[_NTHREADS]; // 0: suspended, 1: interrupted, 2: success

sigset_t mask;

static pid_t gettid( void );
static void *thread_function(void *arg);
static void signalHandler(int);

int main() {

  cpu_set_t cpuset;
  int nproc;
  int i;
  thread_arg_t thread_args[_NTHREADS];
  int id;

  CPU_ZERO( &cpuset );
  CallErr(sched_getaffinity,
      (gettid(), sizeof( cpu_set_t ), &cpuset));

  nproc = CPU_COUNT(&cpuset);

  for (i=0 ; i < _NTHREADS ; i++) {
    thread_args[i].cpu_id = i % nproc;
    thread_args[i].thread_id = i;
    status[i] = 0;
  }

  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

  pthread_mutex_init(&exit_mutex, NULL);

  // We pray for no locks on buffers and setbuf will work, if not we
  // need to use filelock() on on FILE* access, tricky

  setbuf(stdout, NULL);
  setbuf(stderr, NULL);

  act.sa_flags = SA_NOCLDSTOP | SA_NOCLDWAIT;
  act.sa_handler = signalHandler;
  sigemptyset(&act.sa_mask);
  sigemptyset(&mask);
  sigaddset(&mask, SIGINT);

  if (setjmp(jmp_env)) {

    if (gettid()==tids[0]) {
      // Main Thread
      printf("main thread: waiting for clients to terminate\n");
      for (i = 0; i < _NTHREADS; i++) {
    CallErr(pthread_join, (pids[i], NULL));
    if (status[i] == 1)
      printf("thread %d: terminated\n",i+1);
      }
      // On linux this can be done immediate after creation
      CallErr(pthread_attr_destroy, (&attr));
      CallErr(pthread_mutex_destroy, (&exit_mutex));

      return 0;
    }
    else {
      // Should never happen
      printf("worker thread received signal");
    }
    return -1;
  }

  // Install handler
  CallErr(sigaction, (SIGINT, &act, &oact));

  // Block SIGINT
  CallErr(pthread_sigmask, (SIG_BLOCK, &mask, NULL));

  tids[0] = gettid();
  srand ( time(NULL) );

  for (i = 0; i < _NTHREADS; i++) {
    // Inherits main threads signal handler, they are blocking
    CallErr(pthread_create,
        (&pids[i], &attr, thread_function,
         (void *)&thread_args[i]));
  }

  if (pthread_sigmask(SIG_UNBLOCK, &mask, NULL)) {
    fprintf(stderr, "main thread: can't block SIGINT");
  }
  printf("Infinite loop started - CTRL-C to exit\n");

  for (i = 0; i < _NTHREADS; i++) {
    CallErr(pthread_join, (pids[i], NULL));
    //printf("%d\n",status[i]);
    if (status[i] == 2)
      printf("thread %d: finished succesfully\n",i+1);
  }

  // Clean up and exit 
  CallErr(pthread_attr_destroy, (&attr));
  CallErr(pthread_mutex_destroy, (&exit_mutex));

  return 0;

}

static void signalHandler(int sig) {

  int i;
  pthread_t id;

  id = pthread_self();

  for (i = 0; i < _NTHREADS; i++)
    if (pids[i] == id) {
      // Exits if worker thread
      printf("Worker thread caught signal");
      break;
    }

  if (sig==2) {
    sigaction(SIGINT, &oact, &act);
  }

  pthread_mutex_lock(&exit_mutex);
  if (!exitnow)
    exitnow = 1;
  pthread_mutex_unlock(&exit_mutex);

  longjmp(jmp_env, 1); 
}

void *thread_function(void *arg) {
  cpu_set_t set;

  thread_arg_t* threadarg;

  int thread_id;

  threadarg = (thread_arg_t*) arg;

  thread_id = threadarg->thread_id+1;

  tids[thread_id] = gettid();

  CPU_ZERO( &set );
  CPU_SET( threadarg->cpu_id, &set );

  CallErrExit(sched_setaffinity, (gettid(), sizeof(cpu_set_t), &set ),
          NULL);

  int k = 8;
  // While loop waiting for exit condition
  while (k>0) {
    sleep(rand() % 3);
    pthread_mutex_lock(&exit_mutex);
    if (exitnow) {
      status[threadarg->thread_id] = 1;
      pthread_mutex_unlock(&exit_mutex);
      pthread_exit(NULL);
    }
    pthread_mutex_unlock(&exit_mutex);
    k--;
  }

  status[threadarg->thread_id] = 2;
  pthread_exit(NULL);
}

static pid_t gettid( void ) {
  pid_t pid;
  CallErr(pid = syscall, (__NR_gettid));
  return pid;
}

Answer 3

From sigwait() documentation :

The sigwait() function suspends execution of the calling thread until one of the signals specified in the signal set becomes pending.

A pending signal means a blocked signal waiting to be delivered to one of the thread/process. Therefore, you need not to unblock the signal like you did with your pthread_sigmask(SIG_UNBLOCK, &signal_set, NULL) call.

This should work :

static void* WaitForAbortThread(void* v){
    sigset_t signal_set;

    sigemptyset(&signal_set);
    sigaddset(&signal_set, SIGABRT); 

    sigwait( &signal_set, &sig  );

    TellAllThreadsWeAreGoingDown();

    sleep(10);

    return null;
}

Answer 4

I got some information from this <link>

It says :

To allow a thread to wait for asynchronously generated signals, the threads library provides the sigwait subroutine. The sigwait subroutine blocks the calling thread until one of the awaited signals is sent to the process or to the thread. There must not be a signal handler installed on the awaited signal using the sigwait subroutine.

I will remove the sigaction() handler and try only sigwait().

Answer 5

From the code snippet you've posted, it seems you got the use of sigwait() wrong. AFAIU, you need WaitForAbortThread like below:

     sigemptyset( &signal_set); // change it from sigfillset()
     for (;;) {
           stat = sigwait(&signal_set, &sig);

           if (sig == SIGABRT) {
          printf("here's sigbart.. do whatever you want.\n");
          pthread_kill(tid, signal); // thread id and signal
         }
       }

I don't think pthread_sigmask() is really needed. Since you only want to handle SIGABRT, first init signal_set as empty then simply add SIGABRT, then jump into the infinite loop, sigwait will wait for the particular signal that you're looking for, you check the signal if it's SIGABRT, if yes - do whatever you want. NOTE the uses of pthread_kill(), use it to sent any signal to other threads specified via tid and the signal you want to sent, make sure you know the tid of other threads you want to sent signal. Hope this will help!