signal handler in child thread

Question

I tried to install SIGINT handler for the child thread in the code below. I expect the child thread to print hello when it receives SIGINT from the parent process. However, nothing comes out and the program exits immediately.

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

typedef struct proxy_node_t{
    pthread_t sub_thread;
    pthread_t p_self;
}proxy_node;

proxy_node* proxy;

static void proxy_singnal_handler(){
    printf("Hello\n");
    return;
}

static void* t_consensus(void *arg){
    signal(SIGINT,proxy_singnal_handler);
    sleep(1);
    return NULL;
}

int main(int argc, char **argv)
{
    proxy = (proxy_node*)malloc(sizeof(proxy_node));
    proxy->p_self = pthread_self();
    pthread_create(&proxy->sub_thread,NULL,t_consensus,NULL);
    pthread_kill(proxy->sub_thread,SIGINT);
    sleep(1);
    return 0;
}

Answer 1

There are several problems.

1) The signal handler signature is not correct. It should take an int whereas you define it with no parameter. ie

static void proxy_singnal_handler(){

should be

static void proxy_singnal_handler(int sig){

2) You can't call functions that are not async-signal-safe from a signal handler ( printf() in your case). See signal(7) for details. You can instead use write(2) to print that message:

printf("Hello\n");

can be:

write(1, "Hello\n", 6);

3) When main thread sends SIGINT , the t_consensus thread might not have even started. So, signal() may not have been installed yet. So, you need to make sure signal() is installed before pthread_kill() could send SIGINT .

---

Just to demonstrate it, I have added some sleep calls (see comments in the code). But please note that sleep() is not a good way to synchronization and if you intend to adapt this example then you should use a conditional variable instead.

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

typedef struct proxy_node_t{
    pthread_t sub_thread;
    pthread_t p_self;
}proxy_node;

proxy_node* proxy;

static void proxy_singnal_handler(int sig){
    write(1, "Hello\n", 6);
    return;
}

static void* t_consensus(void *arg){
    signal(SIGINT,proxy_singnal_handler);
    while(1); /* infinite loop */
    return NULL;
}

int main(int argc, char **argv)
{
    proxy = (proxy_node*)malloc(sizeof(proxy_node));
    proxy->p_self = pthread_self();
    pthread_create(&proxy->sub_thread,NULL,t_consensus,NULL);
    sleep(2); /* delay to ensure signal handler is installed */
    pthread_kill(proxy->sub_thread,SIGINT);
    sleep(2); /* delay to ensure signal gets executed before the process exits */
    return 0;
}

Answer 2

Since @Maxim Egorushkin wanted to see a solution that exits gracefully and uses semaphores:

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

typedef struct proxy_node_t{
    pthread_t sub_thread;
    pthread_t p_self;
}proxy_node;

proxy_node* proxy;

static void proxy_singnal_handler(int sig)
{
    write(1, "Hello\n", 6);
    return;
}

sem_t sema1;
sem_t sema2;

static void* t_consensus(void *arg)
{
  signal(SIGINT,proxy_singnal_handler);
  sem_post(&sema1);   /*notify main thread that signal-handler is installed*/
  sem_wait(&sema2);   /*ensure thread exists to be pthread_kill'ed, could use sigsuspend instead*/
  return NULL;
}
 int main(int argc, char **argv)
{
  sem_init(&sema1, 0, 0);
  sem_init(&sema2, 0, 0);
  proxy = (proxy_node*)malloc(sizeof(proxy_node));
  proxy->p_self = pthread_self();
  pthread_create(&proxy->sub_thread,NULL,t_consensus,NULL);
  sem_wait(&sema1);    /*wait until the thread has installed the signal handler*/
  pthread_kill(proxy->sub_thread,SIGINT);
  sem_post(&sema2);    /*not strictly necessary if the thread uses sigsuspend*/
  pthread_join(proxy->sub_thread, NULL);
  free(proxy);         /*not strictly necessary before exiting*/
  sem_destroy(&sema1);
  sem_destroy(&sema2);
  return 0;
}