c - Signal handling in pthreads

Question

I have created a pthread, and installed a signal handler inside that, same way as we do in main( ) function. The thread's signal handler is a separate function. Surprisingly, it is not working, that is the thread's signal handler is not able to catch signals.

Here is the code:

#include <pthread.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <signal.h>

typedef struct data
{
 char name[10];
 int age;
}data;

void sig_func(int sig)
{
 printf("Caught signal: %d
",sig);
 signal(SIGSEGV,sig_func);
}

void func(data *p)
{
 printf("This is from thread function
");
 signal(SIGSEGV,sig_func); // Register signal handler inside thread
 strcpy(p->name,"Mr. Linux");
 p->age=30;
 sleep(2); // Sleep to catch the signal
}

int main()
{
 pthread_t tid;
 pthread_attr_t attr;
 data *ptr;

 pthread_attr_init(&attr);
 pthread_create(&tid,&attr,(void*)func,ptr);
 pthread_kill(tid,SIGSEGV);

 pthread_join(tid,NULL);
 printf("Name:%s
",ptr->name);
 printf("Age:%d
",ptr->age);
}

Output:

Segmentation fault (which means the signal is not caught by handler)

Answer 1

There are several problems with your code:

• ptr is not initialised, so all the ptr-> parts will crash the program
• you are calling pthread_kill() immediately, very likely before the signal handler has been installed, and in a thread (which has unspecified behaviour)
• you call printf() from a signal handler, which is not guaranteed to work (see man 7 signal for a list of safe functions)

This will work a lot better, though you'd still need proper thread synchronisation, and as stated elsewhere, you should use sigaction():

#include <pthread.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <signal.h>

typedef struct data
{
 char name[10];
 int age;
}data;

void sig_func(int sig)
{
 write(1, "Caught signal 11
", 17);
 signal(SIGSEGV,sig_func);
}

void func(data *p)
{
 fprintf(stderr, "This is from thread function
");
 strcpy(p->name,"Mr. Linux");
 p->age=30;
 sleep(2); // Sleep to catch the signal
}

int main()
{
 pthread_t tid;
 pthread_attr_t attr;
 data d;
 data *ptr = &d;

 signal(SIGSEGV,sig_func); // Register signal handler before going multithread
 pthread_attr_init(&attr);
 pthread_create(&tid,&attr,(void*)func,ptr);
 sleep(1); // Leave time for initialisation
 pthread_kill(tid,SIGSEGV);

 pthread_join(tid,NULL);
 fprintf(stderr, "Name:%s
",ptr->name);
 fprintf(stderr, "Age:%d
",ptr->age);
}

Edit: install sighandler in main thread