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> #include <string.h>  typedef struct data {  char name[10];  int age; }data;  void sig_func(int sig) {  printf("Caught signal: %d\n",sig);  signal(SIGSEGV,sig_func); }  void func(data *p) {  printf("This is from thread function\n");  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\n",ptr->name);  printf("Age:%d\n",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> #include <string.h>  typedef struct data {  char name[10];  int age; }data;  void sig_func(int sig) {  write(1, "Caught signal 11\n", 17);  signal(SIGSEGV,sig_func); }  void func(data *p) {  fprintf(stderr, "This is from thread function\n");  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\n",ptr->name);  fprintf(stderr, "Age:%d\n",ptr->age); } 

Edit: install sighandler in main thread