c++ - Signal handling in mutlti-threaded process -

i have basic problem of handling signal in multi-threaded process.

in code, create 1 sub-thread main thread, listen sigalrm later trigger main thread (using other function timer_create gives me same result, please don't focus on this).

the problem is, instead of catching signal, whole process terminated strange "alarm clock" output on console.

this code:

#include <iostream> #include <sys/time.h> #include <unistd.h> #include <csignal>  using namespace std;  void* run_something(void* args){     //unblock sigalrm catched     sigset_t sig;     sigemptyset(&sig);     sigaddset(&sig, sigalrm);     sigprocmask(sig_unblock, &sig, null); //tried pthread_sigmask      //wait sigalrm     int catchedsig;     sigwait(&sig, &catchedsig);     cout<<"in sub-thread, sigalrm catched, return"<<endl; }  int main(int argc, char** argv){     //block sigalrm in main thread     sigset_t sig;     sigemptyset(&sig);     sigaddset(&sig, sigalrm);     sigprocmask(sig_block, &sig, null);      //create new thread     pthread_t thread;     pthread_attr_t attr;     pthread_attr_init(&attr);     pthread_create(&thread, &attr, run_something, null);      //trigger sigarlm after 2s     alarm(2); //tried timer_create/sigevent      //wait     cout<<"in main thread, waiting sub-thread terminate"<<endl;     pthread_join(thread, null);     cout<<"in main thread, terminating"<<endl;      return exit_success; } 

expected result

• in main thread, waiting sub-thread terminate
• in sub-thread, sigalrm getting caught, return
• in main thread, terminating

observed result

• in main thread, waiting sub-thread terminate
• alarm clock

additional info: i'm using g++ (debian 5.4.0-4) 5.4.0 20160609.

your run_something thread unblocks sigalrm before calling sigwait signal, this undefined behavior. sigwait removes signal set of pending (i.e., blocked) signals.

don't unblock in thread , you'll see behavior expect.