Not able to catch SIGINT signal while using select()

Question

I'm trying to handle signals while listen socket in syscall select.

Problem: I have the working loop with select call. select waits for socket descriptor is ready. There is need to break loop by SIGINT or SIGQUIT and correct close resources and exit the programm. Below code is

#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <error.h>
#include <errno.h>
#include <signal.h>
#include <syslog.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <netinet/in.h>


bool bBreakJob = false;

void sig_handler(int sig)
{
    switch(sig)
    {
    case SIGHUP:
        //rneed to reload config
        break;
    case SIGINT:
        printf("SIGINT \n");
        bBreakJob = true;
        openlog("mydaemon", LOG_PID | LOG_CONS, LOG_DAEMON);
        syslog(LOG_INFO, "Catched SIGINT");
        closelog();
        break;
    case SIGQUIT:
        printf("SIGQUIT \n");
        openlog("mydaemon", LOG_PID | LOG_CONS, LOG_DAEMON);
        syslog(LOG_INFO, "Catched SIGQUIT");
        bBreakJob = true;
        break;
    case SIGPIPE:
        printf("SIGPIPE \n");
        break;
    }
}

int main(int argc, char** argv)
{
    struct sigaction act, oact;
    sigset_t set;
    sigemptyset(&set);
    sigaddset(&set, SIGINT);
    sigaddset(&set, SIGHUP);
    sigaddset(&set, SIGPIPE);
    sigaddset(&set, SIGQUIT);
    sigprocmask(SIG_UNBLOCK, &set, NULL);
    act.sa_mask = set;
    act.sa_handler = sig_handler;
    act.sa_flags = 0;
    sigaction(SIGINT, &act, NULL);
    sigaction(SIGHUP, &act, NULL);
    sigaction(SIGPIPE, &act, NULL);
    sigaction(SIGQUIT, &act, NULL);

    int fds[0], res, fmax;
    fd_set wset;
    fd_set rset;

    //next line code to open socket 
    int listen_socket = socket(AF_INET, SOCK_STREAM, 0); 
    int iFlags = fcntl(listen_socket, F_GETFL);
    iFlags |= O_NONBLOCK;
    fcntl(listen_socket, F_SETFL, iFlags);
    struct sockaddr_in sin;
    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = htonl(INADDR_ANY);
    sin.sin_port = htons(4000);
    bind(listen_socket, (struct sockaddr *)&sin, sizeof(sin));
    listen(listen_socket, 20);

    fds[0] = listen_socket;
    FD_ZERO(&wset);
    FD_SET(fds[0], &wset);
    fmax = fds[0] + 1;
    while (FD_ISSET(fds[0], &wset))
    {
        rset = wset;
        res = select(fmax, &rset, NULL, NULL, NULL);
        if (res < 0)
        {
            if (errno == EINTR)
            {   //debug message  
                printf("Loop broken by select's result EINTR");
                break;
            } else
            {
                printf("select(...) fails in listed loop. errno %d (%s)", errno, strerror(errno));
                exit(1);
            }

        }
        else if (res == 0)
        {
          //if timeout is handled
        }
        else if (res > 0)
        {
            if (FD_ISSET(fds[0], &rset))
            {
                //Handle socket input 
            }
        }
        if(bBreakJob)
        {
          printf("Loop broken by signal handler");
          break; 
        }
    } //while( 1 );
    FD_CLR(fds[0], &wset);
    if(bBreakJob)
    { //debug message 
      printf("signal SIGINT is handled ");
    }

}

SIGINT never reaches the sig_handler. In IDE QtCreator I've tried to debug. select just interrupted and then return to listen. The condition "if (errno == EINTR)" is not reached even. Ther is no debug messages either in console either no in syslog. And in the same time SIGQUIT works fine: sig_handler is called and the condition "if (errno == EINTR)" is reached too.

As you can see, I've tried to check SIGINT in to ways: with flag from signal handler, and from result of select

I've tried to found answer in topic Not able to catch SIGINT signal while using select(). But cannot found the solution. This problem I meet in other WEB-resources, but there is no solution too.

SIGINT signal is sened from command line: "kill -s 2 (PID)"

UPD Problem has solved. The issue was in debugger. Under debugger SIGINT does not working properly. Running the programm without debugger working fine as expected.

Answer 1

The interaction of select and signals is tricky, because the signal could always arrive right before you call select. Here are two ways to wake up a select loop from a signal handler:

• The "self-pipe trick": Create a pipe and add the read end to your select read set. From your signal handler, write one byte to the write end of this pipe, and it will make the select return immediately (because input is ready).

• Rather than pass NULL as the final argument to select, pass a pointer to a timeval that is a global variable. Within your signal handler, make the timeval 0 seconds. Hence, if a signal arrives before you call select, select will ask for a 0 timeout and return immediately.