Linux Programming Blog

/* An example of a free of signal race program using sigprocmask() and
 * pselect(). */
#include <sys/select.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <arpa/inet.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
 
/* Flag that tells the daemon to exit. */
static volatile int exit_request = 0;
 
/* Signal handler. */
static void hdl (int sig)
{
	exit_request = 1;
}
 
/* Accept client on listening socket lfd and close the connection
 * immediatelly. */
static void handle_client (int lfd)
{
	int sock = accept (lfd, NULL, 0);
	if (sock < 0) {
		perror ("accept");
		exit (1);
	}
 
	puts ("accepted client");
 
	close (sock);
}
 
int main (int argc, char *argv[])
{
	int lfd;
	struct sockaddr_in myaddr;
	int yes = 1;
	sigset_t mask;
	sigset_t orig_mask;
	struct sigaction act;
 
	memset (&act, 0, sizeof(act));
	act.sa_handler = hdl;
 
	/* This server should shut down on SIGTERM. */
	if (sigaction(SIGTERM, &act, 0)) {
		perror ("sigaction");
		return 1;
	}
 
	sigemptyset (&mask);
	sigaddset (&mask, SIGTERM);
 
	if (sigprocmask(SIG_BLOCK, &mask, &orig_mask) < 0) {
		perror ("sigprocmask");
		return 1;
	}
 
	lfd = socket (AF_INET, SOCK_STREAM, 0);
	if (lfd < 0) {
		perror ("socket");
		return 1;
	}
 
	if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR,
				&yes, sizeof(int)) == -1) {
		perror ("setsockopt");
		return 1;
	}
 
	memset (&myaddr, 0, sizeof(myaddr));
	myaddr.sin_family = AF_INET;
	myaddr.sin_addr.s_addr = INADDR_ANY;
	myaddr.sin_port = htons (10000);
 
	if (bind(lfd, (struct sockaddr *)&myaddr, sizeof(myaddr)) < 0) {
		perror ("bind");
		return 1;
	}
 
	if (listen(lfd, 5) < 0) {
		perror ("listen");
		return 1;
	}
 
	while (!exit_request) {
		fd_set fds;
		int res;
 
		/* BANG! we can get SIGTERM at this point, but it will be
		 * delivered while we are in pselect(), because now
		 * we block SIGTERM.
		 */
 
		FD_ZERO (&fds);
		FD_SET (lfd, &fds);
 
		res = pselect (lfd + 1, &fds, NULL, NULL, NULL, &orig_mask);
		if (res < 0 && errno != EINTR) {
			perror ("select");
			return 1;
		}
		else if (exit_request) {
			puts ("exit");
			break;
		}
		else if (exit_request)
			break;
		else if (res == 0)
			continue;
 
		if (FD_ISSET(lfd, &fds)) {
			handle_client (lfd);
		}
	}
 
	return 0;
}