TheMangaStand TheMangaStand - 1 year ago 62
C Question

Signal Handler Not working On my C http Server

Problem: I want my single handler to work as intended and print out "EXITED NICELY" when I press ctrl C. This is a assignment and we must use signal handlers. As you can see I also experimented with sigaction but to the same results.

Current Behaviour: "work" is printing out suggesting the signal handler is working, however it must be getting stuck somewhere because it doesn't cancel the program. Although if I press ctrl c and then send a http request to the server such as curl http:/localhost:port/ it will then exit gracefully and print out my desired message. However I would like it to do that without me having to send a request.

Edit through further research. I put a print after and before my accept call. print before will print once and then the accept will just hold it until it receives a connection. So that where the problem is but how do we fix that?

#include "http_common.h"
#define CONNMAX 1000
#define BYTES 1024

char *ROOT;
int verbose;
int signalReceived = 1;
int listenfd, clients[CONNMAX];
void error(char *);

static void clean(int arg)
if(arg == SIGINT) {
signalReceived = 0;
//signal(SIGINT, clean);
else if(arg == SIGHUP) {
signalReceived = 0;

int main(int argc, char *argv[])
//struct sigaction act;
//memset (&act, '\0', sizeof(act));
//act.sa_handler = clean;
//act.sa_flags = SA_RESTART;
signal(SIGINT, clean);
//signal(SIGHUP, clean);
//if (sigaction(SIGINT, &act, NULL) == -1)
// printf("doing something\n");
struct sockaddr_in clientaddr;
socklen_t addrlen;
char c;
char PORT[6];
ROOT = getenv("PWD");
strcpy(PORT, "8888");
int slot;
while ((c = getopt (argc, argv, "p:v")) != -1)

switch (c) {
verbose = 1;
strcpy(PORT, optarg);
fprintf(stderr, "Wrong arguments given\n");
printf("Listening on port %s%s%s, root is %s%s%s\n", "\033[92m", PORT, "\033[0m", "\033[92m", ROOT, "\033[0m");
int i = 0;
for (i = 0; i < CONNMAX; i++)
clients[i] = -1;
startServer(PORT, &listenfd);
while (signalReceived == 1) {
addrlen = sizeof(clientaddr);
clients[slot] = accept (listenfd, (struct sockaddr *) &clientaddr, &addrlen);
if (clients[slot] < 0)
else {
if (fork() == 0) {
respond(slot, verbose, ROOT, clients);
while (clients[slot] != -1)
slot = (slot + 1) % CONNMAX;

printf("EXITED NICLEY\n");//ients[slot] = accept (listenfd, (struct sockaddr *) &clientaddr, &addrlen);
return 0;

Answer Source

You have to register the signal with sigaction() and without the SA_RESTART flag.

When you register a signal handler with signal(), it will set the SA_RESTART flag. See the glibc manual:

In the GNU C Library, establishing a handler with signal sets all the flags to zero except for SA_RESTART, whose value depends on the settings you have made with siginterrupt. See Interrupted Primitives, to see what this is about.

When SA_RESTART is set, the signal will not interrupt (most) system calls, but it will instead restart them. See the signal man page:

If a signal handler is invoked while a system call or library function call is blocked, then either:

  • the call is automatically restarted after the signal handler returns; or
  • the call fails with the error EINTR.

Which of these two behaviors occurs depends on the interface and whether or not the signal handler was established using the SA_RESTART flag (see sigaction(2)).