Handling signals in different ways

Can anyone help me to understand what are the differences between the following three ways of handling a signal? I'm operating in a client/server in C.

Edit: I've understood that the 1st example is wrong, now I'd like to know which is better/possible, the 2nd one ore the third one? I've also read around the web that someone does a mix between them, using the "struct sigaction sig" in "sigemptyset" and "sigaddset", without any sigprocmask. Is this better than both my solutions?

Handler:

void closeSig(){
    close(socketAccept);
    close(socket);
    exit(1);
}

1st example:

signal(SIGINT, closeSig);

2nd example:

sigset_t set;
struct sigaction sig;
sigemptyset(&set);
sigaddset(&set, SIGINT);
sig.sa_sigaction = &closeSig;
sig.sa_flags = SA_SIGINFO;
sig.sa_mask = set;
sigaction(SIGINT, &sig, NULL);
sigprocmask(SIG_UNBLOCK, &set, NULL);

3rd example:

struct sigaction sig;
sig.sa_sigaction = &closeSig;
sig.sa_flags = SA_SIGINFO;
sigaction(SIGINT, &sig, NULL);

---

According to POSIX sigaction() , the function closeSig() is not strictly a suitable callback for signals. It should be:

void closeSig(int signum);

for a regular callback, but as you're using SA_SIGINFO , it needs to be:

void closeSig(int signum, siginfo_t *info, void *context);

You also need to look at the POSIX specification for sigprocmask() .

Let's look at the second example:

sigset_t set;
struct sigaction sig;
sigemptyset(&set);
sigaddset(&set, SIGINT);
sig.sa_sigaction = &closeSig;
sig.sa_flags = SA_SIGINFO;
sig.sa_mask = set;
sigaction(SIGINT, &sig, NULL);
sigprocmask(SIG_UNBLOCK, &set, NULL);

The & on &closeSig is unnecessary, but harmless. However, the type of closeSig() shown in the question is wrong. The sigset_t says that SIGINT (only) will be blocked when the interrupt handler is called, but that would happen anyway since the flags don't include SA_NODEFER . So, the code handling set is superfluous to sigaction() .

The call to sigprocmask() unblocks interrupts ( SIGINT ). It is not clear why this is called; there would have to be some previous call to sigprocmask() that blocked interrupts for it to make a difference. However, after this, if an interrupt is sent to the process, the closeSig() function will be called with arguments that it does not expect.

The third example is:

struct sigaction sig;
sig.sa_sigaction = &closeSig;
sig.sa_flags = SA_SIGINFO;
sigaction(SIGINT, &sig, NULL);

This is fairly close to the minimal operations described in dissecting and simplifying the second example. The main problem is that sig.sa_mask is set to an indeterminate value. The code should be:

struct sigaction sig;
sig.sa_sigaction = &closeSig;
sig.sa_flags = SA_SIGINFO;
sigemptyset(&sig.sa_mask);
sigaction(SIGINT, &sig, NULL);

This is now equivalent to the second example.

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