Set a custom function as handler to all signals - c

Question

I have this assignment to do:
Write a function void myfunct(void(*f)(int sig)) which sets f as handler to all the possible signals.
I have two problems:
How can a get all the possible signals? Is there a function for this? Can I iterate through them in some way?
Will it really work to set the function f as handler given that it takes a parameter? Shouldn't it not have any parameters?
Thank you.

Answer 1

I would personally instead iterate over a static list of signal numbers, and use preprocessor directives to detect which ones are supported (at compile time). For example:
#include <signal.h>
static const all_signals[] = {
#ifdef SIGHUP
SIGHUP, /* POSIX.1 */
#endif
#ifdef SIGQUIT
SIGQUIT, /* POSIX.1 */
#endif
#ifdef SIGTRAP
SIGTRAP, /* POSIX.1 */
#endif
#ifdef SIGIO
SIGIO, /* BSD/Linux */
#endif
/*
* Other signal names omitted for brevity
*/
/* C89/C99/C11 standard signals: */
SIGABRT,
SIGFPE,
SIGILL,
SIGINT,
SIGSEGV,
/* SIGTERM (C89/C99/C11) is also the terminating signal number */
SIGTERM
};
with SIGTERM being the last entry in the array for which a signal handler is installed:
struct sigaction act;
int i = 0;
memset(&act, 0, sizeof act);
sigemptyset(&act.sa_mask);
act.sa_handler = your_signal_handler;
act.sa_flags = 0;
do {
if (sigaction(all_signals[i], &act, NULL)) {
fprintf(stderr, "Cannot install signal %d handler: %s.\n", all_signals[i], strerror(errno));
exit(EXIT_FAILURE);
}
} while (all_signals[i++] != SIGTERM);
This way your code does not require POSIX etc. support to work, but does support POSIX signals if available at compile time.
You can check the Wikipedia Unix signal article and man 7 signal for known signal names.
You can also install the signal handler for POSIX realtime signals using
#if SIGRTMAX-0 > SIGRTMIN-0
for (i = SIGRTMIN; i <= SIGRTMAX; i++)
if (sigaction(i, &act, NULL)) {
fprintf(stderr, "Cannot install realtime signal %d handler: %s.\n", i, strerror(errno));
exit(EXIT_FAILURE);
}
#endif

Answer 2

How can a get all the possible signals? Is there a function for this? Can I iterate through them in some way?
Most implementation provide a constant such as NSIG (Glibc provides NSIG) or _NSIG (Linux provides _NSIG). So, you can loop through that constant and set the same signal handling function for all of them.
There's no POSIX defined value for "highest signal number". There's been a proposal in POSIX to add a macro NSIG_MAX.
{NSIG_MAX}
Maximum possible return value of sysconf(_SC_NSIG). See [cross-ref to XSH sysconf()]. The value of {NSIG_MAX} shall be no greater than the number of signals that the sigset_t type (see [cross-ref to ]) is capable of representing, ignoring any restrictions imposed by sigfillset() or sigaddset().
But it hasn't made it to POSIX yet (most probably it'll a part of the POSIX version - issue 8).
Will it really work to set the function f as handler given that it takes a parameter? Shouldn't it not have any parameters?
The parameter that the signal handling function takes doesn't matter when you are setting a signal disposition. It takes the signal number but that doesn't prevent you from using it as a handler for multiple signals.
But there are special cases you need to handle. Certain signals that can't caught or ignored (SIGKILL and SIGSTOP). There are other signals (SIGFPE, SIGILL and SIGSEGV) for which, while allowed to caught, the signal handler can't return to its caller (i.e. you need exit from the signal handler).

Answer 3

1. How can a get all the possible signals? Is there a function for this? Can I iterate through them in some way?
Seems like there isn't a standardized list you can iterate, but we actually don't need such a list for your task. Signal numbers are always positive and at most SIGRTMAX. In between 1 and SIGRTMAX there might be some numbers that do not correspond to any signal, but you can still try to set a handler for these. From the posix specification of sigaction(sig, ...) (emphasis mine):
RETURN VALUE
Upon successful completion, sigaction() shall return 0; otherwise, -1 shall be returned, errno shall be set to indicate the error, and no new signal-catching function shall be installed.
ERRORS
The sigaction() function shall fail if:
[EINVAL]
The sig argument is not a valid signal number or an attempt is made to catch a signal that cannot be caught or ignore a signal that cannot be ignored.
So it is perfectly fine to call sigaction(123, ...) where 123 is not a signal.
2. Will it really work to set the function f as handler given that it takes a parameter? Shouldn't it not have any parameters?
Why do you think it should have no parameters?
Again, from the posix specification of sigaction():
Member Type
Member Name
Description
void(*) (int)
sa_handler
Pointer to a signal-catching function
The signal handler is supposed to have an int parameter, just like your function f has, so there is no problem.
Putting everything together
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
struct sigaction siga;
void f(int sig) {
printf("Caught signal %d\n", sig);
}
// sets f as handler to all the possible signals.
void myfunct(void(*f)(int sig)) {
siga.sa_handler = f;
for (int sig = 1; sig <= SIGRTMAX; ++sig) {
// this might return -1 and set errno, but we don't care
sigaction(sig, &siga, NULL);
}
}
int main() {
myfunct(f);
pause(); // wait for signal
return 0;
}