sigaction()

QNX SDP8.0C Library ReferenceAPIDeveloper

Examine or specify the action associated with a signal

Synopsis:

#include <signal.h>

int sigaction( int sig,
               const struct sigaction * act,
               struct sigaction * oact );

Arguments:

sig: The signal number (defined in <signal.h>). For more information, see POSIX and QNX OS signals in the documentation for SignalAction().
If the signal terminates a process, the cleanup of the terminated process occurs by default at the priority of the thread that sent the signal. As a QNX OS extension, if you OR the SIG_TERMER_NOINHERIT flag (defined in <signal.h>) into sig, the cleanup occurs at the priority of the thread that received the signal.
act: NULL, or a pointer to a sigaction structure that specifies how you want to modify the action for the given signal.
oact: NULL, or a pointer to a sigaction structure that the function fills with information about the current action for the signal.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

You can use sigaction() to examine or specify (or both) the action that's associated with a specific signal:

If act isn't NULL, the specified signal is modified.
If oact isn't NULL, the previous action is stored in the structure it points to.

Signal handlers and actions are defined for the process and affect all threads in the process. For example, if one thread ignores a signal, then all threads ignore the signal.

You can target a signal at a thread, process, or process group (see SignalKill()). When targeted at a process, at most one thread receives the signal. This thread must have the signal unblocked (see SignalProcmask()) to be a candidate for receiving it. All synchronously generated signals (e.g., SIGSEGV) are always delivered to the thread that caused them.

Note:

If you use longjmp() to return from a signal handler, the signal remains masked. You can use siglongjmp() to restore the mask to the state saved by a previous call to sigsetjmp().
The kernel saves and restores the FPU context on entering and leaving signal handlers, so it's safe to use floating-point operations in them.

Returns:

0: Success.
-1: An error occurred (errno is set).

Errors:

EAGAIN: Insufficient system resources are available to set up the signal's action.
EFAULT: A fault occurred trying to access the buffers provided.
EINVAL: The signal sig isn't valid.

Examples:

#include <signal.h>
#include <stdio.h>
#include <unistd.h>

volatile unsigned sig_count;

void sig_handler(int signo, siginfo_t *info, void *unused) {
	(void) signo;
	(void) info;
	(void) unused;

	sig_count++;
}

int main(void) {

	struct sigaction sact = {
		.sa_sigaction = sig_handler,
	};
	sigaction(SIGINT, &sact, NULL);

	while(1) {
		pause();
		printf("Count: %u\n", sig_count);
	}
}

Classification:

POSIX 1003.1


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: March 05, 2025