Synopsis

Do not rely on indeterminate values of errno.

Enabled by default

Yes

Severity/Certainty

Low/Low

Full description

A signal handler is allowed to call signal(); if that fails, signal() returns SIG_ERR and sets errno to a positive value. However, if the event that caused a signal was external (not the result of the program calling abort() or raise()), the only functions the signal handler may call are _Exit() or abort(), or it may call signal() on the signal currently being handled; if signal() fails, the value of errno is indeterminate. Using this value results in undefined behavior.

Coding standards

Code examples

The following code example fails the check and will give a warning:

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

typedef void (*pfv)(int);

void handler(int signum) {
    pfv old_handler = signal(signum, SIG_DFL);
    if (old_handler == SIG_ERR) {
        perror("SIGINT handler"); /* Undefined behavior */
        /* Handle error */
    }
}

int main(void) {
    pfv old_handler = signal(SIGINT, handler);
    if (old_handler == SIG_ERR) {
        perror("SIGINT handler");
        /* Handle error */
    }

    /* Main code loop */

    return EXIT_SUCCESS;
}

The following code example passes the check and will not give a warning about this issue:

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

typedef void (*pfv)(int);

void handler(int signum) {
    pfv old_handler = signal(signum, SIG_DFL);
    if (old_handler == SIG_ERR) {
        abort();
    }
}

int main(void) {
    pfv old_handler = signal(SIGINT, handler);
    if (old_handler == SIG_ERR) {
        perror("SIGINT handler");
        /* Handle error */
    }

    /* Main code loop */

    return EXIT_SUCCESS;
}