A signal handler should not re-assert its desire to handle its own signal. This is often done on non-persistent platforms, that is, when a signal handler receives a signal that is bound to a handler, they unbind the signal to default behavior before calling the handler.
A signal handler may only call signal() to reset the signal, or change the current way signals are handled. if it does not need to be async-safe (in other words, all relevant signals are masked, and it may therefore not be interrupted.)
Non-Compliant Code Example
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| Code Block | ||
|---|---|---|
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void handler(int signum) {
/* handling code */
}
/* ... */
/* Equivalent to signal( signum, handler);
but make signal persistent */
struct sigaction act;
act.sa_handler = &handler;
act.sa_flags = 0;
if (sigemptyset( &act.sa_mask) != 0) {
/* handle error */
}
if (sigaction(signum, &act, NULL) != 0) {
/* handle error */
}
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While the handler in this example does not call signal(), it safely can, since relevant signals are masked and so the handler can not be interrupted.
In fact, POSIX recommends sigaction(2) and deprecates signal(2). Unfortunately, sigaction(2) is not C99-compliant, and is not supported on some platforms, including Windows.
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