Signals

Overview

The principal function used in C to adjust signals is signal. Note that use of this function in multithreaded programs is explicitly regarded as undefined behavior.

typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);

The signal function is used to change the action associated with a signal signum in one of three ways:

• If handler is SIG_IGN, then signals of type signum are ignored.
• If handler is SIG_DFL, then the action for signals of type signum reverts to the default action.
• Other handler is the address of a user-defined function.

On Linux machines, it's generally preferred to instead use sigaction:

int sigaction(int signum,
              const struct sigaction *restrict act,
              struct sigaction *restrict oldact);)

Signal Mask

To update the signal mask (i.e. the blocked bit vector), sigprocmask() is typically used:

int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

Other functions include:

int sigemptyset(sigset_t *set);
int sigfillset(sigset_t *set);
int sigaddset(sigset_t *set, int signum);
int sigdelset(sigset_t *set, int signum);
int sigismember(const sigset_t *set, int signum);

Safety

A function is async-signal-safe if and only if it can be safely called from a signal handler. This holds either because it is reentrant or because it cannot be interrupted by a signal handler.

Atomicity

The <signal.h> header also exports the sig_atomic_t type. This is an integer type with a minimal width of 8 bits. Memory-load (evaluation) and store (assignment) are guaranteed atomic; other operations are not.