signal() is a simplified interface to the more general sigvec(3UCB) facility. Programs that use signal() in preference to sigvec() are more likely to be portable to all systems.

A signal is generated by some abnormal event, initiated by a user at a terminal (quit, interrupt, stop), by a program error (bus error, etc.), by request of another program (kill), or when a process is stopped because it wishes to access its control terminal while in the background (see termio(7I)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals cause termination of the receiving process if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the SIGKILL and SIGSTOP signals, the signal() call allows signals either to be ignored or to interrupt to a specified location. See sigvec(3UCB) for a complete list of the signals.

If func is SIG_DFL, the default action for signal sig is reinstated; this default is termination (with a core image for starred signals) except for signals marked with o or a dagger.. Signals marked with o are discarded if the action is SIG_DFL; signals marked with a dagger cause the process to stop. If func is SIG_IGN the signal is subsequently ignored and pending instances of the signal are discarded. Otherwise, when the signal occurs further occurrences of the signal are automatically blocked and func is called.

A return from the function unblocks the handled signal and continues the process at the point it was interrupted.

If a caught signal occurs during certain functions, terminating the call prematurely, the call is automatically restarted. In particular this can occur during a read(2) or write(2) on a slow device (such as a terminal; but not a file) and during a wait(2).

The value of signal() is the previous (or initial) value of func for the particular signal.

After a fork(2) or vfork(2) the child inherits all signals. An exec(2) resets all caught signals to the default action; ignored signals remain ignored.

The previous action is returned on a successful call. Otherwise, -1 is returned and errno is set to indicate the error.

signal() will fail and no action will take place if the following occurs:

EINVAL

sig is not a valid signal number, or is SIGKILL or SIGSTOP.

kill(1), exec(2), fcntl(2), fork(2), getitimer(2), getrlimit(2), kill(2), ptrace(2), read(2), sigaction(2), wait(2), write(2), abort(3C), setjmp(3UCB), sigblock(3UCB), sigstack(3UCB), sigvec(3UCB), wait(3UCB), setjmp(3C), signal(3C), signal(3HEAD), termio(7I)

Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-threaded applications is unsupported.

The handler routine, func, can be declared:

void handler( signum) int signum;

Here signum is the signal number. See sigvec(3UCB) for more details.