SIGNAL(3)

NAME
signal - simplified software signal facilities

LIBRARY
libc

SYNOPSIS
#include <signal.h>
void *
signal(int sig, void *funcintint);

DESCRIPTION
This signal() facility is a simplified interface to the more general sigaction(2) facility.

Signals allow the manipulation of a process from outside its domain as well as allowing the process to manipulate itself or copies of itself (children). There are two general types of signals: those that cause termination of a process and those that do not. Signals which cause termination of a program might result from an irrecoverable error or might be the result of a user at a terminal typing the `interrupt' character. Signals are used when a process is stopped because it wishes to access its control terminal while in the background (see tty(4)). 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 result in the termination of the process receiving them 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() function allows for a signal to be caught, to be ignored, or to generate an interrupt. See signal(7) for comprehensive list of supported signals.

The func procedure allows a user to choose the action upon receipt of a signal. To set the default action of the signal to occur as listed above, func should be SIG_DFL. A SIG_DFL resets the default action. To ignore the signal func should be SIG_IGN. This will cause subsequent instances of the signal to be ignored and pending instances to be discarded. If SIG_IGN is not used, further occurrences of the signal are automatically blocked and func is called.

The handled signal is unblocked when the function returns and the process continues from where it left off when the signal occurred. The handler func remains installed after a signal has been delivered.

For some system calls, if a signal is caught while the call is executing and the call is prematurely terminated, the call is automatically restarted. (The handler is installed using the SA_RESTART flag with sigaction(2)). The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a low speed device and during a ioctl(2) or wait(2). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count).

When a process which has installed signal handlers forks, the child process inherits the signals. All caught signals may be reset to their default action by a call to the execve(2) function; ignored signals remain ignored.

Only functions that are async-signal-safe can safely be used in signal handlers; see sigaction(2) for a complete list.

RETURN VALUES
The previous action is returned on a successful call. Otherwise, SIG_ERR is returned and the global variable errno is set to indicate the error.

ERRORS
signal() will fail and no action will take place if one of the following occur:

[EINVAL]
    Specified sig is not a valid signal number; or an attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

SEE ALSO
kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), bsd_signal(3), psignal(3), setjmp(3), strsignal(3), tty(4), signal(7)

HISTORY
The signal() facility first appeared in AT&T UNIX. It used to be a system call that implemented a different semantics, “old signals”, in which a signal was reset to SIG_DFL after being caught.

This “new signals” facility appeared in 4.0BSD as jobs library and became the default signal(3) in 4.2BSD.