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.
Unlike previous signal facilities, 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
signal(7) for a complete list.