signal - simplified software signal facilities
Standard C Library (libc, -lc)
( *signal(int sig, void (*func)(int));)()
This signal() facility is a simplified interface to the more general
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
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 with the function returns and the process
continues from where it left off when the signal occurred. Unlike previ-
ous signal facilities, the handler func() remains installed after a sig-
nal 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
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.
signal() will fail and no action will take place if one of the following
[EINVAL] Specified sig is not a valid signal number.
[EINVAL] An attempt is made to ignore or supply a handler for
SIGKILL or SIGSTOP.
kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2),
sigprocmask(2), sigsuspend(2), setjmp(3), tty(4), signal(7)
This signal() facility appeared in 4.0BSD.
BSD April 19, 1994 BSD
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