| SIGNAL(3) | Library Functions Manual | SIGNAL(3) |
signal —
simplified software signal facilities
Standard C Library (libc, -lc)
#include
<signal.h>
void (*
signal(int
sig, void
(*func)(int)))(int);
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.
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 occur:
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)
The signal() facility first appeared in
Version 4 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.
| June 5, 2016 | NetBSD 11.0 |