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SIGNAL(3) OpenBSD Programmer's Manual SIGNAL(3)
signal - simplified software signal facilities
(*signal(int sigcatch, void (*func)(int sigraised))) (int);
The 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 ter-
mination of a process and those that do not. Signals which cause termi-
nation 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 receiv-
ing 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 any signal to be caught, to be ignored, or to generate an interrupt.
These signals are defined in the file <signal.h>:
Name Default Action Description
SIGHUP terminate process terminal line hangup
SIGINT terminate process interrupt program
SIGQUIT create core image quit program
SIGILL create core image illegal instruction
SIGTRAP create core image trace trap
SIGABRT create core image abort(3) call (formerly SIGIOT)
SIGEMT create core image emulate instruction executed
SIGFPE create core image floating-point exception
SIGKILL terminate process kill program
SIGBUS create core image bus error
SIGSEGV create core image segmentation violation
SIGSYS create core image system call given invalid
SIGPIPE terminate process write on a pipe with no reader
SIGALRM terminate process real-time timer expired
SIGTERM terminate process software termination signal
SIGURG discard signal urgent condition present on
SIGSTOP stop process stop (cannot be caught or
SIGTSTP stop process stop signal generated from
SIGCONT discard signal continue after stop
SIGCHLD discard signal child status has changed
SIGTTIN stop process background read attempted from
SIGTTOU stop process background write attempted to
SIGIO discard signal I/O is possible on a descriptor
SIGXCPU terminate process cpu time limit exceeded (see
SIGXFSZ terminate process file size limit exceeded (see
SIGVTALRM terminate process virtual time alarm (see
SIGPROF terminate process profiling timer alarm (see
SIGWINCH discard signal window size change
SIGINFO discard signal status request from keyboard
SIGUSR1 terminate process user-defined signal 1
SIGUSR2 terminate process user-defined signal 2
The func argument is a function to be called as the action upon receipt
of the signal sigcatch. The function will be called with one argument,
sigraised, which is the signal raised (thus the same function, func, can
be used by more than one signal). To set the default action of the sig-
nal 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 func returns and the process contin-
ues 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 pro-
cess inherits the signals. All caught signals may be reset to their de-
fault action by a call to the execve(2) function; ignored signals remain
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] A specified signal 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)
This signal() facility appeared in 4.0BSD.
OpenBSD 2.6 April 19, 1994 2
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for signal(3) functions
OpenBSD sources for signal(3)
Up to: Process Signals and Events - Sending and handling signals and events.
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