.\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
.\" and Copyright (c) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
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.\" Modified 2004-11-19, mtk:
.\" added pointer to sigaction.2 for details of ignoring SIGCHLD
.\" 2007-06-03, mtk: strengthened portability warning, and rewrote
.\"     various sections.
.\" 2008-07-11, mtk: rewrote and expanded portability discussion.
.\"
.TH SIGNAL 2 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
signal \- ANSI C signal handling
.SH SYNOPSIS
.B #include <signal.h>
.sp
.B typedef void (*sighandler_t)(int);
.sp
.BI "sighandler_t signal(int " signum ", sighandler_t " handler );
.SH DESCRIPTION
The behavior of
.BR signal ()
varies across UNIX versions,
and has also varied historically across different versions of Linux.
\fBAvoid its use\fP: use
.BR sigaction (2)
instead.
See \fIPortability\fP below.

.BR signal ()
sets the disposition of the signal
.I signum
to
.IR handler ,
which is either
.BR SIG_IGN ,
.BR SIG_DFL ,
or the address of a programmer-defined function (a "signal handler").

If the signal
.I signum
is delivered to the process, then one of the following happens:
.TP 3
*
If the disposition is set to
.BR SIG_IGN ,
then the signal is ignored.
.TP
*
If the disposition is set to
.BR SIG_DFL ,
then the default action associated with the signal (see
.BR signal (7))
occurs.
.TP
*
If the disposition is set to a function,
then first either the disposition is reset to
.BR SIG_DFL ,
or the signal is blocked (see \fIPortability\fP below), and then
.I handler
is called with argument
.IR signum .
If invocation of the handler caused the signal to be blocked,
then the signal is unblocked upon return from the handler.
.PP
The signals
.B SIGKILL
and
.B SIGSTOP
cannot be caught or ignored.
.SH RETURN VALUE
.BR signal ()
returns the previous value of the signal handler, or
.B SIG_ERR
on error.
In the event of an error,
.I errno
is set to indicate the cause.
.SH ERRORS
.TP
.B EINVAL
.I signum
is invalid.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99.
.SH NOTES
The effects of
.BR signal ()
in a multithreaded process are unspecified.
.PP
According to POSIX, the behavior of a process is undefined after it
ignores a
.BR SIGFPE ,
.BR SIGILL ,
or
.B SIGSEGV
signal that was not generated by
.BR kill (2)
or
.BR raise (3).
Integer division by zero has undefined result.
On some architectures it will generate a
.B SIGFPE
signal.
(Also dividing the most negative integer by \-1 may generate
.BR SIGFPE .)
Ignoring this signal might lead to an endless loop.
.PP
See
.BR sigaction (2)
for details on what happens when
.B SIGCHLD
is set to
.BR SIG_IGN .
.PP
See
.BR signal (7)
for a list of the async-signal-safe functions that can be
safely called from inside a signal handler.
.PP
The use of
.I sighandler_t
is a GNU extension, exposed if
.B _GNU_SOURCE
is defined;
.\" libc4 and libc5 define
.\" .IR SignalHandler ;
glibc also defines (the BSD-derived)
.I sig_t
if
.B _BSD_SOURCE
(glibc 2.19 and earlier)
or
.BR _DEFAULT_SOURCE
(glibc 2.19 and later)
is defined.
Without use of such a type, the declaration of
.BR signal ()
is the somewhat harder to read:
.in +4n
.nf

.BI "void ( *" signal "(int " signum ", void (*" handler ")(int)) ) (int);"
.fi
.in
.SS Portability
The only portable use of
.BR signal ()
is to set a signal's disposition to
.BR SIG_DFL
or
.BR SIG_IGN .
The semantics when using
.BR signal ()
to establish a signal handler vary across systems
(and POSIX.1 explicitly permits this variation);
.B do not use it for this purpose.

POSIX.1 solved the portability mess by specifying
.BR sigaction (2),
which provides explicit control of the semantics when a
signal handler is invoked; use that interface instead of
.BR signal ().

In the original UNIX systems, when a handler that was established using
.BR signal ()
was invoked by the delivery of a signal,
the disposition of the signal would be reset to
.BR SIG_DFL ,
and the system did not block delivery of further instances of the signal.
This is equivalent to calling
.BR sigaction (2)
with the following flags:

    sa.sa_flags = SA_RESETHAND | SA_NODEFER;

System\ V also provides these semantics for
.BR signal ().
This was bad because the signal might be delivered again
before the handler had a chance to reestablish itself.
Furthermore, rapid deliveries of the same signal could
result in recursive invocations of the handler.

BSD improved on this situation, but unfortunately also
changed the semantics of the existing
.BR signal ()
interface while doing so.
On BSD, when a signal handler is invoked,
the signal disposition is not reset,
and further instances of the signal are blocked from
being delivered while the handler is executing.
Furthermore, certain blocking system calls are automatically
restarted if interrupted by a signal handler (see
.BR signal (7)).
The BSD semantics are equivalent to calling
.BR sigaction (2)
with the following flags:

    sa.sa_flags = SA_RESTART;

The situation on Linux is as follows:
.IP * 2
The kernel's
.BR signal ()
system call provides System\ V semantics.
.IP *
By default, in glibc 2 and later, the
.BR signal ()
wrapper function does not invoke the kernel system call.
Instead, it calls
.BR sigaction (2)
using flags that supply BSD semantics.
This default behavior is provided as long as a suitable
feature test macro is defined:
.B _BSD_SOURCE
on glibc 2.19 and earlier or
.BR _DEFAULT_SOURCE
in glibc 2.19 and later.
(By default, these macros are defined; see
.BR feature_test_macros (7)
for details.)
If such a feature test macro is not defined, then
.BR signal ()
provides System\ V semantics.
.\"
.\" System V semantics are also provided if one uses the separate
.\" .BR sysv_signal (3)
.\" function.
.\" .IP *
.\" The
.\" .BR signal ()
.\" function in Linux libc4 and libc5 provide System\ V semantics.
.\" If one on a libc5 system includes
.\" .I <bsd/signal.h>
.\" instead of
.\" .IR <signal.h> ,
.\" then
.\" .BR signal ()
.\" provides BSD semantics.
.SH SEE ALSO
.BR kill (1),
.BR alarm (2),
.BR kill (2),
.BR killpg (2),
.BR pause (2),
.BR sigaction (2),
.BR signalfd (2),
.BR sigpending (2),
.BR sigprocmask (2),
.BR sigsuspend (2),
.BR bsd_signal (3),
.BR raise (3),
.BR siginterrupt (3),
.BR sigqueue (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR sysv_signal (3),
.BR signal (7)