man-pages/man2/signal.2

.\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
.\" based on work by Rik Faith <faith@cs.unc.edu>
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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.
.\"
.TH SIGNAL 2 2007-06-03 "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 behaviour 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.

The
.BR signal ()
system call installs a new signal handler for the signal with number
.IR signum .
The signal handler is set to
.I handler
which may be a user specified function, or either
.B SIG_IGN
or
.BR SIG_DFL .

Upon arrival of a signal with number
.I signum
the following happens.
If the corresponding handler is set to
.BR SIG_IGN ,
then the signal is ignored.
If the handler is set to
.BR SIG_DFL ,
then the default action associated with the signal (see
.BR signal (7))
occurs.
Finally, if the handler is set to a function
.I handler
then first either the handler is reset to SIG_DFL
or an implementation-dependent blocking of the signal
is performed, and then
.I handler
is called with argument
.IR signum .

Using a signal handler function for a signal
is called "catching the signal".
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.
.SH "CONFORMING TO"
C89, C99, POSIX.1-2001.
.SH NOTES
The effects of this call in a multi-threaded process are unspecified.
.PP
According to POSIX, the behaviour 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 the
.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 inside from inside a signal handler.
.PP
The use of
.B sighandler_t
is a GNU extension.
Various versions of libc predefine this type; libc4 and libc5 define
.IR SignalHandler ,
glibc defines
.I sig_t
and, when
.B _GNU_SOURCE
is defined, also
.IR sighandler_t .
.SS Portability
The original Unix
.BR signal ()
would reset the handler to SIG_DFL, and System V
(and the Linux kernel and libc4,5) does the same.
On the other hand, BSD does not reset the handler, but blocks
new instances of this signal from occurring during a call of the handler.
The glibc2 library follows the BSD behaviour.

If one on a libc5 system includes
.B "<bsd/signal.h>"
instead of
.B "<signal.h>"
then
.BR signal ()
is redefined as
.BR __bsd_signal ()
and
.BR signal ()
has the BSD semantics.
This is not recommended.

If one on a glibc2 system defines a feature test
macro such as
.B _XOPEN_SOURCE
or uses a separate
.BR sysv_signal (3)
function, one obtains classical behaviour.
This is not recommended.
.SH "SEE ALSO"
.BR kill (1),
.BR alarm (2),
.BR kill (2),
.BR pause (2),
.BR sigaction (2),
.BR sigpending (2),
.BR sigprocmask (2),
.BR sigqueue (2),
.BR sigsuspend (2),
.BR bsd_signal (3),
.BR killpg (3),
.BR raise (3),
.BR siginterrupt (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR sysv_signal (3),
.BR feature_test_macros (7),
.BR signal (7)