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[thirdparty/man-pages.git] / man2 / signal.2

.\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
.\" and Copyright (c) 2007 Michael Kerrisk <mtk-manpages@gmx.net>
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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 rerote
.\"     various sections.
.\"
.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 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
.IR 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.
.SH ERRORS
.TP
.B EINVAL
.I signum
is invalid.
.SH "CONFORMING TO"
C89, C99, POSIX.1-2001.
.SH NOTES
The effects of
.BR signal ()
in a multi-threaded 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 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
.BR 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 behavior.

If one on a libc5 system includes
.I <bsd/signal.h>
instead of
.I <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 behavior.
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)