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-.\" 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
-.\"      <mtk.manpages@gmail.com>
-.\" based on work by Rik Faith <faith@cs.unc.edu>
-.\" and Mike Battersby <mike@starbug.apana.org.au>.
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" 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 (date) "Linux man-pages (unreleased)"
-.SH NAME
-signal \- ANSI C signal handling
-.SH LIBRARY
-Standard C library
-.RI ( libc ", " \-lc )
-.SH SYNOPSIS
-.nf
-.B #include <signal.h>
-.PP
-.B typedef void (*sighandler_t)(int);
-.PP
-.BI "sighandler_t signal(int " signum ", sighandler_t " handler );
-.fi
-.SH DESCRIPTION
-.BR WARNING :
-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.
-.PP
-.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").
-.PP
-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.
-On failure, it returns
-.BR SIG_ERR ,
-and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-.TP
-.B EINVAL
-.I signum
-is invalid.
-.SH VERSIONS
-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
-.B _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:
-.PP
-.in +4n
-.EX
-.BI "void ( *" signal "(int " signum ", void (*" handler ")(int)) ) (int);"
-.EE
-.in
-.SS Portability
-The only portable use of
-.BR signal ()
-is to set a signal's disposition to
-.B 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.
-.PP
-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 ().
-.SH STANDARDS
-C11, POSIX.1-2008.
-.SH HISTORY
-C89, POSIX.1-2001.
-.PP
-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:
-.PP
-.in +4n
-.EX
-sa.sa_flags = SA_RESETHAND | SA_NODEFER;
-.EE
-.in
-.PP
-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.
-.PP
-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:
-.PP
-.in +4n
-.EX
-sa.sa_flags = SA_RESTART;
-.EE
-.in
-.PP
-The situation on Linux is as follows:
-.IP \[bu] 3
-The kernel's
-.BR signal ()
-system call provides System\ V semantics.
-.IP \[bu]
-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
-.B _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 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 the disposition
-.B SIGCHLD
-is set to
-.BR SIG_IGN .
-.PP
-See
-.BR signal\-safety (7)
-for a list of the async-signal-safe functions that can be
-safely called from inside a signal handler.
-.SH SEE ALSO
-.BR kill (1),
-.BR alarm (2),
-.BR kill (2),
-.BR pause (2),
-.BR sigaction (2),
-.BR signalfd (2),
-.BR sigpending (2),
-.BR sigprocmask (2),
-.BR sigsuspend (2),
-.BR bsd_signal (3),
-.BR killpg (3),
-.BR raise (3),
-.BR siginterrupt (3),
-.BR sigqueue (3),
-.BR sigsetops (3),
-.BR sigvec (3),
-.BR sysv_signal (3),
-.BR signal (7)