.\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
.\" based on work by Rik Faith <faith@cs.unc.edu>
.\" and Mike Battersby <mike@starbug.apana.org.au>.
.\"
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.TH SIGNAL 2 2000-04-28 "Linux 2.2" "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
.BR signal ()
system call installs a new signal handler for the signal with number
.IR signum .
The signal handler is set to
.I sighandler
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 to the signal (see
.BR signal (7))
occurs.
Finally, if the handler is set to a function
.I sighandler
then first either the handler is reset to SIG_DFL
or an implementation-dependent blocking of the signal
is performed and next
.I sighandler
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"
The
.BR signal ()
function returns the previous value of the signal handler, or
.B SIG_ERR
on error.
.SH 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 
.B signal
is redefined as
.B __bsd_signal
and 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
.B sysv_signal
function, one obtains classical behaviour.  This is not recommended.

Trying to change the semantics of this call using
defines and includes is not a good idea.  It is better to avoid
.B signal
altogether, and use
.BR sigaction (2)
instead.
.SH NOTES
The effects of this call in a multi-threaded process are unspecified.
.PP
The routine
.I handler
must be very careful, since processing elsewhere was interrupted
at some arbitrary point. POSIX has the concept of "safe function".
If a signal interrupts an unsafe function, and
.I handler
calls an unsafe function, then the behavior is undefined. Safe
functions are listed explicitly in the various standards.
The POSIX 1003.1-2003 list is

_Exit()
_exit()
abort()
accept()
access()
aio_error()
aio_return()
aio_suspend()
alarm()
bind()
cfgetispeed()
cfgetospeed()
cfsetispeed()
cfsetospeed()
chdir()
chmod()
chown()
clock_gettime()
close()
connect()
creat()
dup()
dup2()
execle()
execve()
fchmod()
fchown()
fcntl()
fdatasync()
fork()
fpathconf()
fstat()
fsync()
ftruncate()
getegid()
geteuid()
getgid()
getgroups()
getpeername()
getpgrp()
getpid()
getppid()
getsockname()
getsockopt()
getuid()
kill()
link()
listen()
lseek()
lstat()
mkdir()
mkfifo()
open()
pathconf()
pause()
pipe()
poll()
posix_trace_event()
pselect()
raise()
read()
readlink()
recv()
recvfrom()
recvmsg()
rename()
rmdir()
select()
sem_post()
send()
sendmsg()
sendto()
setgid()
setpgid()
setsid()
setsockopt()
setuid()
shutdown()
sigaction()
sigaddset()
sigdelset()
sigemptyset()
sigfillset()
sigismember()
signal()
sigpause()
sigpending()
sigprocmask()
sigqueue()
sigset()
sigsuspend()
sleep()
socket()
socketpair()
stat()
symlink()
sysconf()
tcdrain()
tcflow()
tcflush()
tcgetattr()
tcgetpgrp()
tcsendbreak()
tcsetattr()
tcsetpgrp()
time()
timer_getoverrun()
timer_gettime()
timer_settime()
times()
umask()
uname()
unlink()
utime()
wait()
waitpid()
write().
.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 the
.BR kill (2)
or the
.BR raise (3)
functions.
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
According to POSIX (3.3.1.3) it is unspecified what happens when
.B SIGCHLD
is set to
.BR SIG_IGN .
Here the BSD and SYSV behaviours differ, causing BSD software
that sets the action for
.B SIGCHLD
to
.B SIG_IGN
to fail on Linux.
.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 .
.SH "CONFORMING TO"
ANSI C
.SH "SEE ALSO"
.BR kill (1),
.BR alarm (2),
.BR kill (2),
.BR killpg (2),
.BR pause (2),
.BR sigaction (2),
.BR sigvec (2),
.BR raise (3),
.BR sigsetops (3),
.BR signal (7)