signal(2) -- Linux man page
NAME
signal - ANSI C signal handling
SYNOPSIS
#include <signal.h>
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
DESCRIPTION
The
signal()
system call installs a new signal handler for the signal with number
signum.
The signal handler is set to
sighandler
which may be a user specified function, or either
SIG_IGN
or
SIG_DFL.
Upon arrival of a signal with number
signum
the following happens.
If the corresponding handler is set to
SIG_IGN,
then the signal is ignored.
If the handler is set to
SIG_DFL,
then the default action associated to the signal (see
signal(7))
occurs.
Finally, if the handler is set to a function
sighandler
then first either the handler is reset to SIG_DFL
or an implementation-dependent blocking of the signal
is performed and next
sighandler
is called with argument
signum.
Using a signal handler function for a signal
is called "catching the signal".
The signals
SIGKILL
and
SIGSTOP
cannot be caught or ignored.
RETURN VALUE
The
signal()
function returns the previous value of the signal handler, or
SIG_ERR
on error.
PORTABILITY
The original Unix
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
<bsd/signal.h>
instead of
<signal.h>
then
signal
is redefined as
__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
_XOPEN_SOURCE
or uses a separate
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
signal
altogether, and use
sigaction(2)
instead.
NOTES
The effects of this call in a multi-threaded process are unspecified.
The routine
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
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().
According to POSIX, the behaviour of a process is undefined after it
ignores a
SIGFPE,
SIGILL,
or
SIGSEGV
signal that was not generated by the
kill(2)
or the
raise(3)
functions.
Integer division by zero has undefined result.
On some architectures it will generate a
SIGFPE
signal.
(Also dividing the most negative integer by -1 may generate
SIGFPE.)
Ignoring this signal might lead to an endless loop.
According to POSIX (3.3.1.3) it is unspecified what happens when
SIGCHLD
is set to
SIG_IGN.
Here the BSD and SYSV behaviours differ, causing BSD software
that sets the action for
SIGCHLD
to
SIG_IGN
to fail on Linux.
The use of
sighandler_t
is a GNU extension.
Various versions of libc predefine this type; libc4 and libc5 define
SignalHandler,
glibc defines
sig_t
and, when
_GNU_SOURCE
is defined, also
sighandler_t.
CONFORMING TO
ANSI C
SEE ALSO
kill(1),
kill(2),
killpg(2),
pause(2),
raise(3),
sigaction(2),
signal(7),
sigsetops(3),
sigvec(2),
alarm(2)
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