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SIGNAL(2) Linux Programmer's Manual SIGNAL(2)
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 with 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 sig‐
nal 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 Sys‐
tem 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 func‐
tions are listed explicitly in the various standards. The POSIX.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() fdata‐
sync() fork() fpathconf() fstat() fsync() ftruncate() getegid()
geteuid() getgid() getgroups() getpeername() getpgrp() getpid() getp‐
pid() 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() sig‐
fillset() sigismember() signal() sigpause() sigpending() sigprocmask()
sigqueue() sigset() sigsuspend() sleep() socket() socketpair() stat()
symlink() sysconf() tcdrain() tcflow() tcflush() tcgetattr() tcgetp‐
grp() 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 sig‐
nal. (Also dividing the most negative integer by -1 may generate
SIGFPE.) Ignoring this signal might lead to an endless loop.
See sigaction(2) for details on what happens when SIGCHLD is set to
SIG_IGN.
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
C89, POSIX.1-2001.
SEE ALSO
kill(1), alarm(2), kill(2), pause(2), sigaction(2), sigpending(2), sig‐
procmask(2), sigqueue(2), sigsuspend(2), killpg(3), raise(3), sigse‐
tops(3), sigvec(3), feature_test_macros(7), signal(7)
Linux 2.2 2000-04-28 SIGNAL(2)
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