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signal - ANSI C signal handling
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
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
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.
The signal() function returns the previous value of the signal handler,
or SIG_ERR on error.
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
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.
See sigaction(2) for details on what happens when SIGCHLD is set to
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.
kill(1), alarm(2), kill(2), pause(2), sigaction(2), sigpending(2),
sigprocmask(2), sigqueue(2), sigsuspend(2), killpg(3), raise(3),
sigsetops(3), sigvec(3), signal(7)