SIGNAL(2) Linux Programmer's Manual SIGNAL(2)
NAME
signal - ANSI C signal handling
SYNOPSIS
#include<signal.h>
typedefvoid(*sighandler_t)(int);
sighandler_tsignal(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 han-
dler 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.
RETURNVALUE
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 seman-
tics. 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
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.
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.
CONFORMINGTO
ANSI C
SEEALSO
kill(1), kill(2), killpg(2), pause(2), raise(3), sigaction(2), sig-
nal(7), sigsetops(3), sigvec(2), alarm(2)
Linux 2.2 2000-04-28 SIGNAL(2)
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