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SIGNAL(2) Linux Programmer's Manual SIGNAL(2)
signal - ANSI C signal handling
#include <signal.h>
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
The behavior of signal() varies across UNIX versions, and has also varied
historically across different versions of Linux. Avoid its use: use
sigaction(2) instead. See Portability below.
signal() sets the disposition of the signal signum to handler, which is either
SIG_IGN, SIG_DFL, or the address of a programmer-defined function (a "signal
handler").
If the signal signum is delivered to the process, then one of the following
happens:
* If the disposition is set to SIG_IGN, then the signal is ignored.
* If the disposition is set to SIG_DFL, then the default action associated
with the signal (see signal(7)) occurs.
* If the disposition is set to a function, then first either the disposition
is reset to SIG_DFL, or the signal is blocked (see Portability below), and
then handler is called with argument signum. If invocation of the handler
caused the signal to be blocked, then the signal is unblocked upon return
from the handler.
The signals SIGKILL and SIGSTOP cannot be caught or ignored.
signal() returns the previous value of the signal handler, or SIG_ERR on
error.
EINVAL signum is invalid.
C89, C99, POSIX.1-2001.
The effects of signal() in a multithreaded process are unspecified.
According to POSIX, the behavior of a process is undefined after it ignores a
SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill(2) or
raise(3). 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 SIG_IGN.
See signal(7) for a list of the async-signal-safe functions that can be safely
called from inside a signal handler.
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. Without use of such a
type, the declaration of signal() is the somewhat harder to read:
void ( *signal(int signum, void (*handler)(int)) ) (int);
The only portable use of signal() is to set a signal's disposition to SIG_DFL
or SIG_IGN. The semantics when using signal() to establish a signal handler
vary across systems (and POSIX.1 explicitly permits this variation); do not
use it for this purpose.
POSIX.1 solved the portability mess by specifying sigaction(2), which provides
explicit control of the semantics when a signal handler is invoked; use that
interface instead of signal().
In the original UNIX systems, when a handler that was established using
signal() was invoked by the delivery of a signal, the disposition of the
signal would be reset to SIG_DFL, and the system did not block delivery of
further instances of the signal. System V also provides these semantics for
signal(). This was bad because the signal might be delivered again before the
handler had a chance to reestablish itself. Furthermore, rapid deliveries of
the same signal could result in recursive invocations of the handler.
BSD improved on this situation by changing the semantics of signal handling
(but, unfortunately, silently changed the semantics when establishing a
handler with signal()). On BSD, when a signal handler is invoked, the signal
disposition is not reset, and further instances of the signal are blocked from
being delivered while the handler is executing.
The situation on Linux is as follows:
* The kernel's signal() system call provides System V semantics.
* By default, in glibc 2 and later, the signal() wrapper function does not
invoke the kernel system call. Instead, it calls sigaction(2) using flags
that supply BSD semantics. This default behavior is provided as long as the
_BSD_SOURCE feature test macro is defined. By default, _BSD_SOURCE is
defined; it is also implicitly defined if one defines _GNU_SOURCE, and can
of course be explicitly defined.
On glibc 2 and later, if the _BSD_SOURCE feature test macro is not defined,
then signal() provides System V semantics. (The default implicit definition
of _BSD_SOURCE is not provided if one invokes gcc(1) in one of its standard
modes (-std=xxx or -ansi) or defines various other feature test macros such
as _POSIX_SOURCE, _XOPEN_SOURCE, or _SVID_SOURCE; see
feature_test_macros(7).)
* The signal() function in Linux libc4 and libc5 provide System V semantics.
If one on a libc5 system includes <bsd/signal.h> instead of <signal.h>, then
signal() provides BSD semantics.
kill(1), alarm(2), kill(2), killpg(2), pause(2), sigaction(2), signalfd(2),
sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), bsd_signal(3),
raise(3), siginterrupt(3), sigsetops(3), sigvec(3), sysv_signal(3), signal(7)
This page is part of release 3.32 of the Linux man-pages project. A
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at http://www.kernel.org/doc/man-pages/.
Linux 2008-07-11 SIGNAL(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface