signal - ANSI C signal handling
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
* 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
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
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
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),
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