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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 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.
RETURN VALUE
signal() returns the previous value of the signal handler, or SIG_ERR on error. In the
event of an error, errno is set to indicate the cause.
ERRORS
EINVAL signum is invalid.
CONFORMING TO
C89, C99, POSIX.1-2001.
NOTES
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, exposed if _GNU_SOURCE is defined; glibc also
defines (the BSD-derived) sig_t if _BSD_SOURCE is defined. Without use of such a type,
the declaration of signal() is the somewhat harder to read:
void ( *signal(int signum, void (*handler)(int)) ) (int);
Portability
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. This
is equivalent to calling sigaction(2) with the following flags:
sa.sa_flags = SA_RESETHAND | SA_NODEFER;
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, but unfortunately also changed the semantics of the
existing signal() interface while doing so. 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. Furthermore, certain blocking system
calls are automatically restarted if interrupted by a signal handler (see signal(7)). The
BSD semantics are equivalent to calling sigaction(2) with the following flags:
sa.sa_flags = SA_RESTART;
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.
SEE ALSO
kill(1), alarm(2), kill(2), killpg(2), pause(2), sigaction(2), signalfd(2), sigpending(2),
sigprocmask(2), sigsuspend(2), bsd_signal(3), raise(3), siginterrupt(3), sigqueue(3),
sigsetops(3), sigvec(3), sysv_signal(3), signal(7)
COLOPHON
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