signal(2) System Calls Manual signal(2)
NAME
signal - ANSI C signal handling
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <signal.h>
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
DESCRIPTION
WARNING: 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 func-
tion (a "signal handler").
If the signal signum is delivered to the process, then one of the fol-
lowing 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 asso-
ciated with the signal (see signal(7)) occurs.
* If the disposition is set to a function, then first either the dis-
position is reset to SIG_DFL, or the signal is blocked (see Porta-
bility 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. On failure,
it returns SIG_ERR, and errno is set to indicate the error.
ERRORS
EINVAL signum is invalid.
STANDARDS
POSIX.1-2001, POSIX.1-2008, C99.
NOTES
The effects of signal() in a multithreaded process are unspecified.
According to POSIX, the behavior of a process is undefined after it ig-
nores 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 the disposition
SIGCHLD is set to SIG_IGN.
See signal-safety(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
(glibc 2.19 and earlier) or _DEFAULT_SOURCE (glibc 2.19 and later) 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 in-
voked; 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 de-
livery of further instances of the signal. This is equivalent to call-
ing sigaction(2) with the following flags:
sa.sa_flags = SA_RESETHAND | SA_NODEFER;
System V also provides these semantics for signal(). This was bad be-
cause 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 se-
mantics 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 sigac-
tion(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 a suitable feature test macro is defined:
_BSD_SOURCE on glibc 2.19 and earlier or _DEFAULT_SOURCE in glibc
2.19 and later. (By default, these macros are defined; see fea-
ture_test_macros(7) for details.) If such a feature test macro is
not defined, then signal() provides System V semantics.
SEE ALSO
kill(1), alarm(2), kill(2), pause(2), sigaction(2), signalfd(2), sig-
pending(2), sigprocmask(2), sigsuspend(2), bsd_signal(3), killpg(3),
raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3),
sysv_signal(3), signal(7)
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