signal - ANSI C signal handling
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t
The behavior of signal
() varies across UNIX versions, and has also varied
historically across different versions of Linux. Avoid its use
instead. See Portability
() sets the disposition of the signal signum
, which is either SIG_IGN
, 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))
- 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
cannot be caught or ignored.
() returns the previous value of the signal handler, or
- 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
, or SIGSEGV
signal that was not generated
. Integer division by zero has undefined
result. On some architectures it will generate a SIGFPE
dividing the most negative integer by -1 may generate SIGFPE
this signal might lead to an endless loop.
for details on what happens when SIGCHLD
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
is defined. Without use of such a type, the declaration of
() 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
. The semantics when using signal
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)
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
() 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
- 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
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
- 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.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found