Section: Linux Programmer's Manual (2)
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signal - ANSI C signal handling
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
The behavior of
varies across Unix versions,
and has also varied historically across different versions of Linux.
Avoid its use: use
See Portability below.
sets the disposition of the signal
which is either
or the address of a programmer-defined function (a "signal handler").
If the signal
is delivered to the process, then one of the following happens:
If the disposition is set to
then the signal is ignored.
If the disposition is set to
then the default action associated with the signal (see
If the disposition is set to a function,
then first either the disposition is reset to
or the signal is blocked (see Portability below), and then
is called with argument
If invocation of the handler caused the signal to be blocked,
then the signal is unblocked upon return from the handler.
cannot be caught or ignored.
returns the previous value of the signal handler, or
C89, C99, POSIX.1-2001.
The effects of
in a multithreaded process are unspecified.
According to POSIX, the behavior of a process is undefined after it
signal that was not generated by
Integer division by zero has undefined result.
On some architectures it will generate a
(Also dividing the most negative integer by -1 may generate
Ignoring this signal might lead to an endless loop.
for details on what happens when
is set to
for a list of the async-signal-safe functions that can be
safely called from inside a signal handler.
The use of
is a GNU extension.
Various versions of libc predefine this type; libc4 and libc5 define
is defined, also
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
is to set a signal's disposition to
The semantics when using
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
which provides explicit control of the semantics when a
signal handler is invoked; use that interface instead of
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
and the system did not block delivery of further instances of the signal.
System V also provides these semantics for
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
(but, unfortunately, silently changed the semantics
when establishing a handler with
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:
system call provides System V semantics.
By default, in glibc 2 and later, the
wrapper function does not invoke the kernel system call.
Instead, it calls
using flags that supply BSD semantics.
This default behavior is provided as long as the
feature test macro is defined.
it is also implicitly defined if one defines
and can of course be explicitly defined.
On glibc 2 and later, if the
feature test macro is not defined, then
provides System V semantics.
(The default implicit definition of
is not provided if one invokes
in one of its standard modes
(-std=xxx or -ansi)
or defines various other feature test macros such as
function in Linux libc4 and libc5 provide System V semantics.
If one on a libc5 system includes
provides BSD semantics.
This page is part of release 3.27 of the Linux
A description of the project,
and information about reporting bugs,
can be found at
- RETURN VALUE
- CONFORMING TO
- SEE ALSO
This document was created by
using the manual pages.