SIGNAL
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index
Return to Main Contents
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
POSIX.1-2001, POSIX.1-2008, C89, C99.
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 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 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 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
feature_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),
sigpending(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)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- Portability
-
- SEE ALSO
-
- COLOPHON
-
|
|
- Running on 
-
:
: 