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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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signal(2) System Calls Manual signal(2)
signal - ANSI C signal handling
Standard C library (libc, -lc)
#include <signal.h>
typedef typeof(void (int)) *sighandler_t;
sighandler_t signal(int signum, sighandler_t handler);
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 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.
signal() returns the previous value of the signal handler. On
failure, it returns SIG_ERR, and errno is set to indicate the
error.
EINVAL signum is invalid.
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. The standard definition of signal()
is:
typeof(void (int)) *signal(int signum, typeof(void (int)) *handler);
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().
C11, POSIX.1-2008.
C89, POSIX.1-2001.
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.
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.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.10 2025-01-05 signal(2)
Pages that refer to this page: alarm(2), getitimer(2), kill(2), pause(2), prctl(2), PR_GET_KEEPCAPS(2const), PR_GET_PDEATHSIG(2const), PR_GET_TIMERSLACK(2const), sigaction(2), sigpending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), sigwaitinfo(2), syscalls(2), wait(2), wait4(2), bsd_signal(3), gsignal(3), killpg(3), profil(3), raise(3), siginterrupt(3), sigqueue(3), sigset(3), sigvec(3), sleep(3), sysv_signal(3), systemd.exec(5), fifo(7), signal(7), signal-safety(7)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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