signal(2) - Linux manual page (original) (raw)
signal(2) System Calls Manual signal(2)
NAME top
signal - ANSI C signal handlingLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <signal.h>**
**typedef typeof(void (int)) *sighandler_t;**
**sighandler_t signal(int** _signum_**, sighandler_t** _handler_**);**DESCRIPTION top
**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)](../man2/sigaction.2.html) 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)](../man7/signal.7.html)) 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 top
**signal**() returns the previous value of the signal handler. On
failure, it returns **SIG_ERR**, and _[errno](../man3/errno.3.html)_ is set to indicate the
error.ERRORS top
**EINVAL** _signum_ is invalid.VERSIONS top
The use of _sighandlert_ is a GNU extension, exposed if **_GNU_SOURCE**
is defined; glibc also defines (the BSD-derived) _sigt_ 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)](../man2/sigaction.2.html),
which provides explicit control of the semantics when a signal
handler is invoked; use that interface instead of **signal**().STANDARDS top
C11, POSIX.1-2008.HISTORY top
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)](../man2/sigaction.2.html) 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)](../man7/signal.7.html)). The BSD
semantics are equivalent to calling [sigaction(2)](../man2/sigaction.2.html) 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)](../man2/sigaction.2.html) 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)](../man7/feature%5Ftest%5Fmacros.7.html) for details.)
If such a feature test macro is not defined, then **signal**()
provides System V semantics.NOTES top
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)](../man2/kill.2.html) or [raise(3)](../man3/raise.3.html). 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)](../man2/sigaction.2.html) for details on what happens when the disposition
**SIGCHLD** is set to **SIG_IGN**.
See [signal-safety(7)](../man7/signal-safety.7.html) for a list of the async-signal-safe functions
that can be safely called from inside a signal handler.SEE ALSO top
[kill(1)](../man1/kill.1.html), [alarm(2)](../man2/alarm.2.html), [kill(2)](../man2/kill.2.html), [pause(2)](../man2/pause.2.html), [sigaction(2)](../man2/sigaction.2.html), [signalfd(2)](../man2/signalfd.2.html),
[sigpending(2)](../man2/sigpending.2.html), [sigprocmask(2)](../man2/sigprocmask.2.html), [sigsuspend(2)](../man2/sigsuspend.2.html), [bsd_signal(3)](../man3/bsd%5Fsignal.3.html),
[killpg(3)](../man3/killpg.3.html), [raise(3)](../man3/raise.3.html), [siginterrupt(3)](../man3/siginterrupt.3.html), [sigqueue(3)](../man3/sigqueue.3.html), [sigsetops(3)](../man3/sigsetops.3.html),
[sigvec(3)](../man3/sigvec.3.html), [sysv_signal(3)](../man3/sysv%5Fsignal.3.html), [signal(7)](../man7/signal.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-05 signal(2)
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