NAME¶

std::signal - std::signal

Synopsis¶

Defined in header <csignal>
/* signal-handler */* signal( int sig, /* signal-handler */* (1)
handler );
extern "C" using /* signal-handler */ = void(int); (2) (exposition only*)

Changes handling of the signal sig. Depending on handler, the signal can be ignored,
set to default, or handled by a user-defined function.

When signal handler is set to a function and a signal occurs, it is implementation
defined whether std::signal(sig, SIG_DFL) will be executed immediately before the
start of signal handler. Also, the implementation can prevent some
implementation-defined set of signals from occurring while the signal handler runs.

For some of the signals, the implementation may call std::signal(sig, SIG_IGN) at
the startup of the program. For the rest, the implementation must call
std::signal(sig, SIG_DFL).

(Note: POSIX introduced sigaction to standardize these implementation-defined
behaviors)

Parameters¶

the signal to set the signal handler to. It can be an
implementation-defined value or one of the following values:

SIGABRT
sig - SIGFPE
SIGILL defines signal types
SIGINT (macro constant)
SIGSEGV
SIGTERM
the signal handler. This must be one of the following:
* SIG_DFL macro. The signal handler is set to default signal handler.
* SIG_IGN macro. The signal is ignored.
handler - * A pointer to a function. The signature of the function must be
equivalent to the following:

extern "C" void fun(int sig);

Return value¶

Previous signal handler on success or SIG_ERR on failure (setting a signal handler
can be disabled on some implementations).

Signal handler

The following limitations are imposed on the user-defined function that is installed
as a signal handler.

If the signal handler is called NOT as a result of std::abort or
std::raise (asynchronous signal), the behavior is undefined if

* the signal handler calls any function within the standard library,
except

* std::abort
* std::_Exit (until C++17)
* std::quick_exit
* std::signal with the first argument being the number of the signal
currently handled (async handler can re-register itself, but not
other signals).
* the signal handler refers to any object with static storage
duration that is not
std::atomic or
(since C++11)volatile std::sig_atomic_t.
A plain lock-free atomic operation is an invocation of a function f
from <atomic>
or <stdatomic.h>
(since C++23), such that:

* f is the function std::atomic_is_lock_free,
* f is the member function is_lock_free (e.g.
std::atomic::is_lock_free()),
* f is a non-static member function of std::atomic_flag,
* f is a non-member function, and the first parameter of f has type
cv std::atomic_flag*,
* f is a non-static member function invoked on an object obj, such
that obj.is_lock_free() yields true, or
* f is a non-member function, and for every pointer-to-atomic
argument arg passed to f, std::atomic_is_lock_free(arg) yields
true.

The behavior is undefined if any signal handler performs any of the
following:

* call to any library function, except for plain lock-free atomic
operations and the following signal-safe functions (note, in
particular, dynamic allocation is not signal-safe): (since C++17)

* std::signal with the first argument being the number of the signal
currently handled (signal handler can re-register itself, but not
other signals).
* member functions of std::numeric_limits
* std::_Exit
* std::abort
* std::quick_exit
* The member functions of std::initializer_list and the
std::initializer_list overloads of std::begin and std::end
* std::forward, std::move, std::move_if_noexcept
* All functions from <type_traits>
* std::memcpy and std::memmove
* access to an object with thread storage duration
* a dynamic_cast expression
* a throw expression
* entry to a try block
* initialization of a static variable that performs dynamic
non-local initialization (including delayed until first ODR-use)
* waits for completion of initialization of any variable with static
storage duration due to another thread concurrently initializing
it

If the user defined function returns when handling SIGFPE, SIGILL, SIGSEGV or any
other implementation-defined signal specifying a computational exception, the
behavior is undefined.

If the signal handler is called as a result of std::abort or std::raise (synchronous
signal), the behavior is undefined if the signal handler calls std::raise.

On entry to the signal handler, the state of the floating-point
environment and the values of all objects is unspecified, except for

* objects of type volatile std::sig_atomic_t

* objects of lock-free std::atomic types (until C++14)
* side effects made visible through (since C++11)
std::atomic_signal_fence

On return from a signal handler, the value of any object modified by
the signal handler that is not volatile std::sig_atomic_t or lock-free
std::atomic is indeterminate.
A call to the function signal() synchronizes-with any resulting
invocation of the signal handler.

If a signal handler is executed as a result of a call to std::raise
(synchronously), then the execution of the handler is sequenced-after
the invocation of std::raise and sequenced-before the return from it
and runs on the same thread as std::raise. Execution of the handlers
for other signals is unsequenced with respect to the rest of the
program and runs on an unspecified thread.
(since C++14)
Two accesses to the same object of type volatile std::sig_atomic_t do
not result in a data race if both occur in the same thread, even if
one or more occurs in a signal handler. For each signal handler
invocation, evaluations performed by the thread invoking a signal
handler can be divided into two groups A and B, such that no
evaluations in B happen-before evaluations in A, and the evaluations
of such volatile std::sig_atomic_t objects take values as though all
evaluations in A happened-before the execution of the signal handler
and the execution of the signal handler happened-before all
evaluations in B.

Notes¶

POSIX requires that signal is thread-safe, and specifies a list of async-signal-safe
library functions that may be called from any signal handler.

Signal handlers are expected to have C linkage and, in general, only use the
features from the common subset of C and C++. However, common implementations allow
a function with C++ linkage to be used as a signal handler.

Example¶

// Run this code

#include <csignal>
#include <iostream>

namespace
{
volatile std::sig_atomic_t gSignalStatus;
}

void signal_handler(int signal)
{
gSignalStatus = signal;
}

int main()
{
// Install a signal handler
std::signal(SIGINT, signal_handler);

std::cout << "SignalValue: " << gSignalStatus << '\n';
std::cout << "Sending signal: " << SIGINT << '\n';
std::raise(SIGINT);
std::cout << "SignalValue: " << gSignalStatus << '\n';
}

Possible output:¶

SignalValue: 0
Sending signal: 2
SignalValue: 2

References¶

* C++23 standard (ISO/IEC 14882:2023):

* 17.13.5 Signal handlers [support.signal]
* C++20 standard (ISO/IEC 14882:2020):

* 17.13.5 Signal handlers [support.signal]
* C++17 standard (ISO/IEC 14882:2017):

* 21.10.4 Signal handlers [support.signal]

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3756 C++17 it was unclear whether std::atomic_flag is it is
signal-safe

See also¶

raise runs the signal handler for particular signal
(function)
atomic_signal_fence fence between a thread and a signal handler executed in the same
(C++11) thread
(function)
C documentation for
signal