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# std::signal

From cppreference.com
< cpp‎ | utility‎ | program

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

Sets the error handler for signal `sig`. The signal handler can be set so that default handling will occur, signal is ignored, or a user-defined function is called.

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)

### [edit] Parameters

sig | - | the signal to set the signal handler to. It can be an implementation-defined value or one of the following values:
SIGABRT SIGFPE SIGILL SIGINT SIGSEGV SIGTERM | defines signal types (macro constant) [edit]
handler | - | 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. pointer to a function. The signature of the function must be equivalent to the following:
void fun(int sig); |  |

### [edit] Return value

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

### [edit] Signal handler

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

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, the behavior is undefined if the signal handler calls std::raise.

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

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

  - std::abort
  - std::_Exit
  - 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 or thread-local(since C++11) storage duration that is not std::atomic(since C++11) or volatile std::sig_atomic_t.

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 std::atomic types (since C++11)
- side effects made visible through std::atomic_signal_fence (since C++11)

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 std::atomic is undefined.

The behavior is undefined if `std::signal` is used in a multithreaded program. It is not required to be thread-safe.

### [edit] 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++. It is implementation-defined if a function with C++ linkage can be used as a signal handler.

### [edit] 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

### [edit] See also

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

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