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# signal

From cppreference.com
< c‎ | program

Defined in header `<signal.h>` |  |
void (*signal( int sig, void (*handler) (int))) (int); |  |

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

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

### [edit] Parameters

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

If the user defined function returns when handling SIGFPE, SIGILL or SIGSEGV, the behavior is undefined.

If the signal handler is called as a result of abort or raise, the behavior is undefined if the signal handler calls raise.

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

- the signal handler calls any function within the standard library, except abort, _Exit, quick_exit, or signal with the first argument not being the number of the signal currently handled.
- the signal handler refers to any object with static or thread-local(since C11) storage duration that is not lock-free atomic(since C11) 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 sig_atomic_t
- objects of lock-free atomic types (since C11)
- side effects made visible through atomic_signal_fence (since C11)

On return from a signal handler, the value of any object modified by the signal handler that is not volatile sig_atomic_t or lock-free atomic(since C11) is undefined.

The behavior is undefined if 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.

### [edit] Example

Run this code

    #include <stdio.h>
    #include <signal.h>

    /* A user-defined signal handler can process one or more signals. */
    static void signal_handler(int signo)
    {
        switch(signo)
        {
            case SIGABRT : puts("Process SIGABRT signal here."); break;
            case SIGFPE  : puts("Process SIGFPE  signal here."); break;
            case SIGILL  : puts("Process SIGILL  signal here."); break;
            case SIGINT  : puts("Process SIGINT  signal here."); break;
            case SIGSEGV : puts("Process SIGSEGV signal here."); break;
            case SIGTERM : puts("Process SIGTERM signal here."); break;
        }

        /* some cleanup here */
        /* exit(signo);      */
    }

    int main(void)
    {
        /* Register a user-defined signal handler. */
        signal(SIGABRT, signal_handler);
        signal(SIGFPE,  signal_handler);
        signal(SIGILL,  signal_handler);
        signal(SIGINT,  signal_handler);
        signal(SIGSEGV, signal_handler);
        signal(SIGTERM, signal_handler);

        /* Raise a signal. */
        raise(SIGABRT);
        raise(SIGFPE);
        raise(SIGILL);
        raise(SIGINT);
        raise(SIGSEGV);
        raise(SIGTERM);

        return 0;
    }

Output:

    Process SIGABRT signal here.
    Process SIGFPE  signal here.
    Process SIGILL  signal here.
    Process SIGINT  signal here.
    Process SIGSEGV signal here.
    Process SIGTERM signal here.

### [edit] See also

raise | runs the signal handler for particular signal (function) [edit]
C++ documentation for signal

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