### 9.4 `signal`—specify handler subroutine for a signal ¶

Synopsis

    #include <signal.h>
    void (*signal(int sig, void(*func)(int))) (int);

    void (*_signal_r(void *reent, int sig, void(*func)(int))) (int);

Description
`signal` provides a simple signal-handling implementation for embedded targets.

`signal` allows you to request changed treatment for a particular signal sig. You can use one of the predefined macros `SIG_DFL` (select system default handling) or `SIG_IGN` (ignore this signal) as the value of func; otherwise, func is a function pointer that identifies a subroutine in your program as the handler for this signal.

Some of the execution environment for signal handlers is unpredictable; notably, the only library function required to work correctly from within a signal handler is `signal` itself, and only when used to redefine the handler for the current signal value.

Static storage is likewise unreliable for signal handlers, with one exception: if you declare a static storage location as ‘`volatile sig_atomic_t`’, then you may use that location in a signal handler to store signal values.

If your signal handler terminates using `return` (or implicit return), your program’s execution continues at the point where it was when the signal was raised (whether by your program itself, or by an external event). Signal handlers can also use functions such as `exit` and `abort` to avoid returning.

The alternate function `_signal_r` is the reentrant version. The extra argument reent is a pointer to a reentrancy structure.

Returns
If your request for a signal handler cannot be honored, the result is `SIG_ERR`; a specific error number is also recorded in `errno`.

Otherwise, the result is the previous handler (a function pointer or one of the predefined macros).

Portability
ANSI C requires `signal`.

No supporting OS subroutines are required to link with `signal`, but it will not have any useful effects, except for software generated signals, without an operating system that can actually raise exceptions.