Nested function pointer declarations [duplicate]

Question

I found this declaration in the APUE book in the chapter on signals:

void (*signal(int signo, void (*func)(int)))(int);

I don't fully understand the syntax. The declaration of (*func) is syntactically what I expect for a function parameter/pointer. I don't understand the syntax where "signal" is declared, and the trailing (int). I was wondering if someone could clarify this. -Thanks

Answer 1

The function signal returns a pointer to a function of type void (int), and has two arguments: an int and a pointer to a function of type void (int).

So something like this should work

void foo(int);

void (*signal(int signo, void (*func)(int)))(int)
{
    return func;
}

int main(int argc, char *argv[])
{
    signal(42, foo);

    return 0;
}