What's the meaning of this piece of code? void (*signal(int sig, void (*func)(int)))(int);

Question

I came across this piece of code and completely got lost interpreting its meaning.

#include 
void (*signal(int sig, void (*func)(int)))(int);         


        

Answer 1

Let's take an example of how this nasty declaration could be used:

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

Without too much verbosity, we could say that "signal" is a function with two parameters that returns a function.

#include 

// First function that could be returned by our principal function
// Note that we can point to it using void (*f)(int)
void car_is_red(int color)
{
    printf("[car_is_red] Color %d (red) is my favorite color too !\n", color);
}

// Second function that could be returned by our principal function
// Note that we can point to it using void (*f)(int)
void car_is_gray(int color)
{
    printf("[car_is_gray] I don't like the color %d (gray) either !\n", color);
}

// The function taken as second parameter by our principal function
// Note that we can point to it using void (*func)(int)
void show_car_type(int mod)
{
    printf("[show_car_type] Our car has the type: %d\n",mod);
}

/* Our principal function. Takes two parameters, returns a function. */
void (* show_car_attributes(int color, void (*func)(int)) )(int)
{
    printf("[show_car_attributes] Our car has the color: %d\n",color); // Use the first parameter

    int mod = 11;  // Some local variable of our function show_car_attributes()
    func(mod);  // Call the function pointed by the second parameter (i.e. show_car_type() )

    // Depending on color value, return the pointer to one of two functions
    // Note that we do NOT use braces with function names
    if (color == 1)
        return car_is_red;
    else
        return car_is_gray;
    }


//main() function
int main()
{
    int color = 2;   // Declare our color for the car
    void (*f)(int);  // Declare a pointer to a function with one parameter (int)

    f = show_car_attributes(color, show_car_type); // f will take the return 
           // value of our principal function. Stated without braces, the 
           // parameter  "show_car_types" is a function pointer of type 
           // void (*func)(int).

    f(color);  // Call function that was returned by show_car_attributes()

    return 0;
}

Let's see what will be output:

If color = 1

[show_car_attributes] Our car has the color: 1
[show_car_type] Our car has the type: 11
[car_is_red] Color 1 (red) is my favorite color too !

If color = 2

[show_car_attributes] Our car has the color: 2
[show_car_type] Our car has the type: 11
[car_is_gray] I don't like the color 2 (gray) either !