结构返回功能

Question

In C component selection, what is the benefit of structure-returning function? for example:

struct S {
   int a, b;
} x;

Why is it that I can assign the above struct as a function as shown below, Is there any benefit of doing this?

extern struct S f(); /* Why is this neccesary? */
x = f(); /* Is this accurate */

Open my eyes on this guys.

Answer 1

It's just a function that happens to return a struct . There's nothing more to it than that. You wouldn't be surprised to see a function return an int , why be surprised when one returns a struct?

As an aside, the extern is superfluous here because that is the default storage class for functions.

Answer 2

It is useful so that you can return multiple values from a function.

For example, you can use it like this

struct Point {
   int x;
   int y;
};

struct Point getMousePos()
{
    struct Point pos;
    pos.x = 567;
    pos.y = 343;
    return pos; 
}

int main()
{
    struct Point mouse_pos = getMousePos();
    printf("Mousepos %d,%d\n", mouse_pos.x, mouse_pos.y");   
}

The function can be forward declared with extern (this would normally be done in a header file), so that other functions know its prototype ie its parameters and return type, even if the function is itself defined in another file.

Answer 3

如果你得到一个结构的副本而不是指向它的指针，你知道你永远不必担心free()它，或者是否有任何数据争用，其中一个线程正在写入结构而另一个线程从中读取，或者该函数返回的指针是否会被某些可能超出您控制范围的操作无效。