ANSI C do you have to use malloc() when creating a struct?

Question

Let's say I have this struct in ANSI C:

typedef struct _point
{
    float x;
    float y;
} Point;

and this function to create this struct:

Point createpoint(float x, float y)
{
    Point p;
    p.x = x;
    p.y = y;
    return p; 
}

This allows me to create a struct with this function i.e:

int main()
{
    Point pointOne = createpoint(5, 6);  
    Point pointTwo = createpoint(10, 4);
    float distance = calculatedistancefunc(pointOne, pointTwo);

    /* ...other stuff */

    return 0;
}

Someone told me that this code is not valid, because the struct does not get malloc'd in the createpoint(float x, float y) function before it gets returned, and that the struct will be deleted. However, when I use my struct like this, it doesn't seem to get deleted.

So my question is: do I have to malloc this struct, and why? / why not?

Answer 1

Whatever you are doing is entirely correct. The statement -

return p;

in the function returns a copy of the local variable p. But if you want the same object that was created in the function, then you need to malloc it. However, you need to free it later.

Point createpoint(float x, float y)
{
    Point p;
    p.x = x;
    p.y = y;
    return p; 
} // p is no longer valid from this point. So, what you are returning is a copy of it.

But -

Point* createpoint(float x, float y)
{
    Point *p = malloc(sizeof(Point));
    p->x = x;
    p->y = y;
    return p; 
}// Now you return the object that p is pointing to.