correct way to return two dimensional array from a function c

Question

I have tried this but it won't work:

#include <stdio.h>

    int * retArr()
    {
    int a[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
    return a;
    }

    int main()
    {
    int a[3][3] = retArr();
    return 0;
    }

I get these errors:

Error 3 error C2075: 'a' : array initialization needs curly braces
4 IntelliSense: return value type does not match the function type

What am I doing wrong?

Answer 1

A struct is one approach:

struct t_thing { int a[3][3]; };

then just return the struct by value.

Full example:

struct t_thing {
    int a[3][3];
};

struct t_thing retArr() {
    struct t_thing thing = {
        {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        }
    };

    return thing;
}

int main(int argc, const char* argv[]) {
    struct t_thing thing = retArr();
    ...
    return 0;
}

---

The typical problem you face is that int a[3][3] = {{1,2,3},{4,5,6},{7,8,9}}; in your example refers to memory which is reclaimed after the function returns. That means it is not safe for your caller to read (Undefined Behaviour).

Other approaches involve passing the array (which the caller owns) as a parameter to the function, or creating a new allocation (e.g. using malloc). The struct is nice because it can eliminate many pitfalls, but it's not ideal for every scenario. You would avoid using a struct by value when the size of the struct is not constant or very large.