Unknown return type for a C function

I am writing a library for C, and one function can return either a string (char *), an integer or a double. Next to that, the length of the string is unknown. I really don't know how to deal with this problem. I thought about using pointers as arguments to the function but that is really messy.

Can anyone give me a solution, and maybe some short sample code? Thanks.

Edit

What ab开发者_运维知识库out a void pointer as return type, and a pointer to the size of the returned value as argument:

void* func(int x, int y, int *size) { ... }
/* or */
void* func(int x, int y, int &size) { ... }
/* always confused about them ): */

?

If a function can return multiple types, I would not recommend using a union (not alone, at least). Your function needs to return two pieces of information: the result, and the type of the result. You might get the best mileage out of using a structure like this one:

enum return_val_type {STRING, INT, DOUBLE};
struct return_val {
    enum return_val_type return_type;
    union {
        char*  s;
        int    i;
        double d;
    };
};

That way, the caller can check the value of return_type and determine how they need to interpret the data (that is, which union field to use).

It sounds like really bad design, but if you feel that you have to do this for some reason then you could use a union to return different types.

You're going to have to redesign your function, or have multiple versions based on type.

Just think about it - how would the compiler be able to handle it? Assuming the implementation is in a separate .c file, the function is a black box to the compiler, so it has no idea what to expect.

You could either one version of the function for each type, or return a pointer, or take a pointer as an argument and store the value in there (which could be unsafe if the size of your result is unknown).