Force Specific Struct Size in C

Question

For various reasons, I have some structs I want to force to be specific sizes (in this case 64 bytes and 512 bytes). Both however, are below the somewhat below the sizes I want them to be.

Is there anyway for me to tell the compiler to set them to these specific sizes and pad with zeros, or would I be best off just declaring an array inside the struct that makes up the excess space so that it aligns on the size I want?

Answer 1

You can use a union.

struct mystruct_s {
    ... /* who knows how long */
};

typedef union {
    struct mystruct_s s;
    unsigned char padding[512];
} mystruct;

This will ensure the union is 512 bytes or more. Then, you can ensure that it is no more than 512 bytes using a static assertion somewhere in your code:

/* Causes a compiler error if sizeof(mystruct) != 512 */
char array[sizeof(mystruct) != 512 ? -1 : 1];

If you are using C11, there is a better way to do this. I don't know anybody who uses C11 yet. The standard was published a matter of weeks ago.

_Static_assert(sizeof(mystruct) == 512, "mystruct must be 512 bytes");

Note that the only way to pad with zeroes is to put the zeroes there manually (calloc or memset). The compiler ignores padding bytes.