What value does the compiler assign to an uninitialised member of a const struct in C?

Question

Say I have a struct typedef that is:

typedef struct {
    int32_t memberOne;
    int32_t memberTwo;
} myStruct_t;

I instantiate a const of that type as follows:

const myStruct_t myConst = {.memberTwo = 32};

What does C say the compiler should set memberOne to be? I've tried it, of course, and I happen to get 0 on the compilers I've tried, what I'm after here is does the C standard require uninitialised members of a const struct to be initialised to something or other by the compiler, i.e. would the code above be considered portable? Clause 6.7.9 of C99 says:

If an object that has static or thread storage duration is not initialized explicitly, then:

— if it has pointer type, it is initialized to a null pointer;

— if it has arithmetic type, it is initialized to (positive or unsigned) zero;

— if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

— if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

...but what about consts? Are they considered to be of static storage type, just without the static keyword?

Answer 1

does the C standard require uninitialised members of a const struct to be initialised to something

Yes, they are guaranteed to be set to zero/null pointers as long as you initialize at least one member explicitly. const plays no part in it.

You've already found the relevant part about how objects with static storage duration are initialized. Just keep reading the same chapter:

C17 6.7.9 §19

The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject; all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

C17 6.7.9 §21

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.