MSVC Ignores Empty Initializer List in C

2024-09-30

Before C23, ISO C forbids empty initializer braces. Take the following code as an example:

#include <stdio.h>

typedef struct tagFoo {
    int i;
} Foo;

static Foo foos[3] = {
    {},
    {1},
    {2},
};

int main(void){
    printf("sizeof(foos): %zu\n", sizeof(foos));
    printf("sizeof(Foo): %zu\n", sizeof(Foo));
    printf("foo[0].i: %d\n", foos[0].i);
    printf("foo[1].i: %d\n", foos[1].i);
    printf("foo[2].i: %d\n", foos[2].i);

    return 0;
}

Compile the program in gcc 14.2 with -pedantic, we got:

sizeof(foos): 12
sizeof(Foo): 4
foo[0].i: 0
foo[1].i: 1
foo[2].i: 2

Without -pedantic, the result will not change, despite no warning would be thrown.

However, in MSVC v19.40 (vs 17.10), the code compiles without any warning, but the result is weird:

sizeof(foos): 12
sizeof(Foo): 4
foo[0].i: 1
foo[1].i: 2
foo[2].i: 0

MSVC ignores the empty initializer list, and shifted the struct array.

FYI, currently C23 empty initializers are partially supported in gcc, and not supported in MSVC.