A stupid trick to calculate the nu of entries of a array dynamically

CStupidTrickStructVariableElements.c

#include <stdio.h>
#include <stdint.h>
#include <stddef.h>
#include <assert.h>

#define TEST_STRUCT_ENTRIES(struct_type, start_member, end_member, expected_count) \
    do { \
        size_t start_offset = offsetof(struct_type, start_member); \
        size_t end_offset = offsetof(struct_type, end_member); \
        size_t num_elements = (end_offset - start_offset) / sizeof(uint32_t); \
        printf("Testing %s:\n", #struct_type); \
        printf("  Size of struct: %zu bytes\n", sizeof(struct_type)); \
        printf("  Offset of %s: %zu bytes\n", #start_member, start_offset); \
        printf("  Offset of %s: %zu bytes\n", #end_member, end_offset); \
        printf("  Calculated number of uint32_t elements: %zu\n", num_elements); \
        assert(num_elements == expected_count); \
        assert(num_elements * sizeof(uint32_t) == sizeof(struct_type)); \
        printf("  Test passed.\n\n"); \
    } while (0)

typedef struct {
    uint32_t a;
    uint32_t b;
    uint32_t c;
    uint32_t _end_marker[];
} Struct1;

typedef struct {
    uint32_t x;
    uint32_t y;
    uint32_t z;
    uint32_t w;
    uint32_t _end_marker[];
} Struct2;

typedef struct {
    uint32_t m;
    uint32_t n;
    uint32_t _end_marker[];
} Struct3;

typedef struct {
    uint32_t p;
    uint32_t q;
    uint32_t r;
    uint32_t s;
    uint32_t t;
    uint32_t _end_marker[];
} Struct4;

typedef struct {
    uint32_t u;
    uint32_t v;
    uint32_t _end_marker[];
} Struct5;

int main() {
    TEST_STRUCT_ENTRIES(Struct1, a, _end_marker, 3);
    TEST_STRUCT_ENTRIES(Struct2, x, _end_marker, 4);
    TEST_STRUCT_ENTRIES(Struct3, m, _end_marker, 2);
    TEST_STRUCT_ENTRIES(Struct4, p, _end_marker, 5);
    TEST_STRUCT_ENTRIES(Struct5, u, _end_marker, 2);

    return 0;
}