Learning that you can use unions in C for grouping things into namespaces
Anonymous nested structs are also quite useful for creating struct fields with explicit offsets:    #include 
    #include 
    
    #define YDUMMY(suffix, size) char dummy##suffix[size]
    #define XDUMMY(suffix, size) YDUMMY(suffix, size)
    #define PAD(size) XDUMMY(__COUNTER__, size)
    
    struct ExplicitLayoutStruct {
        union {
            struct __attribute__((packed)) { PAD(3); uint32_t foo; };
            struct __attribute__((packed)) { PAD(5); uint16_t bar; };
            struct __attribute__((packed)) { PAD(13); uint64_t baz; };
        };
    };
    
    int main(void) {
        // offset foo = 3
        // offset bar = 5
        // offset baz = 13
        printf("offset foo = %d\n", offsetof(struct ExplicitLayoutStruct, foo));
        printf("offset bar = %d\n", offsetof(struct ExplicitLayoutStruct, bar));
        printf("offset baz = %d\n", offsetof(struct ExplicitLayoutStruct, baz));
        return 0;
    }