Using zer-length array for shared implementation for byte buffer

buffer.c

/*
 ============================================================================
 Name        : Buffer.c
 Author      : Jooyung Han
 Version     :
 Copyright   : 
 Description : Hello World in C, Ansi-style
 ============================================================================
 */

#include <_mingw.h>
#include <memory.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct tagBuffer {
    uint16_t size;
    uint8_t data[];
} Buffer;

enum {
    PACKET_A_MAX = 256, PACKET_B_MAX = 128,
};

typedef struct tagPacketA {
    uint8_t header1;                // 1
    uint8_t header2;                // 1
                                    // 8 (padded 6)
    Buffer buf;                     // 8
    uint8_t data[PACKET_A_MAX];     // 256
} PacketA;

typedef struct tagPacketB {
    uint8_t header1;                // 1
    uint8_t header2;                // 1
                                    // 8 (padded 6)
    Buffer buf;                     // 8
    uint8_t data[PACKET_B_MAX];     // 128
} PacketB;

void Buffer_Append(Buffer* buf, uint8_t b) {
    buf->data[buf->size++] = b;
}

typedef struct tagReader {
    Buffer* buf;
    size_t ptr;
} Reader;

Reader Buffer_reader(Buffer* buf) {
    Reader r = { buf, 0 };
    return r;
}

uint8_t Reader_get(Reader* r) {
    return r->buf->data[r->ptr++];
}

size_t Reader_ptr(Reader* r) {
    return r->ptr;
}

int main(void) {
    PacketA pa;
    memset(&pa, 0, sizeof(PacketA));

    printf("sizeof(Buffer) = %d\n", (int) sizeof(Buffer));
    printf("sizeof(PacketA) = %d\n", (int) sizeof(PacketA));
    printf("sizeof(PacketB) = %d\n", (int) sizeof(PacketB));

    Buffer_Append(&pa.buf, 'a');
    Buffer_Append(&pa.buf, 'b');

    printf("pa.buf.size = %d\n", pa.buf.size);
    printf("pa.data[0] = %c\n", pa.data[0]);
    printf("pa.data[1] = %c\n", pa.data[1]);
    printf("pa.buf.data[0] = %c\n", pa.buf.data[0]);
    printf("pa.buf.data[1] = %c\n", pa.buf.data[1]);

    Reader r = Buffer_reader(&pa.buf);
    printf("Reader_get(r) = %c\n", Reader_get(&r));
    printf("Reader_get(r) = %c\n", Reader_get(&r));
    printf("Reader_ptr(r) = %d\n", (int) Reader_ptr(&r));

    return EXIT_SUCCESS;
}