Visual Studio C ++位域结构大小问题

Question

I have created and compiled following code in GCC using packed attribute and it worked like as expected. But in Visual Studio results are not same as GCC.

#pragma pack(push, 1)
typedef struct
{
    uint8_t TargetID: 6;
    enum_OPCode OPCode: 3;
    uint8_t CRC7: 7;
} struct_commDataPack_request;
#pragma pack(pop)

As you can see size of all elements must be 16bits = 2Bytes which is true in GCC, but in Visual Studio it returns 3Bytes! and if I decrease size of it to 15bits it will return 2Bytes.

How can I fix it?

Answer 1

Well, packing is just too platform dependent to really rely on it.

In VS, when you do "pack(push, 1)", you essentially set the padding to 8 bits and, as you saw, end up with three bytes (6 bits of which are padding).

Padding on GCC however (in case you did it with __attribute__((packed)) or some alias for it) can be turned off completely. That's why you see just 2 bytes.

To keep it portable, why don't you just write your own small serialization routine? Something like this:

struct_commDataPack_request s;
short wire = s.TargetID | (s.OPCode<<6) | (s.CRC7 << 9);

Answer 2

I used this code to reproduce in VS2017:

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

typedef enum { A,B,C } enum_OPCode;

#pragma pack(push, 1)
typedef struct
{
  uint8_t TargetID : 6;
  uint8_t OPCode : 3;
  uint8_t CRC7 : 7;
} struct_commDataPack_request;
#pragma pack(pop)

int main()
{
  printf("%zd\n", sizeof(struct_commDataPack_request));
}

The size here is 3.

But when I change

uint8_t OPCode : 3;
uint8_t CRC7 : 7;

to

uint8_t OPCode : 2;
uint8_t CRC7 : 8;

(overall size remains 16 bits), the size is 2.

As suggested before, best is to write your own serialisation/deserialisation.

Answer 3

As stated in MS document :

The underlying type of a bit field must be an integral type. If a bit field would overflow the boundary of the declared type (in your case is uint8_t), new units of storage are allocated.

A way to solve the problem is to use a declared type with a bigger boundary (uint16_t).

Here is the code I used:

#include "pch.h"
#include <stdio.h>
#include <stdint.h>

typedef enum { A, B, C } enum_OPCode;

#pragma pack(push, 1)
typedef struct
{
    uint16_t TargetID : 6;
    uint16_t OPCode : 3;
    uint16_t CRC7 : 7;
} struct_commDataPack_request;
#pragma pack(pop)

int main()
{
    struct_commDataPack_request packet;

    packet.TargetID = 0;
    packet.OPCode = 7;
    packet.CRC7 = 0;

    unsigned char * pData = (unsigned char *)&packet;

    printf("Packet size : %zd\n", sizeof(struct_commDataPack_request));
    for (int i = 0; i < sizeof(packet); i++) {
        printf("byte %d is [%02X]\n", i, pData[i] );
    }
}

The results :

Packet size : 2
byte 0 is [C0]
byte 1 is [01]

( 0x01C0 in bits : 0000 0001 1100 0000)