Sizes of bit fields and unions in C

Question

I have the following code:

#pragma pack(push, 1)
typedef struct __attribute__((packed)){
    uint64_t msg: 48;
    uint16_t crc: 12;
    int : 0;
} data_s;
#pragma pack(pop)

typedef union {
    uint64_t tot;
    data_s split;
} data_t;

int main() {
    data_t data;
    printf(
        "Sizes are: union:%d,struct:%d,uint64_t:%d\n",
        sizeof(data),
        sizeof(data.split),
        sizeof(data.tot)
    );
    return 0;
}

The output I get is Sizes are: union:16,struct:10,uint64_t:8.

Here I have two issues,

1. Even though I'm using bit fields and trying to pack it, I am getting 10 bytes even though the number of bits is less than 64(48+12=60) and can be packed into 8 bytes.

2. Even though the maximum size of the two members of the union is 10, why is its size 16?

Also how do I pack the bits into 8 bytes?

Answer 1

This is implementation defined; how bits are laid out depends on your compiler.

Many compilers split bitfields if they are different types. You could try changing type of crc to uint64_t to see if it makes a difference.

If you want to write portable code and layout is important, then don't use bitfields at all.

Answer 2

You are allocating an integral type and then tell how many bits to use.

Then you allocate another integral type and tell how many bits to use.

The compiler places these in their respective integrals. To have them in a single integral field, use comma's to separate them, e.g.:

uint64_t msg: 48, crc: 12;

(But note the implementation defined aspect user694733 mentions)

Answer 3

These are bit-fields. They are very poorly covered by standardization. If you use them - you are on your own.

• #pragma pack(push, 1) typedef struct __attribute__((packed)){
  These are non-standard compiler extensions of the gcc compiler. What happens when you add them is not covered by any standard. The only thing the standard says is that if a compiler doesn't recognize the #pragma, it must ignore that line.
• The C standard only guarantees that the types _Bool, unsigned int and signed int are valid for bit-fields. You use uint64_t and uint16_t. What happens when you do is not covered by the C standard - this is implementation-defined behavior. The standard speaks of "units", but it is not specified how large a "unit" is.
• msg: 48; The C standard does not specify if this is the least significant 48 bits or the most significant ones. It does not specify order of allocation, it does not specify alignment. Add endianess on top of that, and you can't really know what this code does.
  All the C standard guarantees is that msg resides on a lower address than trailing struct members. Unless they are merged into the same bit-field - then the standard guarantees nothing. Completely implementation-defined.
• int : 0; is useless to add at the end of a bit-field, the only purpose of this code is to the compiler not to merge any trailing bit-field into the previous one.
• #pragma pack and similar doesn't, as far as I know, guarantee that there is no trailing padding in the end of the struct/union.
• gcc is known to behave strange together with bit-fields. It has this in common with every single C compiler ever written.

The answer to your questions can thus be summarized as: because bit-fields.

---

An alternative approach which will be 100% deterministic, well-defined portable and safe is something like this:

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

typedef uint64_t data_t;

static inline uint64_t msg (data_t* data)
{
  return *data >> 12;
}

static inline uint64_t crc (data_t* data)
{
  return *data & 0xFFFu;
}

int main() {
    data_t data = 0xFFFFFAAAu;

    printf("msg: %"PRIX64" crc:%"PRIX64, msg(&data), crc(&data));
    return 0;
}

This is even portable across CPU:s of different endianess.

Answer 4

For your question 2. : A union always takes as much space as its largest member. Here it considers the struct split to be of size 10 and then you probably have optimization flag when you compile to align memory (which is recommended), making it a power of 2 (from 10 to 16).

Answer 5

1. Even though I'm using bit fields and trying to pack it, I am getting 10 bytes even though the number of bits is less than 64(48+12=60) and can be packed into 8 bytes.

Note in the first place that #pragma pack, like most #pragmas, is an extension with implementation-defined behavior. The C language does define its behavior.

In the second place, C affords implementations considerable freedom with respect to how they lay out the contents of a structure, especially with respect to bitfields. In fact, supposing that uint64_t is a different type from unsigned int in your implementation, whether you can even have a bitfield of the former type in the first place is implementation-defined.

C does not leave it completely open, however. Here's the key part of the specification for bitfield layout within a structure:

An implementation may allocate any addressable storage unit large enough to hold a bit- field. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. The alignment of the addressable storage unit is unspecified.

(C2011, 6.7.2.1/11; emphasis added)

Note well that C does not say that the declared type of a bitfield member has anything to do with the size of the addressable storage unit in which its bits are stored (neither there nor anywhere else), though in fact some compilers do implement such behavior. On the other hand, what it does say certainly leads me to expect that if C accepts a 48-bit bitfield in the first place then an immediately-following 12-bit bitfield should be stored in the same unit. Implementation-defined packing specifications don't even enter the picture. Thus, your implementation seems to be non-conforming in this regard.

2. Even though the maximum size of the two members of the union is 10, why is its size 16?

Unions can have trailing padding, just like structures can. Padding will have been introduced into the union's layout to support the compiler's idea of optimal alignment for objects of that type and their members. In particular, it is likely that your structure has at least an 8-byte alignment requirement, so the union is padded to a size that is a multiple of that alignment requirement. This is again implementation-defined, and as long as we're there, it's possible that you could avoid the padding by instructing the compiler to pack the union, too.

Also how do I pack the bits into 8 bytes?

It may be that you can't, but you should check your compiler's documentation. Since the observed behavior appears to be non-conforming, it's anyone's guess what you can or must do.

If it were me, though, the first thing I would try is to remove the pragma and attribute; remove the zero-length bitfield, and change the declared type of the crc bitfield to match that of the preceding bitfield (uint64_t). The point of all of these is to clear away details that conceivably might confuse the compiler, so as to try to get it to render the behavior that the standard demands in the first place. If you can get the struct to come out as 8 bytes, then you probably don't need to do anything more to get the union to come out the same size (on account of 8 being a somewhat magic number).