When to use bit-fields in C

Question

On the question 'why do we need to use bit-fields?', searching on Google I found that bit fields are used for flags.

Now I am curious,

1. Is it the only way bit-fields are used practically?
2. Do we need to use bit fields to save space?

A way of defining bit field from the book:

struct {
    unsigned int is_keyword : 1;
    unsigned int is_extern :  1;
    unsigned int is_static : 1;
} flags;

3. Why do we use int?
4. How much space is occupied?

I am confused why we are using int, but not short or something smaller than an int.

5. As I understand only 1 bit is occupied in memory, but not the whole unsigned int value. Is it correct?

Answer 1

A quite good resource is Bit Fields in C.

The basic reason is to reduce the used size. For example, if you write:

struct {
    unsigned int is_keyword;
    unsigned int is_extern;
    unsigned int is_static;
} flags;

You will use at least 3 * sizeof(unsigned int) or 12 bytes to represent three small flags, that should only need three bits.

So if you write:

struct {
    unsigned int is_keyword : 1;
    unsigned int is_extern : 1;
    unsigned int is_static : 1;
} flags;

This uses up the same space as one unsigned int, so 4 bytes. You can throw 32 one-bit fields into the struct before it needs more space.

This is sort of equivalent to the classical home brew bit field:

#define IS_KEYWORD 0x01
#define IS_EXTERN  0x02
#define IS_STATIC  0x04
unsigned int flags;

But the bit field syntax is cleaner. Compare:

if (flags.is_keyword)

against:

if (flags & IS_KEYWORD)

And it is obviously less error-prone.

Answer 2

Now I am curious, [are flags] the only way bitfields are used practically?

No, flags are not the only way bitfields are used. They can also be used to store values larger than one bit, although flags are more common. For instance:

typedef enum {
    NORTH = 0,
    EAST = 1,
    SOUTH = 2,
    WEST = 3
} directionValues;

struct {
    unsigned int alice_dir : 2;
    unsigned int bob_dir : 2;
} directions;

Do we need to use bitfields to save space?

Bitfields do save space. They also allow an easier way to set values that aren't byte-aligned. Rather than bit-shifting and using bitwise operations, we can use the same syntax as setting fields in a struct. This improves readability. With a bitfield, you could write

directions.alice_dir = WEST;
directions.bob_dir = SOUTH;

However, to store multiple independent values in the space of one int (or other type) without bitfields, you would need to write something like:

#define ALICE_OFFSET 0
#define BOB_OFFSET 2
directions &= ~(3<<ALICE_OFFSET); // clear Alice's bits
directions |= WEST<<ALICE_OFFSET; // set Alice's bits to WEST
directions &= ~(3<<BOB_OFFSET);   // clear Bob's bits
directions |= SOUTH<<BOB_OFFSET;  // set Bob's bits to SOUTH

The improved readability of bitfields is arguably more important than saving a few bytes here and there.

Why do we use int? How much space is occupied?

The space of an entire int is occupied. We use int because in many cases, it doesn't really matter. If, for a single value, you use 4 bytes instead of 1 or 2, your user probably won't notice. For some platforms, size does matter more, and you can use other data types which take up less space (char, short, uint8_t, etc.).

As I understand only 1 bit is occupied in memory, but not the whole unsigned int value. Is it correct?

No, that is not correct. The entire unsigned int will exist, even if you're only using 8 of its bits.

Answer 3

Another place where bitfields are common are hardware registers. If you have a 32 bit register where each bit has a certain meaning, you can elegantly describe it with a bitfield.

Such a bitfield is inherently platform-specific. Portability does not matter in this case.

Answer 4

We use bit fields mostly (though not exclusively) for flag structures - bytes or words (or possibly larger things) in which we try to pack tiny (often 2-state) pieces of (often related) information.

In these scenarios, bit fields are used because they correctly model the problem we're solving: what we're dealing with is not really an 8-bit (or 16-bit or 24-bit or 32-bit) number, but rather a collection of 8 (or 16 or 24 or 32) related, but distinct pieces of information.

The problems we solve using bit fields are problems where "packing" the information tightly has measurable benefits and/or "unpacking" the information doesn't have a penalty. For example, if you're exposing 1 byte through 8 pins and the bits from each pin go through their own bus that's already printed on the board so that it leads exactly where it's supposed to, then a bit field is ideal. The benefit in "packing" the data is that it can be sent in one go (which is useful if the frequency of the bus is limited and our operation relies on frequency of its execution), and the penalty of "unpacking" the data is non-existent (or existent but worth it).

On the other hand, we don't use bit fields for booleans in other cases like normal program flow control, because of the way computer architectures usually work. Most common CPUs don't like fetching one bit from memory - they like to fetch bytes or integers. They also don't like to process bits - their instructions often operate on larger things like integers, words, memory addresses, etc.

So, when you try to operate on bits, it's up to you or the compiler (depending on what language you're writing in) to write out additional operations that perform bit masking and strip the structure of everything but the information you actually want to operate on. If there are no benefits in "packing" the information (and in most cases, there aren't), then using bit fields for booleans would only introduce overhead and noise in your code.

Answer 5

To answer the original question »When to use bit-fields in C?« … according to the book "Write Portable Code" by Brian Hook (ISBN 1-59327-056-9, I read the German edition ISBN 3-937514-19-8) and to personal experience:

Never use the bitfield idiom of the C language, but do it by yourself.

A lot of implementation details are compiler-specific, especially in combination with unions and things are not guaranteed over different compilers and different endianness. If there's only a tiny chance your code has to be portable and will be compiled for different architectures and/or with different compilers, don't use it.

We had this case when porting code from a little-endian microcontroller with some proprietary compiler to another big-endian microcontroller with GCC, and it was not fun. :-/

This is how I have used flags (host byte order ;-) ) since then:

# define SOME_FLAG        (1 << 0)
# define SOME_OTHER_FLAG  (1 << 1)
# define AND_ANOTHER_FLAG (1 << 2)

/* test flag */
if ( someint & SOME_FLAG ) {
    /* do this */
}

/* set flag */
someint |= SOME_FLAG;

/* clear flag */
someint &= ~SOME_FLAG;

No need for a union with the int type and some bitfield struct then. If you read lots of embedded code those test, set, and clear patterns will become common, and you spot them easily in your code.

Answer 6

Why do we need to use bit-fields?

When you want to store some data which can be stored in less than one byte, those kind of data can be coupled in a structure using bit fields.

In the embedded word, when one 32 bit world of any register has different meaning for different word then you can also use bit fields to make them more readable.

I found that bit fields are used for flags. Now I am curious, is it the only way bit-fields are used practically?

No, this not the only way. You can use it in other ways too.

Do we need to use bit fields to save space?

Yes.

As I understand only 1 bit is occupied in memory, but not the whole unsigned int value. Is it correct?

No. Memory only can be occupied in multiple of bytes.

Answer 7

A good usage would be to implement a chunk to translate to—and from—Base64 or any unaligned data structure.

struct {
    unsigned int e1:6;
    unsigned int e2:6;
    unsigned int e3:6;
    unsigned int e4:6;
} base64enc; // I don't know if declaring a 4-byte array will have the same effect.

struct {
    unsigned char d1;
    unsigned char d2;
    unsigned char d3;
} base64dec;

union base64chunk {
    struct base64enc enc;
    struct base64dec dec;
};

base64chunk b64c;
// You can assign three characters to b64c.enc, and get four 0-63 codes from b64dec instantly.

This example is a bit naive, since Base64 must also consider null-termination (i.e. a string which has not a length l so that l % 3 is 0). But works as a sample of accessing unaligned data structures.

Another example: Using this feature to break a TCP packet header into its components (or other network protocol packet header you want to discuss), although it is a more advanced and less end-user example. In general: this is useful regarding PC internals, SO, drivers, an encoding systems.

Another example: analyzing a float number.

struct _FP32 {
    unsigned int sign:1;
    unsigned int exponent:8;
    unsigned int mantissa:23;
}

union FP32_t {
    _FP32 parts;
    float number;
}

(Disclaimer: Don't know the file name / type name where this is applied, but in C this is declared in a header; Don't know how can this be done for 64-bit floating-point numbers since the mantissa must have 52 bits and—in a 32 bit target—ints have 32 bits).

Conclusion: As the concept and these examples show, this is a rarely used feature because it's mostly for internal purposes, and not for day-by-day software.

Answer 8

Bit fields can be used for saving memory space (but using bit fields for this purpose is rare). It is used where there is a memory constraint, e.g., while programming in embedded systems.

But this should be used only if extremely required because we cannot have the address of a bit field, so address operator & cannot be used with them.

Answer 9

To answer the parts of the question no one else answered:

Ints, not Shorts

The reason to use ints rather than shorts, etc. is that in most cases no space will be saved by doing so.

Modern computers have a 32 or 64 bit architecture and that 32 or 64 bits will be needed even if you use a smaller storage type such as a short.

The smaller types are only useful for saving memory if you can pack them together (for example a short array may use less memory than an int array as the shorts can be packed together tighter in the array). For most cases, when using bitfields, this is not the case.

Other uses

Bitfields are most commonly used for flags, but there are other things they are used for. For example, one way to represent a chess board used in a lot of chess algorithms is to use a 64 bit integer to represent the board (8*8 pixels) and set flags in that integer to give the position of all the white pawns. Another integer shows all the black pawns, etc.

Answer 10

You can use them to expand the number of unsigned types that wrap. Ordinary you would have only powers of 8,16,32,64... , but you can have every power with bit-fields.

struct a
{
    unsigned int b : 3 ;
} ;

struct a w = { 0 } ;

while( 1 )
{
    printf("%u\n" , w.b++ ) ;
    getchar() ;
}

Answer 11

To utilize the memory space, we can use bit fields.

As far as I know, in real-world programming, if we require, we can use Booleans instead of declaring it as integers and then making bit field.

Answer 12

If they are also values we use often, not only do we save space, we can also gain performance since we do not need to pollute the caches.

However, caching is also the danger in using bit fields since concurrent reads and writes to different bits will cause a data race and updates to completely separate bits might overwrite new values with old values...

Answer 13

Why do we use int? How much space is occupied?

One answer to this question that I haven't seen mentioned in any of the other answers, is that the C standard guarantees support for int. Specifically:

A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation defined type.

It is common for compilers to allow additional bit-field types, but not required. If you're really concerned about portability, int is the best choice.

Answer 14

Bitfields are much more compact and that is an advantage.

But don't forget packed structures are slower than normal structures. They are also more difficult to construct since the programmer must define the number of bits to use for each field. This is a disadvantage.

Answer 15

Nowadays, microcontrollers (MCUs) have peripherals, such as I/O ports, ADCs, DACs, onboard the chip along with the processor.

Before MCUs became available with the needed peripherals, we would access some of our hardware by connecting to the buffered address and data buses of the microprocessor. A pointer would be set to the memory address of the device and if the device saw its address along with the R/W signal and maybe a chip select, it would be accessed.

Oftentimes we would want to access individual or small groups of bits on the device.

Answer 16

In our project, we used this to extract a page table entry and page directory entry from a given memory address:

union VADDRESS {
    struct {
        ULONG64 BlockOffset : 16;
        ULONG64 PteIndex : 14;
        ULONG64 PdeIndex : 14;
        ULONG64 ReservedMBZ : (64 - (16 + 14 + 14));
    };

    ULONG64 AsULONG64;
};

Now suppose, we have an address:

union VADDRESS tempAddress;
tempAddress.AsULONG64 = 0x1234567887654321;

Now we can access PTE and PDE from this address: cout << tempAddress.PteIndex;