How the size of this structure comes out to be 4 byte

问题

I do have a structure having bit-fields in it.Its comes out to be 2 bytes according to me but its coming out to be 4 .I have read some question related to this here on stackoverflow but not able to relate to my problem.This is structure i do have

struct s{
char b;
int c:8;
};
int main()
{
printf("sizeof struct s = %d bytes \n",sizeof(struct s));
return 0;
}

if int type has to be on its memory boundary,then output should be 8 bytes but its showing 4 bytes??

回答1:

Source: http://geeksforgeeks.org/?p=9705

In sum: it is optimizing the packing of bits (that's what bit-fields are meant for) as maximum as possible without compromising on alignment.

A variable’s data alignment deals with the way the data stored in these banks. For example, the natural alignment of int on 32-bit machine is 4 bytes. When a data type is naturally aligned, the CPU fetches it in minimum read cycles.

Similarly, the natural alignment of short int is 2 bytes. It means, a short int can be stored in bank 0 – bank 1 pair or bank 2 – bank 3 pair. A double requires 8 bytes, and occupies two rows in the memory banks. Any misalignment of double will force more than two read cycles to fetch double data.

Note that a double variable will be allocated on 8 byte boundary on 32 bit machine and requires two memory read cycles. On a 64 bit machine, based on number of banks, double variable will be allocated on 8 byte boundary and requires only one memory read cycle.

So the compiler will introduce alignment requirement to every structure. It will be as that of the largest member of the structure. If you remove char from your struct, you will still get 4 bytes.

In your struct, char is 1 byte aligned. It is followed by an int bit-field, which is 4 byte aligned for integers, but you defined a bit-field.

8 bits = 1 byte. Char can be any byte boundary. So Char + Int:8 = 2 bytes. Well, that's an odd byte boundary so the compiler adds an additional 2 bytes to maintain the 4-byte boundary.

For it to be 8 bytes, you would have to declare an actual int (4 bytes) and a char (1 byte). That's 5 bytes. Well that's another odd byte boundary, so the struct is padded to 8 bytes.

What I have commonly done in the past to control the padding is to place fillers in between my struct to always maintain the 4 byte boundary. So if I have a struct like this:

struct s {
    int id;
    char b;
};

I am going to insert allocation as follows:

struct d {
    int id;
    char b;
    char temp[3];
}

That would give me a struct with a size of 4 bytes + 1 byte + 3 bytes = 8 bytes! This way I can ensure that my struct is padded the way I want it, especially if I transmit it somewhere over the network. Also, if I ever change my implementation (such as if I were to maybe save this struct into a binary file, the fillers were there from the beginning and so as long as I maintain my initial structure, all is well!)

Finally, you can read this post on C Structure size with bit-fields for more explanation.

回答2:

int c:8; means that you are declaring a bit-field with the size of 8 bits. Since the alignemt on 32 bit systems is normally 4 bytes (=32 bits) your object will appear to have 4 bytes instead of 2 bytes (char + 8 bit).

回答3:

But if you specify that c should occupy 8 bits, it's not really an int, is it? The size of c + b is 2 bytes, but your compiler pads the struct to 4 bytes.

回答4:

They alignment of fields in a struct is compiler/platform dependent. Maybe your compiler uses 16-bit integers for bitfields less than or equal to 16 bits in length, maybe it never aligns structs on anything smaller than a 4-byte boundary.

Bottom line: If you want to know how struct fields are aligned you should read the documentation for the compiler and platform you are using.

回答5:

In generic, platform-independent C, you can never know the size of a struct/union nor the size of a bit-field. The compiler is free to add as many padding bytes as it likes anywhere inside the struct/union/bit-field, except at the very first memory location.

In addition, the compiler is also free to add any number of padding bits to a bit-field, and may put them anywhere it likes, because which bit is msb and lsb is not defined by C.

When it comes to bit-fields, you are left out in the cold by the C language, there is no standard for them. You must read compiler documentation in detail to know how they will behave on your specific platform, and they are completely non-portable.

The sensible solution is to never ever use bit fields, they are a reduntant feature of C. Instead, use bit-wise operators. Bit-wise operators and in-depth documented bit-fields will generate the same machine code (non-documented bit-fields are free to result in quite arbitrary code). But bit-wise operators are guaranteed to work the same on any compiler/system in the world.

来源：https://stackoverflow.com/questions/7121408/how-the-size-of-this-structure-comes-out-to-be-4-byte

标签

bit-fields