weird struct definition?

Question

Possible Duplicate:
What does 'unsigned temp:3' means

A struct's definition goes like this,

typedef struct
{   
    uint32_t length : 8;  
    uint32_t offset : 24;
    uint32_t type : 8;
} A;

I haven't seen this kind of definition before, what does it mean by :8 and :24?

possible duplicate of http://stackoverflow.com/q/4373839/1369241 — akaHuman, Jul 20 '12 at 09:46

score 6 · Accepted Answer · answered Jul 20 '12 at 09:37

6

It's defining bitfields. This tells the compiler that length is 8 bits, offset is 24 bits and type is also 8 bits.

answered Jul 20 '12 at 09:37

Some programmer dude

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Then why does it define `length` as `uint32_t`? – Alcott Jul 20 '12 at 09:41
@Alcott That could only work if uint32_t was a typedef for 'unsigned' or 'unsigned int'. – user207421 Jul 20 '12 at 09:43
@EJP, sorry, don't quite understand – Alcott Jul 20 '12 at 09:44
@Alcott Usually the type for bitfields is `unsigned int` (or just `unsigned` or even just `int`). You have to ask the author why he choose to use `uint32_t` (which in most cases is a typedef to `unsigned int` anyway). – Some programmer dude Jul 20 '12 at 09:44
Understand, thanks, @JoachimPileborg. – Alcott Jul 20 '12 at 09:51

Jeyaram · Answer 2 · 2012-07-20T09:58:09.607

Refer the following link. They are bit-fields. http://www.cs.cf.ac.uk/Dave/C/node13.html
http://en.wikipedia.org/wiki/Bit_field

#include <stdio.h>

typedef unsigned int uint32_t;

#pragma pack(push, 1)
typedef struct
{
    uint32_t length : 8;
    uint32_t offset : 24;
    uint32_t type : 8;
} A;

typedef struct
{
    uint32_t length;
    uint32_t offset;
    uint32_t type;
} B;
#pragma pack(pop)

int main()
{
  printf("\n Size of Struct: A:%d   B:%d", sizeof(A), sizeof(B));
  return 0;
}

Structure A's size will be 5 Bytes where as B's size will be 12 bytes.

score 1 · Answer 3 · answered Jul 20 '12 at 09:43

1

This notation defines bit fields, i.e. the size in number of bits for that structure variable.

answered Jul 20 '12 at 09:43

akaHuman

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weird struct definition?

3 Answers3