Extra bytes when declaring a member of a struct as uint32_t

Question

I have a problem when using the uint32_t type from the stdint.h library. If I run the following code (on Ubuntu linux 11.10 x86_64, g++ version 4.6.1):

#include          


        

Answer 1

It's because of alignment. On your platform uint32_t needs to be 4 byte aligned. In order to achieve that dest_addr_64_h has to have two bytes of padding right in front of it because the position right after the two uint8_t members is a multiple of 2 but not 4.

You can use the macro offsetof() to figure out exactly where members are placed within a struct, to see that this is true.

You can either try to make the compiler pack the members tighter together, or you can rearrange the members so that padding isn't needed.

Answer 2

Structs are padded to be an integer multiple of 4 bytes¹ so that they are word-aligned. http://en.wikipedia.org/wiki/Data_structure_alignment#Data_structure_padding

See also:

• Why isn't sizeof for a struct equal to the sum of sizeof of each member?
• Structure padding and packing
• Use of struct padding

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¹ As @Mooing Duck commented, this isn't always true:

It's not always a multiple of 4 bytes, it varies (slightly) depending on the members. On the other hand, 99% of the time it's a multiple of 4 bytes.

Answer 3

Data types have different alignment requirements based on platform. The extra bytes are used to align one of the members of your structure to a particular size and/or position. IF you need more precise control, you can specify this alignment with __attribute__ or #pragma pack

Answer 4

You need to declare to the compiler to pack the structure

I believe that this will work for GCC

struct test
    {
            unsigned char  field1;
            unsigned short field2;
            unsigned long  field3;
    } __attribute__((__packed__));

In MS it would be something using the pragma packed

http://www.cplusplus.com/forum/general/14659/

#pragma pack(push, 1) // exact fit - no padding
struct MyStruct
{
  char b; 
  int a; 
  int array[2];
};
#pragma pack(pop) //back to whatever the previous packing mode was