Becoming a console programmer : Structure Alignment

This is an area that all programmers should be familiar with however i've met and worked with a fair few who never had to care... until they had to care.
consider:

struct foo1
{
    char m_char;
};

struct foo2
{
    int  m_int;
    char m_char;
};
struct foo3
{
    long long m_longlong;
    int  m_int;
    char m_char1;
    char m_char2;
};

using default settings under x86 these three structs are 1, 8 and 16 bytes respectively. What is actually allocated is

struct foo2
{
    int  m_int;            // offset 0 : size 4 bytes
    char m_char;           // offset 4 : size 1 byte
    char m_pad[3];         //                             3 bytes padding
                          // 8 bytes
};
struct foo3
{
    long long m_longlong;  // offset 0  : size 8 bytes
    int  m_int;            // offset 8  : size 4 bytes
    char m_char1;          // offset 12 : size 1 byte
    char m_char2;          // offset 13 : size 1 byte
    char m_pad[2];         //                            2 bytes padding
                          // 16 bytes total
};

now lets take foo3 and jumble those members a little.

struct foo4
{
    char m_char1;
    long long m_longlong;
    char m_char2;
    int  m_int;
};

One might expect foo4 to be the same size as foo3 however it is not, it comes out to 24 bytes, the padding is as follows.

struct foo4
{
    char m_char1;           // offset 0  : size 1 byte
    char m_pad7[7];         //                          7 bytes padding
    long long m_longlong;   // offset 8  : 8 bytes
    char m_char2;           // offset 16 : 1 byte
    char m_pad[3];          //                          3 bytes padding
    int  m_int;             // offset 20 : 4 bytes
};                          // 24 bytes total

each type contained within a struct must begin on a natively aligned boundary, so the 8 byte "long long" must start on an 8 byte offset within the struct but be after m_char1 so the compiler inserts 7 bytes of "padding" that the user cannot directly access to ensure this.

to inspect this for yourself use the code

printf("\n(%d,",offsetof(foo4, m_char1));
printf("%d,",offsetof(foo4, m_longlong));
printf("%d,",offsetof(foo4, m_char2));
printf("%d)",offsetof(foo4, m_int));
printf(": %d",sizeof(foo4));

output : "(0, 8, 16, 20): 24"

Manual Alignment
It is often beneficial to align the base of a structure beyond its native requirements. I won't go into the reasons for this here (thats a later subject) but it does change the internals of a struct when you do this.
Consider:

struct _CRT_ALIGN(16) foo_aligned4
{
    char m_char1;
    long long m_longlong;
    char m_char2;
    int  m_int;
};

the

    _CRT_ALIGN(16)

informs the compiler that this structure should be 16 byte aligned (and therefore a multiple of 16 bytes in size), this is actually a wrapper around a declspec (see below for further reading)

In the case of foo4 this doesn't change the offsets of any of the internals but it does change the size of the struct

struct _CRT_ALIGN(16) foo_aligned4
{
    char m_char1;           // offset 0  : size 1 byte
                            //                          7 bytes padding
    long long m_longlong;   // offset 8  : 8 bytes
    char m_char2;           // offset 16 : 1 byte
                            //                          3 bytes padding
    int  m_int;             // offset 20 : 4 bytes
                                                      8 bytes adding
};                          // 32 bytes total

if we were to then include an instance of foo_aligned4 inside another struct, said struct will inherit the same aligment. In general a structure will always be aligned to the largest alignment of its components.

struct foo_composite
{
    char m_char1;                // offset  0: size 1 byte
                                //                             15 bytes padding
    foo_aligned4 m_foo_aligned4; // offset  16: size 32 byte
    long long m_longlong;        // offset  48: size 8 byte
    char m_char2;                // offset  56: size 1 byte
                                //                             3 bytes padding
    int  m_int;                  // offset  60: size 4 byte
                                // total 64 bytes
};

Now given our composite structure of 64 bytes lets see how small we can make it without altering the client facing elements.

struct _CRT_ALIGN(16) foo_aligned4_opt
{
    long long m_longlong;            // offset  0: size 8 bytes
    int  m_int;                      // offset  8: size 4 bytes
    char m_char1;                    // offset  12: size 1 byte
    char m_char2;                    // offset  13: size 1 byte
    char m_pad[2];                   // offset  14: size 2 bytes
};                                   // 16 bytes total


struct foo_composite_opt
{
    foo_aligned4_opt m_foo_aligned4;// offset  0: size 16 bytes
    long long m_longlong;           // offset  16: size 8 byte
    int  m_int;                     // offset  24: size 4 byte
    char m_char1;                   // offset  28: size 1 byte
    char m_char2;                   // offset  29: size 1 byte
    char m_pad[2];                  // offset  30: size 2 bytes
                                   // 32 bytes total
};

Our new composite struct is 32 bytes rather than 64, every member still exists but in a different configuration.
You may notice that I manually inserted the padding into the struct, this isn't required but personally i prefer it in cases where i require specific alignment or optimal structure size.

So key points are

• structures inherit the largest alignment of their elements
• elements will always begin at an offset that is a multiple of their alignment
• the compiler will insert padding into your structure to achieve alignment requirements
• structure size will always be a multiple of its alignment
• to reduce alignment padding, arrange structures largest to smallest in alignement requirements.

Note that normal allocators, such as malloc, C++ operator new, and the Win32 allocators return memory that will most likely not be sufficiently aligned for __declspec(align(#)) structures or arrays of structures. I recommend writing your own allocator either as a wrapper around these or as a region allocator to achieve alignment above default (4 or 8 bytes usually).


further reading -_CRL_ALIGN, __alignof,