For example:

The __alignof value is the same as the value for sizeof for basic types. Consider, however, this example:

typedef struct { int a; double b; } S;
// __alignof(S) == 8

In this case, the __alignof value is the alignment requirement of the largest element in the structure.

Similarly, for

typedef __declspec(align(32)) struct { int a; } S;

__alignof(S) is equal to 32.

One use for __alignof would be as a parameter to one of your own memory-allocation routines. For example, given the following defined structure S, you could call a memory-allocation routine named aligned_malloc to allocate memory on a particular alignment boundary.

typedef __declspec(align(32)) struct { int a; double b; } S;
int n = 50; // array size
S* p = (S*)aligned_malloc(n * sizeof(S), __alignof(S));

For more information on modifying alignment, see:

• pack

• align

• __unaligned

• /Zp (Struct Member Alignment)

• Examples of Structure Alignment (x64 specific)

Reference