两个不一样的结构为什么会一样大？

[ProgrammingRing]

下面的两个结构SS和TT不一样，但sizeof却一样大，都是56，为什么？

#include <stdio.h>
#include <windows.h>
#include <tchar.h>

typedef struct _VM_COUNTERS {
	SIZE_T PeakVirtualSize;
	SIZE_T VirtualSize;
	ULONG PageFaultCount;
	SIZE_T PeakWorkingSetSize;
	SIZE_T WorkingSetSize;
	SIZE_T QuotaPeakPagedPoolUsage;
	SIZE_T QuotaPagedPoolUsage;
	SIZE_T QuotaPeakNonPagedPoolUsage;
	SIZE_T QuotaNonPagedPoolUsage;
	SIZE_T PagefileUsage;
	SIZE_T PeakPagefileUsage;
} VM_COUNTERS;
typedef struct _TT 
{
	LARGE_INTEGER   CreateTime;
	VM_COUNTERS     VmCounters;
} TT;
typedef struct _SS
{
	LARGE_INTEGER   CreateTime;
	VM_COUNTERS     VmCounters;
	ULONG PrivateUsage;
} SS;

int _tmain(int argc, _TCHAR* argv[])
{
	_tprintf(TEXT("%d %d\r\n"), sizeof(SS), sizeof(TT));
	return 0;
}
// Output:
// 56 56
// 请按任意键继续. . .

[threenewbee]

因为有对齐 http://pppboy.blog.163.com/blog/static/30203796201082494026399/

[threenewbee]

我们改变下顺序： typedef struct _SS { ULONG PrivateUsage; LARGE_INTEGER CreateTime; VM_COUNTERS VmCounters; } SS; 就是64了。你看到，它总是8的倍数。说明它按照8字节（64bit）对齐的。