数据结构长度执行时跟定义的不一致，为什么？？

我定义了两个结构：

#define LENGTHOFFILENAME 255
#define TOTALFILENUM 30

typedef struct tagFILEACLENTRY
{
CHAR FileName[LENGTHOFFILENAME];
UCHAR nFileProperity;
} FILEACLENTRY,*PFILEACLENTRY;

typedef struct tagFILEACL
{
DWORD nChekSum;
UCHAR  nAreaNum;
UCHAR  nComputerNum;
DWORD CardSerialNum;
DWORD nFileNum;
DWORD nReserved;
FILEACLENTRY FileAcl[TOTALFILENUM];
}FILEACL,*PFILEACL;
这里FILEACL结够应该是4+1+1+4+4+4+(255+1)*30=7698(bytes)。
但是我在调试中发现sizeof(FILEACL)==7700！！！
多了两个字节，这是怎么啦？？

We can take an experiment in the VC6.0 enviroment:

Press Alt+F7, then jump \"Project Setting\", Click to the page \"C/C++\", choose the \"Category\", Change the \"Struct member alignment\", when choose \"1 byte\" or \"2 bytes\", the sizeof(FileAcl) = 7698; When choose \"4 bytes\" or \"8 bytes *\" or \"16 bytes\", sizeof(FileAcl) = 7700.

The default setting of \"Struct member aliment\" is 8 bytes.

From the above exprement, we can see the alignment format affect the size of structure.

But there are another question: Why when we set the \"Struct member aliment\" to \"8 bytes\" or \"16 bytes\", the sizeof FILEACL is still 7700. As the rule, when the \"Struct member aliment\" is 8 bytes, it should 7704; when the \"Struct member aliment\" is 16 bytes, it should 7712.

Why? Who can explain it? Thanks!

My msn: jgw2008@hotmail.com

确实是个问题：）
VC设置中的N字节对齐就是说变量存放的起始地址的偏移量有两种情况：第一、如果n大于等于该变量所占用的字节数，那么偏移量必须满足默认的对齐方式，第二、如果n小于该变量的类型所占用的字节数，那么偏移量为n的倍数，不用满足默认的对齐方式。

结构的总大小也有个约束条件，分下面两种情况：如果n大于所有成员变量类型所占用的字节数，那么结构的总大小必须为占用空间最大的变量占用的空间数的倍数；否则必须为n的倍数。

在这种情况下，由于上面的结构中最大变量占用的空间数为4字节，对于8字节和16字节来说，情况是相同的，即结构的总大小必须为占用空间最大的变量占用的空间数（现在是4）的倍数。

我已经解决了，是编译时对齐方式造成的。
结构typedef struct tagFILEACL
    {
    DWORD nChekSum;
    UCHAR nAreaNum;
    UCHAR nComputerNum;
    DWORD CardSerialNum;
    DWORD nFileNum;
    DWORD nReserved;
    FILEACLENTRY FileAcl[TOTALFILENUM];
    }FILEACL,*PFILEACL;
中DWORD nChekSum后面只有两个UCHAR变量，由于vc->setting->c/c++->Code Generation->Struct member alignment设置的是“8 Bytes *”，编译是会在UCHAR设置的变量处补齐。
或者把vc->setting->c/c++->Code Generation->Struct member alignment设置的是“1 Bytes *”，
或者更改结构为
typedef struct tagFILEACL
{
DWORD nChekSum;
UCHAR nAreaNum;
UCHAR nComputerNum;
UCHAR nReserved[2];
DWORD CardSerialNum;
DWORD nFileNum;
FILEACLENTRY FileAcl[TOTALFILENUM];
}FILEACL,*PFILEACL;
可以解决此问题！！！
 :D :D :D :D :D :D :D :D :D :D

我定义了两个结构：

#define LENGTHOFFILENAME 255
#define TOTALFILENUM 30

typedef struct tagFILEACLENTRY
{
CHAR FileName[LENGTHOFFILENAME];
UCHAR nFileProperity;
} FILEACLENTRY,*PFILEACLENTRY;

typedef struct tagFILEACL
{
DWORD nChekSum;
UCHAR  nAreaNum;
UCHAR  nComputerNum;
DWORD CardSerialNum;
DWORD nFileNum;
DWORD nReserved;
FILEACLENTRY FileAcl[TOTALFILENUM];
}FILEACL,*PFILEACL;
这里FILEACL结够应该是4+1+1+4+4+4+(255+1)*30=7698(bytes)。
但是我在调试中发现sizeof(FILEACL)==7700！！！
多了两个字节，这是怎么啦？？

将结构这样定义应该就可以了，简单的边界对齐问题：
typedef struct tagFILEACL
{
DWORD nChekSum;
DWORD CardSerialNum;
DWORD nFileNum;
DWORD nReserved;
UCHAR  nAreaNum;
UCHAR  nComputerNum;
FILEACLENTRY FileAcl[TOTALFILENUM];
}FILEACL,*PFILEACL;

We can take an experiment in the VC6.0 enviroment:

Press Alt+F7, then jump \"Project Setting\", Click to the page \"C/C++\", choose the \"Category\", Change the \"Struct member alignment\", when choose \"1 byte\" or \"2 bytes\", the sizeof(FileAcl) = 7698; When choose \"4 bytes\" or \"8 bytes *\" or \"16 bytes\", sizeof(FileAcl) = 7700.

The default setting of \"Struct member aliment\" is 8 bytes.

From the above exprement, we can see the alignment format affect the size of structure.

But there are another question: Why when we set the \"Struct member aliment\" to \"8 bytes\" or \"16 bytes\", the sizeof FILEACL is still 7700. As the rule, when the \"Struct member aliment\" is 8 bytes, it should 7704; when the \"Struct member aliment\" is 16 bytes, it should 7712.

Why? Who can explain it? Thanks!

My msn: jgw2008@hotmail.com

一般都是按字对齐的

这个和字节对齐有关，如果不够会自动补充字节，具体有一些规则
定义时最好把同类型或同大小的放到一起