WinCE 下結(jié)構體占用空間問題的分析
[導讀]年前辭職了，準備年后找工作。所以在網(wǎng)上查找一下 C/C++ 基礎方面的面試題，個人感覺這方面還是要準備一下。雖然后面的應聘沒有用到所準備的這些，算是學習一下。在學習過程中，注意到一些關于結(jié)構體占用內(nèi)存
年前辭職了，準備年后找工作。
所以在網(wǎng)上查找一下 C/C++ 基礎方面的面試題，個人感覺這方面還是要準備一下。
雖然后面的應聘沒有用到所準備的這些，算是學習一下。
在學習過程中，注意到一些關于結(jié)構體占用內(nèi)存空間大小(sizeof 與 padding byte)的問題，覺得挺有趣的。
在 WinCE 以前的小系統(tǒng)(嵌入式系統(tǒng))編程時，特別注意內(nèi)存的使用情況；但到了 WinCE 系統(tǒng)后，由于設備一般都有 128M(或更多)的內(nèi)存，所以在內(nèi)存的使用與優(yōu)化上很多人已經(jīng)不太注意。
嵌入式方面的程序員，還是有必要了解以下關于結(jié)構體占用空間這個問題。
typedef?struct?TwoBytes_S
{
char?cOne;
char?cTwo;
}TwoBytesEml;
typedef?struct?FourBytes_S
{
char?cOne;
char?cTwo;
char?cThree;
char?cFour;
}FourBytesEml;
typedef?struct?EightBytes_S
{
char?cOne;
char?cTwo;
char?cThree;
char?cFour;
char?cFive;
char?cSix;
char?cSeven;
char?cEight;
}EightBytesEml;
//?結(jié)構體實際占用的空間大小與結(jié)構體中占用空間最大的成員有什么關系?
//?結(jié)論:?結(jié)構體的大小為結(jié)構體中占用字節(jié)最大的成員的倍數(shù)。?
//?對比?TestOder,?TestOrder3?和?TestOder2?占用內(nèi)存空間的情況
typedef?struct?TestOrder_S
{
char?cFirst;
char?cSecond;
int?iFirst;
char?cThird;
char?cFourth;
}TestOrder;
typedef?struct?TestOrder2_S
{
int?iFirst;
char?cFirst;
char?cSecond;
char?cThird;
char?cFourth;
}TestOrder2;
typedef?struct?TestOrder3_S
{
int?iFirst;
char?cFirst;
char?cSecond;
char?cThird;
}TestOrder3;
//?對比?TestShort?和?TestShortEml?占用內(nèi)存空間的情況
typedef?struct?TestShort_S
{
char?cOne;
short?sAlign;
}TestShort;
typedef?struct?TestShortEml_S
{
char?cOne;
TwoBytesEml?sAlign;
}TestShortEml;
//?對比?TestInt?和?TestIntEml?占用內(nèi)存空間的情況
typedef?struct?TestInt_S
{
char?cOne;
int?iAlign;
}TestInt;
typedef?struct?TestIntEml_S
{
char?cOne;
FourBytesEml?iAlign;
}TestIntEml;
//?對比?TestDouble?和?TestDoubleEml?占用內(nèi)存空間的情況
typedef?struct?TestDouble_S
{
char?cOne;
double?dfAlign;
}TestDouble;
typedef?struct?TestDoubleEml_S
{
char?cOne;
EightBytesEml?dfAlign;
}TestDoubleEml;
//?#define?_USE_WINDOWS_CE_PLATFORM
void?CalcSizeOfStruct(void)
{
#ifdef?_USE_WINDOWS_CE_PLATFORM
RETAILMSG(1,(L"size?of?char?is?%drn",sizeof(char)));
RETAILMSG(1,(L"size?of?short?is?%drn",sizeof(short)));
RETAILMSG(1,(L"size?of?int?is?%drn",sizeof(int)));
RETAILMSG(1,(L"size?of?long?is?%drn",sizeof(long)));
RETAILMSG(1,(L"size?of?char?*?is?%drn",sizeof(char?*)));
RETAILMSG(1,(L"size?of?float?is?%drn",sizeof(float)));
RETAILMSG(1,(L"size?of?double?is?%drn",sizeof(double));
//?對比?TestOder,?TestOrder3?和?TestOder2?占用內(nèi)存空間的情況
RETAILMSG(1,(L"size?of?struct(char,char,int,char,char)?is?%drn",sizeof(TestOrder)));
RETAILMSG(1,(L"size?of?struct(int,char,char,char,char)?is?%drn",sizeof(TestOrder2)));
RETAILMSG(1,(L"size?of?struct(int,char,char,char)?is?%drn",sizeof(TestOrder3)));
//?對比?TestShort?和?TestShortEml?占用內(nèi)存空間的情況
RETAILMSG(1,(L"size?of?struct(char,short)?is?%drn",sizeof(TestShort)));
RETAILMSG(1,(L"size?of?struct(char,TwoBytesEml(char,char))?is?%drn",sizeof(TestShortEml)));
//?對比?TestInt?和?TestIntEml?占用內(nèi)存空間的情況
RETAILMSG(1,(L"size?of?struct(char,int)?is?%drn",sizeof(TestInt)));
RETAILMSG(1,(L"size?of?struct(char,FourBytesEml(char,char,char,char))?is?%drn",sizeof(TestIntEml)));
//?對比?TestDouble?和?TestDoubleEml?占用內(nèi)存空間的情況
RETAILMSG(1,(L"size?of?struct(char,double)?is?%drn",sizeof(TestDouble)));
RETAILMSG(1,(L"size?of?struct(char,EightBytesEml(char,char,char,char,char,char,char,char))?is?%drn",
sizeof(TestDoubleEml)));
#else
printf("size?of?char?is?%drn",sizeof(char));
printf("size?of?short?is?%drn",sizeof(short));
printf("size?of?int?is?%drn",sizeof(int));
printf("size?of?long?is?%drn",sizeof(long));
printf("size?of?char?*?is?%drn",sizeof(char?*));
printf("size?of?float?is?%drn",sizeof(float));
printf("size?of?double?is?%drn",sizeof(double));
//?對比?TestOder,?TestOrder3?和?TestOder2?占用內(nèi)存空間的情況
printf("size?of?struct(char,char,int,char,char)?is?%drn",sizeof(TestOrder));
printf("size?of?struct(int,char,char,char,char)?is?%drn",sizeof(TestOrder2));
printf("size?of?struct(int,char,char,char)?is?%drn",sizeof(TestOrder3));
//?對比?TestShort?和?TestShortEml?占用內(nèi)存空間的情況
printf("size?of?struct(char,short)?is?%drn",sizeof(TestShort));
printf("size?of?struct(char,TwoBytesEml(char,char))?is?%drn",sizeof(TestShortEml));
//?對比?TestInt?和?TestIntEml?占用內(nèi)存空間的情況
printf("size?of?struct(char,int)?is?%drn",sizeof(TestInt));
printf("size?of?struct(char,FourBytesEml(char,char,char,char))?is?%drn",sizeof(TestIntEml));
//?對比?TestDouble?和?TestDoubleEml?占用內(nèi)存空間的情況
printf("size?of?struct(char,double)?is?%drn",sizeof(TestDouble));
printf("size?of?struct(char,EightBytesEml(char,char,char,char,char,char,char,char))?is?%drn",
sizeof(TestDoubleEml));
#endif
}
//?第二部分?Leo.Zheng?
struct?TstStruct
{
????char?*p;
????char?c;
????long?x;
};
struct?TstStruct2
{
????char?c;??????/*?1?byte?*/
????char?pad[7];?/*?7?bytes?*/
????char?*p;?????/*?4?bytes?*/
????long?x;??????/*?4?bytes?*/
};
struct?TstStruct3
{
????char?*p;?????/*?4?bytes?*/
????char?c;??????/*?1?byte?*/
};
struct?TstStruct4
{
????short?s;?????/*?2?bytes?*/
????char?c;??????/*?1?byte?*/
};
struct?TstStruct5
{
????short?s;
????char?c;
????int?flip:1;
????int?nybble:4;
????int?septet:7;
};
struct?TstStruct6
{
????char?c;
????struct?Struct6_Sub
????{
????????char?*p;
????????short?x;
????}?inner;
};
struct?TstStruct6_R
{
????struct?Struct6_Sub
????{
????????char?*p;
????????short?x;
????}?inner;
????char?c;
????short?s;
};
struct?TstStruct7
{
????char?c;
????struct?TstStruct7?*p;
????short?x;
};
struct?TstStruct8
{
????struct?TstStruct8?*p;
????short?x;
????char?c;
};
struct?TstStruct9
{
????struct?foo7_inner
????{
????????char?*p;
????????short?x;
????}?inner;
????char?c;
};
void?CalcSizeOfStruct2(void)
{
#ifdef?_USE_WINDOWS_CE_PLATFORM
RETAILMSG(1,(L"size?of?(struct?TstStruct)????=?%dn",?sizeof(struct?TstStruct)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct2)???=?%dn",?sizeof(struct?TstStruct2)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct3)???=?%dn",?sizeof(struct?TstStruct3)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct4)???=?%dn",?sizeof(struct?TstStruct4)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct5)???=?%dn",?sizeof(struct?TstStruct5)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct6)???=?%dn",?sizeof(struct?TstStruct6)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct6_R)?=?%dn",?sizeof(struct?TstStruct6_R)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct7)???=?%dn",?sizeof(struct?TstStruct7)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct8)???=?%dn",?sizeof(struct?TstStruct8)));
????RETAILMSG(1,(L"size?of?(struct?TstStruct9)???=?%dn",?sizeof(struct?TstStruct9)));
#else
????printf("size?of?(struct?TstStruct)????=?%dn",?sizeof(struct?TstStruct));
????printf("size?of?(struct?TstStruct2)???=?%dn",?sizeof(struct?TstStruct2));
????printf("size?of?(struct?TstStruct3)???=?%dn",?sizeof(struct?TstStruct3));
????printf("size?of?(struct?TstStruct4)???=?%dn",?sizeof(struct?TstStruct4));
????printf("size?of?(struct?TstStruct5)???=?%dn",?sizeof(struct?TstStruct5));
????printf("size?of?(struct?TstStruct6)???=?%dn",?sizeof(struct?TstStruct6));
????printf("size?of?(struct?TstStruct6_R)?=?%dn",?sizeof(struct?TstStruct6_R));
????printf("size?of?(struct?TstStruct7)???=?%dn",?sizeof(struct?TstStruct7));
????printf("size?of?(struct?TstStruct8)???=?%dn",?sizeof(struct?TstStruct8));
????printf("size?of?(struct?TstStruct9)???=?%dn",?sizeof(struct?TstStruct9));
#endif
}
//?在?WinCE?上運行輸出如下:
/*
size?of?char?is?1
size?of?short?is?2
size?of?int?is?4
size?of?long?is?4
size?of?char?*?is?4
size?of?float?is?4
size?of?double?is?8
size?of?struct(char,char,int,char,char)?is?12
size?of?struct(int,char,char,char,char)?is?8
size?of?struct(int,char,char,char)?is?8
size?of?struct(char,short)?is?4
size?of?struct(char,TwoBytesEml(char,char))?is?3
size?of?struct(char,int)?is?8
size?of?struct(char,FourBytesEml(char,char,char,char))?is?5
size?of?struct(char,double)?is?16
size?of?struct(char,EightBytesEml(char,char,char,char,char,char,char,char))?is?9
size?of?(struct?TstStruct)????=?12
size?of?(struct?TstStruct2)???=?16
size?of?(struct?TstStruct3)???=?8
size?of?(struct?TstStruct4)???=?4
size?of?(struct?TstStruct5)???=?8
size?of?(struct?TstStruct6)???=?12
size?of?(struct?TstStruct6_R)?=?12
size?of?(struct?TstStruct7)???=?12
size?of?(struct?TstStruct8)???=?8
size?of?(struct?TstStruct9)???=?12
*/
//?在?X86?上運行的輸出如下：
/*
size?of?char?is?1
size?of?short?is?2
size?of?int?is?4
size?of?long?is?4
size?of?char?*?is?4
size?of?float?is?4
size?of?double?is?8
size?of?struct(char,char,int,char,char)?is?12
size?of?struct(int,char,char,char,char)?is?8
size?of?struct(int,char,char,char)?is?8
size?of?struct(char,short)?is?4
size?of?struct(char,TwoBytesEml(char,char))?is?3
size?of?struct(char,int)?is?8
size?of?struct(char,FourBytesEml(char,char,char,char))?is?5
size?of?struct(char,double)?is?16
size?of?struct(char,EightBytesEml(char,char,char,char,char,char,char,char))?is?9
size?of?(struct?TstStruct)????=?12
size?of?(struct?TstStruct2)???=?16
size?of?(struct?TstStruct3)???=?8
size?of?(struct?TstStruct4)???=?4
size?of?(struct?TstStruct5)???=?8
size?of?(struct?TstStruct6)???=?12
size?of?(struct?TstStruct6_R)?=?12
size?of?(struct?TstStruct7)???=?12
size?of?(struct?TstStruct8)???=?8
size?of?(struct?TstStruct9)???=?12
*/
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