字节对齐与结构体大小

#include <stdio.h>

#include <string.h>

/*

* Abbreviation:

* NAV - Native Alignment Value

* NAV-of-basic_type: (on 32bit CPU)

* char
- 1

* int
- 4

* short
- 2

* long
- 4

* float
- 4

* double
- 8

* NAV-of-struct: max(NAV-of-members)

* SAV - Specified Alignment Value

* EAV - Effective Alignment Value; EAV = min(NAV, SAV)

* sizeof(struct) % EAV-of-struct == 0

*/

/* default SAV = 4 gcc*/

struct S0_1 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

short s;
/* NAV = 2; EAV = min(NAV, SA V) = 2 */

}; /* NAV = max(NAV-of-members) = 2; EAV = min(NAV, SAV) = 2 */

/*

* memory structure:

* 0xbffff380: 0x11
0x00 0x22
0x22

*

* sizeof(struct S0_1) = 4

*/

struct S0_2 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

char c2;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

short s;
/* NAV = 2; EAV = min(NAV, SAV) = 2 */

}; /* NAV = max(NAV-of-members) = 2; EAV = min(NAV, SAV) = 2 */

/*

* memory structure:

* 0xbffff37c: 0x11
0x22 0x33
0x33

*

* sizeof(struct S0_2) = 4

*/

struct S0_3 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

char a[2];
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

short s;
/* NAV = 2; EAV = min(NAV, SAV) = 2 */

}; /* NAV = max(NAV-of-members) = 2; EAV = min(NAV, SAV) = 2 */

/*

* memory structure:

* 0xbffff376: 0x11
0x22 0x33
0x00 0x44
0x44

*

* sizeof(struct S0_3) = 6

*/

struct S0_4 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

long l;
/* NAV = 4; EAV = min(NAV, SAV) = 4 */

}; /* NAV = max(NAV-of-members) = 4; EAV = min(NAV, SAV) = 4 */

/*

* memory structure:

* 0xbffff358: 0x11
0x00 0x00
0x00 0x22
0x22 0x22
0x22

*

* sizeof(struct S0_4) = 8

*/

struct S0_5 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

double
d; /* NAV = 8; EAV = min(NAV, SAV) = 4 */

}; /* NAV = max(NAV-of-members) = 8; EAV = min(NAV, SAV) = 4 */

/*

* memory structure:

* 0xbffff344: 0x11
0x00 0x00
0x00 0x22
0x22 0x22
0x22

* 0xbffff34c: 0x22
0x22 0x22
0x22

*

* sizeof(struct S0_5) = 12

*/

struct S {

struct S0_1 m1;
/* NAV = 2; EAV = 2; sizeof(struct S0_1) = 4 */

struct S0_2 m2;
/* NAV = 2; EAV = 2; sizeof(struct S0_2) = 4 */

struct S0_3 m3;
/* NAV = 2; EAV = 2; sizeof(struct S0_3) = 6 */

struct S0_4 m4;
/* NAV = 4; EAV = 4; sizeof(struct S0_4) = 8 */

struct S0_5 m5;
/* NAV = 8; EAV = 4; sizeof(struct S0_5) = 12 */

}; /* NAV = max(NAV-of-members) = 8; EAV = min(NAV, SAV) = 4 */

/*

* memory structure:

* 0xbffff314: 0x11
0x00 0x22
0x22 0x11
0x22 0x33
0x33

* 0xbffff31c: 0x11
0x22 0x33
0x00 0x44
0x44 0x00
0x00

* 0xbffff324: 0x11
0x00 0x00
0x00 0x22
0x22 0x22
0x22

* 0xbffff32c: 0x11
0x00 0x00
0x00 0x22
0x22 0x22
0x22

* 0xbffff334: 0x22
0x22 0x22
0x22

*

* sizeof(struct S) = 36

*/

#pragma pack(push)

#pragma pack(1) /* SAV = 1 (1 byte alignment) */

struct S1 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

long l;
/* NAV = 4; EAV = min(NAV, SAV) = 1 */

short s;
/* NAV = 2; EAV = min(NAV, SAV) = 1 */

}; /* NAV = max(NAV-of-members) = 1; EAV = min(NAV, SAV) = 1 */

/*

* memory structure:

* 0xbffff36f: 0x11
0x22 0x22
0x22 0x22
0x33 0x33

*

* sizeof(struct S1) = 7

*/

#pragma pack(pop)

#pragma pack(push)

#pragma pack(2) /* SAV = 2 (2 bytes alignment) */

struct S2 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

long l;
/* NAV = 4; EAV = min(NAV, SAV) = 2 */

short s;
/* NAV = 2; EAV = min(NAV, SAV) = 2 */

}; /* NAV = max(NAV-of-members) = 2; EAV = min(NAV, SAV) = 2 */

/*

* memory structure:

* 0xbffff350: 0x11
0x00 0x22
0x22 0x22
0x22 0x33
0x33

*

* sizeof(struct S2) = 8

*/

#pragma pack(pop)

/* now, restore 4-bytes alignment, SAV = 4 */

struct S4 {

char c;
/* NAV = 1; EAV = min(NAV, SAV) = 1 */

long l;
/* NAV = 4; EAV = min(NAV, SAV) = 4 */

short s;
/* NAV = 2; EAV = min(NAV, SAV) = 2 */

}; /* NAV = max(NAV-of-members) = 4; EAV = min(NAV, SAV) = 4 */

/*

* memory structure:

* 0xbffff338: 0x11
0x00 0x00
0x00 0x22
0x22 0x22
0x22

* 0xbffff340: 0x33
0x33 0x00
0x00

*

* sizeof(struct S2) = 12

*/