#pragma pack(n)

C/C++ 数据成员对齐规则利器 #pragma pack(n)

/* set packed size */
#pragma pack(n)
struct node
{
unsigned char scale;
long long int val;
}
/* recovery packed size */
#pragma pack()

编程实例

/*
* Test pragma pack(n)
* Author: Fly_pig
*/

#include <stdio.h>
#include <inttypes.h>

/* define integer variable */
typedef unsigned char   uint8;
typedef signed   char   int8;
typedef signed   short  int16;
typedef signed   int    int32;
typedef signed long long int64;

/* set packed size to 1 */
#pragma pack(1)

typedef struct
{
uint8 scale;
int8  val;
} bipack8;

typedef struct
{
uint8 scale;
int16 val;
} bipack16;

typedef struct
{
uint8 scale;
int32 val;
} bipack32;

typedef struct
{
uint8 scale;
int64 val;
} bipack64;

/* reset packed size */
#pragma pack()

typedef struct
{
uint8 scale;
int8  val;
} bi8;

typedef struct
{
uint8 scale;
int16 val;
} bi16;

typedef struct
{
uint8 scale;
int32 val;
} bi32;

typedef struct
{
uint8 scale;
int64 val;
} bi64;

int main()
{
/* print packed struct size */
printf("sizeof(bipack8)  = %d\n", sizeof(bipack8));
printf("sizeof(bipack16) = %d\n", sizeof(bipack16));
printf("sizeof(bipack32) = %d\n", sizeof(bipack32));
printf("sizeof(bipack64) = %d\n", sizeof(bipack64));
/* print default packed struct size */
printf("sizeof(bi8)  = %d\n", sizeof(bi8));
printf("sizeof(bi16) = %d\n", sizeof(bi16));
printf("sizeof(bi32) = %d\n", sizeof(bi32));
printf("sizeof(bi64) = %d\n", sizeof(bi64));
return 0;
}

执行结果

fly_pig@ubuntu:~/Study/C$ gcc -o test_pack test_pack.c
fly_pig@ubuntu:~/Study/C$ ./test_pack
sizeof(bipack8)  = 2
sizeof(bipack16) = 3
sizeof(bipack32) = 5
sizeof(bipack64) = 9
sizeof(bi8)  = 2
sizeof(bi16) = 4
sizeof(bi32) = 8
sizeof(bi64) = 12