博客

未对齐时的结构体大小

#include <stdio.h>
#include <stdint.h>

struct aa {
    uint8_t a;
    uint16_t b;
    uint8_t c;
};

int main(int argc, char **argv)
{
    printf("struct size=%d\n", sizeof(struct aa));
}

结果

[~/test]$ gcc test.c
[~/test]$ ./a.out 
struct size=6

对齐后的结构体大小

使用GUN关键字

attribute

((packed))

#include <stdio.h>
#include <stdint.h>

#ifdef __GNUC__
#define STRUCT_PACKED __attribute__ ((packed))
#else
#define STRUCT_PACKED
#endif

struct aa {
    uint8_t a;
    uint16_t b;
    uint8_t c;
} STRUCT_PACKED;

int main(int argc, char **argv)
{
    printf("struct size=%d\n", sizeof(struct aa));
}

结果

[~/test]$ gcc test.c
[~/test]$ ./a.out 
struct size=4

使用#pragma pack(1)

#include <stdio.h>
#include <stdint.h>

#pragma pack(1) // 让编译器做1字节对齐 
struct aa {
    uint8_t a;
    uint16_t b;
    uint8_t c;
};
#pragma pack() // 取消1字节对齐，恢复为默认对齐

int main(int argc, char **argv)
{
    printf("struct size=%d\n", sizeof(struct aa));
}

结果

[~/test]$ gcc test.c
[~/test]$ ./a.out 
struct size=4