c - 为什么将我的 C 结构写入二进制文件会产生比结构包含更多的数据

Question

我正在尝试将复杂的 C 结构（包含结构的结构）写入二进制文件，但不知何故，它生成的数据多于结构包含的数据。这是我的代码：

代码清单 1：

#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>

#include<sys/stat.h>
#include<fcntl.h>
#include<unistd.h>

typedef struct __pbi {
    char pbi_first_entry[3];
    char pbi_second_entry[8];
} PBI;

typedef struct __bpr {
    short bpr_first_entry;
    char bpr_second_entry;
    short bpr_third_entry;
    char bpr_fourth_entry;
    short bpr_fifth_entry;
    short bpr_sixth_entry;
    char bpr_seventh_entry;
    short bpr_eighth_entry;
} BPR;

typedef struct __combo {
    PBI  pbi;
    BPR  bpb;
} COMBO;

int main(int argc, char * argv[])
{
    if(argc < 2)
    {
        printf("Usage : %s device\n", argv[0]);
    }

    int dst;
    char buffer[512];
 
    int writecount;

    if((dst = open(argv[1], O_RDWR | O_CREAT)) == -1)
    {
        printf("Couldn't open file %s for writing : %s\n", argv[1], strerror(errno));
        exit(EXIT_FAILURE);
    }

    COMBO combo;

    memcpy(combo.pbi.pbi_first_entry, (char[]){0, 1, 2}, 3);
    
    printf("sizeof(pbi_first_entry) = %d\n", sizeof(combo.pbi.pbi_first_entry));
    
    memcpy(combo.pbi.pbi_second_entry, (char[]){1, 2, 3, 4, 5, 6, 7, 8}, 8);

    printf("sizeof(pbi_second_entry) = %d\n", sizeof(combo.pbi.pbi_second_entry));

    combo.bpb.bpr_first_entry = 0x0201;

    printf("sizeof(bpr_first_entry) = %d\n", sizeof(combo.bpb.bpr_first_entry));
    
    combo.bpb.bpr_second_entry = 1;

    printf("sizeof(bpr_second_entry) = %d\n", sizeof(combo.bpb.bpr_second_entry));

    combo.bpb.bpr_third_entry = 0x0201;

    printf("sizeof(bpr_third_entry) = %d\n", sizeof(combo.bpb.bpr_third_entry));
    
    combo.bpb.bpr_fourth_entry = 1;

    printf("sizeof(bpr_fourth_entry) = %d\n", sizeof(combo.bpb.bpr_fourth_entry));

    combo.bpb.bpr_fifth_entry = 0x0201;

    printf("sizeof(bpr_fifth_entry) = %d\n", sizeof(combo.bpb.bpr_fifth_entry));
    
    combo.bpb.bpr_sixth_entry = 0x0B0A;

    printf("sizeof(bpr_sixth_entry) = %d\n", sizeof(combo.bpb.bpr_sixth_entry));
    
    combo.bpb.bpr_seventh_entry = 1;

    printf("sizeof(bpr_seventh_entry) = %d\n", sizeof(combo.bpb.bpr_seventh_entry));

    combo.bpb.bpr_eighth_entry = 0x0201;

    printf("sizeof(bpr_eighth_entry) = %d\n", sizeof(combo.bpb.bpr_eighth_entry));
    
    printf("sizeof(combo) = %d\n", sizeof(combo));

    if((writecount = write(dst, &combo, sizeof(combo))) == -1)
    {
        printf("Failed to write the combo file : %s\n", strerror(errno));
        exit(-5);   
    }

    printf("Successfully wrote %d byte(s)!\n", writecount);

    close(dst);

    return(EXIT_SUCCESS);
}

当我编译并运行代码时，我得到以下输出：

输出清单 1：

sizeof(pbi_first_entry) = 3
sizeof(pbi_second_entry) = 8
sizeof(bpr_first_entry) = 2
sizeof(bpr_second_entry) = 1
sizeof(bpr_third_entry) = 2
sizeof(bpr_fourth_entry) = 1
sizeof(bpr_fifth_entry) = 2
sizeof(bpr_sixth_entry) = 2
sizeof(bpr_seventh_entry) = 1
sizeof(bpr_eighth_entry) = 2
sizeof(combo) = 28
Successfully wrote 28 byte(s)!

如果为每个条目添加 sizeof() 返回的值，它们将加起来为 24，但由于某种原因 sizeof(combo) 等于 28。combo 是整个结构。也因为 combo 在某种程度上等于 28 ；28 个字节被写入文件。

这是二进制文件的十六进制内容：

十六进制列表 1：

00 01 02 01 02 03 04 05 06 07 08 28 01 02 01 FF
01 02 01 28 01 02 0A 0B 01 B1 01 02

如你看到的 ; 直到第 11 个字节一切都很好，然后有一个字节的值等于0x28 ，然后一切都恢复正常，直到我们遇到另一个char，然后是 ashort并且它再次发生：我们从0x0201（short）开始，然后是1（char) 然后出现一个神秘的字节，其值为0xFF。每次遇到 achar后跟 a时都会发生这种情况short（0x28和0xB1作为神秘值）。

标签： cstruct