c - 如何在不使用#pragma pack 或 __atribute__((packed)) 的情况下通过结构指针读取 BMP 文件并访问其 header 信息？

Question

这是我的代码。我想知道如何“正确地”读取 BMP 文件，然后在不强制打包结构的情况下读取 header 值。

typedef struct __attribute__((packed)){
uint8_t magic[2];   /* the magic number used to identify the BMP file:
                     0x42 0x4D (Hex code points for B and M).
                     The following entries are possible:
                     BM - Windows 3.1x, 95, NT, ... etc
                     BA - OS/2 Bitmap Array
                     CI - OS/2 Color Icon
                     CP - OS/2 Color Pointer
                     IC - OS/2 Icon
                     PT - OS/2 Pointer. */
uint32_t filesz;    /* the size of the BMP file in bytes */
uint16_t creator1;  /* reserved. */
uint16_t creator2;  /* reserved. */
uint32_t offset;    /* the offset, i.e. starting address,
                     of the byte where the bitmap data can be found. */
} bmp_header_t;

fp = fopen("input.bmp", "r");
bmp_header_p = malloc(sizeof(bmp_header_t));

fread(bmp_header_p, sizeof(char), 14, fp);

printf("magic number = %c%c\n", bmp_header_p->magic[0], bmp_header_p->magic[1]);
printf("file size = %" PRIu32 "\n", bmp_header_p->filesz);

Answer 1

您不能一次fread() 进入整个结构。相反，您 fread() 分别进入其字段，如下所示:

if (fread(&header->magic[0], 2, 1, fp) != 1) {
    // error
}

if (fread(&header->filesz, 4, 1, fp) != 1) {
    // error
}