为什么不将fread（）放入我的结构工作中？

Question

I have the following code that reads from a file into a struct and into an arrary. When I try to print the data in the struct it isn't what I would expect. The array prints what would be expected, the first two characters in the file.

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. */
} bmp_header_t;


bool
bmp_get_header_from_file(FILE *fp, bmpfile_t *bmp)
{
       fseek(fp, 0L, SEEK_SET);
       char magic[1];

       fread(magic, 1, 2, fp);
       printf("magic is: %c, %c\n", magic[0], magic[1]);

       fread(&bmp->header, 1, 2, fp); 
       printf("magic is: %c, %c\n", bmp->header.magic[0], bmp->header.magic[1]);
}

Answer 1

When you do the first fread , you advance the read position of the file, so the second fread will read the next two bytes. You'll need to add another fseek before the second read.

Answer 2

First things first, char magic[1] gives you one byte, not two. Reading two bytes into that is a definite no-no.

Second, the subsequent fread will attempt to read the next two bytes in the file rather than the first two, because the first fread has advanced the file pointer.

Something like this would be better:

   char magic[2];

   fseek(fp, 0L, SEEK_SET);
   fread(magic, 1, 2, fp);
   printf("magic is: %c, %c\n", magic[0], magic[1]);

   fseek(fp, 0L, SEEK_SET);
   fread(&bmp->header, 1, 2, fp); 
   printf("magic is: %c, %c\n", bmp->header.magic[0], bmp->header.magic[1]);