Question

I find myself writing a simple program to extract data from a bmp file. I just got started and I am at one of those WTF moments.

When I run the program and supply this image: http://www.hack4fun.org/h4f/sites/default/files/bindump/lena.bmp

I get the output:

type: 19778
size: 12
res1: 0
res2: 54
offset: 2621440

The actual image size is 786,486 bytes. Why is my code reporting 12 bytes?

The header format specified in,
http://en.wikipedia.org/wiki/BMP_file_format matches my BMP_FILE_HEADER structure. So why is it getting filled with wrong information?

The image file doesn’t appear to be corrupt and other images are giving equally wrong outputs. What am I missing?

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

typedef struct {
    unsigned short type;
    unsigned int size;
    unsigned short res1;
    unsigned short res2;
    unsigned int offset;
} BMP_FILE_HEADER;

int main (int args, char ** argv) {
    char *file_name = argv[1];

    FILE *fp = fopen(file_name, "rb");

    BMP_FILE_HEADER file_header;

    fread(&file_header, sizeof(BMP_FILE_HEADER), 1, fp);

    if (file_header.type != 'MB') {
        printf("ERROR: not a .bmp");
        return 1;
    }

    printf("type: %i\nsize: %i\nres1: %i\nres2: %i\noffset: %i\n", file_header.type, file_header.size, file_header.res1, file_header.res2, file_header.offset);
    fclose(fp);

    return 0;
}

Answer 1

There are two mistakes I could find in your code.

First mistake: You have to pack the structure to 1, so every type size is exactly the size its meant to be, so the compiler doesn’t align it for example in 4 bytes alignment. So in your code, short, instead of being 2 bytes, it was 4 bytes. The trick for this, is using a compiler directive for packing the nearest struct:

#pragma pack(1)

typedef struct {
    unsigned short type;
    unsigned int size;
    unsigned short res1;
    unsigned short res2;
    unsigned int offset;
} BMP_FILE_HEADER;

Now it should be aligned properly.

The other mistake is in here:

if (file_header.type != 'MB')

You are trying to check a short type, which is 2 bytes, with a char type (using ''), which is 1 byte. Probably the compiler is giving you a warning about that, it’s canonical that single quotes contain just 1 character with 1-byte size.

To get this around, you can divide this 2 bytes into 2 1-byte characters, which are known (M and B), and put them together into a word. For example:

if (file_header.type != (('M' << 8) | 'B'))

If you see this expression, this will happen:

'M' (which is 0x4D in ASCII) shifted 8 bits to the left, will result in 0x4D00, now you can just add or or the next character to the right zeroes: 0x4D00 | 0x42 = 0x4D42 (where 0x42 is 'B' in ASCII). Thinking like this, you could just write:

if (file_header.type != 0x4D42)

Then your code should work.