Problem with reading Bitmaps in C

Question

I’m having trouble reading files Bitmap. Results not as expected.

The size is always the same regardless of which image I open

Height and width do not match the size stated in the image viewer

The color values are not always those of the colors that are in the image

bmp. h:

#ifndef  BMP_H
#define  BMP_H

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

__attribute__((packed)) struct RGB{ // Color of Bitmap file
    char red;
    char green;
    char blue;
    char reserved;
};

__attribute__((packed)) struct bmp_header{
    char id[2]; // File identificator
    int size; // Size on bytes
    short int r_area_1; // Reserved area
    short int r_area_2; // Reserved area
    int offset; // Offset of Bitmap
};

__attribute__((packed)) struct bmp_header_map{
    int size; // Size of Header - 40 bytes
    int width; // Width of Bitmap
    int height; // Height of Bitmap
    short int p_quality; // Qualit of planes (default = 1)
    short int bpp; // Bits per Pixel
    int compression; // Compresion method
    int image_size; // Size of image
    int h_resolution; // Horizontal resolution
    int v_resolution; // Vertical resolution
    int p_color; // Number of color in palet
    int i_color; // Number of important colors
};

struct bitmap{
    struct bmp_header *header;
    struct bmp_header_map *map;
    struct RGB *rgb;
};

typedef struct bitmap Bitmap;

extern Bitmap *bmp_read(const char *fname); // read bitmap files
extern char bmp_write(const char *fname, Bitmap *bmp); // write bitmap files

#endif  // BMP_H*/

bmp_read. c

#include "bmp.h"

struct bmp_header *bmp_get_header(FILE *bmp);
struct bmp_header_map *bmp_get_header_map(FILE *bmp);
struct RGB *bmp_get_colors(FILE *bmp, int width, int height);

extern Bitmap *bmp_read(const char *fname){
    struct bmp_header *header;
    struct bmp_header_map *map;
    struct RGB *rgb;

    Bitmap *bmp = (Bitmap *) malloc(sizeof(Bitmap));
    FILE *file = fopen(fname, "r");

    header = bmp_get_header(file);
    map = bmp_get_header_map(file);
    //rgb = bmp_get_colors(file);

    fclose(file);
    bmp->header = header;
    bmp->map = map;
    bmp->rgb = rgb;

    return bmp;
}

struct bmp_header *bmp_get_header(FILE *bmp){
    struct bmp_header *header;
    header = (struct bmp_header *) malloc(sizeof(struct bmp_header));
    bzero(header, sizeof(struct bmp_header));

    fread(header, sizeof(struct bmp_header), 1, bmp);

    return header;
}

struct bmp_header_map *bmp_get_header_map(FILE *bmp){
    struct bmp_header_map *map;
    map = (struct bmp_header_map *) malloc(sizeof(struct bmp_header_map));
    bzero(map, sizeof(struct bmp_header_map));

    fread(map, sizeof(struct bmp_header_map), 1, bmp);

    return map;
}

struct RGB *bmp_get_colors(FILE *bmp, int width, int height){
    struct RGB *rgb = (struct RGB*) malloc(sizeof(struct RGB)*width*height);
    bzero(rgb, sizeof(struct RGB));
    fread(rgb, width*height*3, 1, bmp);

    return rgb;
}

int main(int argc, char *argv[]){
    struct bmp_header *header;
    struct bmp_header_map *map;
    struct RGB *rgb;
    struct bitmap *bmp;

    bmp = bmp_read("teste.bmp");
    header = bmp->header;
    map = bmp->map;
    rgb = bmp->rgb;

    printf("%c%c %d %d\n",header->id[0], header->id[1], header->size, header->offset);
    printf("%dx%d %d %d\n",map->width, map->height, map->image_size, map->i_color);
    printf("%d\n", map->i_color);
    int x;
    for(x=0; x<=30; x++){
        printf("%d %dx%dx%d\n",x,rgb[x].red, rgb[x].green, rgb[x].blue);
    }
    return 0;
}

Output obtained (Ena.bmp):

BM 3 2621440
16777216x65536 185729024 372834304
372834304
0 0x0
1 1x0x0
2 84x8x64
3 0x0
4 0x0
5 0x0
6 109x123x113
7 10x-65x51
8 -48x5x64
9 0x0
10 -96x-68x-105
11 -4x127x0
12 0x0
13 0x0
14 0x0
15 0x0
16 109x123x-111
17 -91x68x-53
18 109x123x-31
19 -27x-120x-75
20 0x0
21 0x0
22 0x0
23 0x0
24 0x0
25 0x0
26 -96x9x64
27 0x0
28 -88x-68x-105
29 -4x127x0
30 1x0x0

Exits obtained (test.bmp):

BM 0 8126464
655360x65536 185794560 -134217728
-134217728
0 0x0
1 1x0x0
2 84x8x64
3 0x0
4 0x0
5 0x0
6 -72x-4x53
7 -39x70x-63
8 -48x5x64
9 0x0
10 32x-19x-51
11 -4x127x0
12 0x0
13 0x0
14 0x0
15 0x0
16 -72x-4x-43
17 -62x91x56
18 -72x-4x-91
19 47x4x-110
20 0x0
21 0x0
22 0x0
23 0x0
24 0x0
25 0x0
26 -96x9x64
27 0x0
28 40x-19x-51
29 -4x127x0
30 1x0x0

Lena.bmp 256x256:

test.bmp 10x10:

Thanks for the help!

Answer 1

I believe that the problem may be in the creation of structs with multiple size of 4 bytes. In the tests I did on my machine I inserted the line

printf("size of bmp header: %ul\n", sizeof(struct bmp_header));

and was returning 16 and not 14 (despite the "attribute((packed)) struct bmp_header").

I then entered the lines:

#pragma pack(push, 1)

and

#pragma pack(pop)

respectively at the beginning and at the end of the bmp. h file and started working. The output of the size test looked like this:

size of bmp header: 14

And the rest of the output was also consistent in my test, although I haven’t tested the excerpt:

int x;
for(x=0; x<=30; x++){
    printf("%d %dx%dx%d\n",x,rgb[x].red, rgb[x].green, rgb[x].blue);
}

Since the line

//rgb = bmp_get_colors(file);

was commented.