How to read BMP file in C?

Question

I am trying to consult the basic information of a BMP. For that, I built a struct for the file header and another for the image header. Referring to a BMP table of values ​​(http://www.dragonwins.com/domains/getteched/bmp/bmpfileformat.htm), I read the values ​​and validated as specified. However, only the bfType is read correctly and other values ​​are filled in with wrong information. On my computer, sizeof(int) = 4

Structs:

typedef struct BmpFileHeader {
   char bfType[2];
   unsigned int bfSize;
   unsigned short int __bfReserved1;
   unsigned short int __bfReserved2;
   unsigned long int bfOffBits;
} BMPFILEHEADER;

typedef struct BmpImageHeader {
   unsigned int biSize;
   int biWidth;
   int biHeight;
   unsigned short int biPlanes;
   unsigned short int biBitCount;
   unsigned int biCompression;
   unsigned int biSizeImage;
   int biXPelsPerMeter;
   int biYPelPerMeter;
   unsigned int biClrUsed;
   unsigned int biClrImportant;  
} BMPIMAGEHEADER;

Print functions:

void printFileHeader(BMPFILEHEADER fileHeader) {
   printf("\nType: %c%c.\n", fileHeader.bfType[0],fileHeader.bfType[1]);
   printf("Size: %d.\n", fileHeader.bfSize);
   printf("Verify (Must be 0 0): %d %d.\n");
   printf("Offset : %d.\n", fileHeader.bfOffBits);
};

void printImageHeader(BMPIMAGEHEADER imageHeader) {
   printf("\nSize of header: %d.\n", imageHeader.biSize);
   printf("Width: %d.\n", imageHeader.biWidth);
   printf("Height: %d.\n", imageHeader.biHeight);
   printf("Color Planes: %d.\n", imageHeader.biPlanes);
   printf("Bits per Pixel: %d.\n", imageHeader.biBitCount);
   printf("Compression: %d.\n", imageHeader.biCompression);
   printf("Image size: %d.\n", imageHeader.biSizeImage);
   printf("Preferred resolution in pixels per meter (X-Y): %d-%d.\n", imageHeader.biXPelsPerMeter, imageHeader.biYPelPerMeter);
   printf("Number color map: %d.\n", imageHeader.biClrUsed);
   printf("Number of significant colors: %d.\n", imageHeader.biClrImportant);
}

Main function:

int main() {
   FILE *image;
   BMPFILEHEADER header;
   BMPIMAGEHEADER imageHeader;

   image = fopen("test.bmp", "rb");

   if(!image) {
      printf("Could not open the file %s.", "test.bmp");
      fclose(image);
      return 1;
   }

   fread(&header, sizeof(BMPFILEHEADER), 1,image);
   
   printf("File header information:");
   printFileHeader(header);

   if(header.bfType[0] != 'B' || header.bfType[1] != 'M') {
      printf("The file %s is not a valid BMP.", "test.bmp");
      return 1;
   }

   fread(&imageHeader, sizeof(BMPIMAGEHEADER), 1, image);
   
   printf("\nImage header information:");
   printImageHeader(imageHeader);

   if(imageHeader.biSize != 40 || imageHeader.biCompression != 0 || imageHeader.biBitCount != 24) {
      printf("The file %s is not a valid BMP.", "test.bmp");
      fclose(image);
      return 1;
   }

   fclose(image);
   return 0;
}

Answer 1

Structure packing and alignment padding are implementation defined, and byte order is platform defined.

If the byte order for your platform is the same as that defined for BMP (little-endian) then you can use whatever compiler extensions your toolchain supports for structure packing. For example in GCC:

typedef struct BmpFileHeader {
   char bfType[2];
   unsigned int bfSize;
   unsigned short int __bfReserved1;
   unsigned short int __bfReserved2;
   unsigned long int bfOffBits;
} __attribute__ ((packed)) BMPFILEHEADER; 

typedef struct BmpImageHeader {
   unsigned int biSize;
   int biWidth;
   int biHeight;
   unsigned short int biPlanes;
   unsigned short int biBitCount;
   unsigned int biCompression;
   unsigned int biSizeImage;
   int biXPelsPerMeter;
   int biYPelPerMeter;
   unsigned int biClrUsed;
   unsigned int biClrImportant;  
} __attribute__ ((packed)) BMPIMAGEHEADER;

Byte order for BMP is little-endian for for integer values; so for x86 and most ARM platforms you may not need to worry about byte-order. Pixel byte order is somewhat less straightforward.

However for a truly portable solution you would have to read the data byte-by-byte and load each member of the structure individually - so called deserialisation.

You would also do well to ensure compliance with the header structure by using stdint.h data types uint8_t, uint16_t, uint32_t, int32_t etc.