Reading and writing bitmaps in c

Question

I'm trying to create an application that inverts the colors of a bitmap file but am having some trouble with actually gathering the data and from the bitmap. I'm using structures to keep the data for the bitmap and it's header. Right now I have:

struct
{
    uint16_t type;
    uint32_t size;
    uint32_t offset;
    uint32_t header_size;
    int32_t  width;
    int32_t  height;
    uint16_t planes;
    uint16_t bits;
    uint32_t compression;
    uint32_t imagesize;
    int32_t  xresolution;
    int32_t  yresolution;
    uint32_t ncolours;
    uint32_t importantcolours;
} header_bmp

struct {
    header_bmp header;
    int data_size;
    int width;
    int height;
    int bytes_per_pixel;
    char *data;
} image_bmp;

Now for actually reading and writing the bitmap I have the following:

image_bmp* startImage(FILE* fp)
{
header_bmp* bmp_h = (struct header_bmp*)malloc(sizeof(struct header_bmp));
ReadHeader(fp, bmp_h, 54);
}

void ReadHeader(FILE* fp, char* header, int dataSize)
{
fread(header, dataSize, 1, fp);
}

From here how do I extract the header information into my header structure?

Also if anyone has any good resources over reading and writing bitmaps, please let me know. I have been searching for hours and can't find much useful information over the topic.

Answer 1

You actually should already have all the data in the correct places. The only issue possibly gone wrong could be endianness. e.g. is the number 256 represented in "short" as 0x01 0x00 or 0x00 0x01.

EDIT: there is something wrong related to the syntax of struct...

 struct name_of_definition { int a; int b; short c; short d; };
 struct name_of_def_2 { struct name_of_definition instance; int a; int b; }
    *ptr_to_instance; // or one can directly allocate the instance it self by
                      // by omitting the * mark.
 struct { int b; int c; } instance_of_anonymous_struct;

 ptr_to_instance = malloc(sizeof(struct name_of_def_2));

also:

 ReadHeader(fp, (char*)&ptr_to_instance->header, sizeof(struct definition));
           //    ^ don't forget to cast to the type accepted by ReadHeader

In this way you can directly read data into the middle of the struct, but the possible issue of endianness still lurks around.