Reading from the header of a bmp file - c

Question

I am writing a program to read a bmp header. I've written some code that was working when it was all in main. How to implement this code as a function of its own and then implementing it onto main?
Here is the whole code :
#include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdin.h>
struct bmp_header {
uint16_t type;
uint32_t size;
uint16_t reserved1;
uint16_t reserved2;
uint32_t offset;
uint32_t dib_size;
uint32_t width;
uint32_t height;
uint16_t planes;
uint16_t bpp;
uint32_t compression;
uint32_t image_size;
uint32_t x_ppm;
uint32_t y_ppm;
uint32_t num_colors;
uint32_t important_colors;
};
void read_bmp(FILE *BMPFile,struct bmp_header* Header) {
fread(&(Header->type), 2, 1, BMPFile);
fread(&(Header->size),4,1,BMPFile);
fread(&(Header->reserved1),2,1,BMPFile);
fread(&(Header->reserverd2),2,1,BMPFile);
fread(&(Header->offset),4,1,BMPFile);
fread(&(Header->dib_size),4,1,BMPFile);
fread(&(Header->width),4,1,BMPFile);
fread(&(Header->height),4,1,BMPFile);
fread(&(Header->planes),2,1,BMPFile);
fread(&(Header->bpp),2,1,BMPFile);
fread(&(Header->compression),4,1,BMPFile);
fread(&(Header->image_size),4,1,BMPFile);
fread(&(Header->x_ppm),4,1,BMPFile);
fread(&(Header->y_pp),4,1,BMPFile);
fread(&(Header->num_colors),4,1,BMPFile);
fread(&(Header->important_colors),4,1,BMPFile);
}
int main() {
FILE *BMPFile = fopen("image.bmp","rb");
if(BMPFile == NULL)
{
return;
}
struct bmp_header* Header;
read_bmp(BMPFile,Header);
fclose(BMPFile);
return 0;
}
The relevant parts of the version of the program with all reading action in main, that worked as expected, is reported below
int main( void )
{
FILE *BMPFile = fopen ("lenna.bmp", "rb");
if (BMPFile == NULL)
{
return 0;
}
struct bmp_header Header;
memset(&Header, 0, sizeof(Header));
fread(&Header.type, 2, 1, BMPFile);
fread(&Header.size),4,1,BMPFile);
fread(&Header.reserved1),2,1,BMPFile);
fread(&Header.reserverd2),2,1,BMPFile);
fread(&Header.offset),4,1,BMPFile);
fread(&Header.dib_size),4,1,BMPFile);
fread(&Header.width),4,1,BMPFile);
fread(&Header.height),4,1,BMPFile);
fread(&Header.planes),2,1,BMPFile);
fread(&Header.bpp),2,1,BMPFile);
fread(&Header.compression),4,1,BMPFile);
fread(&Header.image_size),4,1,BMPFile);
fread(&Header.x_ppm),4,1,BMPFile);
fread(&Header.y_pp),4,1,BMPFile);
fread(&Header.num_colors),4,1,BMPFile);
fread(&Header.important_colors),4,1,BMPFile);
/* Header fields print section */
/* ... */
}

Answer 1

Whenever a working code stops working, it is useful to focus on the changes between the two versions of the code. So, why does your original code work correctly? It looks like this:
int main( void )
{
FILE *BMPFile = fopen ("lenna.bmp", "rb");
if (BMPFile == NULL)
{
return 0;
}
struct bmp_header Header;
memset(&Header, 0, sizeof(Header));
fread(&Header.type, 2, 1, BMPFile);
...
}
You declare Header, of type struct bmp_header, in main's stack (as local variable). In this way the structure will be for sure allocated during all program's lifetime.
You memset it to 0
You pass Header's fields addresses directly to fread
In the new version of the program you have a function defined as
void read_bmp(FILE *BMPFile,struct bmp_header* Header);
so you need a pointer to struct bmp_header to be passed to it. For this reason you declare
struct bmp_header* Header;
and call read_bmp(BMPFile,Header);.
What is the different with the working version? Well, the pointer! Declaring a pointer you say to the compiler that it contains an address, in this case the address of the structure required by read_bmp().
But you never say to the compiler what the address is, so that freads within read_bmp() will write to random location causing a segmentation fault.
What to do
You need to pass to read_bmp() a valid struct bmp_header address, and you have two options.
You can allocate Header in the stack like you did before, and pass its address to read_bmp(), through & operator. It should have been your first attempt, since it was really similar to your working solution.
struct bmp_header Header;
read_bmp(BMPFile, &Header);
You can declare Header as a pointer, but you will need to dynamically allocate its memory through malloc:
struct bmp_header * Header = malloc(sizeof(struct bmp_header));
read_bmp(BMPFile, Header);

Answer 2

You have to create struct bmp_header* type function. Next create struct bmp_header pointer in your function & allocate memory (header size == 54B) for returned reference.