Reading information stored in header array when reading BMP file in c

Question

I am trying to learn image processing using c.

#include <stdio.h>

int main(){
    FILE *streamIn;
    streamIn=fopen("lena512.bmp","r");

    //read imageHeader and colorTable
    unsigned char header[54];//to store the image header
    //unsigned char colorTable[1024];//to store the colorTable,if it exists

    for(int i=0;i<54;i++){
            header[i]=getc(streamIn);
    }
    /*
      width of the image(18th byte)
      height of the image(22nd byte)
      bitDepth of the image(28th byte)
    */
    int width=*(int*)(&header[18]);
    int height=*(int*)(&header[22]);
    int bitDepth=*(int*)(&header[28]);


}

I came across the line which i couldn't understand.

int width=*(int*)(&header[18]);

Why can't we simply do the type casting like int width=(int)(header[18]);?

Answer 1

I came across the line which i couldn't understand.
int width=*(int*)(&header[18]);

Neither did the person who wrote it, because this is blatant undefined behavior in several different ways. It gives This gives a misaligned address (on mainstream systems) as well as a strict aliasing violation. What is the strict aliasing rule?

Those 3 lines at the end are bugs, you cannot write C code like that. The correct way to write this program might be to copy the data into pre-defined structs, provided by the OS API.

Answer 2

You are using *(int*)(&header[18]) and you want to know why (int)(header[18]) can't be used.

Well, these are completely different expressions.

The first one takes the address of header[18] , which is an unsigned char , so this gives a pointer to such. Casting it to a pointer to int and dereferencing it by * in the assignment copies a complete int starting at that address, reading all necessary bytes.

The second one simply reads the single unsigned char in header[18] and casts the value to an int . This will use only the first byte.

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Note: Microsoft's header files provide structure definitions of the BMP header. It's much better to use them. You can for example call fread() to read the header.