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

int main()
{
        //width, height, and bitcount are the key factors:
    int32_t width = 64;
    int32_t height = 64;
    uint16_t bitcount = 24;//<- 24-bit bitmap

    //take padding in to account
    int width_in_bytes = ((width * bitcount + 31) / 32) * 4;

    //total image size in bytes, not including header
    uint32_t imagesize = width_in_bytes * height;

    //this value is always 40, it's the sizeof(BITMAPINFOHEADER)
    const uint32_t biSize = 40;

    //bitmap bits start after headerfile, 
    //this is sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER)
    const uint32_t bfOffBits = 54; 

    //total file size:
    uint32_t filesize = 54 + imagesize;

    //number of planes is usually 1
    const uint16_t biPlanes = 1;

    //create header:
    //copy to buffer instead of BITMAPFILEHEADER and BITMAPINFOHEADER
    //to avoid problems with structure packing
    unsigned char header[54] = { 0 };
    memcpy(header, "BM", 2);
    memcpy(header + 2 , &filesize, 4);
    memcpy(header + 10, &bfOffBits, 4);
    memcpy(header + 14, &biSize, 4);
    memcpy(header + 18, &width, 4);
    memcpy(header + 22, &height, 4);
    memcpy(header + 26, &biPlanes, 2);
    memcpy(header + 28, &bitcount, 2);
    memcpy(header + 34, &imagesize, 4);

    //prepare pixel data:
    unsigned char* buf = malloc(imagesize);
    for(int row = height - 1; row >= 0; row--)
    {
        for(int col = 0; col < width; col++)
        {
            buf[row * width_in_bytes + col * 3 + 0] = rand()%256;//blue
            buf[row * width_in_bytes + col * 3 + 1] = rand()%256;//green
            buf[row * width_in_bytes + col * 3 + 2] = rand()%256;//red
        }
    }

    FILE *fout = fopen("test.bmp", "wb");
    fwrite(header, 1, 54, fout);
    fwrite((char*)buf, 1, imagesize, fout);
    fclose(fout);
    free(buf);
}