Source code for blog post http://thesourcecodenotes.blogspot.co.uk/2015/09/create-image-and-pixel-structure-for.html The code creates a 256x256 PPM image with a white background and only on red pixel located in the center of it.

ppm_minim.c

/*
  Source code for blog post http://thesourcecodenotes.blogspot.co.uk/2015/09/create-image-and-pixel-structure-for.html
  The code creates a 256x256 PPM image with a white background and only one red pixel located in the center of it.
  by Antonio Molinaro, 2015.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    unsigned char r,g,b;
}Pixel;

typedef struct {
	unsigned int width, height;
    Pixel * data;    
}Image;

unsigned int getImgWidth(const Image * i){ return i->width; }

unsigned int getImgHeight(const Image * i){ return i->height; }

void setPixel(int x, int y, Image ** data, const Pixel data_new)
{
  Pixel * p = (*data)->data;
  p[x + y * getImgWidth(*data)].r = data_new.r;
  p[x + y * getImgWidth(*data)].g = data_new.g;
  p[x + y * getImgWidth(*data)].b = data_new.b;

}

int main() {

  char filename[80] = "myimage2.ppm";

  //create and open image file  
  FILE * fp = fopen(filename, "wb");
    
  if (!fp) {
    fprintf(stderr, "Unable to open file '%s'\n", filename);
    exit(1);
  }

  //allocate immage memory
  Image * i = malloc(sizeof(Image));
   
  if(!i) {
    fprintf(stderr, "Unable to allocate memory\n");
    fclose(fp);
    exit(1);
  }
  //initialize image structure
  i->width = 256;
  i->height = 256;

  //allocate pixels memory
  i->data = malloc(sizeof(Pixel) * getImgWidth(i) * getImgHeight(i));

  if(!i->data) {
    fprintf(stderr, "Unable to allocate memory\n");
    fclose(fp);
    exit(1); 
  }
  //set all image's pixels to 255
  memset(i->data, 255, sizeof(Pixel) * getImgWidth(i) * getImgHeight(i));
  
  //initialize a pixel to red
  Pixel red = {255,0,0};
  
  //and set with it the central pixel of the image 
  setPixel(getImgWidth(i)/2, getImgHeight(i)/2, &i, red); 
  
  //write the image header to the file
  fprintf(fp, "P6\n %d\n %d\n 255\n", getImgWidth(i), getImgHeight(i));
  
  //write the image data to file
  fwrite(i->data, 3, getImgWidth(i) * getImgHeight(i), fp);
  
  fclose(fp);
  return 0;
}