I62 Answer ― Random Pixels

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

/* randomPixels -- create an image with pixels of random color

The red, green, and blue values for each pixel are choosen
randomly and independently.

*/

int main(int argc, char *argv[])
{
  int r, nrows, c, ncols, red, grn, blu;
  FILE *image;

  /* check the command line parameters */
  if ( argc != 4 )
  {
    printf("colorChip fileName.ppm nrows ncols\n");
    return 0;
  }

  /* open the image file for writing in binary mode */
  if ( (image = fopen( argv[1], "wb") ) == NULL )
  {
    printf("file %s could not be created\n", argv[1]);
    return 0;
  }

  nrows = atoi( argv[2] );
  if ( nrows < 1 )
  {
    printf("number of rows must be positive\n");
    return 0;
  }

  ncols = atoi( argv[3] );
  if ( ncols < 1 )
  {
    printf("number of columns must be positive\n");
    return 0;
  }

  /* write out the PPM Header information */
  fprintf( image, "P6 ");
  fprintf( image, "%d %d %d ", ncols, nrows, 255 );
  srand( time(NULL) );

  /* write out the pixel data */
  for ( r=0; r<nrows; r++ )
    for ( c=0; c<ncols; c++ )
    {
      red = rand()%256;
      grn = rand()%256;
      blu = rand()%256;
      fputc( red, image );  fputc( grn, image );  fputc( blu, image );
    }

  /* close the file */
  fclose ( image );

  return 1;
}

Comments: Using integer modulo division with rand() does not produce the best random integers. You might want to use randInt() from section B of the puzzles.