92 Answer ― Make Gray

#include <stdlib.h>
#include <stdio.h>
#include "../basicColorImage.c"

void toGray( colorImage cimg, colorImage gimg)
{
  int r, c, avg;
  pixel pix;
  for ( r=0; r<cimg.nrows; r++ )
    for ( c=0; c<cimg.ncols; c++ )
    {
      pix = getColorPixel( cimg, r, c ) ;
      avg = (pix.red + pix.grn + pix.blu)/3;
      pix.red = avg;
      pix.grn = avg;
      pix.blu = avg;
      setColorPixel( gimg, r, c, pix );
    }
}

int main ( int argc, char* argv[] )
{
  colorImage cimg;
  colorImage gimg;

  if ( argc != 3 )
  {
    printf("PPMtoGrayPPM oldImage.ppm newImage.ppm\n");
    system( "pause" );
    exit( EXIT_FAILURE );
  }

  /* read in the old image */
  readPPMimage( &cimg, argv[1]);

  /* create empty color image */
  if ( newColorImage( &gimg, cimg.nrows, cimg.ncols ) == NULL )
  {
    printf(">>error<< newColorImage can't allocate memory\n");
    exit( EXIT_FAILURE );
  }
  
  /* fill in gray level image */
  toGray( cimg, gimg ) ;

  /* write the image to disk and free memory */
  writePPMimage( gimg, argv[2]);
  freeColorImage( &cimg );
  freeColorImage( &gimg );
}

Comments: The average color value will always be less than or equal any of the three RGB values for a pixel, so overflow (values above 255) will never happen.