/* openstr.c */
#include <stdio.h>
#include <stdlib.h>

void main(int argc, char *argv[])
{
  FILE *tietovirta;

  /* ... */

  if(argc == 1)
    tietovirta = stdin;
  else
    if ((tietovirta = fopen(argv[1], "r")) == NULL)
      {
	perror("Syottotiedoston avaus epaonnistui");
	exit(1);
      }

  /* ... */

  fclose(tietovirta);
}