Simple code snipet that will either use stdin, or the open the file received as the first runtime parameter.

stdin_or_file.c

/* Copyright (c) 2012 Mgr. Simon Toth (kontakt@simontoth.cz)
* Licensed under the MIT license: http://www.opensource.org/licenses/mit-license.php
*/

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

int main(int argc, char *argv[])
{
	FILE *input = stdin;

	/* check if there is a command line parameter */
	if (argc >= 2)
	{
		/* if there is, try to open it as file */
		input = fopen(argv[1],"r");
		if (input == NULL)
		{
			fprintf(stderr,"Couldn't open file \"%s\"\n",argv[1]);
			return EXIT_FAILURE;
		}
	}

	/* input is now either an open file, ready to be read from, or standard input */

	/* cleanup */
	if (input != stdin) fclose(input);
	return EXIT_SUCCESS;
}