Using stdin in C exclusively through a piped in file - c

Question

I wrote a file parser for a project that parses a file provided on the command line.
However, I would like to allow the user to enter their input via stdin as well, but exclusively through redirection via the command line.
Using a Linux based command prompt, the following commands should yield the same results:
./check infile.txt (Entering filename via command line)
./check < infile.txt
cat infile.txt | ./check
The executable should accept a filename as the first and only command-line argument. If no filename is specified, it should read from standard input.
Edit: I realized how simple it really was, and posted an answer. I will leave this up for anyone else who might need it at some point.

Answer 1

This is dangerously close to "Please write my program for me". Or perhaps it even crossed that line. Still, it's a pretty simple program.
We assume that you have a parser which takes a single FILE* argument and parses that file. (If you wrote a parsing function which takes a const char* filename, then this is by way of explaining why that's a bad idea. Functions should only do one thing, and "open a file and then parse it" is two things. As soon as you write a function which does two unrelated things, you will immediately hit a situation where you really only wanted to do one of them (like just parse a stream without opening the file.)
So that leaves us with:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "myparser.h"
/* Assume that myparser.h includes
* int parseFile(FILE* input);
* which returns non-zero on failure.
*/
int main(int argc, char* argv[]) {
FILE* input = stdin; /* If nothing changes, this is what we parse */
if (argc > 1) {
if (argc > 2) {
/* Too many arguments */
fprintf(stderr, "Usage: %s [FILE]\n", argv[0]);
exit(1);
}
/* The convention is that using `-` as a filename is the same as
* specifying stdin. Just in case it matters, follow the convention.
*/
if (strcmp(argv[1], "-") != 0) {
/* It's not -. Try to open the named file. */
input = fopen(argv[1], "r");
if (input == NULL) {
fprintf(stderr, "Could not open '%s': %s\n", argv[1], strerror(errno));
exit(1);
}
}
}
return parse(input);
}
It would probably have been better to have packaged most of the above into a function which takes a filename and returns an open FILE*.

Answer 2

I guess my brain is fried because this was a very basic question and I realized it right after I posted it. I will leave it up for others who might need it.
ANSWER:
You can fgets from stdin, then to check for the end of the file you can still use feof for stdin by using the following:
while(!feof(stdin))