How to use UNIX terminal input redirection in C?

Question

I have this C program that gets the input from the user and sends it to a method:

#include <stdio.h>
#include <string.h>
#include "constants.h"
#include "lines.h"
#include "compare.h"

//gets arguments and sends to compare.c
int main() {
   int op = 1;
   char filename[20];

   scanf("%d ", &op);
   gets(filename);
   if ((char) op + '0' < 49 || (char) op + '0' > 57) {
       printf("Error: Invalid input");
       exit(0);
   }
   readstdin(filename, op);
   return 0;
}

but instead of executing the program and reading from stdin, I want it to read from the unix terminal so that:

./sort [field] < input_file

will read into the file. ([field] is option if no input is put in, default is 1).

For example the command to execute the C program in UNIX would look like this:

./sort 1 < test.txt

How do I go about doing this?

Any help is much appreicated Thanks!

Do you want to pass the filename alone or the complete data from command line? — another.anon.coward
– another.anon.coward, Commented Dec 2, 2011 at 5:19
both filename and field from the command line but I want the filename and field as separate values — Kyle Asaff
– Kyle Asaff, Commented Dec 2, 2011 at 5:29
Then just use command line arguments. Use argc & argv in main() — another.anon.coward
– another.anon.coward, Commented Dec 2, 2011 at 5:35
AusCBloke's definitely matches the description of what you wanted to actually accomplish. If you use redirection like you originally supposed then the contents of that file are piped to the program. I've had assignments where the professor wanted an interactive program but gave us a file with space delimited input so we could use redirection like that and from the perspective of the program there is no difference from a user typing in the contents of the file manually. Not pertinent to this situation but might be good to know its possible. — Matt
– Matt, Commented Dec 2, 2011 at 6:12

AusCBloke · Accepted Answer · 2011-12-02 05:36:55Z

For a start, you're getting your arguments the wrong way in your code. If what you're wanting is to run your program such as ./sort <option> <filename>, then you don't use stdin to retrieve these arguments.

The arguments in a C program are passed to main using the following function signature:

int main(int argc, char *argv[])

argc is the number of command line arguments passed to the program, and argv is the array of strings of those arguments.

With a run of ./sort 5 test.in:

argc will equal 3
argv[0] will be "./sort"
argv[1] will be "5"
argv[2] will be "test.in"

You should check that the value of argc is 3 to ensure that 2 arguments have been passed ("5", "test.in"), as well as the filename ("./sort") for a total of 3.

If you want to have optional fields, it would be better to have them after the compulsory ones, or better yet is to use something like getopt where you could instead have something like: ./sort --file test.in or ./sort --opt 5 --file test.in. It's probably unnecessary for this case, but it's an option.

You can parse the integer option using atoi or strtol, however you like, to convert it from a string (char*) to an integral type and fopen, fgets, fclose to read from the input file.

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