Question

I’m new to C and I’d like to ask about running a C program and supplying input at the same time.

What I would like to do is run a program (ex. fileOpener) and also state which file to open

./fileOpener < filename1

I’ve tried it already and it works fine, but what do I use to know what filename1 is? That way I can open the file with

fp = fopen(filename1, "r")

Thanks.

Edit: OK, I’ll try to explain a bit more. If there wasn’t a “<” then I could just use command line arguments as I have done before, but when I tried it with the <, it didn’t work

Specifically: fileOpener code:

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

int main(int argc, char *argv[]){
printf("%s", argv[1]);
}

when I use ./fileOpener < filename1 the output is ./fileOpener

I used gcc -o fileOpener fileOpener.c as the compiler

Answer 1

You can’t do that, if you use redirection (i.e. “< filename”) the file is opened by the system. You could discover the name, but it’s non-portable, and anyway useless since the file is already open. Just use stdin instead of fp, and you need not use fopen() (nor fclose()):

int main()
{
   char buffer[1024];

   // fgets() reads at most 1024 characters unless it hits a newline first
   // STDIN has been already opened by the system, and assigned to data flowing
   // in from our file ( < inputFile ).
   fgets(buffer, 1024, stdin);

   printf("The first line of input was: %s", buffer);
}

A different approach is to use arguments:

int main(int argc, char **argv)
{
   FILE *fp = NULL;
   char buffer[1024];

   if (argc != 2)
   {
       fprintf(stderr, "You need to specify one argument, and only one\n");
       fprintf(stderr, "Example: %s filename\n", argv[0]);
       // Except that argv[0], this program's name, counts.
       // So 1 argument in command line means argc = 2.
       return -1;
   }
   printf("I am %s. You wanted to open %s\n", argv[0], argv[1]);

   fp = fopen(argv[1], "r");

   fgets(buffer, 1024, stdin);

   printf("The first line of input was: %s", buffer);

   fclose(fp); fp = NULL; // paranoid check

   return 0;
}