File input/output is part of Short Circuit's Console Program Basics selection.
Task

Create a file called   "output.txt",   and place in it the contents of the file   "input.txt",   via an intermediate variable.

In other words, your program will demonstrate:

   how to read from a file into a variable
   how to write a variable's contents into a file


Oneliners that skip the intermediate variable are of secondary interest — operating systems have copy commands for that.




#include <stdio.h>
 
int main(int argc, char **argv) {
  FILE *in, *out;
  int c;
 
  in = fopen("input.txt", "r");
  if (!in) {
    fprintf(stderr, "Error opening input.txt for reading.\n");
    return 1;
  }
 
  out = fopen("output.txt", "w");
  if (!out) {
    fprintf(stderr, "Error opening output.txt for writing.\n");
    fclose(in);
    return 1;
  }
 
  while ((c = fgetc(in)) != EOF) {
    fputc(c, out);
  }
 
  fclose(out);
  fclose(in);
  return 0;
}