/*
 * Program to read integers from a file, double them, and write the
 * results to a file.  Names of files are supplied as command-line 
 * arguments.  (Yes, it's silly, but it's an example of using fscanf
 * and fprintf.)
 */
#include <stdio.h>
#include <stdlib.h>

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

    int num;
    int exit_status;

    FILE *infile;
    FILE *outfile;

    if (argc < 3) {
        fprintf(stderr, "usage:  %s infile outfile\n", argv[0]);
        return 1;
    }

    infile = fopen(argv[1], "r");
    if (infile == NULL) {
        fprintf(stderr, "unable to open input file %s\n", argv[1]);
        return EXIT_FAILURE;
    }

    outfile = fopen(argv[2], "w");
    if (outfile == NULL) {
        fprintf(stderr, "unable to open output file %s\n", argv[2]);
        return EXIT_FAILURE;
    }

    while (fscanf(infile, "%d", &num) == 1) {
        fprintf(outfile, "%d\n", num * 2);
    }
    if (feof(infile)) {
        exit_status = EXIT_SUCCESS;
    }
    else {
        fprintf(stderr, "input error\n");
        exit_status = EXIT_FAILURE;
    }

    fclose(infile);
    fclose(outfile);
    return exit_status;
}