Question

Now I do have a hw question for everyone…I’ve been staring at this for a couple of days kind of tinkering and playing around but even with that I end up with a load of errors…

What I’m trying to do is take the program below and change it so that it takes an optional command line argument infile. If infile is given, then copy infile to standard output, otherwise copy standard input to standard output as before.

The trick about this is that the solution must use the original copy loop (lines 9-11) for both cases. One can only insert code, and not change any of the existing code. Any help would be great. Thanks.

/* $begin cpfile */
include "csapp.h"
int main(int argc, char **argv)
{
    int n;
    rio_t rio;
    char buf[MAXLINE];

    Rio_readinitb(&rio, STDIN_FILENO);                  //line 9
    while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) //line 10
        Rio_writen(STDOUT_FILENO, buf, n);              //line 11
        /* $end cpfile */
        exit(0);
        /* $begin cpfile */
    }
/* $end cpfile */

Answer 1

You can insert dup2 before the lines 9 – 11 and it seems that you will not need change code on the lines 9 – 11. This is an example.

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <fcntl.h>


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

    int file_handle;
    int dup2_res;
    if (argc == 2) {
        file_handle = open(argv[1], O_RDONLY);
        dup2_res = dup2 (file_handle, STDIN_FILENO);
    }

    char buffer[100];
    ssize_t read_bytes = 1;
    while (read_bytes)
    {
        read_bytes = read(STDIN_FILENO, &buffer, sizeof(buffer) );
        buffer[read_bytes] = 0;
        printf("%s", buffer);
    }
    close(file_handle);

    return 0;
}