Problem when redirecting standard output with dup and dup2 in c

Question

I have a problem with dup and dup2.

int copy (char * file1, char * file2) {
        int dr = open(file1, O_RDONLY);
        if (dr == -1) {
                return -1;
        }

        int dw = open(file2, O_WRONLY|O_TRUNC|O_CREAT, 0644); 
        if (dw == -1) {
                return -1;
        }

        int save0 = dup(0); 
        int save1 = dup(1); 

        dup2(dr, 0); 
        close(dr); 

        dup2(dw, 1); 
        close(dw); 

        char c;
        while ((c = getchar()) != EOF) {  
                putchar(c); 
        }

        dup2(save0, 0); 
        dup2(save1, 1); 

        printf("Hi");
        return 0;
}

I don't understand why the data from my file file1 is displayed on the screen because dr is standard input and dw is standard output...

Thanks for the help.

You didn't flush stdout before you switched out the underlying file descriptor. The call to `putchar` put the data in a buffer. Then you switch the underlying fd to be the original stdout. Then the buffer is flushed. — William Pursell, Dec 22 '20 at 22:02
And `char c;` is wrong when used in `while ((c = getchar()) != EOF)`. `EOF` is an `int` value that can not match any `char` value, which is why `getchar()` returns and `int` and not a `char`. By cramming the returned value into a `char`, you can not reliably detect `EOF`. You also need to check the return values from all your `dup()` and `dup2()` calls - if you run up against an open file descriptor, they can and will fail. — Andrew Henle, Dec 22 '20 at 22:39

score 3 · Accepted Answer · answered Dec 22 '20 at 22:03

You (probably) just need to flush the buffers:

while ((c = getchar()) != EOF) {  
        putchar(c); 
}
fflush(stdout);   /* Actually write to the file */
dup2(save0, 0); 
dup2(save1, 1);

Since you haven't included the full code, it's hard to be certain. Also, make sure you haven't done any read-like functions (eg fread, fgetc, etc) on stdin before this function is called.

Problem when redirecting standard output with dup and dup2 in c

1 Answers1