Why do we close the file descriptor after calling dup2?

Question

I cannot for the life of me get my head fully around dup2().

    int fd = open("data", O_RDONLY); 
    /* open the disk file */
    int newfd = dup2(fd,0); 
    /* close 0, dup fd to 0 */
    if( newfd != 0)
    {
        fprintf(stderr,"Could not duplicate fd to 0\n");
        exit(1);
    }
    close(fd);

So I understand that dup2() in this case will close 0 (standard keyboard input for stdin), it will then make stdin read from the file data, but why would you then close(fd)? I thought fd was what stdin is reading from now?

Close at the end of the above code, then, after that, I can call for example fgets(line, 100, stdin) and it will get the input from data and not the keyboard. — Juan Pablo, May 09 '19 at 02:10
Well I thought I could ask it on here so if anyone else in the world had a similar question, they could also get the answer. — Juan Pablo, May 09 '19 at 02:12
It's a fair point, and I think PSkocik has an answer for you. — tadman, May 09 '19 at 02:12

Petr Skocik · Accepted Answer · 2023-03-03T23:13:26.043

6

open creates a reference-counted file description (some struct) in the kernel and the returned file descriptor is kind of like a pointer to it (one of the counted references) from userspace, unless it's a negative value in which case open failed. dup2(fd,0) makes 0 also point to the same file description (unless you passed a negative fd or unless you're out of filedescriptors). If your goal was to have 0 point to the filedescription, you can now get rid of the old "pointer"/filedescriptor so you don't waste filedescriptor slots (a process can only allocate a limited number).

(Additionally closing the last reference to a filedescription may lead to certain desirable actions such as the file being dropped if it's an unliked file or the read end of a pipe receiving EOF if you just closed the last reference to the corresponding write end. Unnecessary references (filedescriptors) lying around get in the way of this.)

edited Mar 03 '23 at 23:13

answered May 09 '19 at 02:11

Petr Skocik

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Ah ha! That makes a lot of sense, thank you for your answer PSkocik. – Juan Pablo May 09 '19 at 02:16
1

So close does not remove the kernel's description, just invalidates the number in userspace? Unless it's the last one? So like reference counting? – Mad Physicist May 09 '19 at 02:24
1

@MadPhysicist Exactly. FDs are very much like refcounted (smart) pointers from userspace. – Petr Skocik May 09 '19 at 02:24
1

Wow. Been using it for years never had to know how it works. Thanks for the lesson. Very +1. – Mad Physicist May 09 '19 at 02:28

Why do we close the file descriptor after calling dup2?

1 Answers1