Why are stdin and stdout seemingly interchangeable?

Question

I understand that stdin and stdout (at least in UNIX parlance) are stream buffers, and that stdout is used to output from a program to the console (or to then be piped by a shell, etc), and that stdin is for standard input to a program..

So why is it, at least on macOS, that they can be used interchangeably (stdout as stdin, and vice versa?

Examples:

If you run cat /dev/stdin then type something in, and it echoes it back. Running the command as cat /dev/stdout does the same thing.
Similarly, echo "Hey There" > /dev/stdout and echo "Hey There" > /dev/stdin both output 'Hey There' back to the terminal.
It also works in C++:

example:

#include <iostream>
#include <string>
#include <fstream>

int main(int argc, const char * argv[]) {
    std::string echoString;
    std::fstream stdoutFile;
    stdoutFile.open("/dev/stdout");
    stdoutFile << "Hey look! I'm using stdout properly!\nNow You trying using it wrongly: " << std::endl;
    stdoutFile >> echoString;
    stdoutFile << "You Typed: " << echoString << std::endl;
}

When prompted, typing a single word, followed by EOF (Ctrl+D) works as expected.

Note that the fact that /dev/stdin and /dev/stdout are interchangeable does not imply that stdin and stdout are interchangeable. In particular, if you start a process with > /dev/stdin then it will not be possible to read from the process's standard output stream. — Brian Bi
@Brian what do you mean? This seems to work: pastebin.com/Su3yuJax — TheInnerParty
That's not the same as what I said. std::fstream opens in read/write mode by default. If you do > foo on the terminal, it will open foo in write-only mode and the file descriptor will not be usable for reading. — Brian Bi

Sam Varshavchik Sam Varshavchik · Accepted Answer · 2016-11-23T02:36:15

Because, typically, when a program is invoked from an interactive terminal, with no redirection, both standard input and standard output are connected to the same terminal device, such as /dev/tty (the actual device name varies based on the operating system).

The terminal device is a read/write device. Reading from the terminal device reads terminal input. Writing to the terminal device generates output on the terminal.

You still have discrete file descriptors, 0 and 1, but they're connected to the same device.

Think of it as single, bi-directional pipe, that's duped to both file descriptors 0 and 1.

Linux behaves the same way (you can echo Foo >/dev/stdin and see the output):

$ ls -al /proc/self/fd/[01]
lrwx------. 1 mrsam mrsam 64 Nov 22 21:34 /proc/self/fd/0 -> /dev/pts/1
lrwx------. 1 mrsam mrsam 64 Nov 22 21:34 /proc/self/fd/1 -> /dev/pts/1

So, for this process, file descriptors 0 and 1 is connected to the /dev/pts/1, the same pseudo-terminal device. Whether you are reading from file descriptor 0 or file descriptor 1, you end up reading from the same underlying /dev device, so it makes no difference which actual file descriptor you use.

This is, of course, operating system-dependent. Other POSIX-based operating systems may implement their standard input and output in other ways, where you can't actually write to standard input and read from standard output.

Why are stdin and stdout seemingly interchangeable?

2 Answers