c - Strange behavior performing library functions on STDOUT and STDIN's file descriptors

Question

Throughout my years as a C programmer, I've always been confused about the standard stream file descriptors. Some places, like Wikipedia^[1], say:

In the C programming language, the standard input, output, and error streams are attached to the existing Unix file descriptors 0, 1 and 2 respectively.

This is backed up by unistd.h:

/* Standard file descriptors.  */
#define STDIN_FILENO    0       /* Standard input.  */
#define STDOUT_FILENO   1       /* Standard output.  */
#define STDERR_FILENO   2       /* Standard error output.  */

However, this code (on any system):

write(0, "Hello, World!
", 14);

Will print Hello, World! (and a newline) to STDOUT. This is odd because STDOUT's file descriptor is supposed to be 1. write-ing to file descriptor 1 also prints to STDOUT.

Performing an ioctl on file descriptor 0 changes standard input^[2], and on file descriptor 1 changes standard output. However, performing termios functions on either 0 or 1 changes standard input^[3][4].

I'm very confused about the behavior of file descriptors 1 and 0. Does anyone know why:

• writeing to 1 or 0 writes to standard output?
• Performing ioctl on 1 modifies standard output and on 0 modifies standard input, but performing tcsetattr/tcgetattr on either 1 or 0 works for standard input?

Answer 1

I guess it is because in my Linux, both 0 and 1 are by default opened with read/write to the /dev/tty which is the controlling terminal of the process. So indeed it is possible to even read from stdout.

However this breaks as soon as you pipe something in or out:

#include <unistd.h>
#include <errno.h>
#include <stdio.h>

int main() {
    errno = 0;
    write(0, "Hello world!
", 14);
    perror("write");
}

and run with

% ./a.out 
Hello world!
write: Success
% echo | ./a.out
write: Bad file descriptor

termios functions always work on the actual underlying terminal object, so it doesn't matter whether 0 or 1 is used for as long as it is opened to a tty.