Writing to stdin and reading from stdout (UNIX/LINUX/C Programming)

Question

I was working on an assignment where a program took a file descriptor as an argument (generally from the parent in an exec call) and read from a file and wrote to a file descriptor, and in my testing, I realized that the program would work from the command-line and not give an error if I used 0, 1 or 2 as the file descriptor. That made sense to me except that I could write to stdin and have it show on the screen.

Is there an explanation for this? I always thought there was some protection on stdin/stdout and you certainly can't fprintf to stdin or fgets from stdout.

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
int main()
{
    char message[20];
    read(STDOUT_FILENO, message, 20);
    write(STDIN_FILENO, message, 20);

    return 0;
}

Answer 1

Attempting to write on a file marked readonly or vice-versa would cause write and read to return -1, and fail. In this specific case, stdin and stdout are actually the same file. In essence, before your program executes (if you don't do any redirection) the shell goes:

  if(!fork()){
       <close all fd's>
       int fd = open("/dev/tty1", O_RDWR);
       dup(fd);
       dup(fd);
       execvp("name", argv);
  }

So, stdin, out, and err are all duplicates of the same file descriptor, opened for reading and writing.