Kernel, Process creation

static int init(void * unused)
{
	lock_kernel();
	do_basic_setup();

	/*
	 * Ok, we have completed the initial bootup, and
	 * we're essentially up and running. Get rid of the
	 * initmem segments and start the user-mode stuff..
	 */
	free_initmem();
	unlock_kernel();

	if (open("/dev/console", O_RDWR, 0) < 0)
		printk("Warning: unable to open an initial console.\n");

	(void) dup(0);
	(void) dup(0);
	
	/*
	 * We try each of these until one succeeds.
	 *
	 * The Bourne shell can be used instead of init if we are 
	 * trying to recover a really broken machine.
	 */

	if (execute_command)
		execve(execute_command,argv_init,envp_init);
	execve("/sbin/init",argv_init,envp_init);
	execve("/etc/init",argv_init,envp_init);
	execve("/bin/init",argv_init,envp_init);
	execve("/bin/sh",argv_init,envp_init);
	panic("No init found.  Try passing init= option to kernel.");
}

In this init/main.c, after basic kernel initialization things, open("/dev/console") and two dup(0) opens three file descriptor 0, 1, 2 as stdin, stdout, stderr. Later when execve() or fork() happens, it simply copies calling process’s task_struct as process context, so all file descriptors are also passed down as-is.

All processes’ file descriptor 0, 1, 2 originates to init’s file descriptor.

If calling process opens PTY or TTY, which is common when we opens new terminal to run a process, it is also passed down to exec/forked process.