2023-11-26 07:44:51
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.