On Tue, 6 Sep 2005, Sat. wrote:
> here is a snip in 0.11 version linux ,
> in linux/init/main.c
>
>
> 179 if (!(pid=fork())) {
> 180 close(0);
> 181 if (open( "/etc/rc",O_RDONLY,0))
> 182 _exit(1);
> 183 execve( "/bin/sh",argv_rc,envp_rc);
> 184 _exit(2);
> 185 }
>
> natually, the code from 180 to 184 is runned by the new process, what
> I can understand is why the new process know that the next code will
> run is close(0) and why it know It will end at line 184 ?
>
> so ,I feel that there should be some connection between them . but
> what the relationship in depth is ?
>
> thanks your help :)
>
After the fork() system call, fork() returns the child's pid
(or an error) to the parent, and a ZERO (0) to the child. That
way the code 'knows' if the child or the parent is executing.
switch ((pid = fork()))
{
case 0: // Child runs here
printf("Child\n");
sleep(1);
exit(0);
case -1: // This is for errors
fprintf(stderr, "fork() failed (%s)\n", strerror(errno));
exit(1);
default: // The parent executes here
printf("Parent waits for child with pid %d\n", pid);
wait(&status);
printf("Child executed with %d status\n", status >> 8);
}
>
> 2005/9/6, Dirk Gerdes <[email protected]>:
>> There is no connection between a child an its parent.
>> The child only gets a copy of the code.
>> If there were a pointer to a child or to the parent, you wouldn't need any
>> signals.
>> The processes could communicate directly.
>>
>> regards
>>
>> ----- Original Message -----
>> From: "Sat." <[email protected]>
>> To: <[email protected]>
>> Sent: Tuesday, September 06, 2005 11:15 AM
>> Subject: what will connect the fork() with its following code ? a simple
>> example below:
>>
>>
>>> if(!(pid=fork())){
>>> ......
>>> printk("in child process");
>>> ......
>>> }else{
>>> .....
>>> printk("in father process");
>>> .....
>>> }
>>>
>>> this is a classical example, when the fork() system call runs, it will
>>> build a new process and active it . while the schedule() select the
>>> new process it will run. this is rather normal.
>>>
>>> but there is always a confusion in my minds.
>>> because , sys_fork() only copies father process and configure some new
>>> values., and do nothing . so the bridge between the new process and
>>> its following code, printk("in child process"), seems disappear . so I
>>> always believe that the new process should have a pointer which point
>>> the code "printk("in child process");". except this , there are not
>>> any connection between them ?
>>>
>>> very confused :(
>>>
>>> any help will appreciate !
sys_fork() should never be executed from inside the kernel because
it is going to copy a process' code and data. The kernel isn't a
process. To make a process from inside the kernel, you need to use:
//
// Code to execute as a kernel thread.
//
int thread_code(void *ptr)
{
daemonize("%s", "Kernel Thread!");
allow_signal(SIGTERM);
for(;;) // Run forever
{
do_something();
set_current_state(TASK_INTERRUPTIBLE);
if(signal_pending(current))
complete_and_exit(&exit_flag);
}
}
//
// Code to start the kernel thread.
//
pid = kernel_thread(thread_code, NULL, CLONE_FS|CLONE_FILES);
//
// Code to stop the kernel thread.
//
kill_proc(pid, SIGTERM);
wait_for_completion(&exit_flag);
>>>
>>>
>>>
>>> --
>>> Sat.
Cheers,
Dick Johnson
Penguin : Linux version 2.6.13 on an i686 machine (5589.54 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
I apologize for the following. I tried to kill it with the above dot :
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