what will connect the fork() with its following code ? a simple example below:

what will connect the fork() with its following code ? a simple example below:

[Sat.]

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  !



-- 
Sat.

Re: what will connect the fork() with its following code ? a simple example below:

[Bernd Petrovitsch]

On Tue, 2005-09-06 at 17:15 +0800, Sat. wrote:
> 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

No, it is the same point as in the parent process - just the fork()
call. In fact the fork() returns twice - once within the parent process
(returning as result the pid of the child) and once within the child
(returning 0) [ and we ignore the error case for simplicity ].
Basically you are not forced to do a if() or switch() on the return
value(s) of fork()
----  snip  ----
printf("1. fork() in process %d returned %d\n", getpid(), fork());
printf("2. fork() in process %d returned %d\n", getpid(), fork());
printf("3. fork() in process %d returned %d\n", getpid(), fork());
printf("4. fork() in process %d returned %d\n", getpid(), fork());
printf("5. fork() in process %d returned %d\n", getpid(), fork());
----  snip  ----
Put this in a main() function and try it out.

> 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 ?

The kernel doesn't know (or care) about the C code (e.g. if(), ...) in
your application.
The "pointer" (if you want to call it) is the normal instruction pointer
(or what else it is called on your CPU).

> very confused :( 

	Bernd
-- 
Firmix Software GmbH                   http://www.firmix.at/
mobil: +43 664 4416156                 fax: +43 1 7890849-55
          Embedded Linux Development and Services

Re: what will connect the fork() with its following code ? a simple example below:

[Sat.]

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 :) 


2005/9/6, Dirk Gerdes <mail@dirk-gerdes.de>:
> 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." <walking.to.remember@gmail.com>
> To: <linux-kernel@vger.kernel.org>
> 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  !
> >
> >
> >
> > --
> > Sat.
> > -
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
> >
> 
> 


-- 
Sat.

Re: what will connect the fork() with its following code ? a simple example below:

[linux-os (Dick Johnson)]

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 <mail@dirk-gerdes.de>:
>> 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." <walking.to.remember@gmail.com>
>> To: <linux-kernel@vger.kernel.org>
>> 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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Thank you.

Re: what will connect the fork() with its following code ? a simple example below:

[Dirk Gerdes]

fork returns 0 to the child and the pid of the child to the parent.

both child and parent get the same code, so the child gets true in the 
if-statement and the parent gets false.

it would be the same as

pid = fork();
if (pid == 0){
// child
}
else{
// parent
}


----- Original Message ----- 
From: "Sat." <walking.to.remember@gmail.com>
To: "Dirk Gerdes" <mail@dirk-gerdes.de>
Cc: <linux-kernel@vger.kernel.org>
Sent: Tuesday, September 06, 2005 1:59 PM
Subject: Re: what will connect the fork() with its following code ? a simple 
example below:


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 :)


2005/9/6, Dirk Gerdes <mail@dirk-gerdes.de>:
> 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." <walking.to.remember@gmail.com>
> To: <linux-kernel@vger.kernel.org>
> 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  !
> >
> >
> >
> > --
> > Sat.
> > -
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" 
> > in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
> >
>
>


-- 
Sat.

Re: what will connect the fork() with its following code ? a simple example below:

[Valdis.Kletnieks]

[-- Attachment #1: Type: text/plain, Size: 803 bytes --]

On Tue, 06 Sep 2005 17:15:51 +0800, "Sat." said:

Not a kernel problem, please consult an intro-to-C list next time....

> if(!(pid=fork())){
>      ......
>      printk("in child process");
>      ......
> }else{
>      .....
>      printk("in father process"); 
>      .....
> }
> 

> values., and do nothing . so the bridge  between the new process and
> its following code, printk("in child process"), seems disappear 

I'm assuming you actually meant printf() (which is the userspace stdio call)
rather than printk() (which is the inside-the-kernel variant).

'man setbuf' - most likely the output of the child process is buffered and
never actually output before it exits.  You want to set stdout to be
line-buffered or unbuffered, or write to stderr (unbuffered by default) rather
than stdout. 


[-- Attachment #2: Type: application/pgp-signature, Size: 226 bytes --]

Re: what will connect the fork() with its following code ? a simple example below:

[Bernd Petrovitsch]

On Tue, 2005-09-06 at 12:58 -0400, Valdis.Kletnieks@vt.edu wrote:
> On Tue, 06 Sep 2005 17:15:51 +0800, "Sat." said:
> 
> Not a kernel problem, please consult an intro-to-C list next time....
> 
> > if(!(pid=fork())){
> >      ......
> >      printk("in child process");
> >      ......
> > }else{
> >      .....
> >      printk("in father process"); 
> >      .....
> > }
> > 
> 
> > values., and do nothing . so the bridge  between the new process and
> > its following code, printk("in child process"), seems disappear 
> 
> I'm assuming you actually meant printf() (which is the userspace stdio call)
> rather than printk() (which is the inside-the-kernel variant).

I actually assumed the same.

> 'man setbuf' - most likely the output of the child process is buffered and
> never actually output before it exits.  You want to set stdout to be

It is buffered since the above printf() lacks a terminating "\n". So
either put a "\n" at the end or call "fflush(stdout);"

> line-buffered or unbuffered, or write to stderr (unbuffered by default) rather
> than stdout. 

	Bernd
-- 
Firmix Software GmbH                   http://www.firmix.at/
mobil: +43 664 4416156                 fax: +43 1 7890849-55
          Embedded Linux Development and Services