Re: clone(2)

[Ángel González <keisial at gmail dot com>]

TL;DR: You need to provide child_stack.

On 18/12/13 19:14, vijay nag wrote:
> Hello,
>
> I want to create a new process using clone(2) system call.  The
> clone(2) is invoked the following way "clone(NULL, NULL, SIGCHLD |
> CLONE_PARENT, NULL);" and seems to be returning EINVAL.  I RTFM of
> clone(2) which suggested me to use NULL for child_stack when creating
> a new task. It also suggests that EINVAL is returned when child_stack
> argument is NULL. I'm little bit confused about its usage. Can someone
> illustrate a simple invocation of clone(2) for creating a child task ?
The man page mentions two things: the clone() method provided by libc 
and the raw syscall interface which differs slightly. You are only 
interested in the first one, and it doesn't allow a NULL child_stack.


> Excerpts from man for sys_clone
>
> "Another difference for sys_clone is that the child_stack argument may
> be zero, in which case copy-on-write semantics ensure that the child
> gets separate copies of stack pages when either process modifies the
> stack. In this case, for correct operation, the CLONE_VM option should
> not be specified."
This is at the «The raw system call interface» section, and is comparing 
the clone syscall (let's call it sys_clone) to clone(). So ignore this 
section.


> EINVAL
>
> Returned by clone() when a zero value is specified for child_stack.


Finally, there's anothe relevant to child_stack (maybe it's missing in 
your version of the man page).
>        The  child_stack  argument specifies the location of the stack 
> used by the child process.  Since the child and calling process may 
> share memory, it is not possible for the child
>        process to execute in the same stack as the calling process.  
> The calling process must therefore set up memory space for the child 
> stack and pass a  pointer  to  this  space  to
>        clone().   Stacks  grow  downward on all processors that run 
> Linux (except the HP PA processors), so child_stack usually points to 
> the topmost address of the memory space set up
>        for the child stack.


Merry Christmas