Subject: Re: __clone function
To: None <Richard.Earnshaw@arm.com>
From: Chris Gilbert <firstname.lastname@example.org>
Date: 07/20/2001 18:03:18
On Friday 20 July 2001 2:22 pm, Richard Earnshaw wrote:
> > Hi,
> > I was wondering if anyone has the time to check over the attached __clone
> > function. I've not had time to do optimise it, but it should do the
> > right thing (famous last words :) I've only had time to actually compile
> > it, but thought it would save everyone doing their own version.
> > Basically drop the attached file in lib/libc/arch/arm/sys and make
> > dependall in libc (libc now requires __clone.S)
> It's hard to know whether this is correct or not without knowing what
> "clone" is supposed to do. Is there a manual page for this function?
Ahh, yes it's very new :)
CLONE(2) NetBSD Programmer's Manual CLONE(2)
clone - spawn new process with options
Standard C Library (libc, -lc)
clone(int (*func)(void *arg), void *stack, int flags, void *arg);
__clone(int (*func)(void *arg), void *stack, int flags, void *arg);
The clone system call (and associated library support code) creates a new
process in a way that allows the caller to specify several options for
the new process creation.
Unlike fork(2) or vfork(2), in which the child process returns to the
call site, clone causes the child process to begin execution at the func-
tion specified by func. The argument arg is passed to the entry point,
as a means for the parent to provide context to the child. The stack
pointer for the child process will be set to stack. Note that the clone
interface requires that the application know the stack direction for the
architecture, and that the caller initialize the stack argument as appro-
priate for the stack direction.
The flags argument specifies several options that control how the child
process is created. The lower 8 bits of flags specify the signal that is
to be sent to the parent when the child exits. The following flags may
also be specified by bitwise-or'ing them with the signal value:
CLONE_VM Share the virtual address space with the parent. The ad-
dress space is shared in the same way as vfork(2).
CLONE_FS Share the ``file system information'' with the parent.
This include the current working directory and file cre-
CLONE_FILES Share the file descriptor table with the parent.
CLONE_SIGHAND Share the signal handler set with the parent. Note that
the signal mask is never shared between the parent and the
child, even if CLONE_SIGHAND is set.
CLONE_VFORK Preserve the synchronization semanics of vfork(2); the
parent blocks until the child exits.
The clone call returns the pid of the child in the parent's context. The
child is provided no return value, since it begins execution at a differ-
If the child process's entry point returns, the value it returns is
passed to _exit(2), and the child process exits. Note that if the child
process wants to exit directly, it should use _exit(2), and not exit(3),
since exit(3) will flush and close standard I/O channels, and thereby
corrupt the parent process's standard I/O data structures (even with
fork(2) it is wrong to call exit(3) since buffered data would then be
Note that clone is not intended to be used for new native NetBSD applica-
tions. It is provided as a means to port software originally written for
the Linux operating system to NetBSD.
RETURN VALUES and ERRORS
Same as for fork(2).
chdir(2), chroot(2), fork(2), vfork(2), sigaction(2), sigprocmask(2),