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 function 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 appropriate 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 address 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 creation mask.
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 semantics 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 different address.
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 flushed twice).
Note that
clone is not intended to be used for new native
NetBSD applications. It is provided as a means to port software originally written for the Linux operating system to
NetBSD.