.\"     $NetBSD: cpu_lwp_fork.9,v 1.6 2017/03/10 15:30:45 wiz Exp $
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.\" by Gregory McGarry; and Rui Paulo.
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.Dd March 10, 2017
.Dt CPU_LWP_FORK 9
.Os
.Sh NAME
.Nm cpu_lwp_fork ,
.Nm child_return ,
.Nm lwp_trampoline
.Nd finish a fork operation
.Sh SYNOPSIS
.In sys/proc.h
.Ft void
.Fn cpu_lwp_fork "struct lwp *l1" "struct lwp *l2" "void *stack" \
"size_t stacksize" "void (*func)(void *)" "void *arg"
.Ft void
.Fn child_return "void *arg"
.Sh DESCRIPTION
.Fn cpu_lwp_fork
is the machine-dependent portion of
.Fn fork1
which finishes a fork operation, with child lwp
.Fa l2
nearly set up.
It copies and updates the PCB and trap frame from the parent
.Fa l1 ,
making the child ready to run.
.Pp
.Fn cpu_lwp_fork
rigs the child's kernel stack so that it will start in
.Fn lwp_trampoline .
.Fn lwp_trampoline
does not have a normal calling sequence and is entered by
.Fn cpu_switch .
If an alternate user-level stack is requested (with non-zero values
in both the
.Fa stack
and
.Fa stacksize
arguments), the user stack pointer is set up accordingly.
.Pp
After being entered by
.Fn cpu_switch
and while running in user context (within the kernel)
.Fn lwp_trampoline
will invoke the function
.Fa func
with the argument
.Fa arg .
If a kernel thread is being created, the return path and argument
are specified with
.Fa func
and
.Fa arg .
If a user process is being created,
.Fn fork1
will pass
.Fn child_return
and
.Fa l2
to
.Fn cpu_lwp_fork
as
.Fa func
and
.Fa arg
respectively.
This causes the newly-created child process to go directly to user
level with an apparent return value of 0 from
.Xr fork 2 ,
while the parent process returns normally.
.Sh SEE ALSO
.Xr fork 2 ,
.Xr cpu_switchto 9 ,
.Xr fork1 9