.\"	$OpenBSD: kthread.9,v 1.7 2004/04/17 14:05:16 jmc Exp $
.\"
.\" Copyright (c) 1999 Marc Espie
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.Dd September 1, 1999
.Dt KTHREAD 9
.Os
.Sh NAME
.Nm kthread_create ,
.Nm kthread_exit ,
.Nm kthread_create_deferred
.Nd kernel threads
.Sh SYNOPSIS
.Fd #include <sys/kthread.h>
.Ft int
.Fn kthread_create "void (*func)(void *)" "void *arg" "struct proc **newpp" "const char *fmt" ...
.Ft void
.Fn kthread_exit "int ecode"
.Ft void
.Fn kthread_create_deferred "void (*func)(void *)" "void *arg"
.Sh DESCRIPTION
Kernel threads are system light-weight processes: cloned from process 0
(the swapper), sharing its memory map and limits, but with a copy of its
file descriptor table.
They don't receive broadcast nor group signals and they can't be swapped.
.Pp
Any process can call
.Fn kthread_create
to create a kernel thread.
The new process starts up executing
.Fa func
with argument
.Fa arg .
If
.Fa newpp
is not
.Dv NULL ,
it is filled with the address of the new process.
.Fa fmt
and the remaining arguments are used to name the process.
.Pp
A kernel thread will terminate by calling
.Fn kthread_exit ,
with exit code
.Fa ecode .
.Pp
Since the system has to be up and running for creating
new processes, device drivers that want to create kernel threads early
(e.g., at attach time) may use
.Fn kthread_create_deferred
instead.
The system will call back the function
.Fa func
with argument
.Fa arg
when it can create threads, so it is up to
.Fa func
to call
.Fn kthread_create
at that point.
.Sh RETURN VALUES
Upon successful completion,
.Fn kthread_create
returns 0.
Otherwise, the following error values are returned:
.Bl -tag -width [EAGAIN]
.It Bq Er EAGAIN
The limit on the total number of system processes would be exceeded.
.El
.Sh SEE ALSO
.Xr fork1 9
.Sh BUGS
There is currently no way to use
.Va ecode
to any sensible purpose from
.Fn kthread_exit .