.\"	$OpenBSD: amd64_get_ldt.2,v 1.4 2007/05/31 19:19:27 jmc Exp $
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.\"     from: @(#)fork.2	6.5 (Berkeley) 3/10/91
.\"	$NetBSD: amd64_get_ldt.2,v 1.3 1996/01/05 14:56:44 pk Exp $
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt AMD64_GET_LDT 2 amd64
.Os
.Sh NAME
.Nm amd64_get_ldt ,
.Nm amd64_set_ldt
.Nd manage amd64 per-process Local Descriptor Table entries
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <machine/segments.h>
.Fd #include <machine/sysarch.h>
.Ft int
.Fn amd64_get_ldt "int start_sel" "union descriptor *descs" "int num_sels"
.Ft int
.Fn amd64_set_ldt "int start_sel" "union descriptor *descs" "int num_sels"
.Sh DESCRIPTION
.Fn amd64_get_ldt
returns a list of the amd64 descriptors in the current process'
LDT.
.Fn amd64_set_ldt
sets a list of amd64 descriptors in the current process'
LDT.
For both routines,
.Fa start_sel
specifies the index of the selector in the LDT at which to begin and
.Fa descs
points to an array of
.Fa num_sels
descriptors to be set or returned.
.Pp
Each entry in the
.Fa descs
array can be either a segment_descriptor or a gate_descriptor,
as defined in
.Aq Pa amd64/segments.h .
These structures are defined by the architecture
as disjoint bit-fields, so care must be taken in constructing them.
.Pp
Before this API can be used the functionality has to be enabled
using the machdep.userldt
.Xr sysctl 8
variable.
.Pp
.Sy Note:
Code using the
.Fn amd64_get_ldt
and
.Fn amd64_set_ldt
functions must be compiled using
.Cm -lamd64 .
.Sh RETURN VALUES
Upon successful completion,
.Fn amd64_get_ldt
returns the number of amd64 descriptors copied into
.Fa descs
from the current process' LDT.
Otherwise, a value of \-1 is returned and the global
variable
.Va errno
is set to indicate the error.
.Pp
Upon successful completion,
.Fn amd64_set_ldt
returns the first selector set;
if the kernel allocated a descriptor in the LDT,
the allocated index is returned.
Otherwise, a value of \-1 is returned and the global
variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn amd64_get_ldt
and
.Fn amd64_set_ldt
will fail if:
.Bl -tag -width [EINVAL]
.It Bq Er EINVAL
An inappropriate parameter was used for
.Fa start_sel
or
.Fa num_sels .
.It Bq Er EACCES
The caller attempted to use a descriptor that would
circumvent protection or cause a failure.
.El
.Sh REFERENCES
.Rs
.%A Intel
.%T AMD64 Microprocessor Programmer's Reference Manual
.Re
.Sh WARNING
You can really hose your process using this.