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.\"     from: @(#)fork.2	6.5 (Berkeley) 3/10/91
.\" $FreeBSD: src/lib/libc/i386/sys/i386_get_ldt.2,v 1.24.10.1.6.1 2010/12/21 17:09:25 kensmith Exp $
.\"
.Dd October 14, 2006
.Dt I386_GET_LDT 2
.Os
.Sh NAME
.Nm i386_get_ldt ,
.Nm i386_set_ldt
.Nd manage i386 per-process Local Descriptor Table entries
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In machine/segments.h
.In machine/sysarch.h
.Ft int
.Fn i386_get_ldt "int start_sel" "union descriptor *descs" "int num_sels"
.Ft int
.Fn i386_set_ldt "int start_sel" "union descriptor *descs" "int num_sels"
.Sh DESCRIPTION
The
.Fn i386_get_ldt
system call
returns a list of the i386 descriptors in the current process'
LDT.
The
.Fn i386_set_ldt
system call
sets a list of i386 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 gate_descriptor and are defined in
.In i386/segments.h .
These structures are defined by the architecture
as disjoint bit-fields, so care must be taken in constructing them.
.Pp
If
.Fa start_sel
is
.Em LDT_AUTO_ALLOC ,
.Fa num_sels
is 1 and the descriptor pointed to by
.Fa descs
is legal, then
.Fn i386_set_ldt
will allocate a descriptor and return its
selector number.
.Pp
If
.Fa num_descs
is 1,
.Fa start_sels
is valid, and
.Fa descs
is NULL, then
.Fn i386_set_ldt
will free that descriptor
(making it available to be reallocated again later).
.Pp
If
.Fa num_descs
is 0,
.Fa start_sels
is 0 and
.Fa descs
is NULL then, as a special case,
.Fn i386_set_ldt
will free all descriptors.
.Sh RETURN VALUES
Upon successful completion,
.Fn i386_get_ldt
returns the number of descriptors currently in the LDT.
The
.Fn i386_set_ldt
system call
returns the first selector set on success.
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
The
.Fn i386_get_ldt
and
.Fn i386_set_ldt
system calls
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
An inappropriate value 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 SEE ALSO
i386 Microprocessor Programmer's Reference Manual, Intel
.Sh WARNING
You can really hose your process using this.