.\" Copyright (c) 1995 Michael Chastain (mec@duracef.shout.net), 22 July 1995.
.\"
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.TH MODIFY_LDT 2 2012-07-13 "Linux" "Linux Programmer's Manual"
.SH NAME
modify_ldt \- get or set ldt
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.sp
.BI "int modify_ldt(int " "func" ", void *" "ptr" ", unsigned long " "bytecount" );
.fi

.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.BR modify_ldt ()
reads or writes the local descriptor table (ldt) for a process.
The ldt is a per-process memory management table used by the i386 processor.
For more information on this table, see an Intel 386 processor handbook.
.PP
When
.I func
is 0,
.BR modify_ldt ()
reads the ldt into the memory pointed to by
.IR ptr .
The number of bytes read is the smaller of
.I bytecount
and the actual size of the ldt.
.PP
When
.I func
is 1,
.BR modify_ldt ()
modifies one ldt entry.
.I ptr
points to a
.I user_desc
structure
and
.I bytecount
must equal the size of this structure.
.\"
.\" FIXME Should this page say something about func == 2 and func == 0x11?
.\" In Linux 2.4, func == 2 returned "the default ldt"
.\" In Linux 2.6, func == 2 is a nop, returning a zeroed out structure.
.\" Linux 2.4 and 2.6 implement an operation for func == 0x11

The
.I user_desc
structure is defined in \fI<asm/ldt.h>\fP as:
.in +4n
.nf

struct user_desc {
    unsigned int  entry_number;
    unsigned long base_addr;
    unsigned int  limit;
    unsigned int  seg_32bit:1;
    unsigned int  contents:2;
    unsigned int  read_exec_only:1;
    unsigned int  limit_in_pages:1;
    unsigned int  seg_not_present:1;
    unsigned int  useable:1;
};
.fi
.in
.PP
In Linux 2.4 and earlier, this structure was named
.IR modify_ldt_ldt_s .
.\" .PP
.\" The ldt is specific for the calling process. Any attempts to change
.\" the ldt to include the address space of another process or the kernel
.\" will result in a segmentation violation when trying to access the memory
.\" outside of the process address space. The memory protection is enforced
.\" at the paging layer.
.SH RETURN VALUE
On success,
.BR modify_ldt ()
returns either the actual number of bytes read (for reading)
or 0 (for writing).
On failure,
.BR modify_ldt ()
returns \-1 and sets
.I errno
to indicate the error.
.SH ERRORS
.TP
.B EFAULT
.I ptr
points outside the address space.
.TP
.B EINVAL
.I ptr
is 0,
or
.I func
is 1 and
.I bytecount
is not equal to the size of the structure
.IR modify_ldt_ldt_s ,
or
.I func
is 1 and the new ldt entry has invalid values.
.TP
.B ENOSYS
.I func
is neither 0 nor 1.
.SH CONFORMING TO
This call is Linux-specific and should not be used in programs intended
to be portable.
.SH NOTES
Glibc does not provide a wrapper for this system call; call it using
.BR syscall (2).
.SH SEE ALSO
.BR vm86 (2)
.SH COLOPHON
This page is part of release 3.79 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%http://www.kernel.org/doc/man\-pages/.