MODIFY_LDT

Section: Linux Programmer's Manual (2)
Updated: 2007-06-01
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NAME

modify_ldt - get or set ldt

SYNOPSIS

#include <sys/types.h>

int modify_ldt(int func, void *ptr, unsigned long bytecount);

DESCRIPTION

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.

When func is 0, modify_ldt() reads the ldt into the memory pointed to by ptr. The number of bytes read is the smaller of bytecount and the actual size of the ldt.

When func is 1, modify_ldt() modifies one ldt entry. ptr points to a user_desc structure and bytecount must equal the size of this structure.

The user_desc structure is defined in <asm/ldt.h> as:


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;
};

In Linux 2.4 and earlier, this structure was named modify_ldt_ldt_s.

RETURN VALUE

On success, modify_ldt() returns either the actual number of bytes read (for reading) or 0 (for writing). On failure, modify_ldt() returns -1 and sets errno to indicate the error.

ERRORS

EFAULT: ptr points outside the address space.
EINVAL: ptr is 0, or func is 1 and bytecount is not equal to the size of the structure modify_ldt_ldt_s, or func is 1 and the new ldt entry has invalid values.
ENOSYS: func is neither 0 nor 1.

CONFORMING TO

This call is Linux-specific and should not be used in programs intended to be portable.

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2).

COLOPHON

This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.