#include <sys/types.h>

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

       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 struc-

       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.

       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.

       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.

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

Linux                             2007-06-01                     MODIFY_LDT(2)
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