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     i386_get_ldt, i386_set_ldt — manage i386 per-process Local Descriptor
     Table entries


     Standard C Library (libc, -lc)


     #include <machine/segments.h>
     #include <machine/sysarch.h>

     i386_get_ldt(int start_sel, union descriptor *descs, int num_sels);

     i386_set_ldt(int start_sel, union descriptor *descs, int num_sels);


     The i386_get_ldt() system call returns a list of the i386 descriptors in
     the current process' LDT.  The i386_set_ldt() system call sets a list of
     i386 descriptors in the current process' LDT.  For both routines,
     start_sel specifies the index of the selector in the LDT at which to
     begin and descs points to an array of num_sels descriptors to be set or

     Each entry in the descs array can be either a segment_descriptor or
     gate_descriptor and are defined in <i386/segments.h>.  These structures
     are defined by the architecture as disjoint bit-fields, so care must be
     taken in constructing them.

     If start_sel is LDT_AUTO_ALLOC, num_sels is 1 and the descriptor pointed
     to by descs is legal, then i386_set_ldt() will allocate a descriptor and
     return its selector number.

     If num_descs is 1, start_sels is valid, and descs is NULL, then
     i386_set_ldt() will free that descriptor (making it available to be
     reallocated again later).

     If num_descs is 0, start_sels is 0 and descs is NULL then, as a special
     case, i386_set_ldt() will free all descriptors.


     Upon successful completion, i386_get_ldt() returns the number of
     descriptors currently in the LDT.  The 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 errno is set to indicate the error.


     The i386_get_ldt() and i386_set_ldt() system calls will fail if:

     [EINVAL]           An inappropriate value was used for start_sel or

     [EACCES]           The caller attempted to use a descriptor that would
                        circumvent protection or cause a failure.


     i386 Microprocessor Programmer's Reference Manual, Intel


     You can really hose your process using this.