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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 num_sels.

     [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.