The i386_get_ldt system call will return the list of i386 descriptors that the process has in its LDT. The i386_set_ldt system call will set a list of i386 descriptors for the current process in its LDT. Both routines accept a starting selector number start_sel, an array of memory that will contain the descriptors to be set or returned descs, and the number of entries to set or return num_sels.
The argument descs can be either segment_descriptor or gate_descriptor and are defined in
.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. In the case when a descriptor is allocated by the kernel, its number will be 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: