i386_set_ldt(2) manage i386 per-process Local Descriptor Table entries

Other Alias



Lb libc


In machine/segments.h In machine/sysarch.h Ft int Fn i386_get_ldt int start_sel union descriptor *descs int num_sels Ft int Fn i386_set_ldt int start_sel union descriptor *descs int num_sels


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

Each entry in the Fa descs array can be either a 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 Fa start_sel is LDT_AUTO_ALLOC Fa num_sels is 1 and the descriptor pointed to by Fa descs is legal, then Fn i386_set_ldt will allocate a descriptor and return its selector number.

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

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


Upon successful completion, Fn i386_get_ldt returns the number of descriptors currently in the LDT. The Fn 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 Fn i386_get_ldt and Fn i386_set_ldt system calls will fail if:

An inappropriate value was used for Fa start_sel or Fa num_sels .
The caller attempted to use a descriptor that would circumvent protection or cause a failure.


You can really hose your process using this.