Заголовочный файл:
/* Position Modes (fst_posmode) for F_PREALLOCATE */
#define F_PEOFPOSMODE 3 /* Make it past all of the SEEK pos modes so that */
/* we can keep them in sync should we desire */
#define F_VOLPOSMODE 4 /* specify volume starting postion */
Справочная страница:
The position modes (fst_posmode) for the F_PREALLOCATE command indicate how to use the offset field.
The modes are as follows:
F_PEOFPOSMODE Allocate from the physical end of file.
F_VOLPOSMODE Allocate from the volume offset.
Документ трудно понять.