Description

   The ftok function returns a key, based on the path_name and
   project_id parameters, that is usable in subsequent calls to the
   semget function. The ftok function returns the same key for all
   paths that name the same file, when called with the same project_
   id parameter. Different keys are returned for the same file if
   different project_id parameters are used, or if paths are used
   that name different files existing on the same file system at the
   same time. If a file named by path_name is removed and recreated
   with the same name, the ftok function may return a different key
   than the original one.

   Only the low order 8 bits of project_id are significant. The
   behavior of ftok is unspecified if these bits are 0.

   For maximum portability, project_id must be a single-byte
   character.

   Upon successful completion, the ftok function returns a key.
   Otherwise, it returns the value (key_t)-1 and sets errno to
   indicate the error.