VMS Help CC, Language topics, Builtin Functions, Find First Set Bit  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The _FFS function finds the position of the first set bit in a
  field.  The bits are tested for set status starting at bit 0 and
  extending to the highest bit in the field.

  Syntax:

       typedef enum {_ff_bit_not_found, _ff_bit_found} _FF_STATUS;

       _FF_STATUS _FFS(int __start, char __size, const void *__base,
                       int *__position);