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

  The _FFC function finds the position of the first clear bit in a
  field.  The bits are tested for clear 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 _FFC(int __start, char __size, const void * __base,
                       int *__position);