My printf manpage says:

       z      A  following  integer  conversion  corresponds to a
              size_t or ssize_t argument. (Linux libc5 has Z with
              this meaning. Don't use it.)

And the opengroup spec says

z Specifies that a following d , i , o , u , x , or X conversion specifier
  applies to a size_t or the corresponding signed integer type argument; or
  that a following n conversion specifier applies to a pointer to a signed
  integer type corresponding to a size_t argument.


yet our vsnprintf implementation has

				/* 'z' support added 23/7/1999 S.H.    */
				/* 'z' changed to 'Z' --davidm 1/25/99 */


I guess the path of least surprise is to support both.  gcc-3.2.1 doesn't
  seem to care.