proc proto stdcall package=work.proto_ds.cfcns;
  link 'c:\temp\myfactorial.dll';
  int myfactorial(int n) ;
run;

proc fcmp inlib=work.proto_ds outlib=work.fcmp_ds.sasfcns;
  function sas_myfactorial(x);
    return (myfactorial(x));
  endsub;
quit;

options cmplib=(work.proto_ds work.fcmp_ds);

data _null_;
  x=sas_myfactorial(6);
  put x=;
run;