how do i write a C++ dll which a parameter ref the string for powerbuilder?

hello sir/madma

         may i ask question about C/C++ build a api for powerbuilder to consume,i'd like write dll which powerbuilder could call in function which is defined int myfun(string ls_param1,ref string  ls_param2) "mydll.dll;Ansil" ,i just cant figure out the ls_param2 in C++,if should write as

int myfun(char * theparm1,char** ls_param2) ?

do i have to use char ** in the second parameter?

thanks

regards

Ken