very basic c++: seg fault when doing cross class pointer assignment with malloc

In the following code, 'buf' is malloced, but then why accessing its member gives seg fault?

clas开发者_Python百科s tool{
   ...
   void do(char* buf){
        buf = malloc(100);
        ... //init buf[0], buf[1], etc
   }
};

class user{
   ...
   tool  *tl;
   char  *buf;

   user(){

       tl = new tool;
       tl -> do(buf);

       cout<<buf[1]<<endl;  //---> gives seg fault!  Why?
   }
};

You didn't write to the copy of buf in user. All you did was allocate the memory, store it to a local variable in do() and then forget about it when do() returned.

You need do() to receive a char**. Only if you do it this way can do() return the newly allocated memory to the caller.

void _do(char** buf){
    *buf = (char*)malloc(100);
    ... //init buf[0], buf[1], etc
}
...
tl -> _do(&buf);

Of course, since this is C++ I wonder why you don't use references and std::string, but perhaps this is illustrative code.