Allocate memory problems

Some were discussed in former posts. If the parameter of the function is a pointer, don't use it to allocate memory dynamically, the following codes have problem:

void GetMemory(char *p, int num)
{
   p = (char *)malloc(sizeof(char) * num);
}
void foo(void)
{
   char *str = NULL;
   GetMemory(str, 100);// str is still NULL   
   strcpy(str, "hello");   // wrong
}

The compiler generates a temp pointer _p = p. _p get some memory block but p doesn't. The following 2 codes are correct. The first uses pointer to pointer:

void GetMemory(char **p, int num)
{
   *p = (char *)malloc(sizeof(char) * num);
}
void foo(void)
{
   char *str = NULL;
   GetMemory(&str, 100);   
   strcpy(str, "hello");  
   cout<< str << endl;
   free(str);  
}

The second uses 'return pointer'

char *GetMemory(int num)
{
   char *p = (char *)malloc(sizeof(char) * num);
   return p;
}
void foo(void)
{
   char *str = NULL;
   str = GetMemory(100);  
   strcpy(str, "hello");
   cout<< str << endl;
   free(str);  
}

Be careful, don't return a pointer with junk, such like following code. p points to junk after the scope.

char *GetString(void)
{
   char p[] = "hello world";
   return p;   
}
void foo(void)
{
char *str = NULL;
str = GetString();    // str:junk inside
cout<< str << endl;
}

The following codes may get the correct answer but it is still wrong. The pointer points to a constant string and it never changes

char *GetString(void)
{
   char *p = "hello world!";
   return p;
}
void foo(void)
{
   char *str = NULL;
   str = GetString();
   cout<< str << endl;
}