void func(char* buffer)
{
    ...
    buffer = ...
    ...
}
...
char * buffer;
...
func(buffer);

void func(char** buffer)
{
    ...
    *buffer = ...
    ...
}
...
char * buffer;
...
func(&buffer);

void func(char* &buffer)
{
    ...
    buffer = ...
    ...
}
...
char * buffer;
...
func(buffer);