direct access to ns[C]String buffer

Hi,

Is there any way to get direct access to a buffer in a string class 
similar to the MFC CString::GetBuffer()/CString::ReleaseBuffer() pair?

Inside the implementation of an XPCOM interface, I am calling a library 
function that takes a raw pointer and a buffer length and writes a null 
terminated string into it. I want to assign this string to a nsCString 
which is returned from the interface. The easiest way would be to 
request the string to have the max size buffer, write the data directly 
into the strings buffer, then tell the string the actual size of the 
data. This way there is only a single allocation.

e.g.
nsCString str;
int bufsiz = 1000;
str.SetLength( bufsiz );
char * p = str.GetBuffer();
rawpointerfn( p, &bufsiz );
str.SetLength( bufsiz );

Surely this is possible? Or do I have to double allocate (once manually, 
once by the string)?

Regards,
Brodie.