stringstream: rdbuf()->sgetn(buf, rdbuf()->in_avail()) contentsdiffer from str()

[whatdoineed2do@yahoo.co.uk]

hi,

i'm trying avoid uncessary copy of data whilst using the stringstream
so i am using the sgetn() to copy the data into a buf as apposed to
getting the same data via the .str() which will construct a string
object, and a string object copy on return to me.

however i've noticed that if the last thing put onto the stringstream
is not txt (an int for example) then the sgetn() using the in_avail()
DIFFERS from what i get back from .str(). Below illustrates my
problem

Am using SunStudio 10 on sparc 5.10 -- haven't had chance to try g++

any thoughts on where i may be going wrong appreciated
thanks
ray


#include <unistd.h>

#include <iostream>
#include <sstream>
using namespace std;

int main()
{
stringstream s;

s.seekp(0); s.seekg(0);
s << "pid=" << getpid();
cout << "'" << s.str() << "'" << endl; // PID PRINTED AS EXPECTED


char buf[100];
memset(buf, 0, 100);

s.seekp(0); s.seekg(0);
s << "pid=" << getpid();
s.rdbuf()->sgetn(buf, s.rdbuf()->in_avail());
cout << "'" << buf << "'" << endl; // PID NOT PRINTED

memset(buf, 0, 100);

s.seekp(0); s.seekg(0);
s << "pid=" << getpid() << " and some text";
s.rdbuf()->sgetn(buf, s.rdbuf()->in_avail());
cout << "'" << buf << "'" << endl; // AS EXPECTED

return 0;