Re: libstdc++/8636: global ostringstreams lose their data

[Paolo Carlini <pcarlini@unitus.it>]

The following reply was made to PR libstdc++/8636; it has been noted by GNATS.

From: Paolo Carlini <pcarlini@unitus.it>
To: gcc-gnats@gcc.gnu.org,  gcc-prs@gcc.gnu.org,  gcc-bugs@gcc.gnu.org, 
 nobody@gcc.gnu.org
Cc: michael.pronath@gmx.de
Subject: Re: libstdc++/8636: global ostringstreams lose their data
Date: Thu, 21 Nov 2002 11:53:59 +0100

 Yes, the problem is present with g++-3.0, g++-3.2 and g++-3.2.1
 all with glibc2.3.1
 
 Debian system:   libc6_2.3.1-3
                   g++-3.2_3.2.1-0pre2
                   g++-3.0_3.0.4-7
                   libstdc++5-dev_3.2.1-0pre2
 
 Redhat system:   glibc-devel-2.1.3-22
 		 glibc-2.1.3-22
                   g++-3.2   self-made
 
 The original topic was wrong, I found an even simpler example, it's got
 nothing to do with global/local variables; it's probably in the
 allocator:
 
 -----------------------------------------------------------------------
 #include <iostream>
 #include <sstream>
 #include <string>
 
 using namespace std;
 
 int main()
 {
      const char *x;
      ostringstream teststream;
      string teststring="";
      teststream << "World, where are you?";
      x = teststream.str().c_str();
      teststring = "Hello ";
      teststring += x;
      cout << "teststring is '" << teststring << "'." << endl;
      cout << "teststream.str() is '" << teststream.str() << "'." << endl;
      cout << "teststring.data() is at 0x" << hex << (int) teststring.data()
 	 << ", x is at 0x" << hex << (int) x << endl;
      cout << "teststream.str().c_str() is at 0x" << hex
 	 << (int) teststream.str().c_str() << endl;
 }
 ------------------------------------------------------------------------
 
 
 g++-3.2 -o teststream teststream.cc
 
 ldd teststream
 	libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x4001a000)
 	libm.so.6 => /lib/libm.so.6 (0x400c4000)
 	libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x400e5000)
 	libc.so.6 => /lib/libc.so.6 (0x400ed000)
 	/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
 
 Unexpected output:
 teststring is 'Hello Hello Hello Hello Hello Hel'.
 teststream.str() is 'World, where are you?'.
 teststring.data() is at 0x804a99c, x ist at 0x804a99c
 teststream.str().c_str() is at 0x804a94c
 
 
 I thinks it's that the operator+= of string is resizing the string
 because it needs new space to append.  From the allocator, it receives
 the address of "x" as if this space was already freed by the
 ostringstream (don't know why).  Then the string appends its data to
 itself, which leads to the repeating Hello.
 
 
 Regards,
 Michael
 
 
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8636