[Boost-bugs] [Boost C++ Libraries] #3971: basic_random_generator doesn't produce a fully randomized uuid when sizeof(unsigned long) == 4

Subject: [Boost-bugs] [Boost C++ Libraries] #3971: basic_random_generator doesn't produce a fully randomized uuid when sizeof(unsigned long) == 4
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-03-03 10:37:37

#3971: basic_random_generator doesn't produce a fully randomized uuid when
sizeof(unsigned long) == 4
--------------------------------------------------+-------------------------
 Reporter:  gareth.sylvester-bradley@…            |       Owner:  atompkins
     Type:  Patches                               |      Status:  new      
Milestone:  Boost 1.43.0                          |   Component:  uuid     
  Version:  Boost 1.42.0                          |    Severity:  Problem  
 Keywords:                                        |  
--------------------------------------------------+-------------------------
 There is a flaw in basic_random_generator<>
 (boost/uuid/uuid_generators.hpp, line 366-374) that means it will only
 generate about 4 billion different values before a collision on the many
 platforms where sizeof(unsigned long) == 4.

 On these platforms the first 8 bytes of the uuid are populated with random
 data, but the contents of the remaining 8 bytes are undefined.

 In my tests they are populated with repeated data, due to Microsoft's
 specific implementation for the undefined behaviour of >> when its "right
 operand is greater than or equal to the length in bits of the promoted
 left operand."

 For example, try the following test program with a Win32 build by Visual
 C++ 9.0 –

 {{{
 #include <iostreams>
 #include <boost/uuid/uuid_generators.hpp>
 #include <boost/uuid/uuid_io.hpp>

 int main( int, char*[] )
 {
   std::cout << boost::uuids::random_generator()() << std::endl;
   return 0;
 }
 }}}

 This produced for me:

 {{{
 f1633d75-ee94-47b9-ae94-37b9ee9437b9
 }}}

 Or wrapped to show the problem:

 {{{
 f1633d75-
 ee94-47b9-
 ae94-37b9
 ee94 37b9
 }}}

 The simplest patch could be just:

 {{{
 370a370
 >                 i=0;
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/3971>
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