Re: [boost] [interprocess] Segmentation fault with 1.34+ on Linux x86_64

From: Glenn Schrader (gschrad_at_[hidden])
Date: 2007-09-21 11:48:21

I found it. The problem is in offset_ptr.hpp. A diff against the current 
trunk is below. Note that I just moved the ~. The problem is that gcc is 
treating pointers as 64 bit values but int and unsigned as 32 bit 
values. Just to verify this, when I run the program ...

    #include <iostream>
    using namespace std;
    unsigned u;
    int i;
    int main() {
        cout << sizeof(u) << ' ' << sizeof(&u) << endl;
        cout << sizeof(i) << ' ' << sizeof(&i) << endl;
        cout << sizeof(size_t) << endl;
    }

... the output is ...

    4 8
    4 8
    8

In the original code the ~ only complemented the 32 bit unsigned value. 
When the value was expanded into a size_t the value isn't sign extended 
so the upper 32 bits wind up being zero. Since this is used as an 
address mask the upper half of the address is zeroed. This is gcc 4.1.1 
on kernel 2.6.20-1.2948.fc6 in case anybody is interested.

-glenn

% svn diff
Index: boost/interprocess/offset_ptr.hpp
===================================================================
--- boost/interprocess/offset_ptr.hpp   (revision 39447)
+++ boost/interprocess/offset_ptr.hpp   (working copy)
@@ -384,7 +384,7 @@
    typedef boost::interprocess::offset_ptr<T>         pointer;
 
    static pointer get_pointer(const pointer &n)
-   {  return (T*)(std::size_t(n.get()) & std::size_t(~2u));  }
+   {  return (T*)(std::size_t(n.get()) & ~std::size_t(2u));  }
 
    static void set_pointer(pointer &n, pointer p)
    {  n = (T*)(std::size_t(p.get()) | (std::size_t(n.get()) & 
std::size_t(2u))); }
%

Glenn Schrader wrote:
> Ion,
>
> I will attempt to find the check in date where this problem first 
> showed up. This ought to at least narrow down the possibilities.
>
> -glenn
>
> Ion Gaztañaga wrote:
>> Glenn Schrader wrote:
>>  
>>> Using gdb I get the following seg fault. I also included a backtrace.
>>>     
>>
>> Thanks for the info. Unfortunately, the lap from work (Centrino Core 
>> Duo) were I wanted to install 64 bit linux is not a 64bit processor 
>> (it seems that Core Duo 2 is 64 bit but no Core Duo), so I really 
>> don't know how to starting solving this :-(
>>
>> Regards,
>>
>> Ion
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>
>