[Bug libstdc++/16612] empty basic_strings can't live in shared memory

["bangerth at dealii dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From bangerth at dealii dot org  2004-07-21 14:02 -------
Not knowing much about the code in question, but here is where I see 
the problem: in basic_string.h, we have a representation of the empty 
string: 
      struct _Rep : _Rep_base 
      { 
        /*...*/ 
 
	// The following storage is init'd to 0 by the linker, resulting 
        // (carefully) in an empty string with one reference. 
        static size_type _S_empty_rep_storage[]; 
 
This member variable is indeed zero initialized in the linker, see 
basic_string.tcc: 
  // Linker sets _S_empty_rep_storage to all 0s (one reference, empty string) 
  // at static init time (before static ctors are run). 
  template<typename _CharT, typename _Traits, typename _Alloc> 
    typename basic_string<_CharT, _Traits, _Alloc>::size_type 
    basic_string<_CharT, _Traits, _Alloc>::_Rep::_S_empty_rep_storage[ 
    (sizeof(_Rep_base) + sizeof(_CharT) + sizeof(size_type) - 1) / 
      sizeof(size_type)]; 
 
Note that allocation of this memory is done by the linker in the address 
space of each program that uses strings, not by an allocator class. Thus, 
if two programs use shared memory to communicate with each other, their 
_S_empty_rep_storage objects may lie at different addresses. Whether that 
is a problem I don't know, but it seems unsafe indeed. 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16612