[Bug c++/61735] New: basic_string bug when type_size is char.

[Bug c++/61735] New: basic_string bug when type_size is char.

[gkourtis at freemail dot gr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61735

            Bug ID: 61735
           Summary: basic_string bug when type_size is char.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gkourtis at freemail dot gr

I use g++ and I have defined a custom allocator where the size_type is byte. I
am using it with basic_string to create custom strings.

The "basic_string.tcc" code behaves erroneously because in the code of

_S_create(size_type __capacity, size_type __old_capacity, const _Alloc&
__alloc) 

the code checks for

const size_type __extra = __pagesize - __adj_size % __pagesize;

But all the arithmetic are byte arithmetic and so __pagesize that should have a
value 4096, becomes 0 (because 4096 is a multiple of 256 and bigger of 256) and
we have a "division by 0" exception (the code hangs).

I used the subsequent code to eliminate the unwanted effect:
#include <limits>
#define __pagesize_def 4096
{
if(std::numeric_limits<size_type>::max()>=__pagesize_def)
{ const size_type __extra = __pagesize - __adj_size % __pagesize; __capacity +=
__extra / sizeof(_CharT); 
} 

A little bit of discussion at:
http://stackoverflow.com/questions/24595837/bug-in-stdbasic-string-in-special-case-of-allocator

[Bug libstdc++/61735] basic_string bug when type_size is char.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61735

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
#include <string>

template<typename T>
struct tiny_allocator : std::allocator<T>
{
  template<typename U> struct rebind { typedef tiny_allocator<U> other; };

  typedef unsigned char size_type;

  static size_type max_size() { return 255; }
};

int main()
{
  std::basic_string<char, std::char_traits<char>, tiny_allocator<char> > s;
  s += 'a';
}

ubsan notices the error:

/home/jwakely/gcc/4.x/include/c++/4.10.0/bits/basic_string.tcc:597:54: runtime
error: division by zero
Floating point exception (core dumped)

The fix might be as simple as changing the type of __pagesize to unsigned,
although I expect there are other overflow issues lurking for an 8-bit
size_type.

[Bug libstdc++/61735] pre-C++11 basic_string bug when type_size is char (-D_GLIBCXX_USE_CXX11_ABI=0)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61735

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|basic_string bug when       |pre-C++11 basic_string bug
                   |type_size is char.          |when type_size is char
                   |                            |(-D_GLIBCXX_USE_CXX11_ABI=0
                   |                            |)

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I will note that the (default) newer basic_string that was included in GCC 5+
does not have this issue but the pre-c++ ABI basic_string still has this issue.
That is you need -D_GLIBCXX_USE_CXX11_ABI=0 to hit the issue now.