ATTN: g++ maintainer: Using string instances to pass arguments to dlls

Pavel Tsekov
Tue Oct 4 15:02:00 GMT 2005


I noticed the following problem while porting an internal C++ application
from linux to Cygwin. If a std::string instance created in one module
(exe or dll) is passed to another say as an argument of a function call,
the program crashes or hangs. I did debug for a while and it turned
out that it is not the program itself that causes the crash - the cause
lies in the std::string implementation, the fact that libstdc++ is
provided as a static archive and that it is compiled without

What happens is that each module which links agains libstdc++ get its very
personal copy of the class member _S_empty_rep_storage. Now since
_GLIBCXX_FULLY_DYNAMIC_STRING is not defined an empty string instance i.e.
one that is created by the default constructor of std::string gets a
pointer to _S_empty_rep_storage i.e. the actual allocation of memory is
delayed until memory is really needed. If _GLIBCXX_FULLY_DYNAMIC_STRING is
defined each string instance would get a pointer to a newly heap
allocated block of memory instead . Now look at the ouput of gdb and nm
used on the attached testcase:

Administrator@mordor ~/src/testcase/tc
$ nm dll.dll | grep _S_empty_rep_storage
1000c030 d .data$_ZNSs4_Rep20_S_empty_rep_storageE
1000c030 D __ZNSs4_Rep20_S_empty_rep_storageE

Administrator@mordor ~/src/testcase/tc
$ nm main.exe | grep _S_empty_rep_storage
00442120 d .data$_ZNSs4_Rep20_S_empty_rep_storageE
00442120 D __ZNSs4_Rep20_S_empty_rep_storageE

Breakpoint 1, main (argc=1, argv=0x10042980) at
9       {
(gdb) n
10        string s, s1;
12        export1 (s);
(gdb) print s
$1 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>},
    _M_p = 0x44212c ""}}
(gdb) print s1
$2 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>},
    _M_p = 0x44212c ""}}

Here the two strings share _S_empty_rep_storage.

(gdb) step
export1 (s=@0x22eea0) at
7         string s1;
(gdb) n
9         s1.push_back ('A');
(gdb) n
10        s.push_back ('A');
(gdb) print s
$3 = (string &) @0x22eea0: {static npos = 4294967295,
  _M_dataplus = {<allocator<char>> = {<new_allocator<char>> = {<No data
fields>}, <No data fields>}, _M_p = 0x44212c ""}}
(gdb) print s1
$4 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>},
    _M_p = 0x10042aec "A"}}

Here s1 points to the dll local _S_empty_rep_storage.

The _M_p member of _M_dataplus is pointing to different copies of
_S_empty_rep_storage - one stored in the executable which calls the dll
and another one in the dll itself.

Now the second push_back() call in export1 () will end up calling
_M_mutate() to actually allocate storage. _M_mutate() will call
_M_rep()->_M_dispose() which will end up free()-ing the memory reserved
for _S_empty_rep_storage in the main exe. There is a check to prevent free()-ing

          if (__builtin_expect(this != &_S_empty_rep(), false))

... but it doesn't work well in the case when a string instance created
in one module is passed to another and libstdc++ is statically linked
because of the fact that each module has its own copy

Can we get this fixed ?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: dllstr.tar.bz2
Type: application/octet-stream
Size: 423 bytes
URL: <>
-------------- next part --------------
Unsubscribe info:
Problem reports:

More information about the Cygwin mailing list