Hello,
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
_GLIBCXX_FULLY_DYNAMIC_STRING defined.
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:
[EMAIL PROTECTED] ~/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
[EMAIL PROTECTED] ~/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 main.cc:9
9 {
(gdb) n
10 string s, s1;
(gdb)
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 ([EMAIL PROTECTED]) at dll.cc:7
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"}}
(gdb)
===
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
_S_empty_rep_storage:
#ifndef _GLIBCXX_FULLY_DYNAMIC_STRING
if (__builtin_expect(this != &_S_empty_rep(), false))
#endif
.... 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
_S_empty_rep_storage.
Can we get this fixed ?
--
Summary: Using string instances to pass arguments to dlls fails
Product: gcc
Version: 3.4.4
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: libstdc++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: gerrit at gcc dot gnu dot org
GCC build triplet: i686-pc-cygwin
GCC host triplet: i686-pc-cygwin
GCC target triplet: i686-pc-cygwin
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24196
