Memory access violation caused in string class

Question

Hello all,
I have a multithreaded application. In my main thread I create a link list using the STL list container. The list is an doubly link list of structs and each struct contains a string* to identify the name of the struct.
The string* is allocated in the main thread using new as shown below:-
typedef struct{
string* name;
}GRM;
GRM* lgrm = new GRM;
//I have a pre-allocated GRM struct with values filled in and I am making a local copy of it here..
_localCopy(GRM &lgrm, const GRM &grm);
mylist.push_back(*lgrm);
void _localCopy(GRM &lgrm, const GRM &grm)
name = new string( grm.name->c_str());
somewhere else in this I invoke multiple threads with each thread trying to print the name of a single struct.
Now the problem is even before the threads are invoked, the address of the location pointed by the lgrm.name gets changed. Not sure why??. when I trace this variable ( lgrm.name ) inside the list, it remains intact till I start allocating memory for my thread objects:
pthread_t *child = new pthread_t[maxThreads];
The problem completely disappears when I use char* instead of string* ..
Please help!!!!
Is it do with some STL library requirements. I use -mt and -D_REENTRANT option and the library=-lpthread

Answer 1

Yes, I am compiling and linking with -mt option. This is what is happening. I am giving you the debug output. I have function called readXMLInput() where I create a struct called grm of type GSstruct.
typedef struct{
string* groupName;
}_GSstruct;
typedef<_GSstruct> GSlist
typedef struct{
string *rName;
GSlist gslist;
} Combo;
list<Combo> clist;
--------------------------debug output--------------------------
2836 _grm.groupName = new string(uc.groupAddress);
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) next
t@1 (l@1) stopped in MgroupManager::readXMLInput at line 2837 in file "MgroupManager.cc"
2837 _grm.problemCondition=0;
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) print _grm.groupName
_grm.groupName = 0x14d6c0
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) print _grm.groupName
_grm.groupName = {
__data_ = {
__data_ = 0x14d280 "239.2.2.2"
__nullref = struct __rwstd::__null_string_ref_rep<char,std::char_traits<char>,std::allocator<char>,__rwstd::__string_ref_rep<std::allocator<char> > > /* STATIC CLASS */
-----------------------debug output end---------------------
Now somewhere down I pass this struct as reference pointer to another function called
insertToComboList(&grm) where I make a copy of the contents of this struct _grm by copying its contents into another struct lgrm of the same type but allocated on the heap: see below:-
---------------------------debug output-------------------------
GSstruct* lgrm   = new GSstruct;
_localGSCopy(*lgrm,grm);
localGSCopy(GSstruct&lgrm, GSstruct &grm)
595 lgrm.groupName =new string();
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) next
t@1 (l@1) stopped in McastTopo::_localGSCopy at line 596 in file "McastTopo.cc"
596 lgrm.groupName->assign(*(grm.groupName));
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) next
t@1 (l@1) stopped in McastTopo::_localGSCopy at line 597 in file "McastTopo.cc"
597 lgrm.sourceLen = grm.sourceLen;
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) print lgrm.groupName
lgrm.groupName = 0x14d6d0
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) print *lgrm.groupName
*lgrm.groupName = {
__data_ = {
__data_ = 0x14d188 "239.2.2.2"
__nullref = struct __rwstd::__null_string_ref_rep<char,std::char_traits<char>,std::allocator<char>,__rwstd::__string_ref_rep<std::allocator<char> > > /* STATIC CLASS */
-------------------------debug output end-------------------
3. Now this struct is inserted into a list of type GSstruct which belongs to another struct of type Combo as mentioned in step 1. sorry if it is getting complicated and unfortunately it is complicated..
Combo *rs = new Combo;
combo.gslist.pushback(*lgrm);
867 rs->gslist.push_back(*lgrm);
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) next
t@1 (l@1) stopped in McastTopo::_insertToComboList at line 868 in file "McastTopo.cc"
868 rlist.push_back(*rs);(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) st.__node.next.data.groupName <print
*clist.__node->next->data.gslist.__node->next->data.groupName = {
__data_ = {
__data_ = 0x14d188 "239.2.2.2"
__nullref = struct __rwstd::__null_string_ref_rep<char,std::char_traits<char>,std::allocator<char>,__rwstd::__string_ref_rep<std::allocator<char> > > /* STATIC CLASS */
-----------------------end of debug output------------------
4. as you can see from above, the struct still contains the groupName with valid data content.
5. Now down the line we stop before we starts the thread to see if the contents have changed in the above memory location. see below. we are in a method called forkChildren()
----------------------------debug output-----------------------
t@1 (l@1) stopped in MgroupManager::forkChildren at line 2584 in file "MgroupManager.cc"
2584 cout << "printing list 1 in fork children.."<<endl<<endl;
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) print *clist.__node.next.data.gs >
*clist.__node->next->data.gslist.__node->next->data.groupName = {
__data_ = {
__data_ = 0x14d188 "239.2.2.2"
__nullref = struct __rwstd::__null_string_ref_rep<char,std::char_traits<char>,std::allocator<char>,__rwstd::__string_ref_rep<std::allocator<char> > > /* STATIC CLASS */
----------------------output end---------------------------------
6. Now we will trace thru this method to see where the contents are changing. see that I am going to allocate memory for pthread_t and I check the data content of groupName inside the list before I execute this line:-
---------------------------debug out start------------------
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx)next
2620 pthread_t *child = new pthread_t[maxThreads];
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) print *clist.__node.next.data.gs >
*clist.__node->next->data.gslist.__node->next->data.groupName = {
__data_ = {
__data_ = 0x14d188 "239.2.2.2"
__nullref = struct __rwstd::__null_string_ref_rep<char,std::char_traits<char>,std::allocator<char>,__rwstd::__string_ref_rep<std::allocator<char> > > /* STATIC CLASS */
--------------------------debug output end-------------
7. as you can see from above it is stil intact. Now I will go ahead and execute this line and see the contents inside my clist
----------------------------debug starts------------------
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx)next
2621 int *timeArray = new int[maxThreads +1];
(/opt/SUNWspro/bin/../WS6U2/bin/sparcv9/dbx) print *clist.__node.next.data.gs >
*clist.__node->next->data.gslist.__node->next->data.groupName = {
__data_ = {
__data_ = 0x14d708 ""
__nullref = struct __rwstd::__null_string_ref_rep<char,std::char_traits<char>,std::allocator<char>,__rwstd::__string_ref_rep<std::allocator<char> > > /* STATIC CLASS */
------------------------debug ends here.----------------------
As you can see from above as I moved from line number 2620 to 2621, my clist memory content holding the groupName got erased. NOT SURE HOW!!
Please help. I am very tempted to move to char* way of doing things rather than using STL string as I have spend enough time on this.
Thanyou all in advance for help here.. I am frustrated!- could be a silly error on my part but I am clueless.