edit string in multi thread - c++

Question

I am newer for c++, and I know in c++0x multithreading support is poor.
But now I have to edit a string val in multi thread, I use thread_mutex to protect the val. The problem is the program always core dump when runing.
Although after re-design I find a better solution but I can't figure out why it core. So could some one told me about what happend?
The code is like below
using namespace std;
const static int kThreadSize = 48;
map<string, string> gMap;
string gStr = "";
void * func(void * data)
{
while(true)
{
printf("%s\n", gStr.c_str());
pthread_mutex_t m;
pthread_mutex_init(&m, NULL);
pthread_mutex_lock(&m);
gStr = gStr + "a";//core in this line
printf("%x\n", &gStr);//print the address
pthread_mutex_unlock(&m);
pthread_mutex_destroy(&m);
printf("%s\n", gStr.c_str());
}
}
int main()
{
pthread_t threads[kThreadSize];
for(int i = 0; i < kThreadSize; i ++)
{
pthread_create(&threads[i], NULL, &func, &gMap);
}
for(int i = 0; i < kThreadSize; i ++)
{
pthread_join(threads[i], NULL);
}
return 0;
}
EDIT:
By using the global mutex will solve the problem pointed by Mike.Here I don't paste the new source code.
My new question is I also can't understand why it will core when edit a string in multithreading. Because of COW or reference count?

Answer 1

As i can see there is a problem with yours Mutex, because it is create into the method func() and it make that each thread invocation of func()create a new mutex, it is happened by each threat that invokes this method.
As MikeB says, i sugest you review the thread theory but, meanwhile, try to use a unique mutex to synchronize the access to gMap, as the example shows:
using namespace std;
const static int kThreadSize = 48;
map<string, string> gMap;
string gStr = "";
pthread_mutex_t m;
void * func(void * data)
{
while(true)
{
printf("%s\n", gStr.c_str());
pthread_mutex_lock(&m);
gStr = gStr + "a";//core in this line
printf("%x\n", &gStr);//print the address
pthread_mutex_unlock(&m);
printf("%s\n", gStr.c_str());
}
}
int main()
{
pthread_mutex_init(&m, NULL);
pthread_t threads[kThreadSize];
for(int i = 0; i < kThreadSize; i ++)
{
pthread_create(&threads[i], NULL, &func, &gMap);
}
for(int i = 0; i < kThreadSize; i ++)
{
pthread_join(threads[i], NULL);
}
return 0;
}