Question

I experience a segfault after the following line:

this->my_set.insert(my_string);

The set has been initialized and it contains std::string.
The inserting string is not referring to a dandling pointer and it’s null-terminated.
All the other strings contained in the set are not null and terminate with ‘\0’.

This is the backtrace:

#0  0xb7076bdb in std::_Rb_tree_insert_and_rebalance () from /usr/lib/libstdc++.so.6
#1  0xb7480795 in std::_Rb_tree<std::string, std::string, std::_Identity<std::string>, std::less<std::string>, std::allocator<std::string> >::_M_insert_ (
    this=0x97bf918, __x=0x0, __p=0x980aa78, __v=@0xaabfc1c4) at /usr/include/c++/4.3/bits/stl_tree.h:854<br/>
#2  0xb7480b96 in std::_Rb_tree<std::string, std::string, std::_Identity<std::string>, std::less<std::string>, std::allocator<std::string> >::_M_insert_unique (this=0x97bf918, __v=@0xaabfc1c4) at /usr/include/c++/4.3/bits/stl_tree.h:1148
#3  0xb74fab2d in std::set<std::string, std::less<std::string>, std::allocator<std::string> >::insert (this=0x97bf918, __x=@0xaabfc1c4)
    at /usr/include/c++/4.3/bits/stl_set.h:381

Any idea?

---

Edit

More code:

set<string> dangerous_messages; //placed in header file

            qpid::client::Message msg;
            msg = (*mylocal_queues)[queue]->get(10*1e9);
            string msgUID = msg.getHeaders().getAsString("UID");
            if(this->dangerous_messages.count(msgUID))
            {
                       warning("(read_local_queue) Duplicate message \""+msgUID+"\" discarded");
           }
           else
           {
                        msg.getHeaders().setString("BID", bid);
                        this->dangerous_messages.insert(msgUID);
            }

Printing with gdb, I’ve not noticed any corruption in the set or in the string.

Answer 1

Since you stated that the code is multi-threaded without locks, this is probably the result of a race condition. (At least a race condition may result in the exact same stack trace.)

Use proper locks to ensure correct execution. For example, if you are using OpenMP, then change the code you’ve shown us to:

#pragma omp critical
{
    if(this->dangerous_messages.count(msgUID))
    {
        warning("(read_local_queue) Duplicate message \""+msgUID+"\" discarded");
    }
    else
    {
        msg.getHeaders().setString("BID", bid);
        this->dangerous_messages.insert(msgUID);
    }
}

But in general, your code logic is fundamentally flawed if such an error can occur. You must not share state between threads without a clean cross-thread interface. Use predefined cross-thread communication structures to share state between threads, or use read-only state. Cross-thread writes are almost never OK.