MySQL cppconn threads segmentation fault - c++

Question

I am currently developing a small C++ program that uses a database connection.
It is a connection with a MySQL database through CPPCONN connector.
Cause
I am using multiple threads and therefor I have created the following methods:
void Database::startThread()
{
fDriver->threadInit();
}
void Database::stopThread()
{
fDriver->threadEnd();
}
void Database::connect(const string & host, const string & user, const string & password, const string & database)
{
fDriver = sql::mysql::get_driver_instance();
fConnection.reset(fDriver->connect((SQLString)host,(SQLString)user,(SQLString)password));
fConnection->setSchema((SQLString) database);
fStatement.reset(fConnection->createStatement());
fConnection->setClientOption("multi-queries","true");
fConnection->setClientOption("multi-statements","true");
}
The problem is that I encounter a segmentation fault at the fDriver->threadInit() call.
I can assure you that fDriver is properly instantiated at that point through the connect function.
(fDriver is not null either)
The crash
Unfortunately I cannot give much more useful information but this is GDB's backtrace:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff4d66700 (LWP 16786)]
0x0000000000414547 in Database::startThread (this=Unhandled dwarf expression opcode 0xf3
#0 0x0000000000414547 in Database::startThread (this=Unhandled dwarf expression opcode 0xf3) at src/core/database.cpp:73
#1 0x0000000000405443 in Parser::Parser (this=0x7ffff4d659b8) at src/core/sv_parse.cpp:11
#2 0x000000000041e76d in MessageProcessor::MessageProcessor (this=0x7ffff4d659b0, serverStartTime=...) at src/server/messageProcessor.cpp:12
#3 0x000000000041bae8 in Server::__lambda1::operator() (__closure=0x62c740) at src/server/server.cpp:89
#4 0x00007ffff763f550 in execute_native_thread_routine () at ../../../../../libstdc++-v3/src/c++11/thread.cc:84
#5 0x00007ffff6edb851 in start_thread () from /lib64/libpthread.so.0
#6 0x00007ffff6c2994d in clone () from /lib64/libc.so.6
Remark
Now the weird part: this crash does not occur all the time !
Sometimes it works perfectly.
But it is of course extremely annoying if it doesn't.
CPPCONN version is 1.1.3 and we are using g++ version 4.8.1.
I hope someone can shed some light on this mystery !
Giriel

Answer 1

I struggled for hours with the same mysterious segmentation faults.
I found that adding mutex lock around get_driver_instance() solves the problem.
Here is a basic skeleton for a threaded function. This works for selecting from database, might not work for inserting or updating.
#include <mutex>
std::mutex mtx;
void test()
{
sql::Driver *driver;
sql::Connection *con;
try {
mtx.lock();
driver = get_driver_instance();
mtx.unlock();
driver->threadInit();
con = driver->connect(HOST, USER, PASS);
...
con->close();
driver->threadEnd();
} catch(...) { ... }
}