MySQL cppconn threads segmentation fault

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

garancsi · Accepted Answer · 2014-04-24 10:37:24Z

3

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(...) { ... }
}

answered Apr 24, 2014 at 10:37

garancsi

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