Threading Segfault when reading members - c++

Question

I am facing some problems when I am trying to read a class variable in my worker thread (That variable was not created withing that thread). Here is my class header with static worker thread function:
class C1WTempReader
{
public:
C1WTempReader(std::string device);
virtual ~C1WTempReader();
void startTemRead();
double& getTemperature();
private:
std::string device;
std::string path;
double temperature;
pthread_t w1Thread;
std::mutex w1Mutex;
bool fileExists(const std::string& filename);
static void * threadHelper(void * arg)
{
return ((C1WTempReader*) arg)->tempReadRun(NULL);
}
void* tempReadRun(void* arg);
};
Here are the crucial methods I am using:
C1WTempReader::C1WTempReader(std::string device)
{
path = W1_PATH + device + W1_SLAVE;
if (!fileExists(path))
{
std::cout << "File " << path << " doesnt exist!" << std::endl;
path.clear();
return;
}
std::cout << "1 wire termometer device path: " << path << std::endl;
}
void C1WTempReader::startTempRead()
{
if(pthread_create(&w1Thread, NULL, threadHelper, NULL) == -1)
{
std::cout << "1W thread creation failed" << std::endl;
}
}
void* C1WTempReader::tempReadRun(void* arg)
{
w1Mutex.lock(); // SEGFAULT
std::string line;
std::ifstream myfile (path.c_str());
if (myfile.is_open())
{
while ( getline (myfile,line) )
{
size_t found = line.find("t=");
if (found != std::string::npos)
{
found += 2;
std::string temp = line.substr(found, 10);
temperature = atof(temp.c_str());
temperature /= 1000;
std::cout << "Temperature: " << temperature << " *C" << std::endl;
}
line.clear();
}
myfile.close();
}
else
{
std::cout << "Unable to open file" << std::endl;
temperature = 1000; // bad value
}
w1Mutex.unlock();
return NULL;
}
double& C1WTempReader::getTemperature()
{
if (pthread_join(w1Thread, NULL))
{
std::cout << "1W Unable to join thread!" << std::endl;
}
return temperature;
}
Segmentation fault happens in the tempReadRun method, as soon as I try to lock the mutex. I didnt have mutexes before and I noticed that it happens whenever I try to read any of the class variables that are created ruring the class creation and are not created within the new thread. What Am I doing wrong?
And heres how I am trying to run this:
string device = "28-00000713a636";
C1WTempReader tempReader(device);
tempReader.startTempRead();
.
.
.
double temp = tempReader.getTemperature();

Answer 1

I have found what the problem was. In the function C1WTempReader::startTempRead():
if(pthread_create(&w1Thread, NULL, threadHelper, NULL) == -1)
{
std::cout << "1W thread creation failed" << std::endl;
}
The last argument of pthread_create had to be changed to this.

Answer 2

You are not passing argument to pthread_create:
if(pthread_create(&w1Thread, NULL, threadHelper, this) == -1)