Pthread Argument Passing

Question

I am try to create a pthread and then record some time after running my readIn function but when I print the time in main it comes out as an incorrect value . I'm new to threads and I don't know if I'm passing the arguments the correct way. Any insight would be appreciated.

int main(int argc, char **argv){

    int read;
    pthread_t readerThread;

    ReadWrite arglist;       
    arglist.fileName= inputFile;
    arglist.rVal=(RetVal *) malloc(sizeof(struct RetVal));

    read= pthread_create(&readerThread, NULL, readIn,  (void *) &arglist );

    cout << "Total time for reading: "<< (arglist.rVal)->timeUsed << endl;

}

struct ReadWrite {
    string fileName;
    RetVal * rVal;
} ;

struct RetVal{
    int frequency;
    double timeUsed;
};

void * readIn(void *arg)
{

struct timeval now, later;
gettimeofday(&now, NULL);
ReadWrite* aList = static_cast<ReadWrite *>(arg);
RetVal *rVal = aList->rVal;
string fileName = aList->fileName;

.....
.....
.....

rVal->frequency = dataSet.size();
rVal->timeUsed  = getTimeUsed(now, later);

cout << "Correct time: " << dataSet.size(); << endl;

pthread_exit(NULL);

}

**OUTPUT**

Total CPU time for reading: -1.72723e-77
Correct time: 22841

Answer 1

You're not giving your thread any time to run - the printout in main can happen before your thread function even starts, or right in the middle. So you have a data race and undefined behavior.

You should wait for the thread to finish before doing anything with the result in main . Use pthread_join for that.

(Also C++11 has a thread library <thread> which you should look at, you can avoid those ugly casts with it.)