pthread Windows崩溃C ++

Question

I would like to create an array of length 50 with a thread, when this is done I would like to print some of the first values in every X seconds with a second thread. In the meanwhile the first thread can compute the next array.

The threads are functional until I try to copy some values from the computed array in some temporary variables. I have no compilation error but when I run the program I get a windows crash massage.

Without threads the double *newarray(); function works. Returns an array which was manually allocated and filled with data.

What am I missing here?

Thread 1:

 double *newarray();

 void *computingU(void *)
 {
     double * U_tmp;

     while (true)

     {      
         pthread_mutex_lock( &mutexU );

         memcpy(U_tmp,newarray(),sizeof(double)*Ulenght);

         while (!Usent);   

         Usent = false;


         memcpy(Ucmd,U_tmp,sizeof(double)*Ulenght);

         pthread_mutex_unlock( &mutexU );  

         Ucomputed = true;
     }
}

Thread 2:

void *sendingCMD(void * ) {
    double * U_tmp;

    while (true)
    {

        while (!Ucomputed);

        Ucomputed = false;

        pthread_mutex_lock( &mutexU );

        memcpy(U_tmp,Ucmd,sizeof(double)*Ulenght);

        pthread_mutex_unlock( &mutexU );

        Usent = true;

        for (int i = 0; i<Ulenght; i++)
        {

           printf("i= %d, u= %f", i, U_tmp[i]);

           sleep(sleepTime) ;
        }

    }
}

Main:

#include <pthread.h>
#include <time.h>
#include <math.h>
#include <unistd.h>

using namespace std;

bool Ucomputed = false, Usent = true;
double * Ucmd;
pthread_mutex_t mutexU = PTHREAD_MUTEX_INITIALIZER;
unsigned int Ulenght = 1;
int sleepTime = 1;

int main( void )
{
    #ifdef DEBUG_THREAD
    int rc1, rc2;

    pthread_t thread1, thread2;
   /* Create independent threads each of which will execute functionC */
   if( (rc1=pthread_create( &thread1, NULL, &computingU, NULL)) )   {
      printf("Thread creation failed: %d\n", rc1);
   }

   if( (rc2=pthread_create( &thread2, NULL, &sendingCMD, NULL)) )
   {
      printf("Thread creation failed: %d\n", rc2);
   }
    #endif //

    sleep(10);

    while (true);
}

Answer 1

Lets take the first thread in the computingU function, there you have a local variable:

double * U_tmp;

Later you use this variable:

memcpy(U_tmp,newarray(),sizeof(double)*Ulenght);

But nowhere do you initialize the variable, so it doesn't point to anything. As uninitialized (non-static) local variables have indeterminate value, so the pointer U_tmp will point to a seemingly random location. Writing there will lead to undefined behavior and most likely your crash.

And you have the same problem in the other thread.

Answer 2

Look at:

 double * U_tmp;

You never set the pointer to anything, and then attempt to memcpy() data into it. That will 100% crash every time. If it doesn't your OS is broken.