pthread_join hangs accordingly to random global variable value - c

Question

I have built this code utilizing pthreads. The goal is to build an array X[N][D] and assign random values to it. You could read the elements of this array as the coefficients of some points.
On the next step I am trying to calculate an array distances[N]which holds all the distances between the last element (Nth) and each other element. The distances calculation is executed using pthreads.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <math.h>
#define N 10
#define D 2 //works for any d
#define NUM_THREADS 8
//double *distances;
//int global_index = 0;
pthread_mutex_t lock;
double *X;
typedef struct
{
//int thread_id;
double *distances;
int *global_index ;
pthread_mutex_t lock;
double *X;
}parms;
void *threadDistance(void *arg)
{
parms *data = (parms *) arg;
double *distances = data->distances;
double *X = data->X;
int *global_idx = data -> global_index;
int idx,j;
//long id = (long)arg;
pthread_mutex_lock(&lock);
while(*global_idx<N)
{
//printf("Thread #%ld , is calculating\n", id);
idx = *(global_idx);
(*global_idx)++;
pthread_mutex_unlock(&lock);
for(j=0 ; j<D; j++)
{
distances[idx] = pow(X[(j+1)*N-1]-X[j*N+idx], 2);
//printf("dis[%d]= ", dis);
//printf("%f\n",distances[idx]);
}
//printf("global : %d\n", *global_idx);
}
pthread_exit(NULL);
}
void calcDistance(double * X, int n, int d)
{
int i;
int temp=0;
pthread_t threads[NUM_THREADS];
double *distances = malloc(n * sizeof(double));
parms arg;
arg.X = X;
arg.distances = distances;
arg.global_index = &temp;
for (i=0 ; i<NUM_THREADS ; i++)
{
pthread_create(&threads[i], NULL, threadDistance, (void *) &arg);
}
for(i = 0 ; i<NUM_THREADS; i++)
{
pthread_join(threads[i], NULL);
}
/*----print dstances[] array-------*/
printf("--------\n");
for(int i = 0; i<N; i++)
{
printf("%f\n", distances[i]);
}
/*------------*/
free(distances);
}
int main()
{
srand(time(NULL));
//allocate the proper space for X
X = malloc(D*N*(sizeof(double)));
//fill X with numbers in space (0,1)
for(int i = 0 ; i<N ; i++)
{
for(int j=0; j<D; j++)
{
X[i+j*N] = (double) (rand() / (RAND_MAX + 2.0));
}
}
calcDistance(X, N, D);
return 0;
}
The problem is that the code executes completely only when N=100000. If N!=100000 the code just hangs and I have found that the source of the problem is the pthread_join() function. First of all I cannot understand why the hang depends on the value of N.
Secondly, I have tried printf()ing the value of global_index (as you can see it is commented out in this particular sample of code). As soon as I uncomment the printf("global : %d\n", *global_idx); command the program stops hanging, regardless of the value of N.
It seems crazy to me as the differences between hanging and not hanging are so irrelevant.

Answer 1

regarding:
pthread_mutex_lock(&lock);
while(*global_idx<N)
{
// ...
pthread_mutex_unlock(&lock);
The result is that after the first iteration of the loop, the mutex is always unlocked. Suggest moving the call to pthread_mutex_lock() to inside the top of the loop.
after making the above corrections, I then set N to 10000. Then re-compiled, etc. The result was a seg fault event, so the mis-handling of the mutex is not the only problem.
regarding:
* First of all I cannot understand why the hang depends on the value of N.*
it seems the program is actually crashing with a seg fault event, not hanging