pthread_join seg fault core dump - c

Question

I am getting a seg fault error at my pthread_join line. Below is my thread creation and joining code as well as my my_func thread function that I am calling. The program is supposed to create a variable amount of threads to execute a grep like function.
int
parallelSearchStatic(char **argv)
{
pthread_t worker_thread[NUM_THREADS];
ARGS_FOR_THREAD *args_for_thread;
queue_element_t *element;
int i;
int num_occurrences = 0;
int queue_count = 0;
queue_t *queue = createQueue(); /* Create and initialize the queue data structure. */
DIR *d;
struct dirent *dir;
d = opendir(argv[2]);
if (d)
{
while ((dir = readdir(d)) != NULL)
{
element = (queue_element_t *)malloc(sizeof(queue_element_t));
if(element == NULL){
perror("malloc");
exit(EXIT_FAILURE);
}
strcpy(element->path_name, argv[2]);
strcat(element->path_name, "/");
strcat(element->path_name, dir->d_name);
insertElement(queue, element);
queue_count++;
}
closedir(d);
}
int increment = queue_count/NUM_THREADS;
for(i=0;i<NUM_THREADS;i++){
args_for_thread = (ARGS_FOR_THREAD *)malloc(sizeof(ARGS_FOR_THREAD));
args_for_thread->threadID=i;
args_for_thread->queue=createQueue();
args_for_thread->args=argv;
for(i = 0; i < increment; i++)
{
insertElement(args_for_thread->queue, removeElement(queue));
queue_count--;
}
if(i == (NUM_THREADS - 1) && queue_count != 0)
{
for(i = 0; i < queue_count; i++)
{
insertElement(args_for_thread->queue, removeElement(queue));
}
}
if((pthread_create(&worker_thread[i], NULL, my_func, (void *)args_for_thread))!=0){
printf("Cannot create thread \n");
exit(0);
}
}
for(i=0;i<NUM_THREADS;i++)
{
pthread_join(worker_thread[i], NULL);
}
for(i = 0; i < NUM_THREADS; i++)
num_occurrences += countArray[i];
return num_occurrences;
}
void *my_func(void *this_arg)
{
ARGS_FOR_THREAD *args_for_me = (ARGS_FOR_THREAD *)this_arg; // Typecast the argument passed to this function to the appropriate type
int threadID = args_for_me->threadID;
queue_t *queue = args_for_me->queue;
char** args = args_for_me->args;
int count = 0;
while(queue->head != NULL)
{
queue_element_t *element = removeElement(queue);
char *a[5];
a[0] = args[0];
a[1] = args[1];
a[2] = element->path_name;
a[3] = args[3];
a[4] = args[4];
count += serialSearch(a);
}
countArray[threadID] = count;
free((void *)args_for_me); // Free up the structure
pthread_exit(NULL);
}

Answer 1

You are using re-using i in nested loops within an outer loop that also uses i as its counter variable:
for(i = 0; i < increment; i++)
and
for(i = 0; i < queue_count; i++)
This means that your pthread_create() (which follows these inner loops) is using the wrong value of i to access worker_thread[i], so some values of worker_thread[] remain uninitialised and then cause pthread_join() to crash.
Use a different variable (eg. j) for the inner loops.