Question

This is the first time I’ve used pthreads. I’m having trouble because some times my program seg faults and sometimes it does not. I have a few functions in my program that perform some basic tasks (written in C) like creating a linked list, adding an item to a list, and deleting an item from a list. Each function creates it’s own copy of a list so I don’t think they interact with each other and hence don’t need mutexes. Anyway below is my code if anyone has any ideas or if there are any “common” beginner pthread mistakes.

I run each function 1000 times in parallel, some times seg faulting, some times not. I notice it only happens with a combination of these 3 functions.

The process goes like this:
– create thread
– run threads in parallel
– each thread calls dummy function to perform a task a given number of times
– that dummy function also calls other functions

I think it might have to do with memory usage/allocation because all of these functions have to do with creating/deleting linked list nodes. Thanks a lot.

Here are the creates and joins:

 pthread_create(&t7, NULL, (void*)&p4, (void*)var);
 pthread_create(&t8, NULL, (void*)&p5a, (void*)var);
 pthread_create(&t9, NULL, (void*)&p5b, (void*)var);
 pthread_join(t7, NULL);
 pthread_join(t8, NULL);
 pthread_join(t9, NULL);

Here are the dummy functions:

void p4(int *nptr){
  int n = *nptr;
  // Get current time
  struct timeval t0, t1;
  gettimeofday(&t0,0);

  int i = 0;
  LIST *list = (LIST*)malloc(sizeof(LIST));
      for(i=0;i<n;i++){
    f4(list);
    deleteList(list);
  }
  // Get current time and find time elapsed
  gettimeofday(&t1,0);
  float elapsed = (t1.tv_sec-t0.tv_sec)*1000000 + t1.tv_usec-t0.tv_usec;
      printf("Successful execution of p4 in %f microseconds.\n", elapsed);
  free(list);
}
void p5a(int *nptr){
  int n = *nptr;
  LIST *list = (LIST*)malloc(sizeof(LIST));
  f4(list);
  // Get current time
  struct timeval t0, t1;
  gettimeofday(&t0,0);

  int i = 0;
      for(i=0;i<n;i++){
    f5a(list);
  }
  // Get current time and find time elapsed
  gettimeofday(&t1,0);
  float elapsed = (t1.tv_sec-t0.tv_sec)*1000000 + t1.tv_usec-t0.tv_usec;
  printf("Successful execution of p5a in %f microseconds.\n", elapsed);
}
void p5b(int *nptr){
  int n = *nptr;
  LIST *list = (LIST*)malloc(sizeof(LIST));
  f4(list);
  int i = 0;
      for(i=0;i<n;i++){
    f5a(list);
  }
  // Get current time
  struct timeval t0, t1;
  gettimeofday(&t0,0);
  for(i=0;i<n;i++){
    f5b(list);
  }
  // Get current time and find time elapsed
  gettimeofday(&t1,0);
  float elapsed = (t1.tv_sec-t0.tv_sec)*1000000 + t1.tv_usec-t0.tv_usec;
  printf("Successful execution of p5b in %f microseconds.\n", elapsed);
}

Here are the functions used that perform regular tasks:

// FUNCTION: initialize a linked list with pointers and insert a last element
void f4(LIST *L1){
  // initialize an empty linked list if L1 = null
  if(L1->head == NULL){
    NODE *n = (NODE *)malloc(sizeof(NODE));
    L1->head = n;
    L1->tail = n;
    L1->tail->next = NULL;
    n->data = 1;
  }
  // traverse the linked list to the end
  NODE *iter = L1->head;
  while(iter->next != NULL)
    iter = iter->next;
  // insert a new 2 element
  NODE *new = (NODE *)malloc(sizeof(NODE));
  new->data = 2; // arbitrary for testing
  new->next = NULL;
  iter->next = new;
  L1->tail = new;
}

// FUNCTION: add an item to the end of a list (queue)
void f5a(LIST *list){
  NODE *new = (NODE *)malloc(sizeof(NODE));
  new->data = 999;
  new->next = NULL;
  list->tail->next = new;
  list->tail = new;
}

// FUNCTION: remove an item from the beginning of a list (queue)
void f5b(LIST *list){
  NODE *remove = list->head;
  list->head = list->head->next;
  free(remove);
}

Answer 1

If you read malloc man pages you can see that doesn’t initialize allocated memory to 0. So, you malloc memory in p4, p5a and p5b functions. After this, you invoque f4 function with no initialized LIST contens.

In p4 function you check for a valid pointer with if(L1->head == NULL) but it maybe be not null. So you don’t alloc memory for L1->head and, after this function, in f5b function you are freeing a non allocated pointer.

Advices:

• Always initialize allocated regions.
• Always check malloc returned pointers