c - Malloc segmentation fault

Question

Here is the piece of code in which segmentation fault occurs (the perror is not being called):

job = malloc(sizeof(task_t));
if(job == NULL)
    perror("malloc");

To be more precise, gdb says that the segfault happens inside a __int_malloc call, which is a sub-routine call made by malloc.

Since the malloc function is called in parallel with other threads, initially I thought that it could be the problem. I was using version 2.19 of glibc.

The data structures:

typedef struct rv_thread thread_wrapper_t;

typedef struct future
{
  pthread_cond_t wait;
  pthread_mutex_t mutex;
  long completed;
} future_t;

typedef struct task
{
  future_t * f;
  void * data;
  void *
  (*fun)(thread_wrapper_t *, void *);
} task_t;

typedef struct
{
  queue_t * queue;
} pool_worker_t;

typedef struct
{
  task_t * t;
} sfuture_t;

struct rv_thread
{
  pool_worker_t * pool;
};

Now the future implementation:

future_t *
create_future()
{
  future_t * new_f = malloc(sizeof(future_t));
  if(new_f == NULL)
    perror("malloc");
  new_f->completed = 0;
  pthread_mutex_init(&(new_f->mutex), NULL);
  pthread_cond_init(&(new_f->wait), NULL);
  return new_f;
}

int
wait_future(future_t * f)
{
  pthread_mutex_lock(&(f->mutex));
  while (!f->completed)
    {
      pthread_cond_wait(&(f->wait),&(f->mutex));
    }
  pthread_mutex_unlock(&(f->mutex));
  return 0;
}

void
complete(future_t * f)
{
  pthread_mutex_lock(&(f->mutex));
  f->completed = 1;
  pthread_mutex_unlock(&(f->mutex));
  pthread_cond_broadcast(&(f->wait));
}

The thread pool itself:

pool_worker_t *
create_work_pool(int threads)
{
  pool_worker_t * new_p = malloc(sizeof(pool_worker_t));
  if(new_p == NULL)
    perror("malloc");
  threads = 1;
  new_p->queue = create_queue();
  int i;
  for (i = 0; i < threads; i++){
    thread_wrapper_t * w = malloc(sizeof(thread_wrapper_t));
    if(w == NULL)
      perror("malloc");
    w->pool = new_p;
    pthread_t n;
    pthread_create(&n, NULL, work, w);
  }
  return new_p;
}

task_t *
try_get_new_task(thread_wrapper_t * thr)
{
  task_t * t = NULL;
  try_dequeue(thr->pool->queue, t);
  return t;
}

void
submit_job(pool_worker_t * p, task_t * t)
{
  enqueue(p->queue, t);
}

void *
work(void * data)
{
  thread_wrapper_t * thr = (thread_wrapper_t *) data;
  while (1){
    task_t * t = NULL;
    while ((t = (task_t *) try_get_new_task(thr)) == NULL);
    future_t * f = t->f;
    (*(t->fun))(thr,t->data);
    complete(f);
  }
  pthread_exit(NULL);
}

And finally the task.c:

pool_worker_t *
create_tpool()
{
  return (create_work_pool(8));
}

sfuture_t *
async(pool_worker_t * p, thread_wrapper_t * thr, void *
(*fun)(thread_wrapper_t *, void *), void * data)
{
  task_t * job = NULL;
  job = malloc(sizeof(task_t));
  if(job == NULL)
    perror("malloc");
  job->data = data;
  job->fun = fun;
  job->f = create_future();
  submit_job(p, job);
  sfuture_t * new_t = malloc(sizeof(sfuture_t));
  if(new_t == NULL)
    perror("malloc");
  new_t->t = job;
  return (new_t);
}

void
mywait(thread_wrapper_t * thr, sfuture_t * sf)
{
  if (sf == NULL)
    return;
  if (thr != NULL)
    {
      while (!sf->t->f->completed)
        {
          task_t * t_n = try_get_new_task(thr);
          if (t_n != NULL)
            {
          future_t * f = t_n->f;
          (*(t_n->fun))(thr,t_n->data);
          complete(f);
            }
        }
      return;
    }
  wait_future(sf->t->f);
  return ;
}

The queue is the lfds lock-free queue.

#define enqueue(q,t) {                                 
    if(!lfds611_queue_enqueue(q->lq, t))             
      {                                               
        lfds611_queue_guaranteed_enqueue(q->lq, t);  
      }                                               
  }

#define try_dequeue(q,t) {                            
    lfds611_queue_dequeue(q->lq, &t);               
  }

The problem happens whenever the number of calls to async is very high.

Valgrind output:

Process terminating with default action of signal 11 (SIGSEGV)
==12022==  Bad permissions for mapped region at address 0x5AF9FF8
==12022==    at 0x4C28737: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)

Answer 1

A SIGSEGV (segmentation fault) is firing in malloc is usually caused by heap corruption. Heap corruption does not cause a segmentation fault, so you would see that only when malloc tries to access there. The problem is that the code that creates the heap corruption could be in any point even far away from where the malloc is called. It is usually the next-block pointer inside the malloc that is changed by your heap corruption to an invalid address, so that when you call malloc an invalid pointer gets dereferenced and you get a segmentation fault.

I think you may try portions of your code isolated from the rest of the program to reduce the visibility of the bug.

Moreover I see that you never free the memory here and there can be a possible memory leak.

In order to check a memory leak you can run the top command top -b -n 1 and check:

RPRVT - resident private address space size
RSHRD - resident shared address space size
RSIZE - resident memory size
VPRVT - private address space size
VSIZE - total memory size