fuq stands for a Fundamentally Unstable Queue. It is a thread safe, lock-free SPSC (single producer single consumer) queue written in C.

This code started as an academic experiment and eventually turned into a working API. Because of the difficulty to properly test, it is permanently marked as unstable. Though, while only tested on Linux, results have been consistent under prolonged heavy load.

Being SPSC means only a single thread can push items onto the queue, and only one thread can shift items from the queue. Ownership of which thread does each operation can be transferred between threads, but take special care that only a single thread is allowed to perform each operation at any given time.

Community involvement for further testing and hardening is highly encouraged.

fuq_queue
fuq_init
fuq_push
fuq_shift
fuq_dispose

The API has been made minimal and straightforward as possible. The author's background in JS influenced the naming convention. Using push and shift, following the JS Array convention.

The memory construct that contains all the information about an individual queue. For convenience this struct is typedef'd.

To prevent unnecessary memory allocations the queue allocates a slab that is used to store pointers to all the queue items. Defined by FUQ_ARRAY_SIZE, the default size is 511. This is to round out at 512 slots, because 1 slot is added when allocated. The final slot is a pointer to the next slab of queue items. In the case the queue overflows the slab size.

Further unnecessary memory allocation are prevented by storing references to previously allocated slabs not in use. Please note, in order to maintain a lock-free queue these unused slabs are not pooled between all active fuq_queue instances. Instead each instance contains a linked list of all its own unused slabs. So remember that properly disposing of the queue at its end of life is important to cleanup all the linked unused slabs.

To ensure a single instance isn't allowed to store too many slabs, there is a maximum allowed number of slabs that can be stored. Defined by FUQ_MAX_STOR, the default value is 1024.

Pass an uninitialized fuq_queue instance. Instances that have been passed to fuq_dispose are considered uninitialized.

On initialization the initial slab is allocated, and pointers are appropriately set. All these are necessary in order to allow each push and shift to be done in a single instruction.

Place a single void* arg onto the queue. This can be done at any time without the need to lock the queue.

Returns the next item in the queue. If there are no more items in the queue then NULL will be returned. Even so, it is still possible for NULL to be pushed onto the queue without disrupting normal activity. It simply may cause confusion that the queue is empty.

Frees all allocated slabs and stored pointers. The moment this function runs the queue should be considered uninitialized, and no other pulls or shifts should be run on the queue. Though the queue is not fully uninitialized until the function returns.

The following example uses pthread to quickly push and shift values from the queue. This is not an excellent showcase, but at least very simple.

/**
 * Compiled using: -g -Wall -pthread -o sporadic -O3 sporadic.c
 */

#include "./fuq.h"
#include <assert.h>
#include <pthread.h>
#include <stdint.h>
#include <stdio.h>

#define ITER 1e7

fuq_queue queue;
pthread_t thread;


static void* task_runner(void* arg) {
  size_t i;

  /* Pushing values onto the queue as fast as possible. */
  for (i = 1; i < ITER; i++)
    fuq_push(&queue, (void*) i);
  return NULL;
}


int main(void) {
  uint64_t sum = 0;
  uint64_t check = 0;
  void* tmp;
  size_t i;

  fuq_init(&queue);
  assert(pthread_create(&thread, NULL, task_runner, NULL) == 0);

  /* Notice i starts at 1, because 0x0 == NULL. */
  for (i = 1; i < ITER; i++) {
    /* To make sure the final sum is correct. */
    check += i;
    /* Grab a value from the queue. */
    if (NULL != (tmp = fuq_shift(&queue)))
      sum += (uint64_t) tmp;
  }

  fprintf(stderr, "check: %zi\n", check);
  fprintf(stderr, "sum:   %zi\n", sum);

  assert(pthread_join(thread, NULL) == 0);

  /* Finish getting the remaining values from the queue. */
  while (NULL != (tmp = fuq_shift(&queue)))
    sum += (uint64_t) tmp;

  fprintf(stderr, "sum:   %zi\n", sum);
  assert(sum == check);

  fuq_dispose(&queue);
}