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PCQ(9)                 NetBSD Kernel Developer's Manual                 PCQ(9)


NAME

     pcq -- producer/consumer queue


SYNOPSIS

     #include <sys/pcq.h>

     pcq_t *
     pcq_create(size_t maxlen, km_flags_t kmflags);

     void
     pcq_destroy(pcq_t *pcq);

     void *
     pcq_get(pcq_t *pcq);

     size_t
     pcq_maxitems(pcq_t *pcq);

     void *
     pcq_peek(pcq_t *pcq);

     bool
     pcq_put(pcq_t *pcq, void *item);


DESCRIPTION

     The machine-independent pcq interface provides lockless producer/consumer
     queues.  A queue (pcq_t) allows multiple writers (producers), but only a
     single reader (consumer).  The consumer is expected to be protected by a
     lock that covers the structure that the pcq_t is embedded into (e.g.,
     socket lock, ifnet hwlock).  These queues operate in a first-in, first-
     out (FIFO) manner.  The act of inserting or removing an item from a pcq_t
     does not modify the item in any way.  pcq does not prevent an item from
     being inserted multiple times into a single pcq_t.


FUNCTIONS

     pcq_create(maxlen, kmflags)
              Create a queue that can store at most maxlen items at one time.
              kmflags should be either KM_SLEEP, if pcq_create() is allowed to
              sleep until resources are available, or KM_NOSLEEP if it should
              return NULL immediately, if resources are unavailable.

     pcq_destroy(pcq)
              Free the resources held by pcq.

     pcq_get(pcq)
              Remove the next item to be consumed from the queue and return
              it.  If the queue is empty, return NULL.  The caller must pre-
              vent concurrent gets from occuring.

     pcq_maxitems(pcq)
              Return the maximum number of items that the queue can store at
              any one time.

     pcq_peek(pcq)
              Return the next item to be consumed from the queue but do not
              remove it from the queue.  If the queue is empty, return NULL.

     pcq_put(pcq, item)
              Place an item at the end of the queue.  If there is no room in
              the queue for the item, return false; otherwise, return true.
              The item must not have the value of NULL.


CODE REFERENCES

     The pcq interface is implemented within the file sys/kern/subr_pcq.c.


SEE ALSO

     atomic_ops(3), queue(9)


HISTORY

     The pcq interface first appeared in NetBSD 6.0.

NetBSD 6.1.3                   January 22, 2012                   NetBSD 6.1.3