OpenBSD manual page server

Manual Page Search Parameters

POOL(9)                      OpenBSD Kernel Manual                     POOL(9)

     pool_init, pool_destroy, pool_get, pool_put, pool_prime, pool_sethiwat,
     pool_setlowat - resource-pool manager

     #include <sys/pool.h>

     pool_init(struct pool *pool, size_t size, u_int align,
             u_int align_offset, int nitems, char *wmesg,
             struct pool_allocator *palloc);

     pool_destroy(struct pool *pp);

     pool_set_drain_hook(struct pool *pp, void (*fun)(void *, int),
             void *arg);

     void *
     pool_get(struct pool *pp, int flags);

     pool_put(struct pool *pp, void *item);

     pool_prime(struct pool *pp, int nitems, caddr_t storage);

     pool_sethiwat(struct pool *pp, int n);

     pool_setlowat(struct pool *pp, int n);

     pool_sethardlimit(struct pool *pp, int n, const char *warnmess,
             int ratecap);

     These utility routines provide management of pools of fixed-sized areas
     of memory.  Resource pools set aside an amount of memory for exclusive
     use by the resource pool owner.  This can be used by applications to
     guarantee the availability of a minimum amount of memory needed to con-
     tinue operation independent of the memory resources currently available
     from the system-wide memory allocator (malloc(9)). The pool manager ob-
     tains memory by using the special-purpose memory allocator palloc()
     passed to pool_init(), for extra pool items in case the number of alloca-
     tions exceeds the nominal number of pool items managed by a pool re-
     source.  This temporary memory will be automatically returned to the sys-
     tem at a later time.

     The function pool_init() initializes a resource pool.  The arguments are:

           pool          Specifies the pool storage to be initialized.

           size          Specifies the size of the memory items managed by the

           align         Specifies the memory address alignment of the items
                         returned by pool_get().  This argument must be a pow-
                         er of two.  If zero, the alignment defaults to an ar-

                         chitecture-specific natural alignment.

           align_offset  The offset within an item to which the align parame-
                         ter applies.

           nitems        Specifies the number of memory items that are allo-
                         cated to the pool at creation time.  This number may
                         be zero, in which case pool_prime() can be used at a
                         later time to add permanent items to the pool.

           wmesg         The message passed on to tsleep(9) if pool_get() must
                         wait for items to be returned to the pool.

           palloc        The back-end allocator used to manage the memory for
                         the pool.  palloc() may be NULL, in which case the
                         pool manager uses the pool_allocator_kmem allocator
                         which uses uvm_km_kmemalloc(9) and uvm_km_free(9) to
                         allocate and release memory using the kmem_map
                         (seeuvm(9)). It is recommended to set this to
                         pool_allocator_nointr if the pool will never be ac-
                         cessed in interrupt context, since that allocator is
                         much more efficient.

     The pool_destroy() function destroys a resource pool.  It takes a single
     argument pp identifying the pool resource instance.

     The pool_set_drain_hook() function can be used to specify a function that
     will be called when memory is running low.  The callback fun will be
     called with the arguments arg which is the third argument to
     pool_set_drain_hook() and flags which will have PR_WAITOK set if the
     drain hook is allowed to sleep.

     pool_get() allocates an item from the pool and returns a pointer to it.

           pp     The handle identifying the pool resource instance.

           flags  One or more of of PR_URGENT, PR_WAITOK or PR_LIMITFAIL, that
                  define behaviour in case the pooled resources are depleted.
                  If no resources are available and PR_WAITOK is given, this
                  function will wait until items are returned to the pool.
                  Otherwise pool_get() returns NULL. If PR_URGENT is specified
                  and no items are available and palloc() cannot allocate a
                  new page, the system will panic (XXX). If both PR_LIMITFAIL
                  and PR_WAITOK are specified, and the pool has reached its
                  hard limit, pool_get() will return NULL without waiting, al-
                  lowing the caller to do its own garbage collection; however,
                  it will still wait if the pool is not yet at its hard limit.

     pool_put() returns the pool item pointed at by item to the resource pool
     identified by the pool handle pp. If the number of available items in the
     pool exceeds the maximum pool size set by pool_sethiwat() and there are
     no outstanding requests for pool items, the excess items will be returned
     to the system by calling prelease().

           pp    The handle identifying the pool resource instance.

           item  A pointer to a pool item previously obtained by pool_get().

     pool_prime() adds items to the pool.  Storage space for the items is ei-
     ther allocated by using the page allocation routine specified to
     pool_init(), or provided to pool_prime() by the caller through the
     storage parameter.


           pp      The handle identifying the pool resource instance.

           nitems  The number of items to add to the pool.

                   Optional pre-allocated storage.

     This function may return ENOMEM in case the requested number of items
     could not be allocated.  Otherwise, the return value is 0.

     A pool will attempt to increase its resource usage to keep up with the
     demand for its items.  Conversely, it will return unused memory to the
     system should the number of accumulated unused items in the pool exceed a
     programmable limit.  The limits for the minimum and maximum number of
     items which a pool should keep at hand are known as the high and low
     watermarks. The functions pool_sethiwat() and pool_setlowat() set a
     pool's high and low watermarks, respectively.


           pp     The handle identifying the pool resource instance.

           n      The maximum number of items to keep in the pool.  As items
                  are returned and the total number of pages in the pool is
                  larger than the maximum set by this function, any completely
                  unused pages are released immediately (by calling
                  prelease()).  If this function is not used to specify a max-
                  imum number of items, the pages will remain associated with
                  the pool until the system runs low on memory, at which point
                  the VM system will try to reclaim unused pages.


           pp     The handle identifying the pool resource instance.

           n      The minimum number of items to keep in the pool.  The number
                  pages in the pool will not decrease below the required value
                  to accommodate the minimum number of items specified by this
                  function.  Unlike pool_prime(), this function does not allo-
                  cate the necessary memory up-front.

     The function pool_sethardlimit() sets a hard limit on the pool to n
     items.  If the hard limit is reached warnmess will be printed to the con-
     sole, but no more than every ratecap seconds.  Upon successful comple-
     tion, a value of 0 is returned.  The value EINVAL is returned when the
     current size of the pool already exceeds the requested hard limit.

     Note that undefined behaviour results when mixing the storage providing
     methods supported by the pool resource routines.

     The pool resource code uses a per-pool lock to protect its internal
     state.  If any pool functions are called in an interrupt context, the
     caller must block all interrupts that might cause the code to be reen-

     Pool usage logs can be enabled by defining the compile-time option

     The pool manager is implemented in the file sys/kern/subr_pool.c.

     free(9), malloc(9), uvm(9)

     The pool manager first appeared in NetBSD 1.4 and was ported to OpenBSD
     by Artur Grabowski <>.

OpenBSD 3.2                      July 23, 1998                               4