libumem/umem_cache_create.3

'\" te
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.TH UMEM_CACHE_CREATE 3MALLOC "Mar 24, 2008"
.SH NAME
umem_cache_create, umem_cache_destroy, umem_cache_alloc, umem_cache_free \-
allocation cache manipulation
.SH SYNOPSIS
.LP
.nf
cc [ \fIflag \&.\|.\|.\fR ] \fIfile\fR\&.\|.\|. \fB-lumem\fR [ \fIlibrary \&.\|.\|.\fR ]
#include <umem.h>

\fBumem_cache_t *\fR\fBumem_cache_create\fR(\fBchar *\fR\fIdebug_name\fR, \fBsize_t\fR \fIbufsize\fR,
     \fBsize_t\fR \fIalign\fR, \fBumem_constructor_t *\fR\fIconstructor\fR,
     \fBumem_destructor_t *\fR\fIdestructor\fR, \fBumem_reclaim_t *\fR\fIreclaim\fR,
     \fBvoid *\fR\fIcallback_data\fR, \fBvmem_t *\fR\fIsource\fR, \fBint\fR \fIcflags\fR);
.fi

.LP
.nf
\fBvoid\fR \fBumem_cache_destroy\fR(\fBumem_cache_t *\fR\fIcache\fR);
.fi

.LP
.nf
\fBvoid *\fR\fBumem_cache_alloc\fR(\fBumem_cache_t *\fR\fIcache\fR, \fBint\fR \fIflags\fR);
.fi

.LP
.nf
\fBvoid\fR \fBumem_cache_free\fR(\fBumem_cache_t *\fR\fIcache\fR, \fBvoid *\fR\fIbuffer\fR);
.fi

.SH DESCRIPTION
.sp
.LP
These functions create, destroy, and use an "object cache"  An object cache is
a collection of buffers of a single size, with optional content caching enabled
by the use of callbacks (see \fBCache Callbacks\fR).  Object caches are
MT-Safe. Multiple allocations and freeing of memory from different threads can
proceed simultaneously.  Object caches are faster and use less space per buffer
than \fBmalloc\fR(3MALLOC) and \fBumem_alloc\fR(3MALLOC).  For more information
about object caching, see "The Slab Allocator: An Object-Caching Kernel Memory
Allocator" and "Magazines and vmem: Extending the Slab Allocator to Many CPUs
and Arbitrary Resources".
.sp
.LP
The \fBumem_cache_create()\fR function creates object caches. Once a cache has
been created, objects can be requested from and returned to the cache using
\fBumem_cache_alloc()\fR and \fBumem_cache_free()\fR, respectively. A cache
with no outstanding buffers can be destroyed with \fBumem_cache_destroy()\fR.
.SS "Creating and Destroying Caches"
.sp
.LP
The \fBumem_cache_create()\fR function creates a cache of objects and takes as
arguments the following:
.sp
.ne 2
.na
\fB\fIdebug_name\fR\fR
.ad
.RS 17n
A human-readable name for debugging purposes.
.RE

.sp
.ne 2
.na
\fB\fIbufsize\fR\fR
.ad
.RS 17n
The size, in bytes, of the buffers in this cache.
.RE

.sp
.ne 2
.na
\fB\fIalign\fR\fR
.ad
.RS 17n
The minimum alignment required for buffers in this cache. This parameter must
be a power of 2. If 0, it is replaced with the minimum required alignment for
the current architecture.
.RE

.sp
.ne 2
.na
\fB\fIconstructor\fR\fR
.ad
.RS 17n
The callback to construct an object.
.RE

.sp
.ne 2
.na
\fB\fIdestructor\fR\fR
.ad
.RS 17n
The callback to destroy an object.
.RE

.sp
.ne 2
.na
\fB\fIreclaim\fR\fR
.ad
.RS 17n
The callback to reclaim objects.
.RE

.sp
.ne 2
.na
\fB\fIcallback_data\fR\fR
.ad
.RS 17n
An opaque pointer passed to the callbacks.
.RE

.sp
.ne 2
.na
\fB\fIsource\fR\fR
.ad
.RS 17n
This parameter must be \fINULL\fR.
.RE

.sp
.ne 2
.na
\fB\fIcflags\fR\fR
.ad
.RS 17n
This parameter must be either 0 or \fBUMC_NODEBUG\fR. If \fBUMC_NODEBUG\fR, all
debugging features are disabled for this cache. See \fBumem_debug\fR(3MALLOC).
.RE

.sp
.LP
Each cache can have up to three associated callbacks:
.sp
.in +2
.nf
int constructor(void *buffer, void *callback_data, int flags);
void destructor(void *buffer, void *callback_data);
void reclaim(void *callback_data);
.fi
.in -2

.sp
.LP
The \fIcallback_data\fR argument is always equal to the value passed to
\fBumem_cache_create()\fR, thereby allowing a client to use the same callback
functions for multiple caches, but with customized behavior.
.sp
.LP
The reclaim callback is called when the umem function is requesting more memory
from the operating system. This callback can be used by clients who retain
objects longer than they are strictly needed (for example, caching non-active
state).  A typical reclaim callback might return to the cache ten per cent of
the unneeded buffers.
.sp
.LP
The constructor and destructor callbacks enable the management of buffers with
the constructed state. The constructor takes as arguments a buffer with
undefined contents, some callback data, and the flags to use for any
allocations. This callback should transform the buffer into the constructed
state.
.sp
.LP
The destructor callback takes as an argument a constructed object and prepares
it for return to the general pool of memory.  The destructor should undo any
state that the constructor created.  For debugging, the destructor can also
check that the buffer is in the constructed state, to catch incorrectly freed
buffers.  See \fBumem_debug\fR(3MALLOC) for further information on debugging
support.
.sp
.LP
The \fBumem_cache_destroy()\fR function destroys an object cache. If the cache
has any outstanding allocations, the behavior is undefined.
.SS "Allocating Objects"
.sp
.LP
The \fBumem_cache_alloc()\fR function takes as arguments:
.sp
.ne 2
.na
\fB\fIcache\fR\fR
.ad
.RS 9n
a cache pointer
.RE

.sp
.ne 2
.na
\fB\fIflags\fR\fR
.ad
.RS 9n
flags that determine the behavior if \fBumem_cache_alloc()\fR is unable to
fulfill the allocation request
.RE

.sp
.LP
If successful, \fBumem_cache_alloc()\fR returns a pointer to the beginning of
an object of \fIbufsize\fR length.
.sp
.LP
There are three cases to consider:
.RS +4
.TP
.ie t \(bu
.el o
A new buffer needed to be allocated. If the cache was created with a
constructor, it is applied to the buffer and the resulting object is returned.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The object cache was able to use a previously freed buffer.  If the cache was
created with a constructor, the object is returned unchanged from when it was
freed.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The allocation of a new buffer failed. The \fIflags\fR argument determines the
behavior:
.RS

.sp
.ne 2
.na
\fB\fBUMEM_DEFAULT\fR\fR
.ad
.RS 16n
The \fBumem_cache_alloc()\fR function returns \fINULL\fR if the allocation
fails.
.RE

.sp
.ne 2
.na
\fB\fBUMEM_NOFAIL\fR\fR
.ad
.RS 16n
The \fBumem_cache_alloc()\fR function cannot return \fINULL\fR. A callback is
used to determine what action occurs. See \fBumem_alloc\fR(3MALLOC) for more
information.
.RE

.RE

.RE
.SS "Freeing Objects"
.sp
.LP
The \fBumem_cache_free()\fR function takes as arguments:
.sp
.ne 2
.na
\fB\fIcache\fR\fR
.ad
.RS 9n
a cache pointer
.RE

.sp
.ne 2
.na
\fB\fIbuf\fR\fR
.ad
.RS 9n
a pointer previously returned from \fBumem_cache_alloc()\fR. This argument must
not be \fINULL\fR.
.RE

.sp
.LP
If the cache was created with a constructor callback, the object must be
returned to the constructed state before it is freed.
.sp
.LP
Undefined behavior results if an object is freed multiple times, if an object
is modified after it is freed, or if an object is freed to a cache other than
the one from which it was allocated.
.SS "Caches with Constructors"
.sp
.LP
When a constructor callback is in use, there is essentially a contract between
the cache and its clients.  The cache guarantees that all objects returned from
\fBumem_cache_alloc()\fR will be in the constructed state, and the client
guarantees that it will return the object to the constructed state before
handing it to \fBumem_cache_free()\fR.
.SH RETURN VALUES
.sp
.LP
Upon failure, the \fBumem_cache_create()\fR function returns a null pointer.
.SH ERRORS
.sp
.LP
The \fBumem_cache_create()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
There is not enough memory available to allocate the cache data structure.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIdebug_name\fR argument is \fINULL\fR, the \fIalign\fR argument is not a
power of two or is larger than the system pagesize, or the \fIbufsize\fR
argument is 0.
.RE

.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
The \fBlibumem\fR library could not be initialized, or the \fIbufsize\fR
argument is too large and its use would cause integer overflow to occur.
.RE

.SH EXAMPLES
.LP
\fBExample 1 \fRUse a fixed-size structure with no constructor callback.
.sp
.in +2
.nf
#include <umem.h>

typedef struct my_obj {
     long my_data1;
} my_obj_t;

/*
 * my_objs can be freed at any time.  The contents of
 * my_data1 is undefined at allocation time.
 */

umem_cache_t *my_obj_cache;

\&...
my_obj_cache = umem_cache_create("my_obj", sizeof (my_obj_t),
    0, NULL, NULL, NULL, NULL, NULL, 0);
\&...
my_obj_t *cur = umem_cache_alloc(my_obj_cache, UMEM_DEFAULT);
\&...
/* use cur */
\&...
umem_cache_free(my_obj_cache, cur);
\&...
.fi
.in -2

.LP
\fBExample 2 \fRUse an object with a mutex.
.sp
.in +2
.nf
#define _REENTRANT
#include <synch.h>
#include <umem.h>

typedef struct my_obj {
          mutex_t my_mutex;
          long my_data;
} my_obj_t;

/*
 * my_objs can only be freed when my_mutex is unlocked.
 */
int
my_obj_constructor(void *buf, void *ignored, int flags)
{
          my_obj_t *myobj = buf;

          (void) mutex_init(&my_obj->my_mutex, USYNC_THREAD, NULL);

          return (0);
}

void
my_obj_destructor(void *buf, void *ignored)
{
          my_obj_t *myobj = buf;

          (void) mutex_destroy(&my_obj->my_mutex);
}

umem_cache_t *my_obj_cache;

\&...
my_obj_cache = umem_cache_create("my_obj", sizeof (my_obj_t),
    0, my_obj_constructor, my_obj_destructor, NULL, NULL,
         NULL, 0);
\&...
my_obj_t *cur = umem_cache_alloc(my_obj_cache, UMEM_DEFAULT);
cur->my_data = 0;       /* cannot assume anything about my_data */
\&...
umem_cache_free(my_obj_cache, cur);
\&...
.fi
.in -2

.LP
\fBExample 3 \fRUse a more complex object with a mutex.
.sp
.in +2
.nf
#define _REENTRANT
#include <assert.h>
#include <synch.h>
#include <umem.h>

typedef struct my_obj {
          mutex_t my_mutex;
          cond_t my_cv;
          struct bar *my_barlist;
          unsigned my_refcount;
} my_obj_t;

/*
 * my_objs can only be freed when my_barlist == NULL,
 * my_refcount == 0, there are no waiters on my_cv, and
 * my_mutex is unlocked.
 */

int
my_obj_constructor(void *buf, void *ignored, int flags)
{
          my_obj_t *myobj = buf;

          (void) mutex_init(&my_obj->my_mutex, USYNC_THREAD, NULL);
          (void) cond_init(&my_obj->my_cv, USYNC_THREAD, NULL);
          myobj->my_barlist = NULL;
          myobj->my_refcount = 0;

          return (0);
}

void
my_obj_destructor(void *buf, void *ignored)
{
          my_obj_t *myobj = buf;

          assert(myobj->my_refcount == 0);
          assert(myobj->my_barlist == NULL);
          (void) cond_destroy(&my_obj->my_cv);
          (void) mutex_destroy(&my_obj->my_mutex);
}

umem_cache_t *my_obj_cache;

\&...
my_obj_cache = umem_cache_create("my_obj", sizeof (my_obj_t),
    0, my_obj_constructor, my_obj_destructor, NULL, NULL,
         NULL, 0);
\&...
my_obj_t *cur = umem_cache_alloc(my_obj_cache, UMEM_DEFAULT);
\&...
/* use cur */
\&...
umem_cache_free(my_obj_cache, cur);
\&...
.fi
.in -2

.LP
\fBExample 4 \fRUse objects with a subordinate buffer while reusing callbacks.
.sp
.in +2
.nf
#include assert.h>
#include umem.h>

typedef struct my_obj {
          char *my_buffer;
          size_t my_size;
} my_obj_t;

/*
 * my_size and the my_buffer pointer should never be changed
 */

int
my_obj_constructor(void *buf, void *arg, int flags)
{
          size_t sz = (size_t)arg;

          my_obj_t *myobj = buf;

          if ((myobj->my_buffer = umem_alloc(sz, flags)) == NULL)
                return (1);

          my_size = sz;

          return (0);
}

void
my_obj_destructor(void *buf, void *arg)
{
          size_t sz = (size_t)arg;

          my_obj_t *myobj = buf;

          assert(sz == buf->my_size);
          umem_free(myobj->my_buffer, sz);
}

\&...
umem_cache_t *my_obj_4k_cache;
umem_cache_t *my_obj_8k_cache;
\&...
my_obj_cache_4k = umem_cache_create("my_obj_4k", sizeof (my_obj_t),
         0, my_obj_constructor, my_obj_destructor, NULL,
         (void *)4096, NULL, 0);

my_obj_cache_8k = umem_cache_create("my_obj_8k", sizeof (my_obj_t),
         0, my_obj_constructor, my_obj_destructor, NULL,
         (void *)8192, NULL, 0);
\&...
my_obj_t *my_obj_4k = umem_cache_alloc(my_obj_4k_cache,
         UMEM_DEFAULT);
my_obj_t *my_obj_8k = umem_cache_alloc(my_obj_8k_cache,
         UMEM_DEFAULT);
/* no assumptions should be made about the contents
of the buffers */
\&...
/* make sure to return them to the correct cache */
umem_cache_free(my_obj_4k_cache, my_obj_4k);
umem_cache_free(my_obj_8k_cache, my_obj_8k);
\&...
.fi
.in -2

.sp
.LP
See the \fBEXAMPLES\fR section of \fBumem_alloc\fR(3MALLOC) for examples
involving the \fBUMEM_NOFAIL\fR flag.
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.sp
.LP
\fBsetcontext\fR(2), \fBatexit\fR(3C), \fBlibumem\fR(3LIB), \fBlongjmp\fR(3C),
\fBswapcontext\fR(3C), \fBthr_exit\fR(3C), \fBumem_alloc\fR(3MALLOC),
\fBumem_debug\fR(3MALLOC), \fBattributes\fR(5)
.sp
.LP
Bonwick, Jeff, "The Slab Allocator: An Object-Caching Kernel Memory Allocator",
Proceedings of the Summer 1994 Usenix Conference.
.sp
.LP
Bonwick, Jeff and Jonathan Adams, "Magazines and vmem: Extending the Slab
Allocator to Many CPUs and Arbitrary Resources", Proceedings of the Summer 2001
Usenix Conference.
.SH WARNINGS
.sp
.LP
Any of the following can cause undefined results:
.RS +4
.TP
.ie t \(bu
.el o
Destroying a cache that has outstanding allocated buffers.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Using a cache after it has been destroyed.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Calling \fBumem_cache_free()\fR on the same buffer multiple times.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Passing a \fINULL\fR pointer to \fBumem_cache_free()\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Writing past the end of a buffer.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Reading from or writing to a buffer after it has been freed.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Performing \fBUMEM_NOFAIL\fR allocations from an \fBatexit\fR(3C) handler.
.RE
.sp
.LP
Per-cache callbacks can be called from a variety of contexts. The use of
functions that modify the active context, such as \fBsetcontext\fR(2),
\fBswapcontext\fR(3C), and \fBthr_exit\fR(3C), or functions that are unsafe for
use in multithreaded applications, such as \fBlongjmp\fR(3C) and
\fBsiglongjmp\fR(3C), result in undefined behavior.
.sp
.LP
A constructor callback that performs allocations must pass its \fIflags\fR
argument unchanged to \fBumem_alloc\fR(3MALLOC) and \fBumem_cache_alloc()\fR.
Any allocations made with a different flags argument results in undefined
behavior.  The constructor must correctly handle the failure of any allocations
it makes.
.SH NOTES
.sp
.LP
Object caches make the following guarantees about objects:
.RS +4
.TP
.ie t \(bu
.el o
If the cache has a constructor callback, it is applied to every object before
it is returned from \fBumem_cache_alloc()\fR for the first time.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If the cache has a constructor callback, an object passed to
\fBumem_cache_free()\fR and later returned from \fBumem_cache_alloc()\fR is not
modified between the two events.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If the cache has a destructor, it is applied to all objects before their
underlying storage is returned.
.RE
.sp
.LP
No other guarantees are made. In particular, even if there are buffers recently
freed to the cache, \fBumem_cache_alloc()\fR can fail.