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CFAllocator Reference

Derived from
Framework
CoreFoundation/CoreFoundation.h
Companion guide
Declared in
CFBase.h

Overview

CFAllocator is an opaque type that allocates and deallocates memory for you. You never have to allocate, reallocate, or deallocate memory directly for Core Foundation objects—and rarely should you. You pass CFAllocator objects into functions that create objects; these functions have “Create” embedded in their names, for example, CFStringCreateWithPascalString. The creation functions use the allocators to allocate memory for the objects they create.

Functions by Task

Creating an Allocator

Managing Memory with an Allocator

Getting and Setting the Default Allocator

Getting an Allocator's Context

Getting the CFAllocator Type ID

Functions

CFAllocatorAllocate

Allocates memory using the specified allocator.

void * CFAllocatorAllocate (
   CFAllocatorRef allocator,
   CFIndex size,
   CFOptionFlags hint
);

Parameters
allocator

The allocator to use to allocate the memory. Pass NULL or kCFAllocatorDefault to use the current default allocator.

size

The size of the memory to allocate.

hint

A bitfield containing flags that suggest how memory is to be allocated. 0 indicates no hints. No hints are currently defined, so only 0 should be passed for this value.

Return Value

A pointer to the newly allocated memory.

Availability
Related Sample Code
Declared In
CFBase.h

CFAllocatorCreate

Creates an allocator object.

CFAllocatorRef CFAllocatorCreate (
   CFAllocatorRef allocator,
   CFAllocatorContext *context
);

Parameters
allocator

The existing allocator to use to allocate memory for the new allocator. Pass the kCFAllocatorUseContext constant for this parameter to allocate memory using the appropriate function callback specified in the context parameter. Pass NULL or kCFAllocatorDefault to allocate memory for the new allocator using the default allocator.

context

A structure of type CFAllocatorContext. The fields of this structure hold (among other things) function pointers to callbacks used for allocating, reallocating, and deallocating memory.

Return Value

The new allocator object, or NULL if there was a problem allocating memory. Ownership follows the Create Rule.

Discussion

You use this function to create custom allocators which you can then pass into various Core Foundation object-creation functions. You must implement a function callback that allocates memory and assign it to the allocate field of this structure. You typically also implement deallocate, reallocate, and preferred-size callbacks.

Availability
Declared In
CFBase.h

CFAllocatorDeallocate

Deallocates a block of memory with a given allocator.

void CFAllocatorDeallocate (
   CFAllocatorRef allocator,
   void *ptr
);

Parameters
allocator

The allocator that was used to allocate the block of memory pointed to by ptr.

ptr

An untyped pointer to a block of memory to deallocate using allocator.

Discussion

If the allocator does not specify a deallocate callback function, the memory is not deallocated.

Special Considerations

You must use the same allocator to deallocate memory as was used to allocate it.

Availability
Related Sample Code
Declared In
CFBase.h

CFAllocatorGetContext

Obtains the context of the specified allocator or of the default allocator.

void CFAllocatorGetContext (
   CFAllocatorRef allocator,
   CFAllocatorContext *context
);

Parameters
allocator

The allocator to examine. Pass NULL to obtain the context of the default allocator.

context

On return, contains the context of allocator.

Discussion

An allocator's context, a structure of type CFAllocatorContext, holds pointers to various function callbacks (particularly those that allocate, reallocate, and deallocate memory for an object). The context also contains a version number and the info field for program-defined data. To obtain the value of the info field you usually first have to get an allocator's context.

Availability
Declared In
CFBase.h

CFAllocatorGetDefault

Gets the default allocator object for the current thread.

CFAllocatorRef CFAllocatorGetDefault (
   void
);

Return Value

A reference to the default allocator for the current thread. If none has been explicitly set, returns the generic system allocator, kCFAllocatorSystemDefault. Ownership follows the Get Rule.

Discussion

See the discussion for CFAllocatorSetDefault for more detail on the default allocator and for advice on how and when to set a custom allocator as the default.

Availability
Related Sample Code
Declared In
CFBase.h

CFAllocatorGetPreferredSizeForSize

Obtains the number of bytes likely to be allocated upon a specific request.

CFIndex CFAllocatorGetPreferredSizeForSize (
   CFAllocatorRef allocator,
   CFIndex size,
   CFOptionFlags hint
);

Parameters
allocator

The allocator to use, or NULL for the default allocator.

size

The number of bytes to allocate. If the value is 0 or less, the result is the same value.

hint

A bitfield of type CFOptionsFlags. Pass flags to the allocator that suggest how memory is to be allocated. 0 indicates no hints. No hints are currently defined, only 0 should be passed for this argument.

Return Value

The number of bytes likely to be allocated upon a specific request.

Discussion

The return value depends on the allocator's internal allocation strategy, and will be equal to or larger than size. Calling this function may help you better match your memory allocation or reallocation strategy to that of the allocator.

Note that the return value depends on the internal implementation of the allocator and the results may change from release to release or from platform to platform.

If no function callback is assigned to the preferredSize field of the allocator's context (see the CFAllocatorContext structure), then the value of size is returned.

Availability
Declared In
CFBase.h

CFAllocatorGetTypeID

Returns the type identifier for the CFAllocator opaque type.

CFTypeID CFAllocatorGetTypeID (
   void
);

Return Value

The type identifier for the CFAllocator opaque type.

Availability
Declared In
CFBase.h

CFAllocatorReallocate

Reallocates memory using the specified allocator.

void * CFAllocatorReallocate (
   CFAllocatorRef allocator,
   void *ptr,
   CFIndex newsize,
   CFOptionFlags hint
);

Parameters
allocator

The allocator to use for reallocating memory. Pass NULL to request the default allocator.

ptr

An untyped pointer to a block of memory to reallocate to a new size. If ptr is NULL and newsize is greater than 0, memory is allocated (using the allocate function callback of the allocator's context). If ptr is NULL and newsize is 0, the result is NULL.

newsize

The number of bytes to allocate. If you pass 0 and the ptr parameter is non-NULL, the block of memory that ptr points to is typically deallocated. If you pass 0 for this parameter and the ptr parameter is NULL, nothing happens and the result returned is NULL.

hint

A bitfield of type CFOptionsFlags. Pass flags to the allocator that suggest how memory is to be allocated. Zero indicates no hints. No hints are currently defined, only 0 should be passed for this argument.

Discussion

The CFAllocatorReallocate function's primary purpose is to reallocate a block of memory to a new (and usually larger) size. However, based on the values passed in certain of the parameters, this function can also allocate memory afresh or deallocate a given block of memory. The following summarizes the semantic combinations:

The result of the CFAllocatorReallocate function is either an untyped pointer to a block of memory or NULL. A NULL result indicates either a failure to allocate memory or some other outcome, the precise interpretation of which is determined by the values of certain parameters and the presence or absence of callbacks in the allocator context. To summarize, a NULL result can mean one of the following:

Availability
Declared In
CFBase.h

CFAllocatorSetDefault

Sets the given allocator as the default for the current thread.

void CFAllocatorSetDefault (
   CFAllocatorRef allocator
);

Parameters
allocator

The allocator to set as the default for the current thread.

Discussion

The CFAllocatorSetDefault function sets the allocator that is used in the current thread whenever NULL is specified as an allocator argument. Generally, most allocations use the default allocator. Because of this, the default allocator must be prepared to deal with arbitrary memory-allocation requests. In addition, the size and number of requests can change between releases.

A further characteristic of the default allocator is that it can never be released, even if another allocator replaces it as the default. Not only is it impractical to release a default allocator (because there might be caches created somewhere that refer to the allocator) but it is generally safer and more efficient to keep it around.

If you wish to use a custom allocator in a context, the best approach is to specify it in the first parameter of creation functions rather than to set it as the default. Generally, setting the default allocator is not encouraged. If you do set an allocator as the default, either do it for the life time of your application or do it in a nested fashion (that is, restore the previous allocator before you exit your context). The latter approach might be more appropriate for plug-ins or libraries that wish to set the default allocator.

Availability
Declared In
CFBase.h

Callbacks

CFAllocatorAllocateCallBack

A prototype for a function callback that allocates memory of a requested size.

typedef void *(*CFAllocatorAllocateCallBack) (
   CFIndex allocSize,
   CFOptionFlags hint,
   void *info
);

If you name your function MyCallBack, you would declare it like this:

void *MyCallBack (
   CFIndex allocSize,
   CFOptionFlags hint,
   void *info
);

Parameters
allocSize

This function allocates a block of memory of at least allocSize bytes (always greater than 0).

hint

A bitfield that is currently not used (always set to 0).

info

An untyped pointer to program-defined data. Allocate memory for the data and assign a pointer to it. This data is often control information for the allocator. It may be NULL.

Return Value

A pointer to the start of the block.

Availability
Declared In
CFBase.h

CFAllocatorCopyDescriptionCallBack

A prototype for a function callback that provides a description of the specified data.

typedef CFStringRef (*CFAllocatorCopyDescriptionCallBack) (
   const void *info
);

If you name your function MyCallBack, you would declare it like this:

CFStringRef MyCallBack (
   const void *info
);

Parameters
info

An untyped pointer to program-defined data.

Return Value

A CFString object that describes the allocator. The caller is responsible for releasing this object.

Discussion

A prototype for a function callback that provides a description of the data pointed to by the info field. In implementing this function, return a reference to a CFString object that describes your allocator, particularly some characteristics of your program-defined data.

Availability
Declared In
CFBase.h

CFAllocatorDeallocateCallBack

A prototype for a function callback that deallocates a block of memory.

typedef void (*CFAllocatorDeallocateCallBack) (
   void *ptr,
   void *info
);

If you name your function MyCallBack, you would declare it like this:

void MyCallBack (
   void *ptr,
   void *info
);

Parameters
ptr

The block of memory to deallocate.

info

An untyped pointer to program-defined data.

Discussion

A prototype for a function callback that deallocates a given block of memory. In implementing this function, make the block of memory pointed to by ptr available for subsequent reuse by the allocator but unavailable for continued use by the program. The ptr parameter cannot be NULL and if the ptr parameter is not a block of memory that has been previously allocated by the allocator, the results are undefined; abnormal program termination can occur.

Availability
Declared In
CFBase.h

CFAllocatorPreferredSizeCallBack

A prototype for a function callback that gives the size of memory likely to be allocated, given a certain request.

typedef CFIndex (*CFAllocatorPreferredSizeCallBack) (
   CFIndex size,
   CFOptionFlags hint,
   void *info
);

If you name your function MyCallBack, you would declare it like this:

CFIndex MyCallBack (
   CFIndex size,
   CFOptionFlags hint,
   void *info
);

Parameters
size

The amount of memory requested.

hint

A bitfield that is currently not used (always set to 0).

info

An untyped pointer to program-defined data.

Return Value

The actual size the allocator is likely to allocate given this request.

Discussion

A prototype for a function callback that determines whether there is enough free memory to satisfy a request. In implementing this function, return the actual size the allocator is likely to allocate given a request for a block of memory of size size. The hint argument is a bitfield that you should currently not use.

Availability
Declared In
CFBase.h

CFAllocatorReallocateCallBack

A prototype for a function callback that reallocates memory of a requested size for an existing block of memory.

typedef void *(*CFAllocatorReallocateCallBack) (
   void *ptr,
   CFIndex newsize,
   CFOptionFlags hint,
   void *info
);

If you name your function MyCallBack, you would declare it like this:

void *MyCallBack (
   void *ptr,
   CFIndex newsize,
   CFOptionFlags hint,
   void *info
);

Parameters
ptr

The block of memory to resize.

newsize

The size of the new allocation.

hint

A bitfield that is currently not used (always set to 0).

info

An untyped pointer to program-defined data.

Return Value

Pointer to the new block of memory.

Discussion

In implementing this function, change the size of the block of memory pointed to by ptr to the size specified by newsize and return the pointer to the larger block of memory. Return NULL on any reallocation failure, leaving the old block of memory untouched. Also return NULL immediately if any of the following conditions if the ptr parameter is NULL or the newsize parameter is not greater than 0. Leave the contents of the old block of memory unchanged up to the lesser of the new or old sizes. If the ptr parameter is not a block of memory that has been previously allocated by the allocator, the results are undefined; abnormal program termination can occur. The hint argument is a bitfield that you should currently not use (that is, assign 0).

Availability
Declared In
CFBase.h

CFAllocatorReleaseCallBack

A prototype for a function callback that releases the given data.

typedef void (*CFAllocatorReleaseCallBack) (
   const void *info
);

If you name your function MyCallBack, you would declare it like this:

void MyCallBack (
   const void *info
);

Parameters
info

The data to be released.

Discussion

A prototype for a function callback that releases the data pointed to by the info field. In implementing this function, release (or free) the data you have defined for the allocator context.

Availability
Declared In
CFBase.h

CFAllocatorRetainCallBack

A prototype for a function callback that retains the given data.

typedef const void *(*CFAllocatorRetainCallBack) (
   const void *info
);

If you name your function MyCallBack, you would declare it like this:

const void *MyCallBack (
   const void *info
);

Parameters
info

The data to be retained.

Discussion

A prototype for a function callback that retains the data pointed to by the info field. In implementing this function, retain the data you have defined for the allocator context in this field. (This might make sense only if the data is a Core Foundation object.)

Availability
Declared In
CFBase.h

Data Types

CFAllocatorContext

A structure that defines the context or operating environment for an allocator (CFAllocator) object. Every Core Foundation allocator object must have a context defined for it.

struct CFAllocatorContext {
   CFIndex version;
   void *info;
   CFAllocatorRetainCallBack retain;
   CFAllocatorReleaseCallBack release;
   CFAllocatorCopyDescriptionCallBack copyDescription;
   CFAllocatorAllocateCallBack allocate;
   CFAllocatorReallocateCallBack reallocate;
   CFAllocatorDeallocateCallBack deallocate;
   CFAllocatorPreferredSizeCallBack preferredSize;
};
typedef struct CFAllocatorContext CFAllocatorContext;

Fields
version

An integer of type CFIndex. Assign the version number of the allocator. Currently the only valid value is 0.

info

An untyped pointer to program-defined data. Allocate memory for this data and assign a pointer to it. This data is often control information for the allocator. You may assign NULL.

retain

A prototype for a function callback that retains the data pointed to by the info field. In implementing this function, retain the data you have defined for the allocator context in this field. (This might make sense only if the data is a Core Foundation object.) You may set this function pointer to NULL.

release

A prototype for a function callback that releases the data pointed to by the info field. In implementing this function, release (or free) the data you have defined for the allocator context. You may set this function pointer to NULL, but doing so might result in memory leaks.

copyDescription

A prototype for a function callback that provides a description of the data pointed to by the info field. In implementing this function, return a reference to a CFString object that describes your allocator, particularly some characteristics of your program-defined data. You may set this function pointer to NULL, in which case Core Foundation will provide a rudimentary description.

allocate

A prototype for a function callback that allocates memory of a requested size. In implementing this function, allocate a block of memory of at least size bytes and return a pointer to the start of the block. The hint argument is a bitfield that you should currently not use (that is, assign 0). The size parameter should always be greater than 0. If it is not, or if problems in allocation occur, return NULL. This function pointer may not be assigned NULL.

reallocate

A prototype for a function callback that reallocates memory of a requested size for an existing block of memory. In implementing this function, change the size of the block of memory pointed to by ptr to the size specified by newsize and return the pointer to the larger block of memory. Return NULL on any reallocation failure, leaving the old block of memory untouched. Also return NULL immediately if any of the following conditions apply:

  • The ptr parameter is NULL.

  • The newsize parameter is not greater than 0.

    Leave the contents of the old block of memory unchanged up to the lesser of the new or old sizes. If the ptr parameter is not a block of memory that has been previously allocated by the allocator, the results are undefined; abnormal program termination can occur. The hint argument is a bitfield that you should currently not use (that is, assign 0). If you set this callback to NULL the CFAllocatorReallocate function returns NULL in most cases when it attempts to use this allocator.

deallocate

A prototype for a function callback that deallocates a given block of memory. In implementing this function, make the block of memory pointed to by ptr available for subsequent reuse by the allocator but unavailable for continued use by the program. The ptr parameter cannot be NULL and if the ptr parameter is not a block of memory that has been previously allocated by the allocator, the results are undefined; abnormal program termination can occur. You can set this callback to NULL, in which case the CFAllocatorDeallocate function has no effect.

preferredSize

A prototype for a function callback that determines whether there is enough free memory to satisfy a request. In implementing this function, return the actual size the allocator is likely to allocate given a request for a block of memory of size size. The hint argument is a bitfield that you should currently not use.

Discussion

See the “Memory Management” topic for information on creating a custom CFAllocator object and, as part of that procedure, the steps for creating a properly initialized CFAllocatorContext structure.

Availability
Declared In
CFBase.h

CFAllocatorRef

A reference to a CFAllocator object. 

typedef const struct __CFAllocator *CFAllocatorRef;

Discussion

The CFAllocatorRef type is a reference type used in many Core Foundation parameters and function results. It refers to a CFAllocator object, which allocates, reallocates, and deallocates memory for Core Foundation objects.

Availability
Declared In
CFBase.h

Constants

Predefined Allocators

CFAllocator provides the following predefined allocators. In general, you should use kCFAllocatorDefault unless one of the special circumstances exist below.

const CFAllocatorRef kCFAllocatorDefault;
const CFAllocatorRef kCFAllocatorSystemDefault;
const CFAllocatorRef kCFAllocatorMalloc;
const CFAllocatorRef kCFAllocatorMallocZone;
const CFAllocatorRef kCFAllocatorNull;
const CFAllocatorRef kCFAllocatorUseContext;

Constants
kCFAllocatorDefault

This is a synonym for NULL.

Available in Mac OS X v10.0 and later.

Declared in CFBase.h.

kCFAllocatorSystemDefault

Default system allocator.

You rarely need to use this.

Available in Mac OS X v10.0 and later.

Declared in CFBase.h.

kCFAllocatorMalloc

This allocator uses malloc(), realloc(), and free().

Typically you should not use this allocator, use kCFAllocatorDefault instead. This allocator is useful as the bytesDeallocator in CFData or contentsDeallocator in CFString where the memory was obtained as a result of malloc type functions.

Available in Mac OS X v10.0 and later.

Declared in CFBase.h.

kCFAllocatorMallocZone

This allocator explicitly uses the default malloc zone, returned by malloc_default_zone().

You should only use this when an object is safe to be allocated in non-scanned memory.

Available in Mac OS X v10.4 and later.

Declared in CFBase.h.

kCFAllocatorNull

This allocator does nothing—it allocates no memory.

This allocator is useful as the bytesDeallocator in CFData or contentsDeallocator in CFString where the memory should not be freed.

Available in Mac OS X v10.0 and later.

Declared in CFBase.h.

kCFAllocatorUseContext

Special allocator argument to CFAllocatorCreate—it uses the functions given in the context to allocate the allocator.

Available in Mac OS X v10.0 and later.

Declared in CFBase.h.

Declared In
CFBase.h

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© 2003, 2006 Apple Computer, Inc. All Rights Reserved. (Last updated: 2006-12-08)

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