dispatch_data_create(3) BSD Library Functions Manual dispatch_data_create(3)
NAME
dispatch_data_create, dispatch_data_create_concat,
dispatch_data_create_subrange, dispatch_data_create_map,
dispatch_data_apply, dispatch_data_copy_region, dispatch_data_get_size --
create and manipulate dispatch data objects
SYNOPSIS
#include <dispatch/dispatch.h>
dispatch_data_t
dispatch_data_create(const void* buffer, size_t size,
dispatch_queue_t queue, dispatch_block_t destructor);
dispatch_data_t
dispatch_data_create_concat(dispatch_data_t data1,
dispatch_data_t data2);
dispatch_data_t
dispatch_data_create_subrange(dispatch_data_t data, size_t offset,
size_t length);
dispatch_data_t
dispatch_data_create_map(dispatch_data_t data, const void **buffer_ptr,
size_t *size_ptr);
bool
dispatch_data_apply(dispatch_data_t data,
bool (^applier)(dispatch_data_t, size_t, const void *, size_t));
dispatch_data_t
dispatch_data_copy_region(dispatch_data_t data, size_t location,
size_t *offset_ptr);
size_t
dispatch_data_get_size(dispatch_data_t data);
dispatch_data_t dispatch_data_empty;
DESCRIPTION
Dispatch data objects are opaque containers of bytes that represent one
or more regions of memory. They are created either from memory buffers
managed by the application or the system or from other dispatch data
objects. Dispatch data objects are immutable and the memory regions they
represent are required to remain unchanged for the lifetime of all data
objects that reference them. Dispatch data objects avoid copying the
represented memory as much as possible. Multiple data objects can repre-
sent the same memory regions or subsections thereof.
CREATION
The dispatch_data_create() function creates a new dispatch data object of
given size from a buffer. The provided destructor block will be submit-
ted to the specified queue when the object reaches the end of its lifecy-
cle, indicating that the system no longer references the buffer. This
allows the application to deallocate the associated storage. The queue
argument is ignored if one of the following predefined destructors is
passed:
DISPATCH_DATA_DESTRUCTOR_FREE indicates that the provided
buffer can be deallocated with
free(3) directly.
DISPATCH_DATA_DESTRUCTOR_DEFAULT indicates that the provided
buffer is not managed by the
application and should be copied
into memory managed and automati-
cally deallocated by the system.
The dispatch_data_create_concat() function creates a new data object rep-
resenting the concatenation of the memory regions represented by the pro-
vided data objects.
The dispatch_data_create_subrange() function creates a new data object
representing the sub-region of the provided data object specified by the
offset and length parameters.
The dispatch_data_create_map() function creates a new data object by map-
ping the memory represented by the provided data object as a single con-
tiguous memory region (moving or copying memory as necessary). If the
buffer_ptr and size_ptr references are not NULL, they are filled with the
location and extent of the contiguous region, allowing direct read access
to the mapped memory. These values are valid only as long as the newly
created object has not been released.
ACCESS
The dispatch_data_apply() function provides read access to represented
memory without requiring it to be mapped as a single contiguous region.
It traverses the memory regions represented by the data argument in logi-
cal order, invokes the specified applier block for each region and
returns a boolean indicating whether traversal completed successfully.
The applier block is passed the following arguments for each memory
region and returns a boolean indicating whether traversal should con-
tinue:
dispatch_data_t rgn data object representing the region
size_t offset logical position of the region in data
const void *loc memory location of the region
size_t size extent of the region
The rgn data object is released by the system when the applier block
returns. The associated memory location loc is valid only as long as rgn
has not been deallocated; if loc is needed outside of the applier block,
the rgn object must be retained in the block.
The dispatch_data_copy_region() function finds the contiguous memory
region containing the logical position specified by the location argument
among the regions represented by the provided data object and returns a
newly created copy of the data object representing that region. The vari-
able specified by the offset_ptr argument is filled with the logical
position where the returned object starts in the data object.
The dispatch_data_get_size() function returns the logical size of the
memory region or regions represented by the provided data object.
EMPTY DATA OBJECT
The dispatch_data_empty object is the global singleton object represent-
ing a zero-length memory region. It is a valid input to any dis-
patch_data functions that take data object parameters.
MEMORY MODEL
Dispatch data objects are retained and released via calls to
dispatch_retain() and dispatch_release(). Data objects passed as argu-
ments to a dispatch data create or copy function can be released when the
function returns. The newly created object holds implicit references to
their constituent memory regions as necessary.
The functions dispatch_data_create_map() and dispatch_data_apply() return
an interior pointer to represented memory that is only valid as long as
an associated object has not been released. When Objective-C Automated
Reference Counting is enabled, care needs to be taken if that object is
held in a variable with automatic storage. It may need to be annotated
with the objc_precise_lifetime attribute, or stored in a __strong
instance variable instead, to ensure that the object is not released pre-
maturely before memory accesses via the interor pointer have been com-
pleted.
SEE ALSO
dispatch(3), dispatch_object(3), dispatch_io_read(3)
Darwin December 1, 2010 Darwin
Mac OS X 10.8 - Generated Sat Aug 25 07:16:14 CDT 2012