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