dispatch_io_create(3)    BSD Library Functions Manual    dispatch_io_create(3)

NAME
     dispatch_io_create, dispatch_io_create_with_path, dispatch_io_close,
     dispatch_io_set_high_water, dispatch_io_set_low_water,
     dispatch_io_set_interval, dispatch_io_barrier -- open, close and config-
     ure dispatch I/O channels

SYNOPSIS
     #include <dispatch/dispatch.h>

     dispatch_io_t
     dispatch_io_create(dispatch_io_type_t type, int fd,
         dispatch_queue_t queue, void (^cleanup_handler)(int error));

     dispatch_io_t
     dispatch_io_create_with_path(dispatch_io_type_t type, const char *path,
         int oflag, mode_t mode, dispatch_queue_t queue,
         void (^cleanup_handler)(int error));

     void
     dispatch_io_close(dispatch_io_t channel,
         dispatch_io_close_flags_t flags);

     void
     dispatch_io_set_high_water(dispatch_io_t channel, size_t high_water);

     void
     dispatch_io_set_low_water(dispatch_io_t channel, size_t low_water);

     void
     dispatch_io_set_interval(dispatch_io_t channel, uint64_t interval,
         dispatch_io_interval_flags_t flags);

     void
     dispatch_io_barrier(dispatch_io_t channel, void (^barrier)(void));

DESCRIPTION
     The dispatch I/O framework is an API for asynchronous read and write I/O
     operations. It is an application of the ideas and idioms present in the
     dispatch(3) framework to device I/O. Dispatch I/O enables an application
     to more easily avoid blocking I/O operations and allows it to more
     directly express its I/O requirements than by using the raw POSIX file
     API. Dispatch I/O will make a best effort to optimize how and when asyn-
     chronous I/O operations are performed based on the capabilities of the
     targeted device.

     This page provides details on how to create and configure dispatch I/O
     channels. Reading from and writing to these channels is covered in the
     dispatch_io_read(3) page. The dispatch I/O framework also provides the
     convenience functions dispatch_read(3) and dispatch_write(3) for uses
     that do not require the full functionality provided by I/O channels.

FUNDAMENTALS
     A dispatch I/O channel represents the asynchronous I/O policy applied to
     a file descriptor and encapsulates it for the purposes of ownership
     tracking while I/O operations are ongoing.

CHANNEL TYPES
     Dispatch I/O channels can have one of the following types:
           DISPATCH_IO_STREAM  channels that represent a stream of bytes and
                               do not support reads and writes at arbitrary
                               offsets, such as pipes or sockets. Channels of
                               this type perform read and write operations
                               sequentially at the current file pointer posi-
                               tion and ignore any offset specified. Depending
                               on the underlying file descriptor, read opera-
                               tions may be performed simultaneously with
                               write operations.
           DISPATCH_IO_RANDOM  channels that represent random access files on
                               disk. Only supported for seekable file descrip-
                               tors and paths. Channels of this type may per-
                               form submitted read and write operations con-
                               currently at the specified offset (interpreted
                               relative to the position of the file pointer
                               when the channel was created).

CHANNEL OPENING AND CLOSING
     The dispatch_io_create() and dispatch_io_create_with_path() functions
     create a dispatch I/O channel of provided type from a file descriptor fd
     or an absolute pathname, respectively. They can be thought of as analo-
     gous to the fdopen(3) POSIX function and the fopen(3) function in the
     standard C library. For a channel created from a pathname, the provided
     path, oflag and mode parameters will be passed to open(2) when the first
     I/O operation on the channel is ready to execute.

     The provided cleanup_handler block will be submitted to the specified
     queue when all I/O operations on the channel have completed and it is
     closed or reaches the end of its lifecycle. If an error occurs during
     channel creation, the cleanup_handler block will be submitted immediately
     and passed an error parameter with the POSIX error encountered. If an
     invalid type or a non-absolute path argument is specified, these func-
     tions will return NULL and the cleanup_handler will not be invoked. After
     successfully creating a dispatch I/O channel from a file descriptor, the
     application must take care not to modify that file descriptor until the
     associated cleanup_handler is invoked, see FILEDESCRIPTOR OWNERSHIP for
     details.

     The dispatch_io_close() function closes a dispatch I/O channel to new
     submissions of I/O operations. If DISPATCH_IO_STOP is passed in the flags
     parameter, the system will in addition not perform the I/O operations
     already submitted to the channel that are still pending and will make a
     best effort to interrupt any ongoing operations. Handlers for operations
     so affected will be passed the ECANCELED error code, along with any par-
     tial results.

CHANNEL CONFIGURATION
     Dispatch I/O channels have high-water mark, low-water mark and interval
     configuration settings that determine if and when partial results from
     I/O operations are delivered via their associated I/O handlers.

     The dispatch_io_set_high_water() and dispatch_io_set_low_water() func-
     tions configure the water mark settings of a channel.  The system will
     read or write at least the number of bytes specified by low_water before
     submitting an I/O handler with partial results, and will make a best
     effort to submit an I/O handler as soon as the number of bytes read or
     written reaches high_water.

     The dispatch_io_set_interval() function configures the time interval at
     which I/O handlers are submitted (measured in nanoseconds). If
     DISPATCH_IO_STRICT_INTERVAL is passed in the flags parameter, the inter-
     val will be strictly observed even if there is an insufficient amount of
     data to deliver; otherwise delivery will be skipped for intervals where
     the amount of available data is inferior to the channel's low-water mark.
     Note that the system may defer enqueueing interval I/O handlers by a
     small unspecified amount of leeway in order to align with other system
     activity for improved system performance or power consumption.

DATA DELIVERY
     The size of data objects passed to I/O handlers for a channel will never
     be larger than the high-water mark set on the channel; it will also never
     be smaller than the low-water mark, except in the following cases:
           -   the final handler invocation for an I/O operation
           -   EOF was encountered
           -   the channel has an interval with the
               DISPATCH_IO_STRICT_INTERVAL flag set
     Bear in mind that dispatch I/O channels will typically deliver amounts of
     data significantly higher than the low-water mark. The default value for
     the low-water mark is unspecified, but must be assumed to allow interme-
     diate handler invocations. The default value for the high-water mark is
     unlimited (i.e. SIZE_MAX).  Channels that require intermediate results of
     fixed size should have both the low-water and the high-water mark set to
     that size. Channels that do not wish to receive any intermediate results
     should have the low-water mark set to SIZE_MAX.

FILEDESCRIPTOR OWNERSHIP
     When an application creates a dispatch I/O channel from a file descriptor
     with the dispatch_io_create() function, the system takes control of that
     file descriptor until the channel is closed, an error occurs on the file
     descriptor or all references to the channel are released. At that time
     the channel's cleanup handler will be enqueued and control over the file
     descriptor relinquished, making it safe for the application to close(2)
     the file descriptor. While a file descriptor is under the control of a
     dispatch I/O channel, file descriptor flags such as O_NONBLOCK will be
     modified by the system on behalf of the application. It is an error for
     the application to modify a file descriptor directly while it is under
     the control of a dispatch I/O channel, but it may create further I/O
     channels from that file descriptor or use the dispatch_read(3) and
     dispatch_write(3) convenience functions with that file descriptor. If
     multiple I/O channels have been created from the same file descriptor,
     all the associated cleanup handlers will be submitted together once the
     last channel has been closed resp. all references to those channels have
     been released. If convenience functions have also been used on that file
     descriptor, submission of their handlers will be tied to the submission
     of the channel cleanup handlers as well.

BARRIER OPERATIONS
     The dispatch_io_barrier() function schedules a barrier operation on an
     I/O channel. The specified barrier block will be run once, after all cur-
     rent I/O operations (such as read(2) or write(2)) on the underlying file
     descriptor have finished. No new I/O operations will start until the bar-
     rier block finishes.

     The barrier block may operate on the underlying file descriptor with
     functions like fsync(2) or lseek(2).  As discussed in the FILEDESCRIPTOR
     OWNERSHIP section, the barrier block must not close(2) the file descrip-
     tor, and if it changes any flags on the file descriptor, it must restore
     them before finishing.

     There is no synchronization between a barrier block and any
     dispatch_io_read(3) or dispatch_io_write(3) handler blocks; they may be
     running at the same time. The barrier block itself is responsible for any
     required synchronization.

MEMORY MODEL
     Dispatch I/O channel objects are retained and released via calls to
     dispatch_retain() and dispatch_release().

SEE ALSO
     dispatch(3), dispatch_io_read(3), dispatch_object(3), dispatch_read(3),
     fopen(3), open(2)

Darwin                         December 1, 2010                         Darwin