dispatch_io_read(3)      BSD Library Functions Manual      dispatch_io_read(3)

NAME
     dispatch_io_read, dispatch_io_write -- submit read and write operations
     to dispatch I/O channels

SYNOPSIS
     #include <dispatch/dispatch.h>

     void
     dispatch_io_read(dispatch_io_t channel, off_t offset, size_t length,
         dispatch_queue_t queue,
         void (^handler)(bool done, dispatch_data_t data, int error));

     void
     dispatch_io_write(dispatch_io_t channel, off_t offset,
         dispatch_data_t dispatch, dispatch_queue_t queue,
         void (^handler)(bool done, dispatch_data_t data, int error));

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 read from and write to dispatch I/O
     channels. Creation and configuration of these channels is covered in the
     dispatch_io_create(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
     The dispatch_io_read() and dispatch_io_write() functions are used to per-
     form asynchronous read and write operations on dispatch I/O channels.
     They can be thought of as asynchronous versions of the fread(3) and
     fwrite(3) functions in the standard C library.

READ OPERATIONS
     The dispatch_io_read() function schedules an I/O read operation on the
     specified dispatch I/O channel.  As results from the read operation
     become available, the provided handler block will be submitted to the
     specified queue.  The block will be passed a dispatch data object repre-
     senting the data that has been read since the handler's previous invoca-
     tion.

     The offset parameter indicates where the read operation should begin. For
     a channel of DISPATCH_IO_RANDOM type it is interpreted relative to the
     position of the file pointer when the channel was created, for a channel
     of DISPATCH_IO_STREAM type it is ignored and the read operation will
     begin at the current file pointer position.

     The length parameter indicates the number of bytes that should be read
     from the I/O channel. Pass SIZE_MAX to keep reading until EOF is encoun-
     tered (for a channel created from a disk-based file this happens when
     reading past the end of the physical file).

WRITE OPERATIONS
     The dispatch_io_write() function schedules an I/O write operation on the
     specified dispatch I/O channel.  As the write operation progresses, the
     provided handler block will be submitted to the specified queue.  The
     block will be passed a dispatch data object representing the data that
     remains to be written as part of this I/O operation.

     The offset parameter indicates where the write operation should begin. It
     is interpreted as for read operations above.

     The data parameter specifies the location and amount of data to be writ-
     ten, encapsulated as a dispatch data object. The object is retained by
     the system until the write operation is complete.

I/O HANDLER BLOCKS
     Dispatch I/O handler blocks submitted to a channel via the
     dispatch_io_read() or dispatch_io_write() functions will be executed one
     or more times depending on system load and the channel's configuration
     settings (see dispatch_io_create(3) for details). The handler block need
     not be reentrant safe, no new I/O handler instance is submitted until the
     previously enqueued handler block has returned.

     The dispatch data object passed to an I/O handler block will be released
     by the system when the block returns, if access to the memory buffer it
     represents is needed outside of the handler, the handler block must
     retain the data object or create a new (e.g. concatenated) data object
     from it (see dispatch_data_create(3) for details).

     Once an I/O handler block is invoked with the done flag set, the associ-
     ated I/O operation is complete and that handler block will not be run
     again. If an unrecoverable error occurs while performing the I/O opera-
     tion, the handler block will be submitted with the done flag set and the
     appriate POSIX error code in the error parameter. An invocation of a han-
     dler block with the done flag set, zero error and data set to
     dispatch_data_empty indicates that the I/O operation has encountered EOF.

SEE ALSO
     dispatch(3), dispatch_data_create(3), dispatch_io_create(3),
     dispatch_read(3), fread(3)

Darwin                         December 1, 2010                         Darwin