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OSX 10.14 Mojave - man page for dispatch_io_read (mojave section 3)

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

dispatch_io_read, dispatch_io_write -- submit read and write operations to dispatch I/O channels
#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));
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 asynchronous 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.
The dispatch_io_read() and dispatch_io_write() functions are used to perform 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.
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 rep- resenting the data that has been read since the handler's previous invocation. 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 encountered (for a channel created from a disk-based file this happens when reading past the end of the physical file).
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 written, encapsulated as a dispatch data object. The object is retained by the system until the write operation is complete.
/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 associated I/O operation is complete and that handler block will not be run again. If an unrecoverable error occurs while performing the I/O operation, 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 handler block with the done flag set, zero error and data set to dispatch_data_empty indicates that the I/O operation has encountered EOF.
dispatch(3), dispatch_data_create(3), dispatch_io_create(3), dispatch_read(3), fread(3) Darwin December 1, 2010 Darwin