dispatch_read(3)         BSD Library Functions Manual         dispatch_read(3)

NAME
     dispatch_read, dispatch_write -- asynchronously read from and write to
     file descriptors

SYNOPSIS
     #include <dispatch/dispatch.h>

     void
     dispatch_read(int fd, size_t length, dispatch_queue_t queue,
         void (^handler)(dispatch_data_t data, int error));

     void
     dispatch_write(int fd, dispatch_data_t data, dispatch_queue_t queue,
         void (^handler)(dispatch_data_t data, int error)));

DESCRIPTION
     The dispatch_read() and dispatch_write() functions asynchronously read
     from and write to POSIX file descriptors. They can be thought of as asyn-
     chronous, callback-based versions of the fread() and fwrite() functions
     provided by the standard C library. They are convenience functions based
     on the dispatch_io_read(3) and dispatch_io_write(3) functions, intended
     for simple one-shot read or write requests. Multiple request on the same
     file desciptor are better handled with the full underlying dispatch I/O
     channel functions.

BEHAVIOR
     The dispatch_read() function schedules an asynchronous read operation on
     the file descriptor fd.  Once the file descriptor is readable, the system
     will read as much data as is currently available, up to the specified
     length, starting at the current file pointer position. The given handler
     block will be submitted to queue when the operation completes or an error
     occurs. The block will be passed a dispatch data object with the result
     of the read operation. If an error occurred while reading from the file
     descriptor, the error parameter to the block will be set to the appropri-
     ate POSIX error code and data will contain any data that could be read
     successfully. If the file pointer position is at end-of-file, emtpy data
     and zero error will be passed to the handler block.

     The dispatch_write() function schedules an asynchronous write operation
     on the file descriptor fd.  The system will attempt to write the entire
     contents of the provided data object to fd at the current file pointer
     position. The given handler block will be submitted to queue when the
     operation completes or an error occurs. If the write operation completed
     successfully, the error parameter to the block will be set to zero, oth-
     erwise it will be set to the appropriate POSIX error code and the data
     parameter will contain any data that could not be written.

CAVEATS
     The data object passed to a handler block is released by the system when
     the block returns. If data is needed outside of the handler block, it
     must concatenate, copy, or retain it.

     Once an asynchronous read or write operation has been submitted on a file
     descriptor fd, the system takes control of that file descriptor until the
     handler block is executed. During this time the application must not
     manipulate fd directly, in particular it is only safe to close fd from
     the handler block (or after it has returned).

     If multiple asynchronous read or write operations are submitted to the
     same file descriptor, they will be performed in order, but their handlers
     will only be submitted once all operations have completed and control
     over the file descriptor has been relinquished. For details on this and
     on the interaction with dispatch I/O channels created from the same file
     descriptor, see FILEDESCRIPTOR OWNERSHIP in dispatch_io_create(3).

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

Darwin                         December 1, 2010                         Darwin