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 ALSOdispatch(3), dispatch_data_create(3), dispatch_io_create(3),
dispatch_io_read(3), fread(3)Darwin December 1, 2010 Darwin
