.\" Copyright (c) 2010 Apple Inc. All rights reserved. .Dd December 1, 2010 .Dt dispatch_read 3 .Os Darwin .Sh NAME .Nm dispatch_read , .Nm dispatch_write .Nd asynchronously read from and write to file descriptors .Sh SYNOPSIS .Fd #include .Ft void .Fo dispatch_read .Fa "int fd" .Fa "size_t length" .Fa "dispatch_queue_t queue" .Fa "void (^handler)(dispatch_data_t data, int error)" .Fc .Ft void .Fo dispatch_write .Fa "int fd" .Fa "dispatch_data_t data" .Fa "dispatch_queue_t queue" .Fa "void (^handler)(dispatch_data_t data, int error))" .Fc .Sh DESCRIPTION The .Fn dispatch_read and .Fn dispatch_write functions asynchronously read from and write to POSIX file descriptors. They can be thought of as asynchronous, callback-based versions of the .Fn fread and .Fn fwrite functions provided by the standard C library. They are convenience functions based on the .Xr dispatch_io_read 3 and .Xr 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. .Sh BEHAVIOR The .Fn dispatch_read function schedules an asynchronous read operation on the file descriptor .Va fd . Once the file descriptor is readable, the system will read as much data as is currently available, up to the specified .Va length , starting at the current file pointer position. The given .Va handler block will be submitted to .Va queue when the operation completes or an error occurs. The block will be passed a dispatch .Va data object with the result of the read operation. If an error occurred while reading from the file descriptor, the .Va error parameter to the block will be set to the appropriate POSIX error code and .Va data will contain any data that could be read successfully. If the file pointer position is at end-of-file, emtpy .Va data and zero .Va error will be passed to the handler block. .Pp The .Fn dispatch_write function schedules an asynchronous write operation on the file descriptor .Va fd . The system will attempt to write the entire contents of the provided .Va data object to .Va fd at the current file pointer position. The given .Va handler block will be submitted to .Va queue when the operation completes or an error occurs. If the write operation completed successfully, the .Va error parameter to the block will be set to zero, otherwise it will be set to the appropriate POSIX error code and the .Va data parameter will contain any data that could not be written. .Sh CAVEATS The .Va data object passed to a .Va handler block is released by the system when the block returns. If .Va data is needed outside of the handler block, it must concatenate, copy, or retain it. .Pp Once an asynchronous read or write operation has been submitted on a file descriptor .Va fd , the system takes control of that file descriptor until the .Va handler block is executed. During this time the application must not manipulate .Va fd directly, in particular it is only safe to close .Va fd from the handler block (or after it has returned). .Pp 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 .Sx FILEDESCRIPTOR OWNERSHIP in .Xr dispatch_io_create 3 . .Sh SEE ALSO .Xr dispatch 3 , .Xr dispatch_data_create 3 , .Xr dispatch_io_create 3 , .Xr dispatch_io_read 3 , .Xr fread 3