Understanding the Linux Kernel, 3rd Edition

Asynchronous I/O

The POSIX 1003.1 standard defines a set of library functions—listed in Table 16-4—for accessing the files in an asynchronous way. “Asynchronous” essentially means that when a User Mode process invokes a library function to read or write a file, the function terminates as soon as the read or write operation has been enqueued, possibly even before the actual I/O data transfer takes place. The calling process can thus continue its execution while the data is being transferred.

Table 16-4. The POSIX library functions for asynchronous I/O

Function	Description
aio_read( )	Asynchronously reads some data from a file
aio_write( )	Asynchronously writes some data into a file
aio_fsync( )	Requests a flush operation for all outstanding asynchronous I/O operations (does not block)
aio_error( )	Gets the error code for an outstanding asynchronous I/O operation
aio_return( )	Gets the return code for a completed asynchronous I/O operation
aio_cancel( )	Cancels an outstanding asynchronous I/O operation
aio_suspend( )	Suspends the process until at least one of several outstanding I/O operations completes

Using asynchronous I/O is quite simple. The application opens the file by means of the usual open( ) system call. Then, it fills up a control block of type struct aiocb with the information describing the requested operation. The most commonly used fields of the struct aiocb control block are:

aio_fildes

The file descriptor of the file (as returned by the open( ) system call) ...