AIO(3) BSD Library Functions Manual AIO(3)NAMEaio — asynchronous I/O (REALTIME)
LIBRARY
POSIX Real-time Library (librt, -lrt)
SYNOPSIS
#include <aio.h>
DESCRIPTION
The IEEE Std 1003.1-2001 (“POSIX.1”) standard defines an interface for
asynchronous input and output. Although in NetBSD this is provided as
part of the POSIX Real-time Library (librt, -lrt), the implementation
largely resides in the kernel.
Rationale
The rationale can be roughly summarized with the following points.
1. To increase performance by providing a mechanism to carry out I/O
without blocking. Theoretically, if I/O would never block, nei‐
ther at the software nor at the hardware level, the overhead of
I/O would become zero, and processes would no longer be I/O bound.
2. To segregate the different I/O operations into logically distinc‐
tive procedures. Unlike with the standard stdio(3), the aio
interface separates queuing and submitting I/O operations to the
kernel, and receiving notifications of operation completion from
the kernel.
3. To provide an uniform and standardized framework for asynchronous
I/O. For instance, aio avoids the need for (and the overhead of)
extra worker threads sometimes used to perform asynchronous I/O.
Asynchronous I/O Control Block
The Asynchronous I/O Control Block is the basic operational unit behind
aio. This is required since an arbitrary number of operations can be
started at once, and because each operation can be either input or out‐
put. This block is represented by the aiocb structure, which is defined
in the <aio.h> header. The following fields are available for user
applications:
off_t aio_offset;
void *aio_buf;
size_t aio_nbytes;
int aio_fildes;
int aio_lio_opcode;
int aio_reqprio;
struct sigevent aio_sigevent;
The fields are:
1. The aio_offset specifies the implicit file offset at which the
I/O operations are performed. This cannot be expected to be
the actual read/write offset of the file descriptor.
2. The aio_buf member is a pointer to the buffer to which data is
going to be written or to which the read operation stores
data.
3. The aio_nbytes specifies the length of aio_buf.
4. The aio_fildes specifies the used file descriptor.
5. The aio_lio_opcode is used by the lio_listio() function to
initialize a list of I/O requests with a single call.
6. The aio_reqprio member can be used to lower the scheduling
priority of an aio operation. This is only available if
_POSIX_PRIORITIZED_IO and _POSIX_PRIORITY_SCHEDULING are
defined, and the associated file descriptor supports it.
7. The aio_sigevent member is used to specify how the calling
process is notified once an aio operation completes.
The members aio_buf, aio_fildes, and aio_nbytes are conceptually similar
to the parameters ‘buf’, ‘fildes’, and ‘nbytes’ used in the standard
read(2) and write(2) functions. For example, the caller can read
aio_nbytes from a file associated with the file descriptor aio_fildes
into the buffer aio_buf. All appropriate fields should be initialized by
the caller before aio_read() or aio_write() is called.
File Offsets
Asynchronous I/O operations are not strictly sequential; operations are
carried out in arbitrary order and more than one operation for one file
descriptor can be started. The requested read or write operation starts
from the absolute position specified by aio_offset, as if lseek(2) would
have been called with SEEK_SET immediately prior to the operation. The
POSIX standard does not specify what happens after an aio operation has
been successfully completed. Depending on the implementation, the actual
file offset may or may not be updated.
Errors and Completion
Asynchronous I/O operations are said to be complete when:
· An error is detected.
· The I/O transfer is performed successfully.
· The operation is canceled.
If an error condition is detected that prevents an operation from being
started, the request is not enqueued. In this case the read and write
functions, aio_read() and aio_write(), return immediately, setting the
global errno to indicate the cause of the error.
After an operation has been successfully enqueued, aio_error() and
aio_return() must be used to determine the status of the operation and to
determine any error conditions. This includes the conditions reported by
the standard read(2), write(2), and fsync(2). The request remains
enqueued and consumes process and system resources until aio_return() is
called.
Waiting for Completion
The aio interface supports both polling and notification models. The
first can be implemented by simply repeatedly calling the aio_error()
function to test the status of an operation. Once the operation has com‐
pleted, aio_return() is used to free the aiocb structure for re-use.
The notification model is implemented by using the aio_sigevent member of
the Asynchronous I/O Control Block. The operational model and the used
structure are described in sigevent(3).
The aio_suspend() function can be used to wait for the completion of one
or more operations. It is possible to set a timeout so that the process
can continue the execution and take recovery actions if the aio opera‐
tions do not complete as expected.
Cancellation and Synchronization
The aio_cancel() function can be used to request cancellation of an asyn‐
chronous I/O operation. Note however that not all of them can be can‐
celed. The same aiocb used to start the operation may be used as a han‐
dle for identification. It is also possible to request cancellation of
all operations pending for a file.
Comparable to fsync(2), the aio_fsync() function can be used to synchro‐
nize the contents of permanent storage when multiple asynchronous I/O
operations are outstanding for the file or device. The synchronization
operation includes only those requests that have already been success‐
fully enqueued.
FUNCTIONS
The following functions comprise the API of the aio interface:
Function Description
aio_cancel(3) cancel an outstanding asynchronous I/O operation
aio_error(3) retrieve error status of asynchronous I/O operation
aio_fsync(3) asynchronous data synchronization of file
aio_read(3) asynchronous read from a file
aio_return(3) get return status of asynchronous I/O operation
aio_suspend(3) suspend until operations or timeout complete
aio_write(3) asynchronous write to a file
lio_listio(3) list directed I/O
COMPATIBILITY
Unfortunately, the POSIX asynchronous I/O implementations vary slightly.
Some implementations provide a slightly different API with possible
extensions. For instance, the FreeBSD implementation uses a function
‘aio_waitcomplete()’ to wait for the next completion of an aio request.
STANDARDS
The aio interface is expected to conform to the IEEE Std 1003.1-2001
(“POSIX.1”) standard.
HISTORY
The aio interface first appeared in NetBSD 5.0.
CAVEATS
Few limitations can be mentioned:
· Undefined behavior results if simultaneous asynchronous operations
use the same Asynchronous I/O Control Block.
· When an asynchronous read operation is outstanding, undefined behav‐
ior may follow if the contents of aiocb are altered, or if memory
associated with the structure, or the aio_buf buffer, is deallocated.
BSD May 19, 2010 BSD
