NAME
aio - Asynchronous IO
SYNOPSIS
#include <errno.h>
#include <aio.h>
DESCRIPTION
The POSIX.1b standard defines a new set of I/O operations which can
significantly reduce the time an application spends waiting at I/O.
The new functions allow a program to initiate one or more I/O
operations and then immediately resume normal work while the I/O
operations are executed in parallel. This functionality is available
if the unistd.h file defines the symbol _POSIX_ASYNCHRONOUS_IO.
These functions are part of the library with realtime functions named
librt. They are not actually part of the libc binary. The
implementation of these functions can be done using support in the
kernel (if available) or using an implementation based on threads at
userlevel. In the latter case it might be necessary to link
applications with the thread library libpthread in addition to librt
and libaio.
All AIO operations operate on files which were opened previously.
There might be arbitrarily many operations running for one file. The
asynchronous I/O operations are controlled using a data structure named
struct aiocb It is defined in aio.h as follows.
struct aiocb
{
int aio_fildes; /* File desriptor. */
int aio_lio_opcode; /* Operation to be performed. */
int aio_reqprio; /* Request priority offset. */
volatile void *aio_buf; /* Location of buffer. */
size_t aio_nbytes; /* Length of transfer. */
struct sigevent aio_sigevent; /* Signal number and value. */
/* Internal members. */
struct aiocb *__next_prio;
int __abs_prio;
int __policy;
int __error_code;
__ssize_t __return_value;
#ifndef __USE_FILE_OFFSET64
__off_t aio_offset; /* File offset. */
char __pad[sizeof (__off64_t) - sizeof (__off_t)];
#else
__off64_t aio_offset; /* File offset. */
#endif
char __unused[32];
};
The POSIX.1b standard mandates that the struct aiocb structure contains
at least the members described in the following table. There might be
more elements which are used by the implementation, but depending upon
these elements is not portable and is highly deprecated.
int aio_fildes
This element specifies the file descriptor to be used for the
operation. It must be a legal descriptor, otherwise the
operation will fail.
The device on which the file is opened must allow the seek
operation. I.e., it is not possible to use any of the AIO
operations on devices like terminals where an lseek call would
lead to an error.
off_t aio_offset
This element specifies the offset in the file at which the
operation (input or output) is performed. Since the operations
are carried out in arbitrary order and more than one operation
for one file descriptor can be started, one cannot expect a
current read/write position of the file descriptor.
volatile void *aio_buf
This is a pointer to the buffer with the data to be written or
the place where the read data is stored.
size_t aio_nbytes
This element specifies the length of the buffer pointed to by
aio_buf.
int aio_reqprio
If the platform has defined _POSIX_PRIORITIZED_IO and
_POSIX_PRIORITY_SCHEDULING, the AIO requests are processed based
on the current scheduling priority. The aio_reqprio element can
then be used to lower the priority of the AIO operation.
struct sigevent aio_sigevent
This element specifies how the calling process is notified once
the operation terminates. If the sigev_notify element is
SIGEV_NONE, no notification is sent. If it is SIGEV_SIGNAL, the
signal determined by sigev_signo is sent. Otherwise,
sigev_notify must be SIGEV_THREAD. In this case, a thread is
created which starts executing the function pointed to by
sigev_notify_function.
int aio_lio_opcode
This element is only used by the lio_listio and lio_listio64
functions. Since these functions allow an arbitrary number of
operations to start at once, and each operation can be input or
output (or nothing), the information must be stored in the
control block. The possible values are:
LIO_READ
Start a read operation. Read from the file at position
aio_offset and store the next aio_nbytes bytes in the buffer
pointed to by aio_buf.
LIO_WRITE
Start a write operation. Write aio_nbytes bytes starting at
aio_buf into the file starting at position aio_offset.
LIO_NOP
Do nothing for this control block. This value is useful
sometimes when an array of struct aiocb values contains holes,
i.e., some of the values must not be handled although the whole
array is presented to the lio_listio function.
When the sources are compiled using _FILE_OFFSET_BITS == 64 on a
32 bit machine, this type is in fact struct aiocb64, since the
LFS interface transparently replaces the struct aiocb
definition.
For use with the AIO functions defined in the LFS, there is a similar
type defined which replaces the types of the appropriate members with
larger types but otherwise is equivalent to struct aiocb, Particularly,
all member names are the same.
/* The same for the 64bit offsets. Please note that the members aio_fildes
to __return_value have to be the same in aiocb and aiocb64. */
#ifdef __USE_LARGEFILE64
struct aiocb64
{
int aio_fildes; /* File desriptor. */
int aio_lio_opcode; /* Operation to be performed. */
int aio_reqprio; /* Request priority offset. */
volatile void *aio_buf; /* Location of buffer. */
size_t aio_nbytes; /* Length of transfer. */
struct sigevent aio_sigevent; /* Signal number and value. */
/* Internal members. */
struct aiocb *__next_prio;
int __abs_prio;
int __policy;
int __error_code;
__ssize_t __return_value;
__off64_t aio_offset; /* File offset. */
char __unused[32];
};
int aio_fildes
This element specifies the file descriptor which is used for the
operation. It must be a legal descriptor since otherwise the
operation fails for obvious reasons. The device on which the
file is opened must allow the seek operation. I.e., it is not
possible to use any of the AIO operations on devices like
terminals where an lseek call would lead to an error.
off64_t aio_offset
This element specifies at which offset in the file the operation
(input or output) is performed. Since the operation are carried
in arbitrary order and more than one operation for one file
descriptor can be started, one cannot expect a current
read/write position of the file descriptor.
volatile void *aio_buf
This is a pointer to the buffer with the data to be written or
the place where the read data is stored.
size_t aio_nbytes
This element specifies the length of the buffer pointed to by
aio_buf.
int aio_reqprio
If for the platform _POSIX_PRIORITIZED_IO and
_POSIX_PRIORITY_SCHEDULING are defined the AIO requests are
processed based on the current scheduling priority. The
aio_reqprio element can then be used to lower the priority of
the AIO operation.
struct sigevent aio_sigevent
This element specifies how the calling process is notified once
the operation terminates. If the sigev_notify, element is
SIGEV_NONE no notification is sent. If it is SIGEV_SIGNAL, the
signal determined by sigev_signo is sent. Otherwise,
sigev_notify must be SIGEV_THREAD in which case a thread which
starts executing the function pointed to by
sigev_notify_function.
int aio_lio_opcode
This element is only used by the lio_listio and lio_listio64
functions. Since these functions allow an arbitrary number of
operations to start at once, and since each operation can be
input or output (or nothing), the information must be stored in
the control block. See the description of struct aiocb for a
description of the possible values.
When the sources are compiled using _FILE_OFFSET_BITS == 64 on a 32 bit
machine, this type is available under the name struct aiocb64, since
the LFS transparently replaces the old interface.
RETURN VALUES
ERRORS
SEE ALSO
aio_cancel(3), aio_cancel64(3), aio_error(3), aio_error64(3),
aio_fsync(3), aio_fsync64(3), aio_init(3), aio_read(3), aio_read64(3),
aio_return(3), aio_return64(3), aio_suspend(3), aio_suspend64(3),
aio_write(3), aio_write64(3), errno(3).