Asynchronous I/O (The GNU C Library)

### 13.11 Perform I/O Operations in Parallel ¶

The POSIX.1b standard defines a new set of I/O operations which can significantly reduce the time an application spends waiting for 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`.

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` (AIO control block). It is defined in `aio.h` as follows.

Data Type: struct aiocb ¶

  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.

Data Type: struct aiocb64 ¶
  `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 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 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.