简述
linux下异步方式有两种：异步通知和异步IO（AIO），异步通知请参考：linux异步通知
Linux的I/O机制经历了一下几个阶段的演进：
1. 同步阻塞I/O: 用户进程进行I/O操作，一直阻塞到I/O操作完成为止。
2. 同步非阻塞I/O: 用户程序可以通过设置文件描述符的属性O_NONBLOCK，I/O操作可以立即返回，但是并不保证I/O操作成功。
3. 异步事件阻塞I/O: 用户进程可以对I/O事件进行阻塞，但是I/O操作并不阻塞。通过select/poll/epoll等函数调用来达到此目的。
4. 异步时间非阻塞I/O: 也叫做异步I/O(AIO)，用户程序可以通过向内核发出I/O请求命令，不用等待I/O事件真正发生，可以继续做另外的事情，等I/O操作完成，内核会通过函数回调或者信号机制通知用户进程。这样很大程度提高了系统吞吐量。
The POSIX asynchronous I/O (AIO) interface allows applications to initiate one or more I/O operations that are performed asynchronously (i.e., in the background).
The application can elect to be notified of completion of the I/O operation in a variety of ways: by delivery of a signal, by instantiation of a thread, or no notification at all.
函数
The POSIX AIO interface consists of the following functions:
aio_read(3) Enqueue a read request. This is the asynchronous analog of read(2).
aio_write(3) Enqueue a write request. This is the asynchronous analog of write(2).
aio_fsync(3) Enqueue a sync request for the I/O operations on a file descriptor. This is the asynchronous analog of fsync(2) and fdatasync(2).
aio_error(3) Obtain the error status of an enqueued I/O request.
aio_return(3) Obtain the return status of a completed I/O request.
aio_suspend(3) Suspend the caller until one or more of a specified set of I/O requests completes.
aio_cancel(3) Attempt to cancel outstanding I/O requests on a specified file descriptor.
lio_listio(3) Enqueue multiple I/O requests using a single function call.
在异步非阻塞IO中，可以同时发起多个传输操作。这需要每个传输操作有唯一的上下文，这样才能在它们完成时区分到底是哪个传输操作完成了。在AIO中通过aiocb（AIO I/O control Block）结构体区分。
aiocb包含了有关传输的所有信息，以及为数据准备的用户缓冲区。在产生I/O通知（完成）时，aiocb结构就被用来唯一标识所完成的I/O操作。
#include <aiocb.h>
struct aiocb {
/* The order of these fields is implementation-dependent */
int aio_fildes; /* File descriptor */
off_t aio_offset; /* File offset */
volatile void *aio_buf; /* Location of buffer */
size_t aio_nbytes; /* Length of transfer */
int aio_reqprio; /* Request priority */
struct sigevent aio_sigevent; /* Notification method */
int aio_lio_opcode; /* Operation to be performed;
lio_listio() only */
/* Various implementation-internal fields not shown */
};
/* Operation codes for 'aio_lio_opcode': */
enum { LIO_READ, LIO_WRITE, LIO_NOP };
The fields of this structure are as follows:
aio_filedes The file descriptor on which the I/O operation is to be performed.
aio_offset This is the file offset at which the I/O operation is to be performed.
aio_buf This is the buffer used to transfer data for a read or write operation.
aio_nbytes This is the size of the buffer pointed to by aio_buf.
aio_reqprio This field specifies a value that is subtracted from the calling thread's real-time priority in order to determine the priority for execution of this I/O request(see pthread_setschedparam(3)). The specified value must be between 0 and the value returned by sysconf(_SC_AIO_PRIO_DELTA_MAX). This field is ignored for file synchronization operations.
aio_sigevent This field is a structure that specifies how the caller is to be notified when the asynchronous I/O operation completes. Possible values for aio_sigevent.sigev_notify are SIGEV_NONE, SIGEV_SIGNAL, and SIGEV_THREAD. See sigevent(7) for further details.
aio_lio_opcode The type of operation to be performed; used only for lio_listio(3).
应用
应用主要有两种方式：信号机制和回调函数。
参考：
1. Linux 异步IO机制