簡述
linux下異步方式有兩種：異步通知和異步IO（AIO），異步通知請參考：linux異步通知
Linux的I/O機制經歷了一下幾個階段的演進：
1. 同步阻塞I/O: 用戶進程進行I/O操作，一直阻塞到I/O操作完成為止。
2. 同步非阻塞I/O: 用戶程序可以通過設置文件描述符的屬性O_NONBLOCK，I/O操作可以立即返回，但是並不保證I/O操作成功。
3. 異步事件阻塞I/O: 用戶進程可以對I/O事件進行阻塞，但是I/O操作並不阻塞。通過select/poll/epoll等函數調用來達到此目的。
4. 異步時間非阻塞I/O: 也叫做異步I/O(AIO)，用戶程序可以通過向內核發出I/O請求命令，不用等待I/O事件真正發生，可以繼續做另外的事情，等I/O操作完成，內核會通過函數回調或者信號機制通知用戶進程。這樣很大程度提高了系統吞吐量。
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.
函數
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.
在異步非阻塞IO中，可以同時發起多個傳輸操作。這需要每個傳輸操作有唯一的上下文，這樣才能在它們完成時區分到底是哪個傳輸操作完成了。在AIO中通過aiocb（AIO I/O control Block）結構體區分。
aiocb包含了有關傳輸的所有信息，以及為數據准備的用戶緩沖區。在產生I/O通知（完成）時，aiocb結構就被用來唯一標識所完成的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).
應用
應用主要有兩種方式：信號機制和回調函數。
參考：
1. Linux 異步IO機制