libaio介绍

The Linux-native asynchronous I/O facility ("async I/O", or "aio") has a richer API and capability set than the simple POSIX async I/O facility. This library, libaio, provides the Linux-native API for async I/O. The POSIX async I/O facility requires this library in order to provide kernel-accelerated async I/O capabilities, as do applications which require the Linux-native async I/O API.

libaio项目 Overview - libaio - Pagure.iohttps://pagure.io/libaio

centos下安装libaio库

# sudo yum install libaio-devel

libaio提供四个函数如下表：

函数	功能	原型
io_setup	创建一个异步IO上下文（io_context_t是一个句柄）	int io_setup(int maxevents, io_context_t *ctxp);
io_destroy	销毁一个异步IO上下文（如果有正在进行的异步IO，取消并等待它们完成）	int io_destroy(io_context_t ctx);
io_submit	提交异步IO请求	long io_submit(aio_context_t ctx_id, long nr, struct iocb **iocbpp);
io_cancel	取消一个异步IO请求	long io_cancel(aio_context_t ctx_id, struct iocb *iocb, struct io_event *result);
io_getevents	等待并获取异步IO请求的事件（也就是异步请求的处理结果）	long io_getevents(aio_context_t ctx_id, long min_nr, long nr, struct io_event *events, struct timespec *timeout);

五个宏定义：

void io_set_callback(struct iocb *iocb, io_callback_t cb);
void io_prep_pwrite(struct iocb *iocb, int fd, void *buf, size_t count, long long offset);
void io_prep_pread(struct iocb *iocb, int fd, void *buf, size_t count, long long offset);
void io_prep_pwritev(struct iocb *iocb, int fd, const struct iovec *iov, int iovcnt, long long offset);
void io_prep_preadv(struct iocb *iocb, int fd, const struct iovec *iov, int iovcnt, long long offset);

这五个宏定义都是操作struct iocb的结构体。struct iocb是libaio中很重要的一个结构体，用于表示IO，但是其结构略显复杂，为了保持封装性不建议直接操作其元素而用上面五个宏定义操作。 

测试

测试源代码 test.c

#include<stdio.h>
#include<fcntl.h>
#include<string.h>
#include<stdlib.h>
#include<libaio.h>
#include<errno.h>
#include<unistd.h>
int main(void)
{int output_fd;char *content="hello world!";char *outputfile="hello.txt";io_context_t ctx;struct iocb io,*p=&io;struct io_event e;struct timespec timeout;memset(&ctx,0,sizeof(ctx));if(io_setup(10,&ctx)!=0){//initprintf("io_setup error\n");return -1;}if((output_fd=open(outputfile,O_CREAT|O_WRONLY,0644))<0){perror("open error");io_destroy(ctx);return -1;}io_prep_pwrite(&io,output_fd,content,strlen(content),0);io.data=content;if(io_submit(ctx,1,&p)!=1){io_destroy(ctx);printf("io_submit error\n");return -1;}while(1){timeout.tv_sec=0;timeout.tv_nsec=500000000;//0.5sif(io_getevents(ctx,0,1,&e,&timeout)==1){close(output_fd);break;}printf("haven't done\n");sleep(1);}io_destroy(ctx);return 0;
}

Makefile:

test:gcc test.c -o test -laio
clean:rm -rf test

其他参考文档：

Linux Asynchronous I/O (oxnz.github.io)https://oxnz.github.io/2016/10/13/linux-aio/