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linux异步IO编程实例分析( 二 )

异步IO编程

这里注意读写的buf都必须是按扇区对齐的，可以用posix_memalign来分配 。
3.获取完成的IO
long io_getevents (aio_context_t ctx_id, long min_nr, long nr, struct io_event *events, struct timespec *timeout);这里最重要的就是提供一个io_event数组给内核来copy完成的IO请求到这里，数组的大小是io_setup时指定的maxevents 。
timeout是指等待IO完成的超时时间，设置为NULL表示一直等待所有到IO的完成 。
4.销毁IO任务
int io_destroy (io_context_t ctx);注：需要C/C++ Linux服务器开发学习资料私信“资料”（资料包括C/C++，Linux，golang技术，Nginx，ZeroMQ，MySQL，redis，fastdfs，MongoDB，ZK，流媒体，CDN，P2P，K8S，Docker，TCP/IP，协程，DPDK等），免费分享

linux异步IO编程实例分析

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libaio和epoll的结合在异步编程中，任何一个环节的阻塞都会导致整个程序的阻塞，所以一定要避免在io_getevents调用时阻塞式的等待 。还记得io_iocb_common中的flags和resfd吗？看看libaio是如何提供io_getevents和事件循环的结合：
void io_set_eventfd(struct iocb *iocb, int eventfd){        iocb->u.c.flags |= (1 << 0) /* IOCB_FLAG_RESFD */;        iocb->u.c.resfd = eventfd;}这里的resfd是通过系统调用eventfd生成的 。
int eventfd(unsigned int initval, int flags);eventfd是linux 2.6.22内核之后加进来的syscall，作用是内核用来通知应用程序发生的事件的数量，从而使应用程序不用频繁地去轮询内核是否有时间发生，而是由内核将发生事件的数量写入到该fd，应用程序发现fd可读后，从fd读取该数值，并马上去内核读取 。
有了eventfd，就可以很好地将libaio和epoll事件循环结合起来：
1. 创建一个eventfd
efd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);2. 将eventfd设置到iocb中
io_set_eventfd(iocb, efd);3. 交接AIO请求
io_submit(ctx, NUM_EVENTS, iocb);4. 创建一个epollfd，并将eventfd加到epoll中
epfd = epoll_create(1);epoll_ctl(epfd, EPOLL_CTL_ADD, efd, &epevent);epoll_wait(epfd, &epevent, 1, -1);5. 当eventfd可读时，从eventfd读出完成IO请求的数量，并调用io_getevents获取这些IO
【linux异步IO编程实例分析】read(efd, &finished_aio, sizeof(finished_aio);r = io_getevents(ctx, 1, NUM_EVENTS, events, &tms);
linux异步IO编程实例分析

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异步非阻塞IO模型的流程图
一个完整的编程实例#define _GNU_SOURCE#define __STDC_FORMAT_macROS#include <stdio.h>#include <errno.h>#include <libaio.h>#include <sys/eventfd.h>#include <sys/epoll.h>#include <stdlib.h>#include <sys/types.h>#include <unistd.h>#include <stdint.h>#include <sys/stat.h>#include <fcntl.h>#include <inttypes.h>#define TEST_FILE   "aio_test_file"#define TEST_FILE_SIZE  (127 * 1024)#define NUM_EVENTS  128#define ALIGN_SIZE  512#define RD_WR_SIZE  1024struct custom_iocb{    struct iocb iocb;    int nth_request;};void aio_callback(io_context_t ctx, struct iocb *iocb, long res, long res2){    struct custom_iocb *iocbp = (struct custom_iocb *)iocb;    printf("nth_request: %d, request_type: %s, offset: %lld, length: %lu, res: %ld, res2: %ldn",             iocbp->nth_request, (iocb->aio_lio_opcode == IO_CMD_PREAD) ? "READ" : "WRITE",            iocb->u.c.offset, iocb->u.c.nbytes, res, res2);}int main(int argc, char *argv[]){    int efd, fd, epfd;    io_context_t ctx;    struct timespec tms;    struct io_event events[NUM_EVENTS];    struct custom_iocb iocbs[NUM_EVENTS];    struct iocb *iocbps[NUM_EVENTS];    struct custom_iocb *iocbp;    int i, j, r;    void *buf;    struct epoll_event epevent;    efd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);    if (efd == -1) {        perror("eventfd");        return 2;    }    fd = open(TEST_FILE, O_RDWR | O_CREAT | O_DIRECT, 0644);    if (fd == -1) {        perror("open");        return 3;    }    ftruncate(fd, TEST_FILE_SIZE);        ctx = 0;    if (io_setup(8192, &ctx)) {        perror("io_setup");        return 4;    }    if (posix_memalign(&buf, ALIGN_SIZE, RD_WR_SIZE)) {        perror("posix_memalign");        return 5;    }    printf("buf: %pn", buf);    for (i = 0, iocbp = iocbs; i < NUM_EVENTS; ++i, ++iocbp) {        iocbps[i] = &iocbp->iocb;        io_prep_pread(&iocbp->iocb, fd, buf, RD_WR_SIZE, i * RD_WR_SIZE);        io_set_eventfd(&iocbp->iocb, efd);        io_set_callback(&iocbp->iocb, aio_callback);        iocbp->nth_request = i + 1;    }    if (io_submit(ctx, NUM_EVENTS, iocbps) != NUM_EVENTS) {        perror("io_submit");        return 6;    }    epfd = epoll_create(1);    if (epfd == -1) {        perror("epoll_create");        return 7;    }    epevent.events = EPOLLIN | EPOLLET;    epevent.data.ptr = NULL;    if (epoll_ctl(epfd, EPOLL_CTL_ADD, efd, &epevent)) {        perror("epoll_ctl");        return 8;    }    i = 0;    while (i < NUM_EVENTS) {        uint64_t finished_aio;        if (epoll_wait(epfd, &epevent, 1, -1) != 1) {            perror("epoll_wait");            return 9;        }        if (read(efd, &finished_aio, sizeof(finished_aio)) != sizeof(finished_aio)) {            perror("read");            return 10;        }        printf("finished io number: %"PRIu64"n", finished_aio);            while (finished_aio > 0) {            tms.tv_sec = 0;            tms.tv_nsec = 0;            r = io_getevents(ctx, 1, NUM_EVENTS, events, &tms);            if (r > 0) {                for (j = 0; j < r; ++j) {                    ((io_callback_t)(events[j].data))(ctx, events[j].obj, events[j].res, events[j].res2);                }                i += r;                finished_aio -= r;            }        }    }        close(epfd);    free(buf);    io_destroy(ctx);    close(fd);    close(efd);    remove(TEST_FILE);    return 0;}


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