一个epoll/aio/eventfd结合使用的简单例子

#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  1024

struct 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: %ld\n", 

            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: %p\n", 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;

}

说明：

1. 在centos 6.2 (libaio-devel 0.3.107-10) 上运行通过

2. struct io_event中的res字段表示读到的字节数或者一个负数错误码。在后一种情况下，-res表示对应的

   errno。res2字段为0表示成功，否则失败

3. iocb在aio请求执行过程中必须是valid的

4. 在上面的程序中，通过扩展iocb结构来保存额外的信息(nth_request)，并使用iocb.data

   来保存回调函数的地址。如果回调函数是固定的，那么也可以使用iocb.data来保存额外信息。