linux epoll/select使用区分和实例对比-Toy模板网

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Linux内核poll，ppoll，epoll，select代码位置：

poll，ppoll，select相关内核实现在在fs/select.c中;

epoll_ctl和epoll_wait相关函数在fs/eventpoll.c中

epoll实测不支持监听普通文件，select可以，但是发现无论是可读、可写，其一直会返回准备就绪状态，所以select也无法实际用来监控普通文件，可用来监听物理设备如uart对应的ttyS0等。

epoll_ctl控制普通文件会返回-1，即EPERM，添加的fd不支持epoll。比如添加的是普通文件描述符内核设备文件poll file operations定义类似如下：

linux epoll/select使用区分和实例对比,linux,运维,服务器

如下通过epoll实现监控IO变化：

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include <pthread.h>
#include <time.h>
#include <signal.h>
#include <semaphore.h>
#include <stdarg.h>
#include <errno.h>
#include <unistd.h>
#include <termios.h>
#include <fcntl.h>
#include <regex.h>
#include <libgen.h>
#include <poll.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdbool.h>
#include <stdarg.h>
#include <math.h>
#include <getopt.h>
#include <ctype.h>
#include <sys/epoll.h>
#include <sys/timerfd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/syscall.h>
#include <sys/eventfd.h>
#include <sys/socket.h>
#include <sys/reboot.h>
#include <sys/resource.h>
#include <sys/prctl.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <sys/un.h>
#include <sys/shm.h>
#include <stddef.h>

#include <linux/input.h>
#include <linux/types.h>
#include <linux/sockios.h>
#include <linux/ethtool.h>
#include <linux/netlink.h>


static bool thread_start = 0;
static int32_t epoll_fd  = -1;
static int32_t g_test_gpio_fd = -1;
static int32_t g_test_status_fd = -1;
static int test_action  = 1;

typedef struct{
    int8_t              thread_name[OWN_THREAD_NAME_SIZE];     //thread name
    pthread_t     m_thead;                                //thread hanle
    uint32_t            thread_priority;                        //thread priority
    uint32_t            stack_depth;                            //stack size
    void                *thread_func;                           //thread func
    void                *para;                                  
}OWN_thread_t;

#define OWN_THREAD_NAME_SIZE      20
#define TEST_PATH      "/etc/test_value"
#define OWN_WAKE_GPIO_MAX_SIZE             (10)
#define OWN_MIN_STACK         100*1024
#define F_FAIL -1
#define F_SUCCESS 1
#define EPOLL_TIMEOUT (1000)
#define OWN_BUFSIZE_4                  4
#define OWN_THREAD_PRIORITY            3
#define OWN_test_THREAD_STACK_SIZE       (18 * 1024)//byte(18kb)

int32_t OWN_thread_create(OWN_thread_t *threadin)
{
    int32_t ret = -1;

    if(threadin == NULL)
    {
        printf("threadin is null\n");
    }
        pthread_attr_t attr;
        ret = pthread_attr_init(&attr);
        if (ret != 0)
        {
           printf("pthread_attr_init Error\n");
           return -1;
        }
        if(threadin->stack_depth < OWN_MIN_STACK)
        {
            threadin->stack_depth = OWN_MIN_STACK;
        }
        ret = pthread_attr_setstacksize(&attr, threadin->stack_depth);
        if(ret != 0)
        {
           printf("pthread_attr_setstacksize %d Error=%d\n",threadin->stack_depth,ret);
           return -1;
        }
        ret = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
        if(ret != 0)
        {
           printf("pthread_attr_setdetachstate Error=%d\n",ret);
           return -1;
        }
        ret = pthread_create(&(threadin->m_thead), &attr, threadin->thread_func, (void *)(threadin->para));
        pthread_attr_destroy (&attr);
        if(ret != 0)
        {
            printf("pthread_create Error, ret=%d, err:%s\n", ret, strerror(errno));
            return F_FAIL;
        }
        else
        {
            return F_SUCCESS;
        }

    return F_FAIL;
}

static void *OWN_thread_handler(void *arg)
{
    static int32_t first_reload = 1;
    int32_t fd_cnt = 0;
    int32_t ret = 0;
    int32_t i = 0;
    int8_t value[OWN_BUFSIZE_4] = {0};
    struct epoll_event events[OWN_WAKE_GPIO_MAX_SIZE];
    int8_t value_status = 0;

    while (thread_start)
    {
        memset(events, 0x0, sizeof(events));
        fd_cnt = epoll_wait(epoll_fd, events, OWN_WAKE_GPIO_MAX_SIZE, EPOLL_TIMEOUT);
        if(fd_cnt <= 0)
        {
            continue;
        }

        for(i = 0; i < fd_cnt; i++)
        {
            memset(value, 0x0, sizeof(value));
            lseek(events[i].data.fd, 0, SEEK_SET);
            ret = read(events[i].data.fd, value, sizeof(value));
            value_status = atoi((const char *)value);
            printf("value =%s value_status =%d ret : %d\n", value, value_status, ret);

            if (events[i].events & EPOLLPRI  && events[i].data.fd == g_test_gpio_fd)
            {
                printf("gpio test file \r\n");
                if (test_action != NULL)
                {

                    printf("callback test value =%s value_status =%d ret : %d\n", value, value_status, ret);
                }
            }
        }
    }

    return NULL;
}

int main()
{
    int ret = 0;

    struct epoll_event evd = {0};
    OWN_thread_t test_gpio_thread;
    epoll_fd = epoll_create(OWN_WAKE_GPIO_MAX_SIZE);
    if (epoll_fd < 0)
    {
        printf("epoll error \n");
    }
    g_test_gpio_fd = open(TEST_PATH , O_RDONLY | O_NONBLOCK);
    if (g_test_gpio_fd < 0)
    {
        printf("open error \n");
    }

    memset(&evd, 0x0, sizeof(evd));
    evd.data.fd = g_test_gpio_fd;
    evd.events = EPOLLPRI;

    ret = epoll_ctl(g_test_epoll_fd, EPOLL_CTL_ADD, epoll_fd, &evd);
    if (ret < 0)
    {
        printf("epoll_ctl error \n");
    }

    thread_start = 1;
    memset(&test_gpio_thread, 0, sizeof(test_gpio_thread));
    snprintf((char*)test_gpio_thread.thread_name,OWN_THREAD_NAME_SIZE - 1, "Notify handle");
    test_gpio_thread.thread_priority  = OWN_THREAD_PRIORITY;
    test_gpio_thread.stack_depth      = OWN_test_THREAD_STACK_SIZE;
    test_gpio_thread.thread_func      = OWN_thread_handler;
    test_gpio_thread.para             = NULL;

    ret = OWN_thread_create(&test_gpio_thread);
    if (ret != F_SUCCESS) {
        printf("OWN_thread_handler creation failure !\n");
    }
    while(1)
    {
        sleep(2);
    }

    return 0;
}

select应用示例：如下实例为监听普通文件，发现返回值一直大于0，监听意义不大。可以将监听的设备改为ttyS0，则可以正常监控。

函数返回值如下：

>0：就绪描述字的正数目

-1：出错

0 ：超时

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include <pthread.h>
#include <time.h>
#include <signal.h>
#include <semaphore.h>
#include <stdarg.h>
#include <errno.h>
#include <unistd.h>
#include <termios.h>
#include <fcntl.h>
#include <regex.h>
#include <libgen.h>
#include <poll.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdbool.h>
#include <stdarg.h>
#include <math.h>
#include <getopt.h>
#include <ctype.h>


#include <sys/epoll.h>
#include <sys/timerfd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/syscall.h>
#include <sys/eventfd.h>
#include <sys/socket.h>
#include <sys/reboot.h>
#include <sys/resource.h>
#include <sys/prctl.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <sys/un.h>
#include <sys/shm.h>
#include <stddef.h>
#include <linux/input.h>
#include <linux/types.h>
#include <linux/spi/spidev.h>
#include <linux/sockios.h>
#include <linux/ethtool.h>
#include <linux/netlink.h>

static pthread_t test_thread;
int fd = 0;
static void *thread_func(void * arg);

int main(int argc, char * argv [ ])
{
    int ret;
    char buf[] = "hello test! ";

    if (argc < 2)
    {
        return -1;
    }

    setbuf(stdout, NULL);

    fd = open("/etc_rw/test_select", O_RDWR  | O_NONBLOCK);
    if (fd > 0)
    {
        printf("Open success! \n");
    }
    else
    {
        printf("Open failure! \n");
    }

    if (pthread_create(&test_thread, NULL, thread_func, NULL) != 0)
    {
        printf("Failed to creat thread \n");
    }

    while (fd >= 0)
    {
        //ret = write(fd, (uint8_t*)buf, strlen(buf));
        printf("Write fd value = %d, ret value = %d \n", fd, ret);
        sleep(2);
    }

    return 0;
}

static void *thread_func(void * arg)
{
    int ret;
    fd_set fdset;
    struct timeval timeout = {4, 0};
    char buf[100] = {0};

    timeout.tv_sec = 5;
    timeout.tv_usec = 0;
    while (fd >= 0)
    {
        FD_ZERO(&fdset);
        FD_SET(fd, &fdset);
        ret = select(fd + 1, &fdset, NULL, NULL, &timeout);
        if (ret == -1)
        {
            printf("Failed to select \n");
        }
        else if (ret == 0)
        {
            printf("ret value is 0 \n");
            timeout.tv_sec = 4;
            timeout.tv_usec = 0;
        }
        else
        {
            if (FD_ISSET(fd, &fdset))
            {
                //do
               // {
                    memset(buf, 0, 100);
                    ret = read(fd, buf, 100);
                    printf("Read ret = %d buf = %s \n", ret, buf);
               // }while(ret == 200);
            }
        }
    }

    return (void *)1;
}

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