linux epoll/select使用区分和实例对比-编程知识

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：

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 namepthread_t     m_thead;                                //thread hanleuint32_t            thread_priority;                        //thread priorityuint32_t            stack_depth;                            //stack sizevoid                *thread_func;                           //thread funcvoid                *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_LISTEN_TIMEOUT                (1000)
#define OWN_BUFSIZE_4                  4
#define OWN_THREAD_PRIORITY            3
#define OWN_test_THREAD_STACK_SIZE       (16 * 1024)//byte(16kb)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_wake_gpio_monitor_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_LISTEN_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_wake_gpio_monitor_handler;test_gpio_thread.para             = NULL;ret = OWN_thread_create(&test_gpio_thread);if (ret != F_SUCCESS) {printf("OWN_thread_wake_gpio_monitor_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;
}