# FIFO, poll, epoll

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Authors: WhoAmI, CrazyMonkey
email: [e-mail]
Date: 20230513
Copyright: CC BY-NC-SA


e.g.,

---

mkfifo fifo0
mkfifo fifo1

---

terminal 0:
./poll fifo0 fifo1

terminal 1:

echo "termainal 0" > fifo0
echo "termainal 1" > fifo1

**poll:**

On success, poll() returns a nonnegative value which is **the number of elements in the pollfds whose revents fields have been set to a nonzero value (indicating an event or an error).**

A return value of zero indicates that the system call timed out before any file descriptors became read.

On error, -1 is returned, and errno is set to indicate the error.

:::success

如果 您需要處理
Level-triggered and edge-triggered
或 超過 1024 的 connections

您需要用 Linux 特有的

[epoll ](https://man7.org/linux/man-pages/man7/epoll.7.html)

[epoll_wait](https://man7.org/linux/man-pages/man2/epoll_wait.2.html)

**thread 中使用 要特別小心 lock/unlock 的問題!!!**

[Using epoll() For Asynchronous Network Programming](https://kovyrin.net/2006/04/13/epoll-asynchronous-network-programming/)

epoll, epoll_ctl 使用 red-black tree

HW:

[epoll() Tutorial – epoll() In 3 Easy Steps!](https://www.suchprogramming.com/epoll-in-3-easy-steps/)

:::

<h1>
poll
</h1>

poll.c

/*
 *
 *  Based on linux man page: poll_input.c
 * 

   Licensed under GNU General Public License v2 or later.
          
          
   #include <poll.h>

   int poll(struct pollfd *fds, nfds_t nfds, int timeout);
   timeout:  milliseconds
          
*/
       #include <poll.h>
       #include <fcntl.h>
       #include <sys/types.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)
       
       //timout=-1 : infinite timeout
       #define timeout 10
       #define buffersize 255
       int
       main(int argc, char *argv[])
       {
           int nfds, num_open_fds;
           struct pollfd *pfds;

           if (argc < 2) {
              fprintf(stderr, "Usage: %s file...\n", argv[0]);
              exit(EXIT_FAILURE);
           }

           num_open_fds = nfds = argc - 1;
           pfds = calloc(nfds, sizeof(struct pollfd));
           if (pfds == NULL)
               errExit("malloc");

           /* Open each file on command line, and add it 'pfds' array. */

           for (int j = 0; j < nfds; j++) {
               pfds[j].fd = open(argv[j + 1], O_RDONLY);
               if (pfds[j].fd == -1)
                   errExit("open");

               printf("Opened \"%s\" on fd %d\n", argv[j + 1], pfds[j].fd);

               pfds[j].events = POLLIN;
           }

           /* Keep calling poll() as long as at least one file descriptor is
              open. */

           while (num_open_fds > 0) {
               int ready;

               printf("About to poll()\n");
               
               //timout=-1 : infinite timeout
               ready = poll(pfds, nfds, timeout);
               if (ready == -1)
                   errExit("poll");

               printf("Ready: %d\n", ready);

               /* Deal with array returned by poll(). */

               for (int j = 0; j < nfds; j++) {
                   char buf[buffersize+1];

                   if (pfds[j].revents != 0) {
                       printf("  fd=%d; events: %s%s%s\n", pfds[j].fd,
                               (pfds[j].revents & POLLIN)  ? "POLLIN "  : "",
                               (pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
                               (pfds[j].revents & POLLERR) ? "POLLERR " : "");

                       if (pfds[j].revents & POLLIN) {
                           ssize_t s = read(pfds[j].fd, buf, sizeof(buf)-1);
                           if (s == -1)
                               errExit("read");
                           
                            buf[s]=0;   
                           //    size_t or ssize_t argument
                           printf("    read %zd bytes: %s\n", s, buf);
                       } else {                /* POLLERR | POLLHUP */
                           printf("    closing fd %d\n", pfds[j].fd);
                           if (close(pfds[j].fd) == -1)
                               errExit("close");
                           num_open_fds--;
                       }
                   }
               }
           }
           
           free(pfds);
           
           printf("All file descriptors closed; bye\n");
           exit(EXIT_SUCCESS);
       }

---
<h1>epoll</h1>

[epoll - I/O event notification facility](https://man7.org/linux/man-pages/man7/epoll.7.html)

[epoll_create, epoll_create1 - open an epoll file descriptor](https://man7.org/linux/man-pages/man2/epoll_create.2.html)

[epoll_ctl - control interface for an epoll file descriptor](https://man7.org/linux/man-pages/man2/epoll_ctl.2.html)

[epoll_wait](https://man7.org/linux/man-pages/man2/epoll_wait.2.html)

#include <sys/epoll.h>
int epoll_wait(int epfd, struct epoll_event *events,
int maxevents, int timeout);

           typedef union epoll_data {
               void    *ptr;
               int      fd;
               uint32_t u32;
               uint64_t u64;
           } epoll_data_t;

           struct epoll_event {
               uint32_t     events;    /* Epoll events */
               epoll_data_t data;      /* User data variable */
           };

The timeout argument specifies the number of <span class="red">milliseconds </span>that epoll_wait() will block.

Specifying a timeout of -1 causes epoll_wait() to block indefinitely, while specifying a timeout equal to zero cause epoll_wait() to return immediately, even if no events are available.

Maxevents is just the length of the struct epoll_events array pointed to by *events

**epoll.c**

/* Author: WhoAmI
 * email: [e-mail]
 * Licience: GPLv2
 * File: epoll.c
 * /
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/epoll.h>

#define BUFFERLEN 1024
#define perror(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)
#define pinfo(msg)     do { fprintf(stdout,"%s",msg);  } while (0)
#define  MAX_EVENTS 20

 
int main(int argc, char *argv[]) {
    const int EVENTS_SIZE = 20;
    struct epoll_event ev, epev, events[MAX_EVENTS]; 
    char echo[]="Good!!!!";
    char buff[BUFFERLEN];

    
    int socketFd = socket(AF_INET, SOCK_STREAM, 0);

   
   struct  sockaddr_in sockAddr, cli_addr;


    sockAddr.sin_port = htons(8088);
    sockAddr.sin_family = AF_INET;
    sockAddr.sin_addr.s_addr = htons(INADDR_ANY);

    
    if (bind(socketFd, (struct sockaddr *) &sockAddr, sizeof(struct sockaddr)) == -1) {
        printf( "bind error\n");
        return -1;
    }

   
    if (listen(socketFd, 10) == -1) {
        perror( "listen error");
        return -1;
    }

   
    int eFd = epoll_create(1);

   
    
    
    epev.events = EPOLLIN;
    epev.data.fd = socketFd;
    epoll_ctl(eFd, EPOLL_CTL_ADD, socketFd, &epev);
    
   
   

    
    while (!0) {
      
        int eNum = epoll_wait(eFd, events, EVENTS_SIZE, -1);

        if (eNum == -1) {
            perror( "epoll_wait " );
            return -1;
        }
        
        for (int i = 0; i < eNum; i++) {
            
            if (events[i].data.fd == socketFd) {
                if (events[i].events & EPOLLIN) {
                   
                    socklen_t length = sizeof(struct sockaddr);
                    
                    int fd = accept(socketFd, (struct sockaddr *) &cli_addr, &length);
                    if (fd > 0) {
                       
                        epev.events = EPOLLIN | EPOLLET;
                        epev.data.fd = fd;
                        
                        int flags = fcntl(fd, F_GETFL, 0);
                        if (flags < 0) {
                            printf ("set no block error, fd: %d\n",fd); 
                            continue;
                        }
                        if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) {
                            printf("set no block error, fd:" , fd);
                       
                            continue;
                        }
                        //add new connection
                        epoll_ctl(eFd, EPOLL_CTL_ADD, fd, &epev);
                        printf( "client on line fd: %d",   fd);
                        
                    }
                }
            } else {
                
                
                /*
                 *
              POLLRDHUP (since Linux 2.6.17)
              Stream socket peer closed connection, or shut down writing
              half of connection.  The _GNU_SOURCE feature test macro
              must be defined (before including any header files) in
              order to obtain this definition.
              
              
                 *
              POLLERR
              Error condition (only returned in revents; ignored in
              events).  This bit is also set for a file descriptor
              referring to the write end of a pipe when the read end has
              been closed.
                 *
                 */
                
                if (events[i].events & EPOLLERR || events[i].events & EPOLLHUP) {
                    
                    //remove client fd
                    
                    epoll_ctl(eFd, EPOLL_CTL_DEL, events[i].data.fd, events);
                    
                    
                    printf( "client out fd: %d\n" ,events[i].data.fd );;
                   
                    close(events[i].data.fd);
                    
                    
                } else if (events[i].events & EPOLLIN) {
                    
                    
                    int len = read(events[i].data.fd, buff, sizeof(buff)-1);
                    
                    if (len == -1) {
                        epoll_ctl(eFd, EPOLL_CTL_DEL, events[i].data.fd, events);
                        printf("client out fd: %d\n",events[i].data.fd );
                        
                        close(events[i].data.fd);
                    } else {
                        buff[len]=0;
                        printf("%s\n",buff);
                        
                        
                        write(events[i].data.fd, echo, sizeof(echo));
                    }
                }
            }
        }
    }
    
     exit(EXIT_SUCCESS);
}

int exec(char *cmd)
{
    FILE *fp;
    char buffer[256];
    
    
    fp = popen(cmd, "r");
    
    while(fp){
      fgets(buffer, sizeof(buffer), fp);
      if(!feof(fp)){
        printf(">");
        printf("%s", buffer);
      }
      else{
        pclose(fp);
        return 0;
      }        
    }
    
    pclose(fp);
    return 0;
  
}

**將上面兩段程式結合, 可以 telnet 至 server 執行 命令**