linux epoll系列2 利用epoll_wait查看是否可以送信

write函数本来是非阻塞函数，但是当缓存区被写满后，再往缓存区里写的时候，就必须等待缓存区再次变成可写，所以这是write就变成了阻塞了，这个进程或者线程就堵住了，不能被响应了。

epoll_wait函数可以判断出，缓存区是否可写，可写后再调用write函数，这样就避免了write函数被阻塞。

例子1,接收端

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/epoll.h>
#include <arpa/inet.h>
int main(){
  int sock0;
  sockaddr_in addr;
  sockaddr_in client;
  socklen_t len;
  int sock;
  int n;
  char buf[65536];
  int i;
  sock0 = socket(AF_INET, SOCK_STREAM, 0);
  addr.sin_family = AF_INET;
  addr.sin_port = htons(12345);
  addr.sin_addr.s_addr = INADDR_ANY;
  bind(sock0, (sockaddr*)&addr, sizeof(addr));
  listen(sock0, 5);
  len = sizeof(client);
  sock = accept(sock0, (sockaddr*)&client, &len);
  printf("after accept\n");
  for(i = 0; i < 10; ++i){
    sleep(2);
    n = read(sock, buf, sizeof(buf));
    printf("recv data size:[%d] bytes\n", n);
  }
  printf("close socket and finish\n");
  close(sock);
  return 0;
}

github源代码

例子2， 是会发生阻塞的发送端。

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/epoll.h>
#include <arpa/inet.h>
int main(){
  sockaddr_in server;
  int sock;
  char buf[65536];
  int n;
  sock = socket(AF_INET, SOCK_STREAM, 0);
  server.sin_family = AF_INET;
  server.sin_port = htons(12345);
  inet_pton(AF_INET, "127.0.0.1", &server.sin_addr.s_addr);
  n = connect(sock, (sockaddr*)&server, sizeof(server));
  if(n != 0){
    perror("connect");
    return 1;
  }
  int cnt = 0;
  while(1){
    ++cnt;
    printf("[%d]write %ld bytes\n", cnt, sizeof(buf));
    n = write(sock, buf, sizeof(buf));
    if(n <= 0){
      printf("write error:%d\n", n);
      break;
    }
  }
  close(sock);
  return 0;
  
}

github源代码

例子3，利用了epoll_wait,所以是不会发生阻塞的。

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/epoll.h>
#include <arpa/inet.h>
#define EVENTS 12
int main(){
  sockaddr_in server;
  epoll_event ev, ev_ret[EVENTS];
  int sock, epfd;
  char buf[65536];
  int nfds;
  int n;
  sock = socket(AF_INET, SOCK_STREAM, 0);
  server.sin_family = AF_INET;
  server.sin_port = htons(12345);
  inet_pton(AF_INET, "127.0.0.1", &server.sin_addr.s_addr);
  n = connect(sock, (sockaddr*)&server, sizeof(server));
  if(n != 0){
    perror("connect");
    return 1;
  }
  epfd = epoll_create(2);
  if(epfd < 0){
    perror("epfd");
    return 1;
  }
  memset(&ev, 0, sizeof(ev));
  ev.events = EPOLLOUT;//可写
  ev.data.fd = sock;
  if(epoll_ctl(epfd, EPOLL_CTL_ADD, sock, &ev) != 0){
    perror("epoll_clt");
    return 1;
  }
  int cnt = 0;
  while(1){
    cnt++;
    printf("before epoll wait\n");
    nfds = epoll_wait(epfd, ev_ret, EVENTS, -1);
    if(nfds < 0){
      perror("epoll_wait");
      return 1;
    }
    printf("after epoll_wait\n");
    if(ev_ret[0].data.fd == sock){
      printf("[%d]write %ld types\n", cnt, sizeof(buf));
      n = write(sock, buf, sizeof(buf));
      if(n <= 0){
    printf("write error:%d\n", n);
    break;
      }
    }
  }
  close(sock);
  return 0;
}

github源代码

运行方法：先运行接收端，再运行阻塞发送端。

运行方法：先运行接收端，再运行非阻塞发送端。