关于tcp连接的异步connect

关于tcp连接的异步connect实现流程如下：
（1）设置socket连接为NONBLOCK
（2）调用connect函数
（3）poll函数检测fd
（4）判断poll函数的返回值来确定连接是否建立

下面是一个测试代码。

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1. #include <stdio.h>
   
2. #include <stdlib.h>
   
3. #include <poll.h>
   
4. #include <sys/types.h>
   
5. #include <sys/socket.h>
   
6. #include <netinet/ip.h>
   
7. #include <netinet/tcp.h>
   
8. #include <fcntl.h>
   
9. #include <errno.h>
   
10. #include <strings.h>
    
11. #include <string.h>
    
12. int main(int argc, char **argv)
    
13. {
    
14.         int old;
    
15.         int err;
    
16.         int errlen = sizeof(err);
    
17.         struct sockaddr_in addr;
    
18.         socklen_t len = sizeof(struct sockaddr_in);
    
19.         int timeo;
    
20.         int fd;
    
21.         struct pollfd pfd;
    
22.         int ret;
    
23. 
    
24.         if (argc != 4) {
    
25.                 printf("Usage %s ip port timeout\n", argv[0]);
    
26.                 return -1;
    
27.         }
    
28.         timeo = atoi(argv[3]);
    
29.         fd = socket(AF_INET, SOCK_STREAM, 0);
    
30.         if (fd < 0) {
    
31.                 printf("Create socket error erno=%d : %s\n", errno,strerror(errno));
    
32.                 return -1;
    
33.         }
    
34. 
    
35.         bzero(&addr, sizeof(addr));
    
36.         addr.sin_family = AF_INET;
    
37.         addr.sin_addr.s_addr = inet_addr(argv[1]);
    
38.         addr.sin_port = htons(atoi(argv[2]));
    
39. 
    
40.         old = fcntl(fd, F_GETFL, 0);
    
41.         if (fcntl(fd, F_SETFL, old|O_NONBLOCK) < 0) {
    
42.                 printf("setting fd error: errno,%s\n", errno,strerror(errno));
    
43.                 close(fd);
    
44.                  return -1;
    
45.         }
    
46. 
    
47.         if (connect(fd, (struct sockaddr*)&addr, len) == -1 &&
    
48.                         errno != EINPROGRESS) {
    
49.                 printf("Error in connect: %d,%s\n", errno,strerror(errno));
    
50.                 close(fd);
    
51.                 return -1;
    
52.         }
    
53. 
    
54.         pfd.fd = fd;
    
55.         pfd.events = POLLOUT | POLLERR | POLLHUP | POLLNVAL;
    
56.         errno = 0;
    
57. 
    
58.         if (timeo <= 0)
    
59.                 timeo = -1;
    
60. 
    
61.         ret = poll(&pfd, 1, timeo);
    
62. 
    
63.         if (ret < 0) {
    
64.                 if (errno == EINTR) {
    
65.                         close(fd);
    
66.                         return -1;
    
67.                 }
    
68.                 close(fd);
    
69.                 return -1;
    
70. 
    
71.         } else if (0 == ret) {
    
72.                 close(fd);
    
73.                 return -1;
    
74.         }
    
75. 
    
76.         if (pfd.revents & (POLLERR | POLLHUP | POLLNVAL)) {
    
77.                 close(fd);
    
78.                 return -1;
    
79.         }
    
80.         if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, (socklen_t*)&errlen) == -1) {
    
81.                 close(fd);
    
82.                  return -1;
    
83.         }
    
84.         if (err != 0) {//check the SO_ERROR state
    
85.                 errno = err;
    
86.                 close(fd);
    
87.                 return -1;
    
88.         }
    
89. 
    
90.         fcntl(fd, F_SETFL, old);
    
91.         printf("Connect real OK\n");
    
92.         return fd;
    
93. }

在这个代码，如果传入的超时时间小于等于0，那么poll会一直等待。
但是要注意的是，这里所谓的一直等待，绝对不是无限期等下去，具体等待时间与系统配置参数有关。
比如你连接的目的地址不存在，那么syn最多重试发送net.ipv4.tcp_syn_retries次后就会返回POLLERR。