I/O复用的高级应用一：非阻塞connect

非阻塞socket调用connect，而连接没有立即建立时会出错，返回errno值为：EINPROGRESS。

代码

非阻塞connect应用代码示例：

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <assert.h>
#include <stdio.h>
#include <time.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <string.h>

#define BUFFER_SIZE 1023

int setnonblocking(int fd){
    int old_option = fcntl(fd, F_GETFL);
    int new_option = old_option | O_NONBLOCK;
    fcntl(fd, F_SETFL, new_option);
    return old_option;
}

int unblock_connect(const char* ip, int port, int time){
    int ret = 0;
    struct sockaddr_in address;
    bzero(&address, sizeof(address));
    address.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &address.sin_addr);
    address.sin_port = htons(port);

    int sockfd = socket(PF_INET, SOCK_STREAM, 0);
    int fdopt = setnonblocking(sockfd);
    ret = connect(sockfd, (struct sockaddr*)&address, sizeof(address));
    if(ret == 0){
        printf("connect with server immediately\n");
        fcntl(sockfd, F_SETFL, fdopt);
        return sockfd;
    }
    else if(errno != EINPROGRESS){
        printf("unblock connect not support\n");
        return -1;
    }
    fd_set readfds;
    fd_set writefds;
    struct timeval timeout;

    FD_ZERO(&readfds);
    FD_SET(sockfd, &writefds);

    ret = select(sockfd + 1, NULL, &writefds, NULL, &timeout);
    if(ret <= 0){
        printf("connection time out\n");
        close(sockfd);
        return -1;
    }
    if(!FD_ISSET(sockfd, &writefds)){
        printf("no events on sockfd found\n");
        close(sockfd);
        return -1;
    }
    int error = 0;
    socklen_t length = sizeof(error);
    if(getsockopt(sockfd, SOL_SOCKET, SO_ERROR, &error, &length) < 0){
        printf("get socket option failed\n");
        close(sockfd);
        return -1;
    }
    if(error != 0){
        printf("connection failed after select with the error: %d\n", error);
        return -1;
    }
    printf("connection ready after select with the socket: %d\n", sockfd);
    fcntl(sockfd, F_SETFL, fdopt);
    return sockfd;
}
int main(int argc, char* argv[]){
    if(argc < 2){
        printf("usage: %s ip_address port_number\n", basename(argv[0]));
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi(argv[2]);

    int sockfd = unblock_connect(ip, port, 10);
    if(sockfd < 0){
        return 1;
    }
    close(sockfd);
    return 0;
}

编写测试用的server用例：

#include <sys/socket.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <assert.h>
#include <unistd.h>
#include <stdio.h>


int main(int argc, char* argv[]){
    const char* ip = argv[1];
    int port = atoi(argv[2]);
    
    struct sockaddr_in addr;
    bzero(&addr, sizeof(addr));
    addr.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &addr.sin_addr);
    addr.sin_port = htons(port);

    int listenfd = socket(PF_INET, SOCK_STREAM, 0);
    assert(listenfd >= 0);

    int ret = 0;
    ret = bind(listenfd, (struct sockaddr*)&addr, sizeof(addr));
    assert(ret != -1);

    ret = listen(listenfd, 5);

    for(int i = 0; i < 5; i++){
        struct sockaddr_in client;
        socklen_t len = sizeof(client);
        ret = accept(listenfd, (struct sockaddr*)&client, &len);
        printf("accpet %d", i);
        if(ret != -1){
            printf("\terror\n");
        }
        else printf("\tsuccess\n");
        close(ret);
    }
    close(listenfd);
    return 0;
}

运行

运行环境：Ubuntu 18.04.4 LTS

启动server：

modao@modao-hp:~/socket_io$ ./server 127.0.0.1 4523
accpet 0        error
accpet 1        error
accpet 2        error
accpet 3        error
accpet 4        error

使用client尝试connect：

modao@modao-hp:~/socket_io$ ./nonblocking_connect 127.0.0.1 4523
connection time out
modao@modao-hp:~/socket_io$ ./nonblocking_connect 127.0.0.1 4523
connection time out
modao@modao-hp:~/socket_io$ ./nonblocking_connect 127.0.0.1 4523
connection ready after select with the socket: 3
modao@modao-hp:~/socket_io$ ./nonblocking_connect 127.0.0.1 4523
connection time out
modao@modao-hp:~/socket_io$ ./nonblocking_connect 127.0.0.1 4523
connection ready after select with the socket: 3
modao@modao-hp:~/socket_io$ ./nonblocking_connect 127.0.0.1 4523
connection failed after select with the error: 111

client连接五次后server端关闭。

结论

非阻塞的connect存在几处移植性问题：

1. 非阻塞的socket可能导致connect始终失败**（验证确实如此）**
2. select对处于EINPROGRESS状态下的socket可能不起作用
3. 对于出错的socket，getsockopt在有些系统（比如Linux）上返回-1，而在有些系统（比如源自伯克利的UNIX）上则返回0

原书

• Linux高性能服务器编程，游双，9.5