c - 套接字编程客户端/服务器

Question

我是套接字编程的新手，正在尝试弄清楚如何在客户端和服务器之间建立简单的连接。

我找到了一个解释它的网站。 http://www.tutorialspoint.com/unix_sockets/socket_client_example.htm

我尝试在我的 ubuntu 终端中执行代码(gcc -o client client.c)

但是它打印出以下错误信息

client.c In function 'main' :
client.c:32:12: warning: assignment makes pointer from integer without a cast[enabled by defaulted]
client.c:40:25: error : dereferencing pointer to incomplete type
client.c:40:23: error : dereferencing pointer to incomplete type
client.c 46:5: warning: passing argument 2 of 'connect' from incompatible pointer type [enabled by default]

In file included from client c:1:0:
/usr/include/x86_64-linux-gnu/sys/socket.h:138:12: note:expected 'const struct sockaddr *' but argument is of type 'stuct sockaddr_in'

来自网站的client.c代码

#include <stdio.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>

int main(int argc, char *argv[])
{
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;

    char buffer[256];

    if (argc < 3) {
        fprintf(stderr,"usage %s hostname port\n", argv[0]);
        exit(0);
    }
    portno = atoi(argv[2]);
    /* Create a socket point */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
    {
        perror("ERROR opening socket");
       exit(1);
    }

   //warning: assignment makes pointer from integer without a cast[enabled by defaulted]
   server = gethostbyname(argv[1]);

   if (server == NULL) {
        fprintf(stderr,"ERROR, no such host\n");
        exit(0);
   }

    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;

    //dereferencing pointer to incomplete type
    bcopy((char *)server->h_addr,(char *)&serv_addr.sin_addr.s_addr,server->h_length);
    serv_addr.sin_port = htons(portno);

    /* Now connect to the server */
    if (connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr)) < 0) 
    {
         perror("ERROR connecting");
         exit(1);
    }   
    /* Now ask for a message from the user, this message
    * will be read by server
    */
    printf("Please enter the message: ");
    bzero(buffer,256);
    fgets(buffer,255,stdin);
    /* Send message to the server */
    n = write(sockfd,buffer,strlen(buffer));
    if (n < 0) 
    {
         perror("ERROR writing to socket");
         exit(1);
    }
    /* Now read server response */
    bzero(buffer,256);
    n = read(sockfd,buffer,255);
   if (n < 0) 
    {
         perror("ERROR reading from socket");
         exit(1);
    }
    printf("%s\n",buffer);
    return 0;
}

我有点迷茫，需要帮助。提前致谢

Answer 1

不是一个完整的答案，但我的建议是用 more warnings turned on (e.g. with -Wall) 编译:

$ gcc -Wall -o client client.cclient.c: In function `main':client.c:16: warning: implicit declaration of function `exit'client.c:18: warning: implicit declaration of function `atoi'client.c:26: warning: implicit declaration of function `gethostbyname'client.c:26: warning: assignment makes pointer from integer without a castclient.c:32: warning: implicit declaration of function `bzero'client.c:34: warning: implicit declaration of function `bcopy'client.c:34: error: dereferencing pointer to incomplete typeclient.c:34: error: dereferencing pointer to incomplete typeclient.c:38: warning: passing arg 2 of `connect' from incompatible pointer typeclient.c:50: warning: implicit declaration of function `write'client.c:50: warning: implicit declaration of function `strlen'client.c:58: warning: implicit declaration of function `read'$ 

This makes a lot of the problems much clearer.

For all the implicit declaration of function warnings, just look up the relevant manpage to find what header you need:

$ man -s3 exit | grep '#include'
       #include <stdlib.h>
$ man -s2 write | grep '#include'
       #include <unistd.h>
$ man -s3 gethostbyname | grep '#include'
       #include <netdb.h>
       #include <sys/socket.h>       /* for AF_INET */
$ 

一般来说，这些类型的调用将在联机帮助页的第 2 或第 3 部分中，但您可能需要四处寻找。

$ man man | grep calls       2   System calls (functions provided by the kernel)       3   Library calls (functions within program libraries)$