C套接字程序错误

Question

这是我的C客户端代码。不知怎的，它不起作用。当我尝试通过论证时，它起作用了 我希望程序要求用户提供hostname然后它会要求portname然后要发送的消息：

  

输入主机名：localhost
  输入端口名称：56456
  输入消息：嗨用户
  输入消息：内容是什么   输入消息：你好吗

一旦给出它的主机和端口不应该再次要求（直到重启程序）。我尝试使用do while循环，但它不起作用。 在服务器上，它将显示已发送的消息

这是我的代码：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 

void error(const char *msg)
{
    perror(msg);
    exit(0);
}

//int main(int argc, char *argv[])
int main()
{
    char *argv[256];
    int argc;
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;
    printf("\n\nEnter Hostname\n\n");
    fgets(argv[0],256,stdin);
    char buffer[256];
    if (argc < 3) {
       fprintf(stderr,"usage %s hostname port\n", argv[0]);
       exit(0);
    }
    portno = atoi(argv[2]);
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
        error("ERROR opening socket");
    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;
    bcopy((char *)server->h_addr, 
         (char *)&serv_addr.sin_addr.s_addr,
         server->h_length);
    serv_addr.sin_port = htons(portno);
    if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) 
        error("ERROR connecting");


    printf("Please enter the message: ");
    bzero(buffer,256);
    //buffer = tempFunc();
    fgets(buffer,255,stdin);
    printf("\n\nHere Goes the output\n%s",buffer);
    n = write(sockfd,buffer,strlen(buffer));
    if (n < 0) 
         error("ERROR writing to socket");
    bzero(buffer,256);
    n = read(sockfd,buffer,255);
    if (n < 0) 
         error("ERROR reading from socket");
    printf("%s\n",buffer);
    close(sockfd);
    return 0;

}

Answer 1

让我们先解决一些基本的问题 你的argv是一个指针数组，指向内存中的某个任意位置，这里程序可能会崩溃 接下来的事情是，当您使用fgets阅读输入时，您也正在阅读\n。因此localhost\n不是有效的主机名。用二进制零覆盖最后一个字符，删除\n。

int main()
{

    char hostname[256];
    char port[16];
    char buffer[256];
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;
    printf("\n\nEnter Hostname\n\n");
    fgets(hostname, 256,stdin);

    hostname[ strlen(hostname) - 1 ] = '\0';
    fgets(port, 16, stdin);
    port[ strlen(port) - 1] = '\0';

    portno = atoi(port);
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
        error("ERROR opening socket");
    server = gethostbyname(hostname);
    if (server == NULL) {
        fprintf(stderr,"ERROR, no such host\n");
    exit(0);
    //...
}

Answer 2

首先，请勿使用char * argv[256]

char buffer[256];
printf("\n\nEnter Hostname\n\n");
fgets(buffer,256,stdin);

然后检查Removing trailing newline character from fgets() input以处理fgets。

对于无限循环，请勿执行

int a=2; // Useless declaration
do
{
    // Your code
}while(a=2) // I guess you wanted (a == 2)

使用：

while(1)
{
    // Your code
}

或者

for(;;)
{
    // Your code
}

您似乎需要一些培训，尝试一些教程，在C中寻找良好实践，在编译时启用警告标志并学习使用gdb之类的调试器。

P.S：

• https://stackoverflow.com/help/asking
• https://stackoverflow.com/help/answering

Answer 3

对于命令行参数，您不应再次声明argc和argv！试试这个 -

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

void error(const char *msg)
{
        perror(msg);
        exit(0);
}

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]);
        sockfd = socket(AF_INET, SOCK_STREAM, 0);
        if (sockfd < 0)
                error("ERROR opening socket");
        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;
        bcopy((char *)server->h_addr,
                        (char *)&serv_addr.sin_addr.s_addr,
                        server->h_length);
        serv_addr.sin_port = htons(portno);
        if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0)
                error("ERROR connecting");


        printf("Please enter the message: ");
        bzero(buffer,256);
        //buffer = tempFunc();
        fgets(buffer,255,stdin);
        printf("\n\nHere Goes the output\n%s",buffer);
        n = write(sockfd,buffer,strlen(buffer));
        if (n < 0)
                error("ERROR writing to socket");
        bzero(buffer,256);
        n = read(sockfd,buffer,255);
        if (n < 0)
                error("ERROR reading from socket");
        printf("%s\n",buffer);
        close(sockfd);
        return 0;

}

并在你想要的地方添加do while循环！

Answer 4

这是与循环一起使用的代码。感谢@Coconop

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 

void error(const char *msg)
{
    perror(msg);
    exit(0);
}

int main()
{

    char hostname[256];
    char port[16];
    char buffer[256];
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;
    printf("\n\nEnter Hostname\n\n");
    fgets(hostname, 256,stdin);

    hostname[ strlen(hostname) - 1 ] = '\0';
    fgets(port, 16, stdin);
    port[ strlen(port) - 1] = '\0';

    portno = atoi(port);
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
        error("ERROR opening socket");
    server = gethostbyname(hostname);
    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;
    bcopy((char *)server->h_addr, 
         (char *)&serv_addr.sin_addr.s_addr,
         server->h_length);
    serv_addr.sin_port = htons(portno);
    if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) 
        error("ERROR connecting");
int a=2;
do {
    printf("Please enter the message: ");
    bzero(buffer,256);
    //buffer = tempFunc();
    fgets(buffer,255,stdin);
    printf("\n\nHere Goes the output\n%s",buffer);
    n = write(sockfd,buffer,strlen(buffer));
}while(a=2);
    if (n < 0) 
         error("ERROR writing to socket");
    bzero(buffer,256);
    n = read(sockfd,buffer,255);
    if (n < 0) 
         error("ERROR reading from socket");
    printf("%s\n",buffer);
    close(sockfd);
    return 0;

}

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