c 套接字多次 send/recv 调用

Question

我正在学习 C 套接字编程。我对此很陌生，所以如果答案看起来很明显，请原谅我。所以基本上我希望客户端向服务器发送两个字符串（用户名和密码）。问题是服务器只收到用户名然后关闭连接。

谁能解释为什么会这样，我怎样才能让它按预期工作？

客户：

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <signal.h>
#include <ctype.h>          
#include <arpa/inet.h>
#include <netdb.h>

#define PORT 20000
#define LENGTH 512 


void error(const char *msg)
{
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
    /* Variable Definition */
    int sockfd; 
    int nsockfd;
    char revbuf[LENGTH]; 
    struct sockaddr_in remote_addr;

    /* Get the Socket file descriptor */
    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
        fprintf(stderr, "ERROR: Failed to obtain Socket Descriptor! (errno = %d)\n",errno);
        exit(1);
    }

    /* Fill the socket address struct */
    remote_addr.sin_family = AF_INET; 
    remote_addr.sin_port = htons(PORT); 
    inet_pton(AF_INET, "127.0.0.1", &remote_addr.sin_addr); 
    bzero(&(remote_addr.sin_zero), 8);

    /* Try to connect the remote */
    if (connect(sockfd, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr)) == -1)
    {
        fprintf(stderr, "ERROR: Failed to connect to the host! (errno = %d)\n",errno);
        exit(1);
    }
    else 
        printf("[Client] Connected to server at port %d...ok!\n", PORT);

    char username[20], pw[20],buff;
    int rc,rc2;

    printf("username: ");
    scanf("%s",username);  
    rc=send(sockfd, username, strlen(username)+1, 0);
    printf("\n");
    printf("password: ");
    scanf("%s",pw);     
    rc2=send(sockfd, pw, strlen(pw)+1, 0);


    close (sockfd);
    printf("[Client] Connection lost.\n");
    return (0);
}

服务器：

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <signal.h>
#include <ctype.h>          
#include <arpa/inet.h>
#include <netdb.h>

#define PORT 20000 
#define BACKLOG 5
#define LENGTH 512 

void error(const char *msg)
{
    perror(msg);
    exit(1);
}

int main ()
{
    /* Defining Variables */
    int sockfd; 
    int nsockfd; 
    int num;
    int sin_size; 
    struct sockaddr_in addr_local; /* client addr */
    struct sockaddr_in addr_remote; /* server addr */
    char revbuf[LENGTH]; // Receiver buffer

    /* Get the Socket file descriptor */
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1 )
    {
        fprintf(stderr, "ERROR: Failed to obtain Socket Descriptor. (errno = %d)\n", errno);
        exit(1);
    }
    else 
        printf("[Server] Obtaining socket descriptor successfully.\n");

    /* Fill the client socket address struct */
    addr_local.sin_family = AF_INET; // Protocol Family
    addr_local.sin_port = htons(PORT); // Port number
    addr_local.sin_addr.s_addr = INADDR_ANY; // AutoFill local address
    bzero(&(addr_local.sin_zero), 8); // Flush the rest of struct

    /* Bind a special Port */
    if( bind(sockfd, (struct sockaddr*)&addr_local, sizeof(struct sockaddr)) == -1 )
    {
        fprintf(stderr, "ERROR: Failed to bind Port. (errno = %d)\n", errno);
        exit(1);
    }
    else 
        printf("[Server] Binded tcp port %d in addr 127.0.0.1 sucessfully.\n",PORT);

    /* Listen remote connect/calling */
    if(listen(sockfd,BACKLOG) == -1)
    {
        fprintf(stderr, "ERROR: Failed to listen Port. (errno = %d)\n", errno);
        exit(1);
    }
    else
        printf ("[Server] Listening the port %d successfully.\n", PORT);

    int success = 0;
    while(1)
    {
        sin_size = sizeof(struct sockaddr_in);

        if ((nsockfd = accept(sockfd, (struct sockaddr *)&addr_remote, &sin_size)) == -1) 
        {
            fprintf(stderr, "ERROR: Obtaining new Socket Despcritor. (errno = %d)\n", errno);
            exit(1);
        }
        else 
            printf("[Server] Server has got connected from %s.\n", inet_ntoa(addr_remote.sin_addr));

        int rc,rc2;
        char username[20],pw[20];
        rc = recv(nsockfd, username, strlen(username), 0);
        printf(username);
        rc2 = recv(nsockfd, pw, strlen(pw), 0);

        printf("\n");
        printf(pw);


            success = 1;
            close(nsockfd);
            printf("[Server] Connection with Client closed. Server will wait now...\n");
           // while(waitpid(-1, NULL, WNOHANG) > 0);
        //}
    }
}

Answer 1

在服务器中： recv(nsockfd, username, strlen(username), 0);.

第三个参数必须指定缓冲区的大小。但是，username 还不包含任何数据，因此 strlen(username) 是未定义的行为，很可能 returns 是不合适的。请改用 sizeof(username)。密码也一样。

另请注意，recv 将从流中读取最多 20 个字节。因此，除非保证 username 始终为 19 个字符 + '[=16=]'，否则第一个 recv 可能（有点意外）也从 password 中读取一些字符，从第二 recv。 TCP 流中没有消息边界。

c 套接字多次 send/recv 调用

c sockets multiple send/recv calls

c

sockets

tcp

send

recv