c - 在C语言中使用套接字创建服务器和客户端应用程序检查文件

Question

我对C语言了解很浅。实际上，我需要使用套接字客户端服务器编写应用程序来验证文件的存在。我有一个套接字示例，其中客户端发送到服务器字符“A”，服务器接收该字符，递增它并将“B”发送到客户端。客户端打印结果。我想根据我的需要重新制作这个例子。但考虑到我的知识，这并不像我想象的那么容易。我非常感谢您能为我提供的任何帮助。

客户端:

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>

int main(){
    int sockfd;
    int len;
    struct sockaddr_un address;
    int result;
    char ch='A';

    const char *filename = "/home/nikolay/test.txt";

    sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
    address.sun_family = AF_UNIX;
    strcpy(address.sun_path, "server_socket");
    len = sizeof(address);
    result = connect(sockfd, (struct sockaddr *)&address, len);
    if(result == -1){
        perror("oops : client");
        exit(1);
    }

    //want to send filename to server
    write(sockfd, &filename, sizeof(filename));

    //want to receive result int value from server
    read(sockfd, &result, sizeof(result));

    if(result == 0){
        printf("file %s\t exist\n", filename);
        printf("result = %d\n", result);
    }else{
        printf("File %s\t not exist\n", filename);
        printf("result = %d\n", result);
    }

    close(sockfd);
    exit(0);
}

服务器:

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>



int main() {
    int server_sockfd, client_sockfd;
    int server_len, client_len;
    struct sockaddr_un server_address;
    struct sockaddr_un client_address;

    unlink("server_socket");
    server_sockfd = socket(AF_UNIX, SOCK_STREAM, 0);

    server_address.sun_family = AF_UNIX;
    strcpy(server_address.sun_path, "server_socket");
    server_len = sizeof(server_address);
    bind(server_sockfd, (struct sockaddr *)&server_address, server_len);

    listen(server_sockfd, 5);
    while(1) {
        char *filename;
        printf("server waiting\n");

        client_len = sizeof(client_address);
        client_sockfd = accept(server_sockfd,(struct sockaddr *)&client_address, &client_len);

        //read and write client data:
        read(client_sockfd, &filename, sizeof(filename));

            // how receive access to 'filename'?
        int res = access(filename, F_OK);
        //printf("filename = %s\n", filename);
        printf("res = %d\n", res);

            //send to client result check exist file
        write(client_sockfd, &res, sizeof(res));
        close(client_sockfd);
    }
}

我如何发送到服务器文件的名称(字符串)，然后服务器接收该字符串，检查该文件是否存在并将结果(int值)发送到客户端？

使用接收和发送进行一些更改。

客户端

send(sockfd, filename, strlen(filename), 0);

    printf("client: send\n");
    char* res;
    recv(sockfd, res, strlen(res), 0);
    printf("client: reseive\n");
    if(strcasecmp(res, "1") == 0){
        printf("file %s\t exist\n", filename);
    }else if(strcasecmp(res, "-1") == 0){
        printf("file %s\t not exist\n", filename);
    }

服务器:

recv(client_sockfd, filename, strlen(filename), 0);
        int res = access(filename, F_OK);
        //printf("filename = %s\n", filename);
        printf("res = %d\n", res);

        //write(client_sockfd, &res, sizeof(res));
        char* buffer;
        snprintf (buffer, 10, "%d", res);
        printf("convert to string: %s\n", buffer);
        printf("buffer len: %d\n", strlen(buffer));
        send(client_sockfd, buffer, strlen(buffer),0);

发生段错误。为什么会发生这种情况？控制台:

./client
client: send
res = -1
convert to string: -1
buffer len: 2
client: reseive
[1]+  Segmentation fault      ./server

Answer 1

Some changes, using recv and send.

  char* res;
  recv(sockfd, res, strlen(res), 0);

occured Segmentation fault error. Why did this can happened?

res 未初始化，其值不确定。然而，使用这个不确定的值作为 strlen() 的参数是错误的。让我们放弃失败的更改并让您的工作示例开始工作。

client:

  const char *filename = "/home/nikolay/test.txt";
… 
  //want to send filename to server
  write(sockfd, &filename, sizeof(filename));

这里发送的不是文件名字符，而是指向它们的指针，这在服务器的地址空间中没有意义。简单修复:

    const char filename[] = "/home/nikolay/test.txt";

服务器:

        #include <limits.h>
        char filename[PATH_MAX];

来自alk Dec 28 '13 at 15:21的评论也值得注意。