阻止访问其他目录

Question

问题：实现具有一些基本功能的Web服务器。但是，使用此Web服务器，您不应允许任何访问当前目录上方的目录。这很明显，因为没有人能够访问当前目录，否则会引发很多安全问题。我的问题是如何在我的程序运行时进行检查或阻止更改当前目录。目前，这就是我解析收到的请求的方式。

使用当前实现，用户可以切换到当前目录上方的目录，因为没有检查。但我无法找到一种有效的方法。

void handle_request(char * request, int fd)
  {
     /* Request is processed elsewhere, an example of a request is:
      * GET ./a.out 
      * GET /a.out
      * HEAD ./index.html
      * GET /cgi-like/ls?-l&some_file.txt 
      *
      * These are basic requests, that are all in the current directory or
      * the cgi-like sub-directory.
      */
     char * token_request;
     char * token_filename;
     token_request = strtok(request," "); // Determine if GET or HEAD
     token_filename = strtok(NULL," ");

     /* This is where the check should be
      * Check if user is trying to access above the current working dir
      * (e.g GET ../filename) */

     if(strcmp(token_request, "GET") == 0 && check_cgi(token_filename))
     {
       run_cgi(token_filename,fd);
     }
     else if(strcmp(token_request, "GET") == 0)
     {
        run_get(token_filename,fd);
        exit(1);
     }
     else if(strcmp(token_request,"HEAD") == 0)
     {
        run_head(token_filename ,fd);
        exit(1);
     }
     else
     {
        printf("501 Not Implemented\n");
        return;
     }
  }