求助，《linux程序设计》 的一段代码

求助，以下是《linux程序设计》 的一段代码，单步调试时发现一个很奇怪的现象，程序运行到 36 行  ：    closedir(dp);  再单步运行，程序会跑到 18  行：   while((entry=readdir(dp))!=NULL)，好像不是很符合c语言的逻辑？


1#include<dirent.h>
  2 #include<sys/types.h>
  3 #include<string.h>
  4 #include<stdio.h>
  5 #include<stdlib.h>
  6 #include <sys/stat.h>
  7 #include <unistd.h>
  8 void printfdir(char * dir, int depth)
  9 {       DIR *dp;
 10         struct dirent  *entry;
 11         struct stat  statbuf;
 12         if((dp=opendir(dir) )==NULL)
 13                 {
 14                         fprintf(stderr,"can't open dir%s\n",dir);
 15                         return;
 16                 }
 17         chdir(dir );
 18         while((entry=readdir(dp))!=NULL)
 19                 {
 20                         lstat(entry->d_name,&statbuf);
 21                         if(S_ISDIR(statbuf.st_mode))
 22                                 {
 23                                      if((strcmp(".",entry->d_name)==0)||(    strcmp("..",entry->d_name)==0))
 24
 25
 26                                         continue;
 27                                         printf("%*s%s/\n",depth,"",entry->d_    name);
 28
 29                                         printfdir(entry->d_name,depth+4);
 30                                 }
 31                         else
 32
 33                                         printf("%*s%s/\n",depth,"",entry->d_    name);
 34                 }
 35                  chdir("..");
 36                  closedir(dp);
 37
                   }
 40 int main()
 41 {
 42         printf("scan /share/begin_linux:\n");
 43         printfdir("/share/begin_linux/",0);
 44         exit(0);
 45 }