一道Unix面试题

Unix环境高级编程上的一道课后习题 也是一道面试题

the   original   question:
Does   the   UNIX   System   have   a   fundamental   limitation   on   the   depth   of   a   directory   tree?   To   find  
out,   write   a   program   that   creates   a   directory   and   then   changes   to   that   directory,   in   a   loop.  
Make   certain   that   the   length   of   the   absolute   pathname   of   the   leaf   of   this   directory   is  
greater   than   your   system's   PATH_MAX   limit.   Can   you   call   getcwd   to   fetch   the   directory's  
pathname?   How   do   the   standard   UNIX   System   tools   deal   with   this   long   pathname?   Can   you  
archive   the   directory   using   either   tar   or   cpio?

translate:
UNIX系统的目录树有没有深度上的限制？在一个循环中创建一个目录并且进入到那个目录，写这样的程序
来验证。确认目录分支的绝对路径长度大于系统定义的PATH_MAX限制。你能否调用getcwd去获取此目录的
路径？标准UNIX系统工具是怎么处理这种长路径的？你能否使用tar或cpio访问此目录？

doubt：
1.系统的PATH_MAX限制是多少？PATH_MAX是指目录深度，还是指路径字符串长度？
2.处理长路径的工具有哪些？
3.cpio是什么命令？
    cpio是一个备份工具,具体的可以通过man   cpio来获取帮助.

testing   program:
#include   <dirent.h>
#include   <unistd.h>
#include   <fcntl.h>
#include   <sys/types.h>
#include   <sys/stat.h>

//   (part   1.)
//   if   the   PATH_MAX   is   the   depth   of   the   directory   tree.
#define   PATH_TEST   (PATH_MAX+1)

int   mian(void)
{
int   depth   =   0;
char   pathname[PATH_MAX*4];
char   temp[PATH_MAX*4];
printf("the   PATH_MAX's   value   is   %d./n",   PATH_MAX);
while(depth++   <   PATH_TEST)
{
sprintf(temp,   "//%d",   depth);
strcat(pathname,   temp);
ret   =   mkdir(pathname,   777);
if   (ret   ==   -1)
{
printf("the   max   depth   is   %d./n",   depth);
printf("the   failed   pathname   is   /"%s/"./n",   pathname);
break;
}
}
return   1;
}

//   (part   2.)
//   if   the   PATH_MAX   is   the   length   of   the   directory   pathname.
#define   PATH_TEST   (PATH_MAX*2)

int   main(void)
{
int   length   =   0;
int   depth   =   0;
char   pathname[PATH_TEST];
char   temp[PATH_TEST];
printf("the   PATH_MAX's   value   is   %d./n",   PATH_MAX);
while(length   <   PATH_TEST)
{
sprintf(temp,   "//%d",   depth++);
strcat(pathname,   temp);
length   =   strlen(pathname);
ret   =   mkdir(pathname,   777);
if   (ret   ==   -1)
{
printf("the   max   depth   is   %d./n",   depth);
printf("the   max   length   is   %d./n",   length);
printf("the   failed   pathname   is   /"%s/"./n",   pathname);
break;
}
}
return   1;
}

additional   requirement: 

2.   shell脚本的程序：

path=./
count=1
while   [   1   ]
do
path=$path/1234567890
count=`   expr   $count   +   1   `
echo   $count
mkdir   $path
if   [   !   $?   -eq   0   ]
then
break
fi

count是间目录的深度,1234567890是目录名,一层一层往下建,有的机器上可以建378层.
PATH_MAX=4096,为路径字符串长度。
但可以在控制台命令行环境下进入到最底层，建立下级目录，此时目录绝对路径字符串长度已经超过PATH_MAX。

内核对目录的深度没有内在的限制,但是如果路径名的长度超出了PATH_MAX,则有许多命令会失败.