int mkdir(char * dir,int mode):目录创建函数
运用条件：只能在已存在的目录下建立一级子目录
返回值：
返回0表示成功，返回-1表述出错。
头文件：sys/stat.h
mode 表示新目录的权限，可以取以下值：

S_IRUSR
S_IREAD
Read permission bit for the owner of the file. On many systems this bit is 0400. S_IREAD is an obsolete synonym provided for BSD compatibility.
//这是对文件用户的读权限赋予

S_IWUSR
S_IWRITE
Write permission bit for the owner of the file. Usually 0200. S_IWRITE is an obsolete synonym provided for BSD compatibility.
//这是对文件用户的写权限赋予

S_IXUSR
S_IEXEC
Execute (for ordinary files) or search (for directories) permission bit for the owner of the file. Usually 0100. S_IEXEC is an obsolete synonym provided for BSD compatibility.
//这是对文件用户执行权限赋予

S_IRWXU
This is equivalent to (S_IRUSR | S_IWUSR | S_IXUSR).
//这包含了以上三种用户权限

S_IRGRP
Read permission bit for the group owner of the file. Usually 040.
//这是对文件用户组读权限赋予

S_IWGRP
Write permission bit for the group owner of the file. Usually 020.
//这是对文件用户组写权限赋予

S_IXGRP
Execute or search permission bit for the group owner of the file. Usually 010.
//这是对文件用户组执行权限赋予

S_IRWXG
This is equivalent to (S_IRGRP | S_IWGRP | S_IXGRP).
//这包含了以上三种组用户权限

S_IROTH
Read permission bit for other users. Usually 04.
//对其他用户的读操作赋予

S_IWOTH
Write permission bit for other users. Usually 02.
//对其他用户的写操作赋予

S_IXOTH
Execute or search permission bit for other users. Usually 01.
//对其他用户的执行操作赋予

S_IRWXO
This is equivalent to (S_IROTH | S_IWOTH | S_IXOTH).
//这包含了以上三种组其他用户权限赋予

S_ISUID
This is the set-user-ID on execute bit,usually 04000. See How Change Persona.
//这下面就不说明了,因为这又涉及set uid,set gid,sticky bit这三个权限,后面有一篇转贴再说明,不过这也只有我这样的菜鸟不知道的啦.

S_ISGID
This is the set-group-ID on execute bit,usually 02000. See How Change Persona.

S_ISVTX This is the sticky bit,usually 01000.