原型：int mkdir (const char *filename, mode_t mode)
返回0表示成功，返回-1表述出错。使用该函数需要包含头文件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.
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.
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.
S_IRWXU
This is equivalent to (S_IRUSR | S_IWUSR | S_IXUSR).
S_IRGRP
Read permission bit for the group owner of the file. Usually 040.
S_IWGRP
Write permission bit for the group owner of the file. Usually 020.
S_IXGRP
Execute or search permission bit for the group owner of the file. Usually 010.
S_IRWXG
This is equivalent to (S_IRGRP | S_IWGRP | S_IXGRP).
S_IROTH
Read permission bit for other users. Usually 04.
S_IWOTH
Write permission bit for other users. Usually 02.
S_IXOTH
Execute or search permission bit for other users. Usually 01.
S_IRWXO
This is equivalent to (S_IROTH | S_IWOTH | S_IXOTH).
S_ISUID
This is the set-user-ID on execute bit, usually 04000. See How Change Persona.
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.
