功能是判断一个路径是否为目录

在sys/stat.h中还定义了更多相关的函数

The following symbolic names for the values of type mode_t shall also be defined.

File type:

S_IFMT
Type of file.
S_IFBLK
Block special.
S_IFCHR
Character special.
S_IFIFO
FIFO special.
S_IFREG
Regular.
S_IFDIR
Directory.
S_IFLNK
Symbolic link.
S_IFSOCK
Socket. 
File mode bits:

S_IRWXU
Read, write, execute/search by owner.
S_IRUSR
Read permission, owner.
S_IWUSR
Write permission, owner.
S_IXUSR
Execute/search permission, owner.
S_IRWXG
Read, write, execute/search by group.
S_IRGRP
Read permission, group.
S_IWGRP
Write permission, group.
S_IXGRP
Execute/search permission, group.
S_IRWXO
Read, write, execute/search by others.
S_IROTH
Read permission, others.
S_IWOTH
Write permission, others.
S_IXOTH
Execute/search permission, others.
S_ISUID
Set-user-ID on execution.
S_ISGID
Set-group-ID on execution.
S_ISVTX
[XSI]  On directories, restricted deletion flag. 
The bits defined by S_IRUSR, S_IWUSR, S_IXUSR, S_IRGRP, S_IWGRP, S_IXGRP, S_IROTH, S_IWOTH, S_IXOTH, S_ISUID, S_ISGID, [XSI]   and S_ISVTX   shall be unique.

S_IRWXU is the bitwise-inclusive OR of S_IRUSR, S_IWUSR, and S_IXUSR.

S_IRWXG is the bitwise-inclusive OR of S_IRGRP, S_IWGRP, and S_IXGRP.

S_IRWXO is the bitwise-inclusive OR of S_IROTH, S_IWOTH, and S_IXOTH.

Implementations may OR other implementation-defined bits into S_IRWXU, S_IRWXG, and S_IRWXO, but they shall not overlap any of the other bits defined in this volume of IEEE Std 1003.1-2001. The file permission bits are defined to be those corresponding to the bitwise-inclusive OR of S_IRWXU, S_IRWXG, and S_IRWXO.

The following macros shall be provided to test whether a file is of the specified type. The value m supplied to the macros is the value of st_mode from a stat structure. The macro shall evaluate to a non-zero value if the test is true; 0 if the test is false.

S_ISBLK(m)
Test for a block special file.
S_ISCHR(m)
Test for a character special file.
S_ISDIR(m)
Test for a directory.
S_ISFIFO(m)
Test for a pipe or FIFO special file.
S_ISREG(m)
Test for a regular file.
S_ISLNK(m)
Test for a symbolic link.
S_ISSOCK(m)
Test for a socket.