GNU Info

(libc.info)Testing File Type

---

Next: File Owner Prev: Reading Attributes Up: File Attributes

---

Enter node , (file) or (file)node

Testing the Type of a File
--------------------------

   The "file mode", stored in the `st_mode' field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits.  This section discusses only the type code,
which you can use to tell whether the file is a directory, socket,
symbolic link, and so on.  For details about access permissions see
Note: Permission Bits.

   There are two ways you can access the file type information in a file
mode.  Firstly, for each file type there is a "predicate macro" which
examines a given file mode and returns whether it is of that type or
not.  Secondly, you can mask out the rest of the file mode to leave
just the file type code, and compare this against constants for each of
the supported file types.

   All of the symbols listed in this section are defined in the header
file `sys/stat.h'.

   The following predicate macros test the type of a file, given the
value M which is the `st_mode' field returned by `stat' on that file:

 - Macro: int S_ISDIR (mode_t M)
     This macro returns non-zero if the file is a directory.

 - Macro: int S_ISCHR (mode_t M)
     This macro returns non-zero if the file is a character special
     file (a device like a terminal).

 - Macro: int S_ISBLK (mode_t M)
     This macro returns non-zero if the file is a block special file (a
     device like a disk).

 - Macro: int S_ISREG (mode_t M)
     This macro returns non-zero if the file is a regular file.

 - Macro: int S_ISFIFO (mode_t M)
     This macro returns non-zero if the file is a FIFO special file, or
     a pipe.  Note: Pipes and FIFOs.

 - Macro: int S_ISLNK (mode_t M)
     This macro returns non-zero if the file is a symbolic link.  Note:
     Symbolic Links.

 - Macro: int S_ISSOCK (mode_t M)
     This macro returns non-zero if the file is a socket.  Note:
     Sockets.

   An alternate non-POSIX method of testing the file type is supported
for compatibility with BSD.  The mode can be bitwise AND-ed with
`S_IFMT' to extract the file type code, and compared to the appropriate
constant.  For example,

     S_ISCHR (MODE)

is equivalent to:

     ((MODE & S_IFMT) == S_IFCHR)

 - Macro: int S_IFMT
     This is a bit mask used to extract the file type code from a mode
     value.

   These are the symbolic names for the different file type codes:

`S_IFDIR'
     This is the file type constant of a directory file.

`S_IFCHR'
     This is the file type constant of a character-oriented device file.

`S_IFBLK'
     This is the file type constant of a block-oriented device file.

`S_IFREG'
     This is the file type constant of a regular file.

`S_IFLNK'
     This is the file type constant of a symbolic link.

`S_IFSOCK'
     This is the file type constant of a socket.

`S_IFIFO'
     This is the file type constant of a FIFO or pipe.

   The POSIX.1b standard introduced a few more objects which possibly
can be implemented as object in the filesystem.  These are message
queues, semaphores, and shared memory objects.  To allow
differentiating these objects from other files the POSIX standard
introduces three new test macros.  But unlike the other macros it does
not take the value of the `st_mode' field as the parameter.  Instead
they expect a pointer to the whole `struct stat' structure.

 - Macro: int S_TYPEISMQ (struct stat *S)
     If the system implement POSIX message queues as distinct objects
     and the file is a message queue object, this macro returns a
     non-zero value.  In all other cases the result is zero.

 - Macro: int S_TYPEISSEM (struct stat *S)
     If the system implement POSIX semaphores as distinct objects and
     the file is a semaphore object, this macro returns a non-zero
     value.  In all other cases the result is zero.

 - Macro: int S_TYPEISSHM (struct stat *S)
     If the system implement POSIX shared memory objects as distinct
     objects and the file is an shared memory object, this macro
     returns a non-zero value.  In all other cases the result is zero.