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
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
The following predicate macros test the type of a file, given the value
m which is the
st_mode field returned by
This macro returns non-zero if the file is a character special file (a device like a terminal).
This macro returns non-zero if the file is a block special file (a device like a disk).
This macro returns non-zero if the file is a FIFO special file, or a pipe. See Pipes and FIFOs.
This macro returns non-zero if the file is a symbolic link. See Symbolic Links.
This macro returns non-zero if the file is a socket. See 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,
is equivalent to:
((mode & S_IFMT) == S_IFCHR)
These are the symbolic names for the different file type codes:
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
struct stat structure.
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.
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.