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
section The Mode Bits for Access Permission.
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:
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)
These are the symbolic names for the different file type codes:
S_IFDIR
S_IFCHR
S_IFBLK
S_IFREG
S_IFLNK
S_IFSOCK
S_IFIFO
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.
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