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14.9.3 Testing the Type of a File
The file mode, stored in the st_mode field of the fileattributes, contains two kinds of information: the file type code, andthe 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 seeThe Mode Bits for Access Permission.
There are two ways you can access the file type information in a filemode. Firstly, for each file type there is a predicate macrowhich examines a given file mode and returns whether it is of that typeor not. Secondly, you can mask out the rest of the file mode to leavejust the file type code, and compare this against constants for each ofthe supported file types.
All of the symbols listed in this section are defined in the header filesys/stat.h.
The following predicate macros test the type of a file, given the valuem which is the st_mode field returned by stat onthat file:
- Macro: int S_ISDIR (mode_t m) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This macro returns non-zero if the file is a directory.
- Macro: int S_ISCHR (mode_t m) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This macro returns non-zero if the file is a character special file (adevice like a terminal).
- Macro: int S_ISBLK (mode_t m) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This macro returns non-zero if the file is a block special file (a devicelike a disk).
- Macro: int S_ISREG (mode_t m) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This macro returns non-zero if the file is a regular file.
- Macro: int S_ISFIFO (mode_t m) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This macro returns non-zero if the file is a FIFO special file, or apipe. See Pipes and FIFOs.
- Macro: int S_ISLNK (mode_t m) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This macro returns non-zero if the file is a symbolic link.See Symbolic Links.
- Macro: int S_ISSOCK (mode_t m) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
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 forcompatibility with BSD. The mode can be bitwise AND-ed withS_IFMT to extract the file type code, and compared to theappropriate 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 canbe implemented as objects in the filesystem. These are message queues,semaphores, and shared memory objects. To allow differentiating theseobjects from other files the POSIX standard introduced three new testmacros. But unlike the other macros they do not take the value of thest_mode field as the parameter. Instead they expect a pointer tothe whole struct stat structure.
- Macro: int S_TYPEISMQ (struct stat *s) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
If the system implements POSIX message queues as distinct objects and thefile 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) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
If the system implements POSIX semaphores as distinct objects and thefile 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) ¶
Preliminary:| MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
If the system implements POSIX shared memory objects as distinct objectsand the file is a shared memory object, this macro returns a non-zerovalue. In all other cases the result is zero.
