Section 2.6. Options

2.6. Options

We saw the list of POSIX.1 options in Figure 2.5 and discussed XSI option groups in Section 2.2.3. If we are to write portable applications that depend on any of these optionally-supported features, we need a portable way to determine whether an implementation supports a given option.

Just as with limits (Section 2.5), the Single UNIX Specification defines three ways to do this.

1. Compile-time options are defined in <unistd.h>.

2. Runtime options that are not associated with a file or a directory are identified with the sysconf function.

3. Runtime options that are associated with a file or a directory are discovered by calling either the pathconf or the fpathconf function.

The options include the symbols listed in the third column of Figure 2.5, as well as the symbols listed in Figures 2.17 and 2.18. If the symbolic constant is not defined, we must use sysconf, pathconf, or fpathconf to determine whether the option is supported. In this case, the name argument to the function is formed by replacing the _POSIX at the beginning of the symbol with _SC or _PC. For constants that begin with _XOPEN, the name argument is formed by prepending the string with _SC or _PC. For example, if the constant _POSIX_THREADS is undefined, we can call sysconf with the name argument set to _SC_THREADS to determine whether the platform supports the POSIX threads option. If the constant _XOPEN_UNIX is undefined, we can call sysconf with the name argument set to _SC_XOPEN_UNIX to determine whether the platform supports the XSI extensions.

Figure 2.17. Options and name arguments to sysconf

Name of option	Description	name argument
_POSIX_JOB_CONTROL	indicates whether the implementation supports job control	_SC_JOB_CONTROL
_POSIX_READER_WRITER_LOCKS	indicates whether the implementation supports readerwriter locks	_SC_READER_WRITER_LOCKS
_POSIX_SAVED_IDS	indicates whether the implementation supports the saved set-user-ID and the saved set-group-ID	_SC_SAVED_IDS
_POSIX_SHELL	indicates whether the implementation supports the POSIX shell	_SC_SHELL
_POSIX_VERSION	indicates the POSIX.1 version	_SC_VERSION
_XOPEN_CRYPT	indicates whether the implementation supports the XSI encryption option group	_SC_XOPEN_CRYPT
_XOPEN_LEGACY	indicates whether the implementation supports the XSI legacy option group	_SC_XOPEN_LEGACY
_XOPEN_REALTIME	indicates whether the implementation supports the XSI real-time option group	_SC_XOPEN_REALTIME
_XOPEN_REALTIME_THREADS	indicates whether the implementation supports the XSI real-time threads option group	_SC_XOPEN_REALTIME_THREADS
_XOPEN_VERSION	indicates the XSI version	_SC_XOPEN_VERSION

Figure 2.18. Options and name arguments to pathconf and fpathconf

Name of option	Description	name argument
_POSIX_CHOWN_RESTRICTED	indicates whether use of chown is restricted	_PC_CHOWN_RESTRICTED
_POSIX_NO_TRUNC	indicates whether pathnames longer than NAME_MAX generate an error	_PC_NO_TRUNC
_POSIX_VDISABLE	if defined, terminal special characters can be disabled with this value	_PC_VDISABLE
_POSIX_ASYNC_IO	indicates whether asynchronous I/O can be used with the associated file	_PC_ASYNC_IO
_POSIX_PRIO_IO	indicates whether prioritized I/O can be used with the associated file	_PC_PRIO_IO
_POSIX_SYNC_IO	indicates whether synchronized I/O can be used with the associated file	_PC_SYNC_IO

If the symbolic constant is defined by the platform, we have three possibilities.

1. If the symbolic constant is defined to have the value 1, then the corresponding option is unsupported by the platform.

2. If the symbolic constant is defined to be greater than zero, then the corresponding option is supported.

3. If the symbolic constant is defined to be equal to zero, then we must call sysconf, pathconf, or fpathconf to determine whether the option is supported.

Figure 2.17 summarizes the options and their symbolic constants that can be used with sysconf, in addition to those listed in Figure 2.5.

The symbolic constants used with pathconf and fpathconf are summarized in Figure 2.18. As with the system limits, there are several points to note regarding how options are treated by sysconf, pathconf, and fpathconf.

1. The value returned for _SC_VERSION indicates the four-digit year and two-digit month of the standard. This value can be 198808L, 199009L, 199506L, or some other value for a later version of the standard. The value associated with Version 3 of the Single UNIX Specification is 200112L.

2. The value returned for _SC_XOPEN_VERSION indicates the version of the XSI that the system complies with. The value associated with Version 3 of the Single UNIX Specification is 600.

3. The values _SC_JOB_CONTROL, _SC_SAVED_IDS, and _PC_VDISABLE no longer represent optional features. As of Version 3 of the Single UNIX Specification, these features are now required, although these symbols are retained for backward compatibility.

4. _PC_CHOWN_RESTRICTED and _PC_NO_TRUNC return 1 without changing errno if the feature is not supported for the specified pathname or filedes.

5. The referenced file for _PC_CHOWN_RESTRICTED must be either a file or a directory. If it is a directory, the return value indicates whether this option applies to files within that directory.

6. The referenced file for _PC_NO_TRUNC must be a directory. The return value applies to filenames within the directory.

7. The referenced file for _PC_VDISABLE must be a terminal file.

In Figure 2.19 we show several configuration options and their corresponding values on the four sample systems we discuss in this text. Note that several of the systems haven't yet caught up to the latest version of the Single UNIX Specification. For example, Mac OS X 10.3 supports POSIX threads but defines _POSIX_THREADS as

    #define _POSIX_THREADS 

without specifying a value. To conform to Version 3 of the Single UNIX Specification, the symbol, if defined, should be set to -1, 0, or 200112.

Figure 2.19. Examples of configuration options

Limit	FreeBSD 5.2.1	Linux 2.4.22	Mac OS X 10.3	Solaris 9
				UFS file system	PCFS file system
_POSIX_CHOWN_RESTRICTED	1	1	1	1	1
_POSIX_JOB_CONTROL	1	1	1	1	1
_POSIX_NO_TRUNC	1	1	1	1	unsupported
_POSIX_SAVED_IDS	unsupported	1	unsupported	1	1
_POSIX_THREADS	200112	200112	defined	1	1
_POSIX_VDISABLE	255	0	255	0	0
_POSIX_VERSION	200112	200112	198808	199506	199506
_XOPEN_UNIX	unsupported	1	undefined	1	1
_XOPEN_VERSION	unsupported	500	undefined	3	3

An entry is marked as "undefined" if the feature is not defined, i.e., the system doesn't define the symbolic constant or its corresponding _PC or _SC name. In contrast, the "defined" entry means that the symbolic constant is defined, but no value is specified, as in the preceding _POSIX_THREADS example. An entry is "unsupported" if the system defines the symbolic constant, but it has a value of -1, or it has a value of 0 but the corresponding sysconf or pathconf call returned -1.

Note that pathconf returns a value of 1 for _PC_NO_TRUNC when used with a file from a PCFS file system on Solaris. The PCFS file system supports the DOS format (for floppy disks), and DOS filenames are silently truncated to the 8.3 format limit that the DOS file system requires.