Advanced Programming in the UNIX® Environment

ptg10805159

54 UNIX Standardization and Implementations Chapter 2

For each option, we have three possibilities for a platform’s support status.

1. If the symbolic constant is either undefined or defined to have the value−1, then
   the corresponding option is unsupported by the platform at compile time. It is
   possible to run an old application on a newer system wherethe optionis
   supported, so a runtime check might indicate the option is supported even
   though the option wasn’t supported at the time the application was compiled.


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.

The symbolic constants used withpathconfandfpathconfaresummarized in

Figure2.18. Figure2.19 summarizes the nonobsolete options and their symbolic

constants that can be used withsysconf, in addition to those listed in Figure2.5. Note

that we omit options associated with utility commands.

As with the system limits, thereare several points to note regarding how options

aretreated bysysconf,pathconf,andfpathconf.

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 (the 2001 edition of
   POSIX.1). The value associated with Version 4 of the Single UNIX Specification
   (the 2008 edition of POSIX.1) is 200809L.


2. The value returned for_SC_XOPEN_VERSIONindicates the version of the XSI
   that the system supports. The value associated with Version 3 of the Single
   UNIX Specification is 600. The value associated with Version 4 of the Single
   UNIX Specification (the 2008 edition of POSIX.1) is 700.


3. The values_SC_JOB_CONTROL, _SC_SAVED_IDS,and_PC_VDISABLEno
   longer represent optional features. Although XPG4 and prior versions of the
   Single UNIX Specification required that these features be supported, Version 3
   of the Single UNIX Specification is the earliest version wherethese features are
   no longer optional in POSIX.1. These symbols areretained for backward
   compatibility.


4. Platforms conforming to POSIX.1-2008 arealso required to support the
   following options:
   
   
   • _POSIX_ASYNCHRONOUS_IO
   
   
   • _POSIX_BARRIERS
   
   
   • _POSIX_CLOCK_SELECTION
   
   
   • _POSIX_MAPPED_FILES
   
   
   • _POSIX_MEMORY_PROTECTION