11.5. System Options

As well as specifying limits for various system resources, SUSv3 also specifies various options that a UNIX implementation may support. These include support for features such as realtime signals, POSIX shared memory, job control, and POSIX threads. With a few exceptions, implementations are not required to support these options. Instead, SUSv3 allows an implementation to advise—at both compile time and run time—whether it supports a particular feature.

An implementation can advertise support for a particular SUSv3 option at compile time by defining a corresponding constant in <unistd.h>. Each such constant starts with a prefix that indicates the standard from which it originates (e.g., _POSIX_ or _XOPEN_).