libroot: unistd.h defines constants for unsupported POSIX features

[simonsouth]

Haiku does not yet support certain features related to POSIX threads. For this reason, unistd.h defines the _POSIX_THREAD_ATTR_STACKADDR and _POSIX_THREAD_PRIORITY_SCHEDULING constants to -1, meant to signal the absence of these features.

However, a careful reading of ​the POSIX spec suggests these constants should actually be left undefined. Referring to _POSIX_THREAD_ATTR_STACKADDR:

The following symbolic constants are always defined to unspecified values to indicate that this functionality from the POSIX Threads Extension is always present on XSI-conformant systems:

And to _POSIX_THREAD_PRIORITY_SCHEDULING:

If _XOPEN_REALTIME_THREADS is defined to have a value other than -1, then the following symbolic constants will be defined to an unspecified value to indicate that the features are supported:

Emphasis mine. The implication is that if the features are not available the constants should not be defined to any value; that is, they should be left undefined.

This seems to match ​how these constants are actually used in the wild.

The attached patch corrects the implementation by modifying

• unistd.h to undefine the two constants (they are left in the file to indicate they have not simply been forgotten) and
• conf.cpp so sysconf returns -1 when the presence of these features is tested (which matches the existing behaviour; POSIX says simply "​the results are unspecified" in this case).

These changes are needed to build ​Apache Thrift on Haiku.

[simonsouth]

libroot: Undefine constants for unsupported POSIX features

[waddlesplash]

Applied in hrev49935. Thanks!

On a side node, you're welcome to join us in the #haiku channel on Freenode.