Profiles use.mask File

The use.mask file can be used to mark USE flags as unavailable on a particular profile. This can be useful for various reasons:

• Masking hardware-specific feature flags. For example, mmx and sse are only available on x86, altivec is only available on ppc and vis is only available on sparc v9.
• Disabling unavailable soft dependencies. A simple hypothetical example — say fooapp works on mips, but has an optional dependency (controlled by the bar flag) upon libbar, which doesn't work on mips. Then by adding the bar flag to profiles/default-linux/mips/use.mask, fooapp could be made available to mips users with the unresolvable dependency forcibly disabled.

Note that use.mask is a per-flag thing, not per package's use of a given flag. This is one of the reasons that USE flags must have a specific well defined purpose.

Updates to use.mask should be handled via the relevant arch team.

See noblah USE Flags for more discussion.