Diagnosing Missing Features in Gentoo After USE Flag Changes

You enabled a USE flag, rebuilt the package, and the feature is still absent. In Gentoo, that usually means the flag never reached the compiler, or the installed files predate the configuration change. The useful takeaway: emerge -pv shows what Portage would build, while equery uses shows what the installed package actually contains. If those two disagree, a rebuild is pending.

Recognizable condition

The symptom is a mismatch between configuration and behavior. For example, after adding ssl to /etc/portage/package.use for net-misc/curl, the tool still cannot open HTTPS URLs, or a library that should link against OpenSSL does not. The same pattern appears when a driver, Python binding, or Qt frontend is missing after a world update.

Quick cause map

Symptom	Likely cause	Quick check
Package lacks expected feature (e.g., no HTTPS in curl)	USE flag not enabled at build time	emerge -pv net-misc/curl
Build fails after editing make.conf	Incompatible CFLAGS or removed dependency	Review the compile error and CFLAGS in /etc/portage/make.conf
New dependency not pulled in	--newuse not run after flag change	emerge -pv --update --deep --newuse @world
Flag set but installed binary unchanged	Package not rebuilt since configuration edit	equery uses net-misc/curl

Ordered diagnostic checks

1. Inspect global defaults

Run as root:

cat /etc/portage/make.conf

Look for USE and CFLAGS. Global USE applies to every package unless overridden. Aggressive CFLAGS (for example, unstable optimization levels combined with -march=native) can also cause silent miscompilation or hard build failures on specific hardware.

2. Inspect per-package overrides

Gentoo reads either the single file /etc/portage/package.use or the directory /etc/portage/package.use/. A concrete entry looks like:

net-misc/curl ssl

Check for typos in the category/package name and for conflicting entries spread across multiple files.

3. Preview the pending build

Run as root (or a user with Portage permissions):

emerge -pv net-misc/curl

Expected check: the USE line in the output includes ssl. If it does not, the flag is not being applied to this package. Risk: none; this is read-only.

4. Verify active flags on installed files

Install app-portage/gentoolkit if needed, then run:

equery uses net-misc/curl

Expected check: ssl appears in the list of active flags. If emerge -pv shows the flag but equery uses does not, the installed version was built before the change.

5. Determine whether a rebuild is pending

Run as root:

emerge -pv --update --deep --newuse @world

If net-misc/curl appears in the calculated list, the system knows the flag changed and a rebuild is required. If it does not appear, the flag may be masked, forced by the profile, or already satisfied.

Fixes tied to findings

Finding: the flag is absent from configuration

Add it to /etc/portage/package.use (or a file in /etc/portage/package.use/):

net-misc/curl ssl

Then rebuild only that package:

emerge --update --deep --newuse net-misc/curl

Risk: this changes system state and may pull in new dependencies. There is no atomic rollback; reverting means removing the flag and rebuilding again.

Finding: the flag is set but the package was not rebuilt

Run the world update as root:

emerge --update --deep --newuse @world

This rebuilds every package whose active USE set changed. Risk: long compile times and broad system changes. Verify disk space and review the merge list before confirming.

Finding: a dependency blocks the flag

Enabling ssl on curl may require a compatible dev-libs/openssl build. If Portage reports a conflict, adjust the dependency's USE flags in /etc/portage/package.use rather than forcing the parent package. Example:

dev-libs/openssl bindist

Then re-run the preview to confirm the conflict resolves.

Finding: aggressive CFLAGS cause build failures

If the build fails with compiler errors after a make.conf edit, comment out experimental flags and restore a conservative baseline such as:

CFLAGS="-O2 -pipe"

Rebuild the failed package. Risk: performance may change; this is a correctness-first trade-off.

Escalation criteria

• Portage reports an unresolvable dependency cycle or a masked USE flag that the profile forces.
• The package builds successfully but the feature is still missing at runtime, suggesting an upstream bug.
• Compiler errors persist after reverting CFLAGS to defaults.
• You need to override a profile-forced flag; this often requires a custom profile or a justified package.use entry, and is best discussed on Gentoo support channels or Bugzilla.

Limitations and verification

USE flags only affect future builds. Already-installed packages keep their compiled feature set until rebuilt. emerge -pv is a preview, not a report of the current installation. Some flags are controlled by the selected profile and cannot be changed without explicit overrides.

To verify the fix, re-run emerge -pv net-misc/curl and equery uses net-misc/curl after the rebuild. Both should list ssl. Then confirm the runtime behavior—for example, check that the tool's version or help output advertises the expected protocol or library support.