Ccache

Article description::helps avoid repeated recompilation for the same C and C++ object files by fetching result from a cache directory.

Compiler cache is typically useful for:


 * Developers who rebuild the same/similar codebase multiple times and use to test patches.
 * Users who frequently play with USE-flag changes and end up rebuilding the same packages multiple times.
 * Users who use live ebuilds extensively.

Initial setup
Create the cache directory:

Set initial configuration file:

Enable support in :

Done! From now on all builds will try to reuse object files from cache.

Man page
Manual page for (see ) is a great source of various knobs to make caching more robust and aggressive.

General notes
works by prepending to  variable:

triggers the same behavior in.

You can also enable for your current user and reuse the same cache directory:

Useful variables and commands
Some variables:


 * Variable points to cache root directory.
 * Variable allows evicting old cache entries with new entries:

See for many more variables.

Some commands:


 * Command shows cache hit statistics:


 * Command drops all caches:

See for many more commands.

is a wrapper
For before   used to be a wrapper binary. This means by default ccache is not safe to use and leads to errors like. will fix it.

To pass through a binary I suggest using

in all your files. also has a nice side-effect of not invalidating your cache if compiler itself was rebuilt without version changes.

Caveats
Before using advanced options make sure you understand what is being used as a cache key by. By default these are:


 * Timestamp and size of a compiler binary (beware of shell and binary wrappers)
 * Compiler options used
 * Contents of a source file
 * Contents of all include files used for compilation