buildroot/docs/manual/ccache-support.adoc

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[[ccache]]
==== Using +ccache+ in Buildroot

http://ccache.samba.org[ccache] is a compiler cache. It stores the
object files resulting from each compilation process, and is able to
skip future compilation of the same source file (with same compiler
and same arguments) by using the pre-existing object files. When doing
almost identical builds from scratch a number of times, it can nicely
speed up the build process.

+ccache+ support is integrated in Buildroot. You just have to enable
+Enable compiler cache+ in +Build options+. This will automatically
build +ccache+ and use it for every host and target compilation.

The cache is located in the directory defined by the +BR2_CCACHE_DIR+
configuration option, which defaults to
+$HOME/.buildroot-ccache+. This default location is outside of
Buildroot output directory so that it can be shared by separate
Buildroot builds. If you want to get rid of the cache, simply remove
this directory.

You can get statistics on the cache (its size, number of hits,
misses, etc.) by running +make ccache-stats+.

The make target +ccache-options+ and the +CCACHE_OPTIONS+ variable
provide more generic access to the ccache. For example

----
# set cache limit size
make CCACHE_OPTIONS="--max-size=5G" ccache-options

# zero statistics counters
make CCACHE_OPTIONS="--zero-stats" ccache-options
----

+ccache+ makes a hash of the source files and of the compiler options.
If a compiler option is different, the cached object file will not be
used. Many compiler options, however, contain an absolute path to the
staging directory. Because of this, building in a different output
directory would lead to many cache misses.

To avoid this issue, buildroot has the +Use relative paths+ option
(+BR2_CCACHE_USE_BASEDIR+). This will rewrite all absolute paths that
point inside the output directory into relative paths. Thus, changing
the output directory no longer leads to cache misses.

A disadvantage of the relative paths is that they also end up to be
relative paths in the object file. Therefore, for example, the debugger
will no longer find the file, unless you cd to the output directory
first.

See https://ccache.samba.org/manual.html#_compiling_in_different_directories[the
ccache manual's section on "Compiling in different directories"] for
more details about this rewriting of absolute paths.

When +ccache+ is enabled in Buildroot using the +BR2_CCACHE=y+ option:

* +ccache+ is used during the Buildroot build itself

* +ccache+ is not used when building outside of Buildroot, for example
  when directly calling the cross-compiler or using the SDK

One can override this behavior using the +BR2_USE_CCACHE+ environment
variable: when set to +1+, usage of ccache is enabled (default during
the Buildroot build), when unset or set to a value different from +1+,
usage of ccache is disabled.