CCACHE(1)
=========
:man source:  ccache
:man version: {revnumber}
:man manual:  ccache Manual


Name
----

ccache - a fast C/C++ compiler cache


Synopsis
--------

[verse]
*ccache* [_options_]
*ccache* _compiler_ [_compiler options_]
_compiler_ [_compiler options_]                   (via symbolic link)


Description
-----------

Ccache is a compiler cache. It speeds up recompilation by caching the result of
previous compilations and detecting when the same compilation is being done
again.

Ccache has been carefully written to always produce exactly the same compiler
output that you would get without the cache. The only way you should be able to
tell that you are using ccache is the speed. Currently known exceptions to this
goal are listed under <<_caveats,CAVEATS>>. If you discover an undocumented case
where ccache changes the output of your compiler, please let us know.


Run modes
---------

There are two ways to use ccache. You can either prefix your compilation
commands with *ccache* or you can let ccache masquerade as the compiler by
creating a symbolic link (named as the compiler) to ccache. The first method is
most convenient if you just want to try out ccache or wish to use it for some
specific projects. The second method is most useful for when you wish to use
ccache for all your compilations.

To use the first method, just make sure that *ccache* is in your *PATH*.

To use the symlinks method, do something like this:

-------------------------------------------------------------------------------
cp ccache /usr/local/bin/
ln -s ccache /usr/local/bin/gcc
ln -s ccache /usr/local/bin/g++
ln -s ccache /usr/local/bin/cc
ln -s ccache /usr/local/bin/c++
-------------------------------------------------------------------------------

And so forth. This will work as long as the directory with symlinks comes
before the path to the compiler (which is usually in `/usr/bin`). After
installing you may wish to run ``which gcc'' to make sure that the correct link
is being used.

WARNING: The technique of letting ccache masquerade as the compiler works well,
but currently doesn't interact well with other tools that do the same thing.
See _<<_using_ccache_with_other_compiler_wrappers,Using ccache with other
compiler wrappers>>_.

WARNING: Use a symbolic links for masquerading, not hard links.

Command line options
--------------------

These command line options only apply when you invoke ccache as ``ccache''.
When invoked as a compiler (via a symlink as described in the previous
section), the normal compiler options apply and you should refer to the
compiler's documentation.


Common options
~~~~~~~~~~~~~~

*`-c`*, *`--cleanup`*::

    Clean up the cache by removing old cached files until the specified file
    number and cache size limits are not exceeded. This also recalculates the
    cache file count and size totals. Normally, there is no need to initiate
    cleanup manually as ccache keeps the cache below the specified limits at
    runtime and keeps statistics up to date on each compilation. Forcing a
    cleanup is mostly useful if you manually modify the cache contents or
    believe that the cache size statistics may be inaccurate.

*`-C`*, *`--clear`*::

    Clear the entire cache, removing all cached files, but keeping the
    configuration file.

*`--config-path`* _PATH_::

    Let the subsequent command line options operate on configuration file
    _PATH_ instead of the default. Using this option has the same effect as
    setting the environment variable `CCACHE_CONFIGPATH` temporarily.

*`-d`*, *`--directory`* _PATH_::

    Let the subsequent command line options operate on cache directory _PATH_
    instead of the default. For example, to show statistics for a cache
    directory at `/shared/ccache` you can run `ccache -d /shared/ccache -s`.
    Using this option has the same effect as setting the environment variable
    `CCACHE_DIR` temporarily.

*`--evict-older-than`* _AGE_::

    Remove files older than _AGE_ from the cache. _AGE_ should be an unsigned
    integer with a `d` (days) or `s` (seconds) suffix.

*`-h`*, *`--help`*::

    Print a summary of command line options.

*`-F`* _NUM_, *`--max-files`* _NUM_::

    Set the maximum number of files allowed in the cache to _NUM_. Use 0 for no
    limit. The value is stored in a configuration file in the cache directory
    and applies to all future compilations.

*`-M`* _SIZE_, *`--max-size`* _SIZE_::

    Set the maximum size of the files stored in the cache. _SIZE_ should be a
    number followed by an optional suffix: k, M, G, T (decimal), Ki, Mi, Gi or
    Ti (binary). The default suffix is G. Use 0 for no limit. The value is
    stored in a configuration file in the cache directory and applies to all
    future compilations.

*`-X`* _LEVEL_, *`--recompress`* _LEVEL_::

    Recompress the cache to level _LEVEL_ using the Zstandard algorithm. The
    level can be an integer, with the same semantics as the
    <<config_compression_level,*compression_level*>> configuration option), or
    the special value *uncompressed* for no compression. See
    _<<_cache_compression,Cache compression>>_ for more information. This can
    potentionally take a long time since all files in the cache need to be
    visited. Only files that are currently compressed with a different level
    than _LEVEL_ will be recompressed.

*`-o`* _KEY=VALUE_, *`--set-config`* _KEY_=_VALUE_::

    Set configuration option _KEY_ to _VALUE_. See
    _<<_configuration,Configuration>>_ for more information.

*`-x`*, *`--show-compression`*::

    Print cache compression statistics. See _<<_cache_compression,Cache
    compression>>_ for more information. This can potentionally take a long
    time since all files in the cache need to be visited.

*`-p`*, *`--show-config`*::

    Print current configuration options and from where they originate
    (environment variable, configuration file or compile-time default) in
    human-readable format.

*`-s`*, *`--show-stats`*::

    Print a summary of configuration and statistics counters in human-readable
    format.

*`-V`*, *`--version`*::

    Print version and copyright information.

*`-z`*, *`--zero-stats`*::

    Zero the cache statistics (but not the configuration options).


Options for scripting or debugging
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

*`--checksum-file`* _PATH_::

    Print the checksum (64 bit XXH3) of the file at _PATH_.

*`--dump-manifest`* _PATH_::

    Dump manifest file at _PATH_ in text format to standard output. This is
    only useful when debugging ccache and its behavior.

*`--dump-result`* _PATH_::

    Dump result file at _PATH_ in text format to standard output. This is only
    useful when debugging ccache and its behavior.

*`--extract-result`* _PATH_::

    Extract data stored in the result file at _PATH_. The data will be written
    to *ccache-result.** files in to the current working directory. This is
    only useful when debugging ccache and its behavior.

*`-k`* _KEY_, *`--get-config`* _KEY_::

    Print the value of configuration option _KEY_. See
    _<<_configuration,Configuration>>_ for more information.

*`--hash-file`* _PATH_::

    Print the hash (160 bit BLAKE3) of the file at _PATH_. This is only useful
    when debugging ccache and its behavior.

*`--print-stats`*::

    Print statistics counter IDs and corresponding values in machine-parsable
    (tab-separated) format.



Extra options
~~~~~~~~~~~~~

When run as a compiler, ccache usually just takes the same command line options
as the compiler you are using. The only exception to this is the option
*--ccache-skip*. That option can be used to tell ccache to avoid interpreting
the next option in any way and to pass it along to the compiler as-is.

NOTE: *--ccache-skip* currently only tells ccache not to interpret the next
option as a special compiler option -- the option will still be included in the
direct mode hash.

The reason this can be important is that ccache does need to parse the command
line and determine what is an input filename and what is a compiler option, as
it needs the input filename to determine the name of the resulting object file
(among other things). The heuristic ccache uses when parsing the command line
is that any argument that exists as a file is treated as an input file name. By
using *--ccache-skip* you can force an option to not be treated as an input
file name and instead be passed along to the compiler as a command line option.

Another case where *--ccache-skip* can be useful is if ccache interprets an
option specially but shouldn't, since the option has another meaning for your
compiler than what ccache thinks.


Configuration
-------------

ccache's default behavior can be overridden by options in configuration files,
which in turn can be overridden by environment variables with names starting
with *CCACHE_*. Ccache normally reads configuration from two files: first a
system-level configuration file and secondly a cache-specific configuration
file. The priorities of configuration options are as follows (where 1 is
highest):

1. Environment variables.
2. The primary (cache-specific) configuration file (see below).
3. The secondary (system-wide read-only) configuration file
   *_<sysconfdir>_/ccache.conf* (typically */etc/ccache.conf* or
    */usr/local/etc/ccache.conf*).
4. Compile-time defaults.

As a special case, if the the environment variable *CCACHE_CONFIGPATH* is set
it specifies the primary configuration file and the secondary (system-wide)
configuration file won't be read.


Location of the primary configuration file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The location of the primary (cache-specific) configuration is determined like
this:

1. If *CCACHE_CONFIGPATH* is set, use that path.
2. Otherwise, if the environment variable *CCACHE_DIR* is set then use
   *$CCACHE_DIR/ccache.conf*.
3. Otherwise, if <<config_cache_dir,*cache_dir*>> is set in the secondary
   (system-wide) configuration file then use *<cache_dir>/ccache.conf*.
4. Otherwise, if there is a legacy *$HOME/.ccache* directory then use
   *$HOME/.ccache/ccache.conf*.
5. Otherwise, if *XDG_CONFIG_HOME* is set then use
   *$XDG_CONFIG_HOME/ccache/ccache.conf*.
6. Otherwise, use *%APPDATA%/ccache/ccache.conf* (Windows),
   *$HOME/Library/Preferences/ccache/ccache.conf* (macOS) or
   *$HOME/.config/ccache/ccache.conf* (other systems).


Configuration file syntax
~~~~~~~~~~~~~~~~~~~~~~~~~

Configuration files are in a simple ``key = value'' format, one option per
line. Lines starting with a hash sign are comments. Blank lines are ignored, as
is whitespace surrounding keys and values. Example:

-------------------------------------------------------------------------------
# Set maximum cache size to 10 GB:
max_size = 10G
-------------------------------------------------------------------------------

Boolean values
~~~~~~~~~~~~~~

Some configuration options are boolean values (i.e. truth values). In a
configuration file, such values must be set to the string *true* or *false*.
For the corresponding environment variables, the semantics are a bit different:

* A set environment variable means ``true'' (even if set to the empty string).
* The following case-insensitive negative values are considered an error
  (instead of surprising the user): *0*, *false*, *disable* and *no*.
* An unset environment variable means ``false''.

Each boolean environment variable also has a negated form starting with
*CCACHE_NO*. For example, *CCACHE_COMPRESS* can be set to force compression and
*CCACHE_NOCOMPRESS* can be set to force no compression.


Configuration options
~~~~~~~~~~~~~~~~~~~~~~

Below is a list of available configuration options. The corresponding
environment variable name is indicated in parentheses after each configuration
option key.

[[config_absolute_paths_in_stderr]] *absolute_paths_in_stderr* (*CCACHE_ABSSTDERR*)::

    This option specifies whether ccache should rewrite relative paths in the
    compiler's standard error output to absolute paths. This can be useful if
    you use <<config_base_dir,*base_dir*>> with a build system (e.g. CMake with
    the "Unix Makefiles" generator) that executes the compiler in a different
    working directory, which makes relative paths in compiler errors or
    warnings incorrect. The default is false.

[[config_base_dir]] *base_dir* (*CCACHE_BASEDIR*)::

    This option should be an absolute path to a directory. If set, ccache will
    rewrite absolute paths into paths relative to the current working
    directory, but only absolute paths that begin with *base_dir*. Cache
    results can then be shared for compilations in different directories even
    if the project uses absolute paths in the compiler command line. See also
    the discussion under _<<_compiling_in_different_directories,Compiling in
    different directories>>_. If set to the empty string (which is the
    default), no rewriting is done.
+
A typical path to use as *base_dir* is your home directory or another directory
that is a parent of your project directories. Don't use `/` as the base
directory since that will make ccache also rewrite paths to system header
files, which typically is contraproductive.
+
For example, say that Alice's current working directory is
`/home/alice/project1/build` and that she compiles like this:
+
-------------------------------------------------------------------------------
ccache gcc -I/usr/include/example -I/home/alice/project2/include -c /home/alice/project1/src/example.c
-------------------------------------------------------------------------------
+
Here is what ccache will actually execute for different *base_dir* values:
+
-------------------------------------------------------------------------------
# Current working directory: /home/alice/project1/build

# With base_dir = /:
gcc -I../../../../usr/include/example -I../../project2/include -c ../src/example.c

# With base_dir = /home or /home/alice:
gcc -I/usr/include/example -I../../project2/include -c ../src/example.c

# With base_dir = /home/alice/project1 or /home/alice/project1/src:
gcc -I/usr/include/example -I/home/alice/project2/include -c ../src/example.c
-------------------------------------------------------------------------------
+
If Bob has put `project1` and `project2` in `/home/bob/stuff` and both users
have set *base_dir* to `/home` or `/home/$USER`, then Bob will get a cache hit
(if they share ccache directory) since the actual command line will be
identical to that of Alice:
+
-------------------------------------------------------------------------------
# Current working directory: /home/bob/stuff/project1/build

# With base_dir = /home or /home/bob:
gcc -I/usr/include/example -I../../project2/include -c ../src/example.c
-------------------------------------------------------------------------------
+
Without *base_dir* there will be a cache miss since the absolute paths will
differ. With *base_dir* set to `/` there will be a cache miss since the
relative path to `/usr/include/example` will be different. With *base_dir* set
to `/home/bob/stuff/project1` there will a cache miss since the path to
project2 will be a different absolute path.

[[config_cache_dir]] *cache_dir* (*CCACHE_DIR*)::

    This option specifies where ccache will keep its cached compiler outputs.
    The default is *$XDG_CACHE_HOME/ccache* if *XDG_CACHE_HOME* is set,
    otherwise *$HOME/.cache/ccache*. Exception: If the legacy directory
    *$HOME/.ccache* exists then that directory is the default.
+
See also _<<_location_of_the_primary_configuration_file,Location of the primary
configuration file>>_.
+
If you want to use another *CCACHE_DIR* value temporarily for one ccache
invocation you can use the `-d/--directory` command line option instead.

[[config_compiler]] *compiler* (*CCACHE_COMPILER* or (deprecated) *CCACHE_CC*)::

    This option can be used to force the name of the compiler to use. If set to
    the empty string (which is the default), ccache works it out from the
    command line.

[[config_compiler_check]] *compiler_check* (*CCACHE_COMPILERCHECK*)::

    By default, ccache includes the modification time (``mtime'') and size of
    the compiler in the hash to ensure that results retrieved from the cache
    are accurate. This option can be used to select another strategy. Possible
    values are:
+
--
*content*::
    Hash the content of the compiler binary. This makes ccache very slightly
    slower compared to *mtime*, but makes it cope better with compiler upgrades
    during a build bootstrapping process.
*mtime*::
    Hash the compiler's mtime and size, which is fast. This is the default.
*none*::
    Don't hash anything. This may be good for situations where you can safely
    use the cached results even though the compiler's mtime or size has changed
    (e.g. if the compiler is built as part of your build system and the
    compiler's source has not changed, or if the compiler only has changes that
    don't affect code generation). You should only use *none* if you know what
    you are doing.
*string:value*::
    Hash *value*. This can for instance be a compiler revision number or
    another string that the build system generates to identify the compiler.
_a command string_::
    Hash the standard output and standard error output of the specified
    command. The string will be split on whitespace to find out the command and
    arguments to run. No other interpretation of the command string will be
    done, except that the special word *%compiler%* will be replaced with the
    path to the compiler. Several commands can be specified with semicolon as
    separator. Examples:
+
--

----
%compiler% -v
----

----
%compiler% -dumpmachine; %compiler% -dumpversion
----

You should make sure that the specified command is as fast as possible since it
will be run once for each ccache invocation.

Identifying the compiler using a command is useful if you want to avoid cache
misses when the compiler has been rebuilt but not changed.

Another case is when the compiler (as seen by ccache) actually isn't the real
compiler but another compiler wrapper -- in that case, the default *mtime*
method will hash the mtime and size of the other compiler wrapper, which means
that ccache won't be able to detect a compiler upgrade. Using a suitable
command to identify the compiler is thus safer, but it's also slower, so you
should consider continue using the *mtime* method in combination with the
*prefix_command* option if possible. See
_<<_using_ccache_with_other_compiler_wrappers,Using ccache with other compiler
wrappers>>_.
--
--

[[config_compiler_type]] *compiler_type* (*CCACHE_COMPILERTYPE*)::

    Ccache normally guesses the compiler type based on the compiler name. The
    *compiler_type* option lets you force a compiler type. This can be useful
    if the compiler has a non-standard name but is actually one of the known
    compiler types. Possible values are:
+
--
*auto*::
    Guess one of the types below based on the compiler name (following
    symlinks). This is the default.
*clang*::
    Clang-based compiler.
*gcc*::
    GCC-based compiler.
*nvcc*::
    NVCC (CUDA) compiler.
*other*::
    Any compiler other than the known types.
*pump*::
    distcc's "pump" script.
--

[[config_compression]] *compression* (*CCACHE_COMPRESS* or *CCACHE_NOCOMPRESS*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will compress data it puts in the cache. However, this
    option has no effect on how files are retrieved from the cache; compressed
    and uncompressed results will still be usable regardless of this option.
    The default is true.
+
Compression is done using the Zstandard algorithm. The algorithm is fast enough
that there should be little reason to turn off compression to gain performance.
One exception is if the cache is located on a compressed file system, in which
case the compression performed by ccache of course is redundant.
+
Compression will be disabled if file cloning (the
<<config_file_clone,*file_clone*>> option) or hard linking (the
<<config_hard_link,*hard_link*>> option) is enabled.

[[config_compression_level]] *compression_level* (*CCACHE_COMPRESSLEVEL*)::

    This option determines the level at which ccache will compress object files
    using the real-time compression algorithm Zstandard. It only has effect if
    <<config_compression,*compression*>> is enabled (which it is by default).
    Zstandard is extremely fast for decompression and very fast for compression
    for lower compression levels. The default is 0.
+
Semantics of *compression_level*:
+
--
*> 0*::
    A positive value corresponds to normal Zstandard compression levels. Lower
    levels (e.g. *1*) mean faster compression but worse compression ratio.
    Higher levels (e.g. *19*) mean slower compression but better compression
    ratio. The maximum possible value depends on the libzstd version, but at
    least up to 19 is available for all versions. Decompression speed is
    essentially the same for all levels. As a rule of thumb, use level 5 or
    lower since higher levels may slow down compilations noticeably. Higher
    levels are however useful when recompressing the cache with command line
    option *-X/--recompress*.
*< 0*::
    A negative value corresponds to Zstandard's ``ultra-fast'' compression
    levels, which are even faster than level 1 but with less good compression
    ratios. For instance, level *-3* corresponds to ``--fast=3'' for the *zstd*
    command line tool. In practice, there is little use for levels lower than
    *-5* or so.
*0* (default)::
    The value *0* means that ccache will choose a suitable level, currently
    *1*.
--
+
See the http://zstd.net[Zstandard documentation] for more information.

[[config_cpp_extension]] *cpp_extension* (*CCACHE_EXTENSION*)::

    This option can be used to force a certain extension for the intermediate
    preprocessed file. The default is to automatically determine the extension
    to use for intermediate preprocessor files based on the type of file being
    compiled, but that sometimes doesn't work. For example, when using the
    ``aCC'' compiler on HP-UX, set the cpp extension to *i*.

[[config_debug]] *debug* (*CCACHE_DEBUG* or *CCACHE_NODEBUG*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, enable the debug mode. The debug mode creates per-object debug
    files that are helpful when debugging unexpected cache misses. Note however
    that ccache performance will be reduced slightly. See
    _<<_cache_debugging,Cache debugging>>_ for more information. The default is
    false.

[[config_debug_dir]] *debug_dir* (*CCACHE_DEBUGDIR*)::

    Specifies where to write per-object debug files if the _<<config_debug,debug
    mode>>_ is enabled. If set to the empty string, the files will be written
    next to the object file. If set to a directory, the debug files will be
    written with full absolute paths in that directory, creating it if needed.
    The default is the empty string.
+
For example, if *debug_dir* is set to `/example`, the current working directory
is `/home/user` and the object file is `build/output.o` then the debug log will
be written to `/example/home/user/build/output.o.ccache-log`. See also
_<<_cache_debugging,Cache debugging>>_.

[[config_depend_mode]] *depend_mode* (*CCACHE_DEPEND* or *CCACHE_NODEPEND*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, the depend mode will be used. The default is false. See
    _<<_the_depend_mode,The depend mode>>_.

[[config_direct_mode]] *direct_mode* (*CCACHE_DIRECT* or *CCACHE_NODIRECT*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, the direct mode will be used. The default is true. See
    _<<_the_direct_mode,The direct mode>>_.

[[config_disable]] *disable* (*CCACHE_DISABLE* or *CCACHE_NODISABLE*, see _<<_boolean_values,Boolean values>>_ above)::

    When true, ccache will just call the real compiler, bypassing the cache
    completely. The default is false.

[[config_extra_files_to_hash]] *extra_files_to_hash* (*CCACHE_EXTRAFILES*)::

    This option is a list of paths to files that ccache will include in the the
    hash sum that identifies the build. The list separator is semicolon on
    Windows systems and colon on other systems.

[[config_file_clone]] *file_clone* (*CCACHE_FILECLONE* or *CCACHE_NOFILECLONE*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will attempt to use file cloning (also known as ``copy on
    write'', ``CoW'' or ``reflinks'') to store and fetch cached compiler results.
    *file_clone* has priority over <<config_hard_link,*hard_link*>>. The
    default is false.
+
Files stored by cloning cannot be compressed, so the cache size will likely be
significantly larger if this option is enabled. However, performance may be
improved depending on the use case.
+
Unlike the <<config_hard_link,*hard_link*>> option, *file_clone* is completely
safe to use, but not all file systems support the feature. For such file
systems, ccache will fall back to use plain copying (or hard links if
<<config_hard_link,*hard_link*>> is enabled).

[[config_hard_link]] *hard_link* (*CCACHE_HARDLINK* or *CCACHE_NOHARDLINK*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will attempt to use hard links to store and fetch cached
    object files. The default is false.
+
Files stored via hard links cannot be compressed, so the cache size will likely
be significantly larger if this option is enabled. However, performance may be
improved depending on the use case.
+
WARNING: Do not enable this option unless you are aware of these caveats:
+
* If the resulting file is modified, the file in the cache will also be
  modified since they share content, which corrupts the cache entry. As of
  version 4.0, ccache makes stored and fetched object files read-only as a
  safety measure guard. Furthermore, a simple integrity check is made for
  cached object files by verifying that their sizes are correct. This means
  that mistakes like `strip file.o` or `echo >file.o` will be detected even if
  the object file is made writeable, but a modification that doesn't change the
  file size will not.
* Programs that don't expect that files from two different identical
  compilations are hard links to each other can fail.
* Programs that rely on modification times (like ``make'') can be confused if
  several users (or one user with several build trees) use the same cache
  directory. The reason for this is that the object files share i-nodes and
  therefore modification times. If *file.o* is in build tree A (hard-linked
  from the cache) and *file.o* then is produced by ccache in build tree B by
  hard-linking from the cache, the modification timestamp will be updated for
  *file.o* in build tree A as well. This can retrigger relinking in build tree
  A even though nothing really has changed.

[[config_hash_dir]] *hash_dir* (*CCACHE_HASHDIR* or *CCACHE_NOHASHDIR*, see _<<_boolean_values,Boolean values>>_ above)::

    If true (which is the default), ccache will include the current working
    directory (CWD) in the hash that is used to distinguish two compilations
    when generating debug info (compiler option *-g* with variations).
    Exception: The CWD will not be included in the hash if
    <<config_base_dir,*base_dir*>> is set (and matches the CWD) and the
    compiler option *-fdebug-prefix-map* is used. See also the discussion under
    _<<_compiling_in_different_directories,Compiling in different
    directories>>_.
+
The reason for including the CWD in the hash by default is to prevent a problem
with the storage of the current working directory in the debug info of an
object file, which can lead ccache to return a cached object file that has the
working directory in the debug info set incorrectly.
+
You can disable this option to get cache hits when compiling the same source
code in different directories if you don't mind that CWD in the debug info
might be incorrect.

[[config_ignore_headers_in_manifest]] *ignore_headers_in_manifest* (*CCACHE_IGNOREHEADERS*)::

    This option is a list of paths to files (or directories with headers) that
    ccache will *not* include in the manifest list that makes up the direct
    mode. Note that this can cause stale cache hits if those headers do indeed
    change. The list separator is semicolon on Windows systems and colon on
    other systems.

[[config_ignore_options]] *ignore_options* (*CCACHE_IGNOREOPTIONS*)::

    This option is a space-delimited list of compiler options that ccache will
    exclude from the hash. Excluding a compiler option from the hash can be
    useful when you know it doesn't affect the result (but ccache doesn't know
    that), or when it does and you don't care. If a compiler option in the list
    is suffixed with an asterisk (`*`) it will be matched as a prefix. For
    example, `-fmessage-length=*` will match both `-fmessage-length=20` and
    `-fmessage-length=70`.

[[config_inode_cache]] *inode_cache* (*CCACHE_INODECACHE* or *CCACHE_NOINODECACHE*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, enables caching of source file hashes based on device, inode and
    timestamps. This will reduce the time spent on hashing included files as
    the result can be resused between compilations.
+
The feature is still experimental and thus off by default. It is currently not
available on Windows.
+
The feature requires *temporary_dir* to be located on a local filesystem.

[[config_keep_comments_cpp]] *keep_comments_cpp* (*CCACHE_COMMENTS* or *CCACHE_NOCOMMENTS*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will not discard the comments before hashing preprocessor
    output. This can be used to check documentation with *-Wdocumentation*.

[[config_limit_multiple]] *limit_multiple* (*CCACHE_LIMIT_MULTIPLE*)::

    Sets the limit when cleaning up. Files are deleted (in LRU order) until the
    levels are below the limit. The default is 0.8 (= 80%). See
    _<<_automatic_cleanup,Automatic cleanup>>_ for more information.

[[config_log_file]] *log_file* (*CCACHE_LOGFILE*)::

    If set to a file path, ccache will write information on what it is doing to
    the specified file. This is useful for tracking down problems.
+
If set to *syslog*, ccache will log using `syslog()` instead of to a file. If
you use rsyslogd, you can add something like this to `/etc/rsyslog.conf` or a
file in `/etc/rsyslog.d`:
+
-------------------------------------------------------------------------------
# log ccache to file
:programname, isequal, "ccache"         /var/log/ccache
# remove from syslog
& ~
-------------------------------------------------------------------------------

[[config_max_files]] *max_files* (*CCACHE_MAXFILES*)::

    This option specifies the maximum number of files to keep in the cache. Use
    0 for no limit (which is the default). See also
    _<<_cache_size_management,Cache size management>>_.

[[config_max_size]] *max_size* (*CCACHE_MAXSIZE*)::

    This option specifies the maximum size of the cache. Use 0 for no limit.
    The default value is 5G. Available suffixes: k, M, G, T (decimal) and Ki,
    Mi, Gi, Ti (binary). The default suffix is G. See also
    _<<_cache_size_management,Cache size management>>_.

[[config_path]] *path* (*CCACHE_PATH*)::

    If set, ccache will search directories in this list when looking for the
    real compiler. The list separator is semicolon on Windows systems and colon
    on other systems. If not set, ccache will look for the first executable
    matching the compiler name in the normal *PATH* that isn't a symbolic link
    to ccache itself.

[[config_pch_external_checksum]] *pch_external_checksum* (*CCACHE_PCH_EXTSUM* or *CCACHE_NOPCH_EXTSUM*, see _<<_boolean_values,Boolean values>>_ above)::

    When this option is set, and ccache finds a precompiled header file,
    ccache will look for a file with the extension ``.sum'' added
    (e.g. ``pre.h.gch.sum''), and if found, it will hash this file instead
    of the precompiled header itself to work around the performance
    penalty of hashing very large files.

[[config_prefix_command]] *prefix_command* (*CCACHE_PREFIX*)::

    This option adds a list of prefixes (separated by space) to the command
    line that ccache uses when invoking the compiler. See also
    _<<_using_ccache_with_other_compiler_wrappers,Using ccache with other
    compiler wrappers>>_.

[[config_prefix_command_cpp]] *prefix_command_cpp* (*CCACHE_PREFIX_CPP*)::

    This option adds a list of prefixes (separated by space) to the command
    line that ccache uses when invoking the preprocessor.

[[config_read_only]] *read_only* (*CCACHE_READONLY* or *CCACHE_NOREADONLY*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will attempt to use existing cached results, but it will not
    add new results to the cache. Statistics counters will still be updated,
    though, unless the <<config_stats,*stats*>> option is set to *false*.
+
If you are using this because your ccache directory is read-only, you need to
set <<config_temporary_dir,*temporary_dir*>> since ccache will fail to create
temporary files otherwise. You may also want to set <<config_stats,*stats*>> to
*false* make ccache not even try to update stats files.

[[config_read_only_direct]] *read_only_direct* (*CCACHE_READONLY_DIRECT* or *CCACHE_NOREADONLY_DIRECT*, see _<<_boolean_values,Boolean values>>_ above)::

    Just like <<config_read_only,*read_only*>> except that ccache will only try
    to retrieve results from the cache using the direct mode, not the
    preprocessor mode. See documentation for <<config_read_only,*read_only*>>
    regarding using a read-only ccache directory.

[[config_recache]] *recache* (*CCACHE_RECACHE* or *CCACHE_NORECACHE*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will not use any previously stored result. New results will
    still be cached, possibly overwriting any pre-existing results.

[[config_run_second_cpp]] *run_second_cpp* (*CCACHE_CPP2* or *CCACHE_NOCPP2*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will first run the preprocessor to preprocess the source
    code (see _<<_the_preprocessor_mode,The preprocessor mode>>_) and then on a
    cache miss run the compiler on the source code to get hold of the object
    file. This is the default.
+
If false, ccache will first run preprocessor to preprocess the source code and
then on a cache miss run the compiler on the _preprocessed source code_ instead
of the original source code. This makes cache misses slightly faster since the
source code only has to be preprocessed once. The downside is that some
compilers won't produce the same result (for instance diagnostics warnings)
when compiling preprocessed source code.
+
A solution to the above mentioned downside is to set *run_second_cpp* to false
and pass *-fdirectives-only* (for GCC) or *-frewrite-includes* (for Clang) to
the compiler. This will cause the compiler to leave the macros and other
preprocessor information, and only process the *#include* directives. When run
in this way, the preprocessor arguments will be passed to the compiler since it
still has to do _some_ preprocessing (like macros).

[[config_sloppiness]] *sloppiness* (*CCACHE_SLOPPINESS*)::

    By default, ccache tries to give as few false cache hits as possible.
    However, in certain situations it's possible that you know things that
    ccache can't take for granted. This option makes it possible to tell
    ccache to relax some checks in order to increase the hit rate. The value
    should be a comma-separated string with one or several of the following
    values:
+
--
*clang_index_store*::
    Ignore the Clang compiler option *-index-store-path* and its argument when
    computing the manifest hash. This is useful if you use Xcode, which uses an
    index store path derived from the local project path. Note that the index
    store won't be updated correctly on cache hits if you enable this
    sloppiness.
*file_stat_matches*::
    Ccache normally examines a file's contents to determine whether it matches
    the cached version. With this sloppiness set, ccache will consider a file
    as matching its cached version if the mtimes and ctimes match.
*file_stat_matches_ctime*::
    Ignore ctimes when *file_stat_matches* is enabled. This can be useful when
    backdating files' mtimes in a controlled way.
*include_file_ctime*::
    By default, ccache will not cache a file if it includes a header whose ctime
    is too new. This sloppiness disables that check. See also
    _<<_handling_of_newly_created_header_files,Handling of newly created header
    files>>_.
*include_file_mtime*::
    By default, ccache will not cache a file if it includes a header whose
    mtime is too new. This sloppiness disables that check. See also
    _<<_handling_of_newly_created_header_files,Handling of newly created header
    files>>_.
*ivfsoverlay*::
    Ignore the Clang compiler option *-ivfsoverlay* and its argument. This is
    useful if you use Xcode, which uses a virtual file system (VFS) for things
    like combining Objective-C and Swift code.
*locale*::
    Ccache includes the environment variables *LANG*, *LC_ALL*, *LC_CTYPE* and
    *LC_MESSAGES* in the hash by default since they may affect localization of
    compiler warning messages. Set this sloppiness to tell ccache not to do
    that.
*pch_defines*::
    Be sloppy about **#define**s when precompiling a header file. See
    _<<_precompiled_headers,Precompiled headers>>_ for more information.
*modules*::
    By default, ccache will not cache compilations if *-fmodules* is used since
    it cannot hash the state of compiler's internal representation of relevant
    modules. This sloppiness allows caching in such a case. See
    _<<_c_modules,C++ modules>>_ for more information.
*system_headers*::
    By default, ccache will also include all system headers in the manifest.
    With this sloppiness set, ccache will only include system headers in the
    hash but not add the system header files to the list of include files.
*time_macros*::
    Ignore `__DATE__`, `__TIME__` and `__TIMESTAMP__` being present in the
    source code.
--
+
See the discussion under _<<_troubleshooting,Troubleshooting>>_ for more
information.

[[config_stats]] *stats* (*CCACHE_STATS* or *CCACHE_NOSTATS*, see _<<_boolean_values,Boolean values>>_ above)::

    If true, ccache will update the statistics counters on each compilation.
    The default is true.

[[config_temporary_dir]] *temporary_dir* (*CCACHE_TEMPDIR*)::

    This option specifies where ccache will put temporary files. The default is
    */run/user/<UID>/ccache-tmp* if */run/user/<UID>* exists, otherwise
    *<cache_dir>/tmp*.
+
NOTE: In previous versions of ccache, *CCACHE_TEMPDIR* had to be on the same
filesystem as the *CCACHE_DIR* path, but this requirement has been relaxed.)

[[config_umask]] *umask* (*CCACHE_UMASK*)::

    This option specifies the umask for files and directories in the cache
    directory. This is mostly useful when you wish to share your cache with
    other users.


Cache size management
---------------------

By default, ccache has a 5 GB limit on the total size of files in the cache and
no limit on the number of files. You can set different limits using the command
line options *-M*/*--max-size* and *-F*/*--max-files*. Use *ccache
-s/--show-stats* to see the cache size and the currently configured limits (in
addition to other various statistics).

Cleanup can be triggered in two different ways: automatic and manual.


Automatic cleanup
~~~~~~~~~~~~~~~~~

Ccache maintains counters for various statistics about the cache, including the
size and number of all cached files. In order to improve performance and reduce
issues with concurrent ccache invocations, there is one statistics file for
each of the sixteen subdirectories in the cache.

After a new compilation result has been written to the cache, ccache will
update the size and file number statistics for the subdirectory (one of
sixteen) to which the result was written. Then, if the size counter for said
subdirectory is greater than *max_size / 16* or the file number counter is
greater than *max_files / 16*, automatic cleanup is triggered.

When automatic cleanup is triggered for a subdirectory in the cache, ccache
will:

1. Count all files in the subdirectory and compute their aggregated size.
2. Remove files in LRU (least recently used) order until the size is at most
   *limit_multiple * max_size / 16* and the number of files is at most
   *limit_multiple * max_files / 16*, where
   <<config_limit_multiple,*limit_multiple*>>, <<config_max_size,*max_size*>>
   and <<config_max_files,*max_files*>> are configuration options.
3. Set the size and file number counters to match the files that were kept.

The reason for removing more files than just those needed to not exceed the max
limits is that a cleanup is a fairly slow operation, so it would not be a good
idea to trigger it often, like after each cache miss.


Manual cleanup
~~~~~~~~~~~~~~

You can run *ccache -c/--cleanup* to force cleanup of the whole cache, i.e. all
of the sixteen subdirectories. This will recalculate the statistics counters
and make sure that the configuration options *max_size* and
<<config_max_files,*max_files*>> are not exceeded. Note that
<<config_limit_multiple,*limit_multiple*>> is not taken into account for manual
cleanup.


Cache compression
-----------------

Ccache will by default compress all data it puts into the cache using the
compression algorithm http://zstd.net[Zstandard] (zstd) using compression level
1. The algorithm is fast enough that there should be little reason to turn off
compression to gain performance. One exception is if the cache is located on a
compressed file system, in which case the compression performed by ccache of
course is redundant. See the documentation for the configuration options
<<config_compression,*compression*>> and
<<config_compression_level,*compression_level*>> for more information.

You can use the command line option *-x/--show-compression* to print
information related to compression. Example:

-------------------------------------------------------------------------------
Total data:              14.8 GB (16.0 GB disk blocks)
Compressed data:         11.3 GB (30.6% of original size)
  - Original data:       36.9 GB
  - Compression ratio:  3.267 x  (69.4% space savings)
Incompressible data:      3.5 GB
-------------------------------------------------------------------------------

Notes:

* The ``disk blocks'' size is the cache size when taking disk block size into
  account. This value should match the ``cache size'' value from ``ccache
  --show-stats''. The other size numbers refer to actual content sizes.
* ``Compressed data'' refers to result and manifest files stored in the cache.
* ``Incompressible data'' refers to files that are always stored uncompressed
  (triggered by enabling <<config_file_clone,*file_clone*>> or
  <<config_hard_link,*hard_link*>>) or unknown files (for instance files
  created by older ccache versions).
* The compression ratio is affected by
  <<config_compression_level,*compression_level*>>.

The cache data can also be recompressed to another compression level (or made
uncompressed) with the command line option *-X/--recompress*. If you choose to
disable compression by default or to use a low compression level, you can
(re)compress newly cached data with a higher compression level after the build
or at another time when there are more CPU cycles available, for instance every
night. Full recompression potentially takes a lot of time, but only files that
are currently compressed with a different level than the target level will be
recompressed.


Cache statistics
----------------

*ccache -s/--show-stats* can show the following statistics:

[options="header",cols="30%,70%"]
|==============================================================================
|Name | Description
| autoconf compile/link |
Uncachable compilation or linking by an autoconf test.

| bad compiler arguments |
Malformed compiler argument, e.g. missing a value for a compiler option that
requires an argument or failure to read a file specified by a compiler option
argument.

| cache file missing |
A file was unexpectedly missing from the cache. This only happens in rare
situations, e.g. if one ccache instance is about to get a file from the cache
while another instance removed the file as part of cache cleanup.

| cache hit (direct) |
A result was successfully found using <<_the_direct_mode,the direct mode>>.

| cache hit (preprocessed) |
A result was successfully found using <<_the_preprocessor_mode,the preprocessor
mode>>.

| cache miss |
No result was found.

| cache size |
Current size of the cache.

| called for link |
The compiler was called for linking, not compiling. Ccache only supports
compilation of a single file, i.e. calling the compiler with the *-c* option to
produce a single object file from a single source file.

| called for preprocessing |
The compiler was called for preprocessing, not compiling.

| can't use precompiled header |
Preconditions for using <<_precompiled_headers,precompiled headers>> were not
fulfilled.

| can't use modules |
Preconditions for using <<_c_modules,C++ modules>> were not fulfilled.

| ccache internal error |
Unexpected failure, e.g. due to problems reading/writing the cache.

| cleanups performed |
Number of cleanups performed, either implicitly due to the cache size limit
being reached or due to explicit *ccache -c/--cleanup* calls.

| compile failed |
The compilation failed. No result stored in the cache.

| compiler check failed |
A compiler check program specified by
<<config_compiler_check,*compiler_check*>> (*CCACHE_COMPILERCHECK*) failed.

| compiler produced empty output |
The compiler's output file (typically an object file) was empty after
compilation.

| compiler produced no output |
The compiler's output file (typically an object file) was missing after
compilation.

| compiler produced stdout |
The compiler wrote data to standard output. This is something that compilers
normally never do, so ccache is not designed to store such output in the cache.

| couldn't find the compiler |
The compiler to execute could not be found.

| error hashing extra file |
Failure reading a file specified by
<<config_extra_files_to_hash,*extra_files_to_hash*>> (*CCACHE_EXTRAFILES*).

| files in cache |
Current number of files in the cache.

| multiple source files |
The compiler was called to compile multiple source files in one go. This is not
supported by ccache.

| no input file |
No input file was specified to the compiler.

| output to a non-regular file |
The output path specified with *-o* is not a file (e.g. a directory or a device
node).

| output to stdout |
The compiler was instructed to write its output to standard output using *-o
-*. This is not supported by ccache.

| preprocessor error |
Preprocessing the source code using the compiler's *-E* option failed.

| stats updated |
When statistics were updated the last time.

| stats zeroed |
When *ccache -z* was called the last time.

| unsupported code directive |
Code like the assembler *.incbin* directive was found. This is not supported
by ccache.

| unsupported compiler option |
A compiler option not supported by ccache was found.

| unsupported source language |
A source language e.g. specified with *-x* was unsupported by ccache.

|==============================================================================


How ccache works
----------------

The basic idea is to detect when you are compiling exactly the same code a
second time and reuse the previously produced output. The detection is done by
hashing different kinds of information that should be unique for the
compilation and then using the hash sum to identify the cached output. Ccache
uses BLAKE3, a very fast cryptographic hash algorithm, for the hashing. On a
cache hit, ccache is able to supply all of the correct compiler outputs
(including all warnings, dependency file, etc) from the cache. Data stored in
the cache is checksummed with XXH3, an extremely fast non-cryptographic
algorithm, to detect corruption.

Ccache has two ways of gathering information used to look up results in the
cache:

* the *preprocessor mode*, where ccache runs the preprocessor on the source
  code and hashes the result
* the *direct mode*, where ccache hashes the source code and include files
  directly

The direct mode is generally faster since running the preprocessor has some
overhead.

If no previous result is detected (i.e., there is a cache miss) using the
direct mode, ccache will fall back to the preprocessor mode unless the *depend
mode* is enabled. In the depend mode, ccache never runs the preprocessor, not
even on cache misses. Read more in _<<_the_depend_mode,The depend mode>>_
below.


Common hashed information
~~~~~~~~~~~~~~~~~~~~~~~~~

The following information is always included in the hash:

* the extension used by the compiler for a file with preprocessor output
  (normally *.i* for C code and *.ii* for C++ code)
* the compiler's size and modification time (or other compiler-specific
  information specified by <<config_compiler_check,*compiler_check*>>)
* the name of the compiler
* the current directory (if <<config_hash_dir,*hash_dir*>> is enabled)
* contents of files specified by
  <<config_extra_files_to_hash,*extra_files_to_hash*>> (if any)


The preprocessor mode
~~~~~~~~~~~~~~~~~~~~~

In the preprocessor mode, the hash is formed of the common information and:

* the preprocessor output from running the compiler with *-E*
* the command line options except those that affect include files (*-I*,
  *-include*, *-D*, etc; the theory is that these command line options will
  change the preprocessor output if they have any effect at all)
* any standard error output generated by the preprocessor

Based on the hash, the cached compilation result can be looked up directly in
the cache.


The direct mode
~~~~~~~~~~~~~~~

In the direct mode, the hash is formed of the common information and:

* the input source file
* the compiler options

Based on the hash, a data structure called ``manifest'' is looked up in the
cache. The manifest contains:

* references to cached compilation results (object file, dependency file, etc)
  that were produced by previous compilations that matched the hash
* paths to the include files that were read at the time the compilation results
  were stored in the cache
* hash sums of the include files at the time the compilation results were
  stored in the cache

The current contents of the include files are then hashed and compared to the
information in the manifest. If there is a match, ccache knows the result of
the compilation. If there is no match, ccache falls back to running the
preprocessor. The output from the preprocessor is parsed to find the include
files that were read. The paths and hash sums of those include files are then
stored in the manifest along with information about the produced compilation
result.

There is a catch with the direct mode: header files that were used by the
compiler are recorded, but header files that were *not* used, but would have
been used if they existed, are not. So, when ccache checks if a result can be
taken from the cache, it currently can't check if the existence of a new header
file should invalidate the result. In practice, the direct mode is safe to use
in the absolute majority of cases.

The direct mode will be disabled if any of the following holds:

* <<config_direct_mode,*direct_mode*>> is false
* a modification time of one of the include files is too new (needed to avoid a
  race condition)
* a compiler option not supported by the direct mode is used:
** a *-Wp,_X_* compiler option other than *-Wp,-MD,_path_*,
   *-Wp,-MMD,_path_* and *-Wp,-D_define_*
** *-Xpreprocessor*
* the string `__TIME__` is present in the source code


The depend mode
~~~~~~~~~~~~~~~

If the depend mode is enabled, ccache will not use the preprocessor at all. The
hash used to identify results in the cache will be based on the direct mode
hash described above plus information about include files read from the
dependency file generated by the compiler with *-MD* or *-MMD*.

Advantages:

* The ccache overhead of a cache miss will be much smaller.
* Not running the preprocessor at all can be good if compilation is performed
  remotely, for instance when using distcc or similar; ccache then won't make
  potentially costly preprocessor calls on the local machine.

Disadvantages:

* The cache hit rate will likely be lower since any change to compiler options
  or source code will make the hash different. Compare this with the default
  setup where ccache will fall back to the preprocessor mode, which is tolerant
  to some types of changes of compiler options and source code changes.
* If -MD is used, the manifest entries will include system header files as
  well, thus slowing down cache hits slightly, just as using -MD slows down
  make.
* If -MMD is used, the manifest entries will not include system header files,
  which means ccache will ignore changes in them.

The depend mode will be disabled if any of the following holds:

* <<config_depend_mode,*depend_mode*>> is false.
* <<config_run_second_cpp,*run_second_cpp*>> is false.
* The compiler is not generating dependencies using *-MD* or *-MMD*.


Handling of newly created header files
--------------------------------------

If modification time (mtime) or status change time (ctime) of one of the include
files is the same second as the time compilation is being done, ccache disables
the direct mode (or, in the case of a <<_precompiled_headers,precompiled
header>>, disables caching completely). This done as a safety measure to avoid a
race condition (see below).

To be able to use a newly created header files in direct mode (or use a newly
precompiled header), either:

* create the include file earlier in the build process, or
* set <<config_sloppiness,*sloppiness*>> to
  *include_file_ctime,include_file_mtime* if you are willing to take the risk,
  for instance if you know that your build system is robust enough not to
  trigger the race condition.

For reference, the race condition mentioned above consists of these events:

1. The preprocessor is run.
2. An include file is modified by someone.
3. The new include file is hashed by ccache.
4. The real compiler is run on the preprocessor's output, which contains data
   from the old header file.
5. The wrong object file is stored in the cache.


Cache debugging
---------------

To find out what information ccache actually is hashing, you can enable the
debug mode via the configuration option <<config_debug,*debug*>> or by setting
*CCACHE_DEBUG* in the environment. This can be useful if you are investigating
why you don't get cache hits. Note that performance will be reduced slightly.

When the debug mode is enabled, ccache will create up to five additional files
next to the object file:

[options="header",cols="30%,70%"]
|==============================================================================
|Filename | Description
| *<objectfile>.ccache-input-c* |
Binary input hashed by both the direct mode and the preprocessor mode.

| *<objectfile>.ccache-input-d* |
Binary input only hashed by the direct mode.

| *<objectfile>.ccache-input-p* |
Binary input only hashed by the preprocessor mode.

| *<objectfile>.ccache-input-text* |
Human-readable combined diffable text version of the three files above.

| *<objectfile>.ccache-log* |
Log for this object file.

|==============================================================================

If <<config_debug_dir,*config_dir*>> (environment variable *CCACHE_DEBUGDIR*) is
set, the files above will be written to that directory with full absolute paths
instead of next to the object file.

In the direct mode, ccache uses the 160 bit BLAKE3 hash of the
*ccache-input-c* + *ccache-input-d* data (where *+* means concatenation), while
the *ccache-input-c* + *ccache-input-p* data is used in the preprocessor mode.

The *ccache-input-text* file is a combined text version of the three
binary input files. It has three sections (``COMMON'', ``DIRECT MODE'' and
``PREPROCESSOR MODE''), which is turn contain annotations that say what kind of
data comes next.

To debug why you don't get an expected cache hit for an object file, you can do
something like this:

1. Build with debug mode enabled.
2. Save the *<objectfile>.ccache-&#42;* files.
3. Build again with debug mode enabled.
4. Compare *<objectfile>.ccache-input-text* for the two builds. This together
   with the *<objectfile>.ccache-log* files should give you some clues about
   what is happening.


Compiling in different directories
----------------------------------

Some information included in the hash that identifies a unique compilation can
contain absolute paths:

* The preprocessed source code may contain absolute paths to include files if
  the compiler option *-g* is used or if absolute paths are given to *-I* and
  similar compiler options.
* Paths specified by compiler options (such as *-I*, *-MF*, etc) on the command
  line may be absolute.
* The source code file path may be absolute, and that path may substituted for
  `__FILE__` macros in the source code or included in warnings emitted to
  standard error by the preprocessor.

This means that if you compile the same code in different locations, you can't
share compilation results between the different build directories since you get
cache misses because of the absolute build directory paths that are part of the
hash.

Here's what can be done to enable cache hits between different build
directories:

* If you build with *-g* (or similar) to add debug information to the object
  file, you must either:
+
--
** use the compiler option *-fdebug-prefix-map=_old_=_new_* for relocating
   debug info to a common prefix (e.g. *-fdebug-prefix-map=$PWD=.*); or
** set *hash_dir = false*.
--
* If you use absolute paths anywhere on the command line (e.g. the source code
  file path or an argument to compiler options like *-I* and *-MF*), you must
  set <<config_base_dir,*base_dir*>> to an absolute path to a ``base
  directory''. Ccache will then rewrite absolute paths under that directory to
  relative before computing the hash.


Precompiled headers
-------------------

Ccache has support for GCC's precompiled headers. However, you have to do some
things to make it work properly:

* You must set <<config_sloppiness,*sloppiness*>> to *pch_defines,time_macros*.
  The reason is that ccache can't tell whether `__TIME__`, `__DATE__` or
  `__TIMESTAMP__` is used when using a precompiled header. Further, it can't
  detect changes in **#define**s in the source code because of how
  preprocessing works in combination with precompiled headers.
* You may also want to include *include_file_mtime,include_file_ctime* in
  <<config_sloppiness,*sloppiness*>>. See
  _<<_handling_of_newly_created_header_files,Handling of newly created header
  files>>_.
* You must either:
+
--
** use the compiler option *-include* to include the precompiled header (i.e.,
   don't use *#include* in the source code to include the header; the filename
   itself must be sufficient to find the header, i.e. *-I* paths are not
   searched); or
** (for the Clang compiler) use the compiler option *-include-pch* to include
   the PCH file generated from the precompiled header; or
** (for the GCC compiler) add the compiler option *-fpch-preprocess* when
   compiling.

If you don't do this, either the non-precompiled version of the header file
will be used (if available) or ccache will fall back to running the real
compiler and increase the statistics counter ``preprocessor error'' (if the
non-precompiled header file is not available).
--


C++ modules
-----------

Ccache has support for Clang's *-fmodules* option. In practice ccache only
additionally hashes *module.modulemap* files; it does not know how Clang
handles its cached binary form of modules so those are ignored. This should not
matter in practice: as long as everything else (including *module.modulemap*
files) is the same the cached result should work. Still, you must set
<<config_sloppiness,*sloppiness*>> to *modules* to allow caching.

You must use both <<_the_direct_mode,*direct mode*>> and
<<_the_depend_mode,*depend mode*>>. When using <<_the_preprocessor_mode,the
preprocessor mode>> Clang does not provide enough information to allow hashing
of *module.modulemap* files.


Sharing a cache
---------------

A group of developers can increase the cache hit rate by sharing a cache
directory. To share a cache without unpleasant side effects, the following
conditions should to be met:

* Use the same cache directory.
* Make sure that the configuration option <<config_hard_link,*hard_link*>> is
  false (which is the default).
* Make sure that all users are in the same group.
* Set the configuration option <<config_umask,*umask*>> to 002. This ensures
  that cached files are accessible to everyone in the group.
* Make sure that all users have write permission in the entire cache directory
  (and that you trust all users of the shared cache).
* Make sure that the setgid bit is set on all directories in the cache. This
  tells the filesystem to inherit group ownership for new directories. The
  following command might be useful for this:
+
--
----
find $CCACHE_DIR -type d | xargs chmod g+s
----
--

The reason to avoid the hard link mode is that the hard links cause unwanted
side effects, as all links to a cached file share the file's modification
timestamp. This results in false dependencies to be triggered by
timestamp-based build systems whenever another user links to an existing file.
Typically, users will see that their libraries and binaries are relinked
without reason.

You may also want to make sure that a base directory is set appropriately, as
discussed in a previous section.


Sharing a cache on NFS
----------------------

It is possible to put the cache directory on an NFS filesystem (or similar
filesystems), but keep in mind that:

* Having the cache on NFS may slow down compilation. Make sure to do some
  benchmarking to see if it's worth it.
* Ccache hasn't been tested very thoroughly on NFS.

A tip is to set <<config_temporary_dir,*temporary_dir*>> to a directory on the
local host to avoid NFS traffic for temporary files.

It is recommended to use the same operating system version when using a shared
cache. If operating system versions are different then system include files
will likely be different and there will be few or no cache hits between the
systems. One way of improving cache hit rate in that case is to set
<<config_sloppiness,*sloppiness*>> to *system_headers* to ignore system
headers.


Using ccache with other compiler wrappers
-----------------------------------------

The recommended way of combining ccache with another compiler wrapper (such as
``distcc'') is by letting ccache execute the compiler wrapper. This is
accomplished by defining <<config_prefix_command,*prefix_command*>>, for
example by setting the environment variable *CCACHE_PREFIX* to the name of the
wrapper (e.g. *distcc*). Ccache will then prefix the command line with the
specified command when running the compiler. To specify several prefix
commands, set <<config_prefix_command,*prefix_command*>> to a colon-separated
list of commands.

Unless you set <<config_compiler_check,*compiler_check*>> to a suitable command
(see the description of that configuration option), it is not recommended to
use the form *ccache anotherwrapper compiler args* as the compilation command.
It's also not recommended to use the masquerading technique for the other
compiler wrapper. The reason is that by default, ccache will in both cases hash
the mtime and size of the other wrapper instead of the real compiler, which
means that:

* Compiler upgrades will not be detected properly.
* The cached results will not be shared between compilations with and without
  the other wrapper.

Another minor thing is that if <<config_prefix_command,*prefix_command*>> is
used, ccache will not invoke the other wrapper when running the preprocessor,
which increases performance. You can use
<<config_prefix_command_cpp,*prefix_command_cpp*>> if you also want to invoke
the other wrapper when doing preprocessing (normally by adding *-E*).


Caveats
-------

* The direct mode fails to pick up new header files in some rare scenarios. See
  _<<_the_direct_mode,The direct mode>>_ above.


Troubleshooting
---------------

General
~~~~~~~

A general tip for getting information about what ccache is doing is to enable
debug logging by setting the configuration option <<config_debug,*debug*>> (or
the environment variable *CCACHE_DEBUG*); see _<<_cache_debugging,Cache
debugging>>_ for more information. Another way of keeping track of what is
happening is to check the output of *ccache -s*.


Performance
~~~~~~~~~~~

Ccache has been written to perform well out of the box, but sometimes you may
have to do some adjustments of how you use the compiler and ccache in order to
improve performance.

Since ccache works best when I/O is fast, put the cache directory on a fast
storage device if possible. Having lots of free memory so that files in the
cache directory stay in the disk cache is also preferable.

A good way of monitoring how well ccache works is to run *ccache -s* before and
after your build and then compare the statistics counters. Here are some common
problems and what may be done to increase the hit rate:

* If ``cache hit (preprocessed)'' has been incremented instead of ``cache hit
  (direct)'', ccache has fallen back to preprocessor mode, which is generally
  slower. Some possible reasons are:
** The source code has been modified in such a way that the preprocessor output
   is not affected.
** Compiler arguments that are hashed in the direct mode but not in the
   preprocessor mode have changed (*-I*, *-include*, *-D*, etc) and they didn't
   affect the preprocessor output.
** The compiler option *-Xpreprocessor* or *-Wp,_X_* (except *-Wp,-MD,_path_*,
   *-Wp,-MMD,_path_*, and *-Wp,-D_define_*) is used.
** This was the first compilation with a new value of the
   <<config_base_dir,base directory>>.
** A modification or status change time of one of the include files is too new
   (created the same second as the compilation is being done). See
   _<<_handling_of_newly_created_header_files,Handling of newly created header
   files>>_.
** The `__TIME__` preprocessor macro is (potentially) being used. Ccache turns
   off direct mode if `__TIME__` is present in the source code. This is done as
   a safety measure since the string indicates that a `__TIME__` macro _may_
   affect the output. (To be sure, ccache would have to run the preprocessor,
   but the sole point of the direct mode is to avoid that.) If you know that
   `__TIME__` isn't used in practise, or don't care if ccache produces objects
   where `__TIME__` is expanded to something in the past, you can set
   <<config_sloppiness,*sloppiness*>> to *time_macros*.
** The `__DATE__` preprocessor macro is (potentially) being used and the date
   has changed. This is similar to how `__TIME__` is handled. If `__DATE__` is
   present in the source code, ccache hashes the current date in order to be
   able to produce the correct object file if the `__DATE__` macro affects the
   output. If you know that `__DATE__` isn't used in practise, or don't care if
   ccache produces objects where `__DATE__` is expanded to something in the
   past, you can set <<config_sloppiness,*sloppiness*>> to *time_macros*.
** The `__TIMESTAMP__` preprocessor macro is (potentially) being used and the
   source file's modification time has changed. This is similar to how
   `__TIME__` is handled. If `__TIMESTAMP__` is present in the source code,
   ccache hashes the string representation of the source file's modification
   time in order to be able to produce the correct object file if the
   `__TIMESTAMP__` macro affects the output. If you know that `__TIMESTAMP__`
   isn't used in practise, or don't care if ccache produces objects where
   `__TIMESTAMP__` is expanded to something in the past, you can set
   <<config_sloppiness,*sloppiness*>> to *time_macros*.
** The input file path has changed. Ccache includes the input file path in the
   direct mode hash to be able to take relative include files into account and
   to produce a correct object file if the source code includes a `__FILE__`
   macro.
* If ``cache miss'' has been incremented even though the same code has been
  compiled and cached before, ccache has either detected that something has
  changed anyway or a cleanup has been performed (either explicitly or
  implicitly when a cache limit has been reached). Some perhaps unobvious
  things that may result in a cache miss are usage of `__TIME__`, `__DATE__` or
  `__TIMESTAMP__` macros, or use of automatically generated code that contains
  a timestamp, build counter or other volatile information.
* If ``multiple source files'' has been incremented, it's an indication that
  the compiler has been invoked on several source code files at once. Ccache
  doesn't support that. Compile the source code files separately if possible.
* If ``unsupported compiler option'' has been incremented, enable debug logging
  and check which compiler option was rejected.
* If ``preprocessor error'' has been incremented, one possible reason is that
  precompiled headers are being used. See _<<_precompiled_headers,Precompiled
  headers>>_ for how to remedy this.
* If ``can't use precompiled header'' has been incremented, see
  _<<_precompiled_headers,Precompiled headers>>_.
* If ``can't use modules'' has been incremented, see _<<_c_modules,C++
  modules>>_.


Corrupt object files
~~~~~~~~~~~~~~~~~~~~

It should be noted that ccache is susceptible to general storage problems. If a
bad object file sneaks into the cache for some reason, it will of course stay
bad. Some possible reasons for erroneous object files are bad hardware (disk
drive, disk controller, memory, etc), buggy drivers or file systems, a bad
<<config_prefix_command,*prefix_command*>> or compiler wrapper. If this
happens, the easiest way of fixing it is this:

1. Build so that the bad object file ends up in the build tree.
2. Remove the bad object file from the build tree.
3. Rebuild with *CCACHE_RECACHE* set.

An alternative is to clear the whole cache with *ccache -C* if you don't mind
losing other cached results.

There are no reported issues about ccache producing broken object files
reproducibly. That doesn't mean it can't happen, so if you find a repeatable
case, please report it.


More information
----------------

Credits, mailing list information, bug reporting instructions, source code,
etc, can be found on ccache's web site: <https://ccache.dev>.


Author
------

Ccache was originally written by Andrew Tridgell and is currently developed and
maintained by Joel Rosdahl. See AUTHORS.txt or AUTHORS.html and
<https://ccache.dev/credits.html> for a list of contributors.
