= ccache(1)
:mansource: Ccache {revnumber}

== Name

ccache - a fast C/C++ compiler cache


== Synopsis

[verse]
*ccache* [_ccache options_]
*ccache* [_KEY_=_VALUE_ ...] _compiler_ [_compiler options_]
_compiler_ [_compiler options_]

The first form takes options described in <<Command line options>> below. The
second form invokes the compiler, optionally using <<Configuration,configuration
options>> as _KEY_=_VALUE_ arguments. In the third form, ccache is masquerading
as the compiler as described in <<Run modes>>.


== Description

Ccache is a compiler cache that speeds up recompilation by storing the results
of previous compilations and reusing them when the same compilation is performed
again.

Ccache is designed to produce exactly the same compiler output as a normal
compilation. The only difference you should notice is faster build times. Any
known exceptions to this behavior are documented in the _<<Caveats>>_ section.
If you find a case where ccache produces different output than expected, please
report it to us.


== Run modes

There are two ways to use ccache to cache compilations:

1. *Prefix method*: Add `ccache` before your compilation command. This is the
   simplest approach when you want to try ccache or use it for specific
   projects. Example:
+
-------------------------------------------------------------------------------
ccache gcc -c example.c
-------------------------------------------------------------------------------
+
2. *Masquerade method*: Make ccache appear as the compiler by creating a
   symbolic link to ccache with the compiler's name. This is useful when you
   want to use ccache for all compilations. To set up ccache to masquerade as
   `gcc` and `g++`:
+
-------------------------------------------------------------------------------
cp ccache /usr/local/bin/
ln -s ccache /usr/local/bin/gcc
ln -s ccache /usr/local/bin/g++
-------------------------------------------------------------------------------
+
On systems that don't support symbolic links, you can copy ccache instead:
+
-------------------------------------------------------------------------------
cp ccache /usr/local/bin/gcc
cp ccache /usr/local/bin/g++
-------------------------------------------------------------------------------
+
This works as long as the directory containing the symbolic links or ccache copies
appears before the real compiler directory (typically `/usr/bin`) in your `PATH`.
+
WARNING: The masquerade method works well but may conflict with other tools that
use the same technique. See _<<Using ccache with other compiler wrappers>>_ for
more information.


== Command line options

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


=== Common options

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

    Clean up the cache by removing not recently used 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 have modified the
    cache contents manually 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 command line options operate on configuration file _PATH_ instead of
    the default. Using this option has the same effect as setting (overriding)
    the environment variable `CCACHE_CONFIGPATH` temporarily.

*-d*, *--dir* _PATH_::

    Let the 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-namespace* _NAMESPACE_::

    Remove files created in the given <<config_namespace,*namespace*>> from the
    cache.

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

    Remove files used less recently than _AGE_ from the cache. _AGE_ should be
    an unsigned integer with a `d` (days) or `s` (seconds) suffix. If combined
    with `--evict-namespace`, only remove files within that namespace.

*-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: kB, MB, GB, TB (decimal), KiB, MiB,
    GiB or TiB (binary). The default suffix is GiB. 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>>_ for more information. This can potentially 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_ in the configuration file. See
    _<<Configuration>>_ for more information.

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

    Print cache compression statistics. See _<<Cache compression>>_ for more
    information. This can potentially 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.

*--show-log-stats*::

    Print statistics counters from the stats log in human-readable format. See
    <<config_stats_log,*stats_log*>>. Use `-v`/`--verbose` once or twice for
    more details.

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

    Print a summary of configuration and statistics counters in human-readable
    format. Use `-v`/`--verbose` once or twice for more details. See
    <<_cache_statistics,Cache statistics>> for more information.

*--stop-storage-helpers*::

    Storage helpers normally stop after a period of client inactivity, but this
    option can be used to stop any running helpers immediately.

*--threads* _THREADS_::

    Use up to _THREADS_ threads for threaded operations. The default is to use
    one thread per CPU.

*-v*, *--verbose*::

    Increase verbosity. The option can be given multiple times.

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

    Print version and copyright information.

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

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


=== Options for remote file-based storage

*--trim-dir* _PATH_::

    Remove not recently used files from directory _PATH_ until it is at most the
    size specified by `--trim-max-size`.
+
WARNING: Don't use this option to trim the local cache. To trim the local cache
directory to a certain size, use `CCACHE_MAXSIZE=_SIZE_ ccache -c`.

*--trim-max-size* _SIZE_::

    Specify the maximum size for `--trim-dir`. _SIZE_ should be a number
    followed by an optional suffix: kB, MB, GB, TB (decimal), KiB, MiB, GiB or
    TiB (binary). The default suffix is GiB. Use 0 for no limit.

*--trim-method* _METHOD_::

    Specify the method to trim a directory with `--trim-dir`. Possible values
    are:
+
--
*atime*::
    LRU (least recently used) using the file access timestamp. This is the
    default.
*mtime*::
    LRU (least recently used) using the file modification timestamp.
--

*--trim-recompress* _LEVEL_::

    Recompress to level _LEVEL_ using the Zstandard algorithm when using
    `--trim-dir`. 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>>_ for more information. This can potentially 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.


=== Options for scripting or debugging

*--checksum-file* _PATH_::

    Print the checksum (128 bit XXH3) of the file at _PATH_ (`-` for standard
    input).

*--extract-result* _PATH_::

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

*--format* _FORMAT_::

    Specify format for `--print-log-stats` and `--print-stats`. Possible values
    are:
+
--
*tab*::
    Tab separated. This is the default.
*json*::
    JSON formatted.
--

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

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

*--hash-file* _PATH_::

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

*--inspect* _PATH_::

    Print the content of a result or manifest file at _PATH_ (`-` for standard
    input) to standard output in human-readable format. File content embedded in
    a result file will however not be printed; use `--extract-result` to extract
    the file content. This option is only useful when debugging ccache and its
    behavior.

*--print-log-stats*::

    Print statistics counters from the stats log in machine-parsable
    (tab-separated or JSON) format. See <<config_stats_log,*stats_log*>> and
    `--format`.

*--print-stats*::

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

*--print-version*::

    Print version and don't do anything else.


=== 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.

See also <<config_ignore_options,*ignore_options*>>.


== Configuration

You can customize ccache's behavior using configuration files and environment
variables. Configuration options are processed in the following order of
priority (highest first):

1. Command line settings in _KEY_=_VALUE_ form. Example:
+
-------------------------------------------------------------------------------
ccache debug=true compiler_check="%compiler% --version" gcc -c example.c
-------------------------------------------------------------------------------
2. Environment variables starting with `CCACHE_`.
3. A <<Directory-specific configuration file,directory-specific configuration
   file>>.
4. The <<Cache-specific configuration file,cache-specific configuration file>>.
5. The system-wide configuration file `<sysconfdir>/ccache.conf`
   (typically `/etc/ccache.conf` or `/usr/local/etc/ccache.conf`).
6. Compile-time defaults.

Exception: If the environment variable `CCACHE_CONFIGPATH` is set, it specifies
the only configuration file that will be read (environment variables and command
line settings still apply).


=== Cache-specific configuration file

The location of the cache-specific configuration file is determined like this on
non-Windows systems:

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

On Windows, this is the method used to find the configuration file:

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 system
   configuration file then use `<cache_dir>\ccache.conf`. The
   system-wide configuration on Windows is
   `%ALLUSERSPROFILE%\ccache\ccache.conf` by default. The `ALLUSERSPROFILE`
   environment variable is usually `C:\ProgramData`.
4. Otherwise, if there is a legacy `%USERPROFILE%\.ccache` directory then use
   `%USERPROFILE%\.ccache\ccache.conf`.
5. Otherwise, use `%LOCALAPPDATA%\ccache\ccache.conf` if it exists.
6. Otherwise, use `%APPDATA%\ccache\ccache.conf`.

See also the <<config_cache_dir,*cache_dir*>> configuration option for how the
cache directory location is determined.


=== Directory-specific configuration file

Ccache searches for a `ccache.conf` file (separate from the cache-specific
configuration file described above) in the current working directory or any
parent directory. The found file must be owned by the effective user and must
not be world-writable; otherwise ccache will abort with an error. The search
stops when reaching:

- a directory listed in a <<config_ceiling_dirs,"ceiling directory">> (by
  default the user's home directory)
- a directory that contains a <<config_ceiling_markers,"ceiling marker">> (by
  default `.git`)
- a directory owned by another user
- a file system boundary (mount point)

By default, the file cannot set "unsafe options", i.e. those that affect which
commands to execute, which files to write (except cache entries) and which
remote storage to use. It is possible to allow unsafe options by adding an entry
to <<config_safe_dirs,*safe_dirs*>>.


=== Configuration value syntax

All configuration values support expansion of environment variables. The syntax
is similar to POSIX shell syntax: `$VAR` or `${VAR}`. Both variants will expand
to the value of the environment variable `VAR`.

Two consecutive dollar signs (`$$`) will expand to a single dollar sign (`$`).


=== 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 = 10GB
-------------------------------------------------------------------------------

==== Multi-line values

Values can span multiple lines using indentation-based continuation. Lines that
start with whitespace (spaces or tabs) are treated as continuation lines and are
joined to the previous value with a single space. Comments and blank lines
within a multi-line value are skipped. For example:

-------------------------------------------------------------------------------
ignore_options =
  -Wall
  -Wextra
  # This is a comment within the value
  -pedantic
# This ends the multi-line value
compiler = gcc
-------------------------------------------------------------------------------

This is equivalent to:

-------------------------------------------------------------------------------
ignore_options = -Wall -Wextra -pedantic
compiler = gcc
-------------------------------------------------------------------------------

Multi-line values are particularly useful for options that accept multiple
items.

=== 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.

Options that define a list of paths have their entries separated by `;` on
Windows and `:` on other systems.

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

    This option specifies whether ccache should rewrite relative paths in the
    compiler's textual output (standard error and standard 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 specifies a list of absolute paths. When set, ccache converts
    absolute paths to relative paths before hashing, but only for paths that
    start with one of the specified base directories.
+
This enables cache sharing between compilations in different directories, even
when the project uses absolute paths. See
<<Compiling in different directories>>_ for more details. When empty (the
default), no path rewriting occurs.
+
Avoid using `/` as the base directory as this will also rewrite system header
paths, which is usually counterproductive.
+
Example scenario: Alice works in `/home/alice/project1/build` and 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* settings:
+
-------------------------------------------------------------------------------
# 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 stores the same projects in `/home/bob/stuff` and both users set
*base_dir* to `/home` or `/home/$USER`, they will share cache hits because the
rewritten command lines will be identical:
+
-------------------------------------------------------------------------------
# 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.
+
WARNING: Rewriting absolute paths to relative is kind of a brittle hack. It
works OK in many cases, but there might be cases where things break. One known
issue is that absolute paths are not reproduced in dependency files, which can
mess up dependency detection in tools like Make and Ninja. If possible, use
relative paths in the first place instead of using *base_dir*.

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

    This option specifies where ccache will keep its cached compiler outputs.
+
On non-Windows systems, the default is `$HOME/.ccache` if such a directory
exists, otherwise `$XDG_CACHE_HOME/ccache` if `XDG_CACHE_HOME` is set, otherwise
`$HOME/Library/Caches/ccache` (macOS) or `$HOME/.cache/ccache` (other systems).
+
On Windows, the default is `%USERPROFILE%\.ccache` if such a directory exists,
otherwise `%LOCALAPPDATA%\ccache`.
+
WARNING: Previous ccache versions defaulted to storing the cache in
`%APPDATA%\ccache` on Windows. This can result in large network file transfers
of the cache in domain environments and similar problems. Please check this
directory for cache directories and either delete them or the whole directory,
or move them to the `%LOCALAPPDATA%\ccache` directory.
+
See also _<<Cache-specific configuration file>>_.

[#config_ceiling_dirs]
*ceiling_dirs* (*CCACHE_CEILING_DIRS*)::

    This option specifies absolute directories where ccache will stop searching
    for a <<Directory-specific configuration file,directory-specific
    configuration file>>. You might want to set this if accessing files in
    parent directories is too slow, e.g. on network file systems. The default is
    to stop looking when reaching the user's home directory.

[#config_ceiling_markers]
*ceiling_markers* (*CCACHE_CEILING_MARKERS*)::

    This option specifies one or more file or directory names (separated by `;`
    on Windows, `:` on other systems). When searching for a <<Directory-specific
    configuration file,directory-specific configuration file>>, ccache will stop
    searching parent directories when it reaches a directory containing any of
    the specified names. The default is `.git`.

[#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. If compiler plugins are used, these plugins will also be
    added to the hash. 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 `;` as the
    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>>_.
--

[#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.
*clang-cl*::
    clang-cl.
*gcc*::
    GCC-based compiler.
*icl*::
    Intel compiler on Windows.
*icx*::
    Intel LLVM-based compiler.
*icx-cl*::
    Intel LLVM-based MSVC-compatible compiler.
*msvc*::
    Microsoft Visual C++ (MSVC).
*nvcc*::
    NVCC (CUDA) compiler.
*other*::
    Any compiler other than the known types.
--

[#config_compression]
*compression* (*CCACHE_COMPRESS* or *CCACHE_NOCOMPRESS*, see _<<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 https://facebook.github.io/zstd/[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>>_ 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>>_
    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>>_.

[#config_debug_level]
*debug_level* (*CCACHE_DEBUGLEVEL*)::

    Specifies the amount of information that is written when the
    <<config_debug,debug mode>> is enabled. See _<<Cache debugging>>_ for more
    information. The default is 2.

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

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

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

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

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

    When true, ccache will just call the real compiler, bypassing the cache
    completely. The default is false.
+
It is also possible to disable ccache for a specific source code file by adding
the string `ccache:disable` in a comment in the first 4096 bytes of the file.

[#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 hash
    sum that identifies the build.

[#config_file_clone]
*file_clone* (*CCACHE_FILECLONE* or *CCACHE_NOFILECLONE*, see _<<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>>_ 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. 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 writable, 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>>_ 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 the
    compiler options `-fdebug-prefix-map` or `-fdebug-compilation-dir` are used
    appropriately. See also the discussion under
    _<<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.

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

    This option is a space-delimited list of compiler options that ccache will
    ignore. Entries in the list can optionally end with an asterisk (`*`) to
    matching any option suffix. For example, `+-fmessage-length=*+` will match
    both `-fmessage-length=20` and `-fmessage-length=70`. A matching compiler
    option will neither be interpreted specially nor be part of the input hash.
    Ignoring a compiler option from the hash can be useful when you know it
    doesn't affect the result (and ccache doesn't know that), or when it does
    and you don't care. See also _<<Extra options>>_.

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

    If true, ccache will cache source file hashes based on device, inode and
    timestamps. This reduces the time spent on hashing include files since the
    result can be reused between compilations. The default is true. The feature
    requires <<config_temporary_dir,*temporary_dir*>> to be located on a local
    filesystem of a supported type.
+
NOTE: Support for the inode cache feature on Windows is experimental. On Windows
the default is false.

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

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

[#config_libexec_dirs]
*libexec_dirs* (*CCACHE_LIBEXEC_DIRS*)::

    If set, ccache will look for helper programs in these directories instead of
    the default.

[#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>>_.

[#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 5GiB. Available suffixes: kB, MB, GB, TB (decimal) and KiB,
    MiB, GiB, TiB (binary). The default suffix is GiB. See also
    _<<Cache size management>>_.

[#config_msvc_dep_prefix]
*msvc_dep_prefix* (*CCACHE_MSVC_DEP_PREFIX*)::

    This option specifies the prefix of included files output for MSVC compiler.
    The default prefix is "`Note: including file:`". If you use a localized
    compiler, this should be set accordingly.

[#config_msvc_utf8]
*msvc_utf8* (*CCACHE_MSVC_UTF8*)::

    This option adds `/utf-8` to the msvc command line when executing the preprocessor to
    ensure that filenames are not garbled for non-ascii characters.
    This implicitly enables `/validate-charset` and treats the source code as utf-8 which
    may cause compilation errors if comments in your code have characters in the [128, 255]
    range for a given Windows system codepage which results in an invalid utf-8 sequence.
    The default is true.

[#config_namespace]
*namespace* (*CCACHE_NAMESPACE*)::

    If set, the namespace string will be added to the hashed data for each
    compilation. This will make the associated cache entries logically separate
    from cache entries with other namespaces, but they will still share the same
    storage space. Cache entries can also be selectively removed from the local
    cache with the command line option `--evict-namespace`, potentially in
    combination with `--evict-older-than`.
+
For instance, if you use the same local cache for several disparate projects,
you can use a unique namespace string for each one. This allows you to remove
cache entries that belong to a certain project if you stop working with that
project.

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

    If set, ccache will search directories in this list when looking for the
    real compiler. 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>>_ 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>>_.

[#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>>_ above)::

    If true, ccache will attempt to use existing cached results, but it will not
    add new results to any cache backend. 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>>_ 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>>_ above)::

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

[#config_remote_only]
*remote_only* (*CCACHE_REMOTE_ONLY* or *CCACHE_NOREMOTE_ONLY*, see _<<Boolean values>>_ above)::

    If true, ccache will only use <<config_remote_storage,remote storage>>. The
    default is false. Note that cache statistics counters will still be kept in
    the local cache directory unless <<config_stats,*stats*>> is false. See also
    _<<Storage interaction>>_.

[#config_remote_storage]
*remote_storage* (*CCACHE_REMOTE_STORAGE*)::

    This option specifies one or several storage backends (separated by
    whitespace) to query after checking the local cache (unless
    <<config_remote_only,*remote_only*>> is true). See
    _<<Remote storage backends>>_ for documentation of syntax and available
    backends.
+
Examples:
+
----
remote_storage = file:/shared/nfs/directory+`
remote_storage =
  file:///shared/nfs/one read-only
  file:///shared/nfs/two
remote_storage = file:///Z:/example/windows/folder
remote_storage = http://example.com/cache
remote_storage = redis://example.com
----
+
NOTE: In previous ccache versions this option was called *secondary_storage*
(*CCACHE_SECONDARY_STORAGE*), which can still be used as an alias.

[#config_reshare]
*reshare* (*CCACHE_RESHARE* or *CCACHE_NORESHARE*, see _<<Boolean values>>_ above)::

    If true, ccache will write results to remote storage even for local storage
    cache hits. The default is false.

[#config_response_file_format]
*response_file_format* (*CCACHE_RESPONSE_FILE_FORMAT*)::

    Ccache normally guesses the response file format based on the compiler type.
    The *response_file_format* option lets you force the response file quoting
    behavior. This can be useful if the compiler supports both POSIX and Windows
    response file quoting. Possible values are:
+
--
*auto*::
    Guess one of the formats below based on the compiler type. This is the
    default.
*posix*::
    POSIX quoting behavior.
*windows*::
    Windows quoting behavior.
--

[#config_safe_dirs]
*safe_dirs* (*CCACHE_SAFE_DIRS*)::

    List of trusted directories for which directory-specific configuration files
    are allowed to set "unsafe options" (see
    <<_directory_specific_configuration_file,Directory-specific configuration
    file>>). The default is empty.
+
Allowed list values: `\*` matches any directory. An absolute path ending with
`/*` matches any subdirectory (at any depth) under that path. An absolute path
not ending in `/*` matches only that directory.

[#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. Available values
    (separated by whitespace or comma):
+
--
*clang_index_store*::

    *Use case*: Xcode projects with varying index store paths. +
    *Effect*: Ignores the `-index-store-path` and `-index-unit-output-path`
     options when hashing. +
    *Trade-off*: Index won't update correctly on cache hits.

*file_stat_matches*::

    *Use case*: When file content checking is too slow. +
    *Effect*: Uses file timestamps instead of content for cache validation. +
    *Trade-off*: May miss content changes with identical timestamps.

*file_stat_matches_ctime*::

    *Use case*: When controlling file timestamps manually. +
    *Effect*: Ignores status change time when `file_stat_matches` is enabled. +
    *Trade-off*: May miss some file system changes.

*gcno_cwd*::

    *Use case*: Code coverage builds in different directories. +
    *Effect*: Ignores current directory when creating `.gcno` files
    (`-ftest-coverage`). Also disables hashing of the current working directory
    if `-fprofile-abs-path` is used. +
    *Trade-off*: Directory information in coverage files may be incorrect.
+
NOTE: No effect with `--coverage` (it implies `-fprofile-arcs`).

*incbin*::

    *Use case*: Projects using assembly `.incbin` directives. +
    *Effect*: Allows caching files with `.incbin` directives. +
    *Trade-off*: Won't detect changes to included binary files.

*include_file_ctime*::

    *Use case*: Build systems that modify file timestamps frequently. +
    *Effect*: Ignores file status change time when checking for recent
    modifications. +
    *Trade-off*: May miss recent changes to source files.

*include_file_mtime*::

    *Use case*: Build systems that modify file timestamps frequently. +
    *Effect*: Ignores file modification time when checking for recent changes. +
    *Trade-off*: May miss recent modifications to source files.

*ivfsoverlay*::

    *Use case*: Xcode projects mixing Objective-C and Swift. +
    *Effect*: Ignores `-ivfsoverlay` virtual filesystem option. +
    *Trade-off*: May not detect VFS-related changes.

*locale*::

    *Use case*: Builds in environments with varying locale settings. +
    *Effect*: Ignores locale environment variables (`LANG`, `LC_ALL`,
    `LC_CTYPE`, `LC_MESSAGES`). +
    *Trade-off*: Compiler warning messages may vary between cached and fresh
     builds.

*modules*::

    *Use case*: Clang builds using `-fmodules`. +
    *Effect*: Allows caching when C++ modules are used. +
    *Trade-off*: May not detect changes in module internal state.
+
See _<<C++ modules>>_ for details.

*pch_defines*::

    *Use case*: Projects with precompiled headers. +
    *Effect*: Relaxes checking of `#define` directives in precompiled headers. +
    *Trade-off*: May not detect some macro definition changes.
+
See _<<Precompiled headers>>_ for details.

*random_seed*::

    *Use case*: Builds with varying `-frandom-seed` values. +
    *Effect*: Ignores random seed values in compilation hash. +
    *Trade-off*: Builds may not be fully reproducible.

*system_headers*::

    *Use case*: Systems with frequently changing system headers. +
    *Effect*: Only tracks non-system headers in direct mode. +
    *Trade-off*: Won't detect system header changes that affect compilation. +
    *Limitations*: Only supported for GCC-like compilers (not MSVC). System
    headers are still checked in preprocessor mode.
+
See also <<config_ignore_headers_in_manifest,*ignore_headers_in_manifest*>>.

*time_macros*::

    *Use case*: Code using time macros but values don't matter. +
    *Effect*: Ignores `+__DATE__+`, `+__TIME__+`, and `+__TIMESTAMP__+` in
    source. +
    *Trade-off*: Time values in output will be from cached compilation.
--

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

    If true, ccache will update the statistics counters on each compilation. The
    default is true. If false, _<<Automatic cleanup,automatic cleanup>>_ will be
    disabled as well.

[#config_stats_log]
*stats_log* (*CCACHE_STATSLOG*)::

    If set to a file path, ccache will write statistics counter updates to the
    specified file. This is useful for getting statistics for individual builds.
    To show a summary of the current stats log, use `ccache --show-log-stats`.
+
NOTE: Lines in the stats log starting with a hash sign (`#`) are comments.

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

    This option specifies where ccache will put temporary files. The default is
    `$XDG_RUNTIME_DIR/ccache-tmp` (typically `/run/user/<UID>/ccache-tmp`) if
    `XDG_RUNTIME_DIR` is set and the directory 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 (an octal integer) 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.


=== Disabling ccache

To disable ccache completely for all invocations, set
<<config_disable,*disable = true*>> (`CCACHE_DISABLE=1`). You can also disable
ccache for a certain source code file by adding the string `ccache:disable` in a
comment in the first 4096 bytes of the file. In the latter case the `Ccache
disabled` statistics counter will be increased.


== Remote storage backends

The <<config_remote_storage,*remote_storage*>> option lets you configure ccache
to use one or several remote storage backends. By default, the local cache
directory located in <<config_cache_dir,*cache_dir*>> will be queried first and
remote storage second, but <<config_remote_only,*remote_only*>> can be set to
true to disable local storage. Note that cache statistics counters will still be
kept in the local cache directory -- remote storage backends only store
compilation results and manifests.

=== Storage helper process

ccache can spawn a long-lived local storage helper process (installed
separately) to handle communication with the remote storage. Storage helpers
keep remote connections up for efficiency and terminate after a while on
inactivity.

The storage helper program is called `ccache-storage-<scheme>`, where `<scheme>`
is the scheme part of a URL, e.g. `https`. The program can be placed in these
locations (in priority order):

1. In a location specified by the *helper* property (see below).
2. In a <<config_libexec_dirs,libexec directory>>.
3. Next to the ccache executable.
4. In `$PATH`.

If the program is not found, ccache falls back to one of the builtin backends if
available, otherwise ccache exits with an error.

=== Configuration syntax

A remote storage backend is specified by a URL and can be followed by a
whitespace-separated list of properties (`_key_=_value_`) and custom attributes
(`@_key_=_value_`). A missing `=_value_` is shorthand for setting the value to
*true*. Values must be
https://en.wikipedia.org/wiki/Percent-encoding[percent-encoded] if they contain
`%`, `|` or whitespace characters. For compatibility with older ccache versions,
`|` characters are treated as spaces.

==== Properties

* *data-timeout*: Timeout for send/receive data transfer. Resets whenever data
  is received or can be sent. Default: 10s.
* *helper*: Override which storage helper to spawn (either a filename or a full
  path). This is mainly useful when developing or testing new helpers.
* *idle-timeout*: Timeout for the helper process to wait before exiting after
  client inactivity (0s: stay up indefinitely). Default: 10m.
* *read-only*: If *true*, only read from this backend, don't write. The default
  is *false*.
* *request-timeout*: Timeout for the whole request. Default: 1m.
* *shards*: A comma-separated list of names for sharding (partitioning) the
  cache entries using
  https://en.wikipedia.org/wiki/Rendezvous_hashing[Rendezvous hashing],
  typically to spread the cache over a server cluster. When set, the storage URL
  must contain an asterisk (`+*+`), which will be replaced by one of the shard
  names to form a real URL. A shard name can optionally have an appended weight
  within parentheses to indicate how much of the key space should be associated
  with that shard. A shard with weight *w* will contain *w*/*S* of the cache,
  where *S* is the sum of all shard weights. A weight could for instance be set
  to represent the available memory for a memory cache on a specific server. The
  default weight is *1*.
+
Examples:
+
--
* `+http://cache-*.example.com shards=a(3),b(1),c(1.5)+` will put 55% (3/5.5)
  of the cache on `+http://cache-a.example.com+`, 18% (1/5.5) on
  `+http://cache-b.example.com+` and 27% (1.5/5.5) on
  `+http://cache-c.example.com+`.
* `+http://example.com/* shards=alpha,beta+` will put 50% of the cache on
  `+http://example.com/alpha+` and 50% on `+http://example.com/beta+`.
--

Timeouts can be specified with an ms (milliseconds), s (seconds), m (minutes), h
(hours) or d (days) suffix.

==== Custom attributes

Custom attributes on the form `_@key_=_value_` are specific to each storage
helper implementation. See the backend's documentation for which attributes are
available. Examples:

* `+file:///example/directory @umask=002 @update-mtime=true+`
* `+https://example.com/path @bearer-token=deadbeef+`

=== Storage interaction

The table below describes the interaction between local and remote storage on
cache hits and misses if <<config_remote_only,*remote_only*>> is false (which is
the default):

[options="header",cols="20%,20%,60%"]
|==============================================================================
| *Local storage* | *Remote storage* | *What happens*

| miss | miss | Compile, write to local, write to remote^[1]^
| miss | hit  | Read from remote, write to local
| hit  | -    | Read from local, don't write to remote^[2]^

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

^[1]^ Unless remote storage has property `read-only=true`. +
^[2]^ Unless local storage is set to share its cache hits with the
<<config_reshare,*reshare*>> option.

If <<config_remote_only,*remote_only*>> is true:

[options="header",cols="20%,20%,60%"]
|==============================================================================
| *Local storage* | *Remote storage* | *What happens*

| - | miss | Compile, write to remote, don't write to local
| - | hit  | Read from remote, don't write to local

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

=== File storage backend

URL format: `+file:DIRECTORY+` or `+file://[HOST]DIRECTORY+`

This backend stores data as separate files in a directory structure below
*DIRECTORY*, similar (but not identical) to the local cache storage. A typical
use case for this backend would be sharing a cache on an NFS directory.
*DIRECTORY* must start with a slash. *HOST* can be the empty string or
localhost. On Windows, *HOST* can also be the name of a server hosting a shared
folder.

IMPORTANT: ccache will not perform any cleanup of the storage -- that has to be
done by other means, for instance by running `ccache --trim-dir` periodically.

Examples:

* `+file:/shared/nfs/directory+`
* `+file:///shared/nfs/directory @umask=002 @update-mtime=true+`
* `+file:///Z:/example/windows/folder+`
* `+file://example.com/shared/ccache%20folder+`

Optional attributes:

* *@layout*: How to store file under the cache directory. Available values:
+
--
* *flat*: Store all files directly under the cache directory.
* *subdirs*: Store files in 256 subdirectories of the cache directory.
--
+
The default is *subdirs*.
* *@umask*: This attribute (an octal integer) overrides the umask to use for
  files and directories in the cache directory.
* *@update-mtime*: If *true*, update the modification time (mtime) of cache
  entries that are read. The default is *false*.


=== CRSH storage backend

URL format: `+crsh:IPC_ENDPOINT+`

This backend connects to an already running <<Storage helper process,ccache
storage helper>> (CRSH) process listening on `IPC_ENDPOINT` (Unix socket path or
Windows named pipe). The use case for this is mainly when developing or
debugging a storage helper, or in special setups where the helper program is
started and managed externally.

Examples:

* `+crsh:/path/to/your.sock+` (Unix socket)
* `+crsh:name-of-your-pipe+` (Windows named pipe)


=== HTTP storage backend

WARNING: Built-in support for the HTTP protocol will likely be removed in a
future ccache release.

URL format: `+http://HOST[:PORT][/PATH]+`

This backend stores data in an HTTP-compatible server. The required HTTP methods
are `GET`, `PUT` and `DELETE`.

IMPORTANT: ccache will not perform any cleanup of the storage -- that has to be
done by other means, for instance by running `ccache --trim-dir` periodically.

NOTE: HTTPS is not supported.

TIP: See https://ccache.dev/howto/http-storage.html[How to set up HTTP storage]
for hints on how to set up an HTTP server for use with ccache.

Examples:

* `+http://localhost+`
* `+http://someusername:p4ssw0rd@example.com/cache/+`
* `+http://localhost:8080 @layout=bazel+`

Optional attributes:

* *@bearer-token*: Bearer token used to authorize the HTTP requests.
* *@keep-alive*: If *true*, keep the HTTP connection to the storage server open
  to avoid reconnects. The default is *true*.
* *@layout*: How to map key names to the path part of the URL. Available values:
+
--
* *bazel*: Store values in a format compatible with the Bazel HTTP caching
   protocol. More specifically, the entries will be stored as 64 hex digits
   under the `/ac/` part of the cache.
+
NOTE: You may have to disable verification of action cache values in the server
for this to work since ccache entries are not valid action result metadata
values.
* *flat*: Append the key directly to the path part of the URL (with a leading
   slash if needed).
* *subdirs*: Append the first two characters of the key to the URL (with a
  leading slash if needed), followed by a slash and the rest of the key. This
  divides the entries into 256 buckets.
--
+
The default is *subdirs*.
* *@header*: Add the key=value pair to the HTTP headers of the request. For
   example: `+@header=Content-Type=application/octet-stream+` adds
   "Content-Type: application/octet-stream" to the HTTP headers of the request.

=== Redis storage backend

WARNING: Built-in support for the Redis protocol will likely be removed in a
future ccache release.

URL formats:

`+redis://[[USERNAME:]PASSWORD@]HOST[:PORT][/DBNUMBER]+` +
`+redis+unix:SOCKET_PATH[?db=DBNUMBER]+` +
`+redis+unix://[[USERNAME:]PASSWORD@localhost]SOCKET_PATH[?db=DBNUMBER]+`

This backend stores data in a https://redis.io[Redis] (or Redis-compatible)
server. There are implementations for both memory-based and disk-based storage.
*PORT* defaults to *6379* and *DBNUMBER* defaults to *0*.

NOTE: ccache will not perform any cleanup of the Redis storage, but you can
https://redis.io/topics/lru-cache[configure LRU eviction].

TIP: See https://ccache.dev/howto/redis-storage.html[How to set up Redis
storage] for hints on setting up a Redis server for use with ccache.

TIP: You can set up a cluster of Redis servers using the `shards` property
described in _<<Remote storage backends>>_.

Examples:

* `+redis://localhost+`
* `+redis://p4ssw0rd@cache.example.com:6379/0+`
* `+redis+unix:/run/redis.sock+`
* `+redis+unix:///run/redis.sock+`
* `+redis+unix://p4ssw0rd@localhost/run/redis.sock?db=0+`

== 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 the
`-s`/`--show-stats` option 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

After a new compilation result has been stored in the local cache, ccache will
trigger an automatic cleanup if <<config_max_size,*max_size*>> or
<<config_max_files,*max_files*>> is exceeded. The cleanup removes cache entries
in approximate LRU (least recently used) order based on the modification time
(mtime) of files in the cache. For this reason, ccache updates mtime of the
cache files read on a cache hit to mark them as recently used.

For performance reasons only entries in a subset of the cache are considered
when automatic cleanup is triggered, so the oldest entries aren't always removed
first but the overall behavior approximates LRU over time.


=== Manual cleanup

Run `ccache --cleanup` to force cleanup of the whole cache. This will
recalculate the cache size information and make sure that the cache size does
not exceed <<config_max_size,*max_size*>> and <<config_max_files,*max_files*>>.

Note that there is no guarantee that only the oldest entries are evicted, as
discussed in _<<Automatic cleanup>>_ above. To evict based on age use `ccache
--evict-older-than AGE`.


== Cache compression

By default, ccache compresses all cached data using the
http://zstd.net[Zstandard] (zstd) algorithm at compression level 1. This
algorithm is fast enough that compression rarely impacts performance. You might
want to disable compression only if your cache is stored on an
already-compressed filesystem, where ccache's compression would be redundant.

For configuration details, see <<config_compression,*compression*>> and
<<config_compression_level,*compression_level*>>.

Use `ccache --show-compression` to display compression information. Example
output:

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

Notes:

* *Disk blocks*: The actual cache size accounting for filesystem block size.
  This should match the "Cache size" from `ccache --show-stats`.
* *Compressed data*: Result and manifest files stored in the cache.
* *Incompressible data*: Files stored without compression (due to
  <<config_file_clone,*file_clone*>> or <<config_hard_link,*hard_link*>>
  settings) or legacy files from older ccache versions.
* *Compression ratio*: Affected by
  <<config_compression_level,*compression_level*>>.

You can recompress cached data to different compression levels using `ccache
--recompress`. Only files with different compression levels than the target will
be recompressed.


== Cache statistics

`ccache --show-stats` shows a summary of statistics, including cache size,
cleanups (number of performed cleanups, either implicitly due to a cache size
limit being reached or due to explicit `ccache -c` calls), overall hit rate, hit
rate for <<The direct mode,direct>>/<<The preprocessor mode,preprocessed>> modes
and hit rate for local and <<config_remote_storage,remote storage>>.

The statistics counters are not used by ccache itself during builds. This means
that you can safely reset them at any time using `ccache --zero-stats` without
affecting the build process. For example, you might reset them before a build so
that `ccache --show-stats` will only summarize the results from that specific
build. Alternatively, you can set <<config_stats_log,*stats_log*>> before
starting the build and then run `ccache --show-log-stats` afterward to view
build-specific statistics. This approach allows the statistics counters to
continue tracking the entire lifetime of the cache while still giving you
detailed information for individual builds. Another advantage of *stats_log* is
that it collects statistics without interference from other concurrent builds
that access the same cache.

The summary also includes counters called "`Errors`" and "`Uncacheable`", which
are sums of more detailed counters. To see those detailed counters, use the
`-v`/`--verbose` flag. The verbose mode can show the following counters:

[options="header",cols="30%,70%"]
|==============================================================================
| *Counter* | *Description*

| Autoconf compile/link |
Uncacheable 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.

| Called for linking |
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.

| Ccache disabled |
Ccache was disabled by a `ccache:disable` string in the source code file.

| Could not use modules |
Preconditions for using <<C++ modules>> were not fulfilled.

| Could not use precompiled header |
Preconditions for using <<Precompiled headers,precompiled headers>> were not
fulfilled.

| Could not read or parse input file |
An input file could not be read or parsed (see the debug log for details).

| Could not write to output file |
The output path specified with `-o` could not be written to.

| Compilation 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 output file missing |
One of the files expected to be produced by the compiler was missing after
compilation.

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

| Could not 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*).

| Forced recache |
<<config_recache,*CCACHE_RECACHE*>> was used to overwrite an existing result.

| Input file modified during compilation |
An input file was modified during compilation.

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

| Missing cache file |
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.

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

| Preprocessing failed |
Preprocessing the source code using the compiler's `-E` option failed.

| 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 environment variable |
An environment variable not supported by ccache was set.

| Unsupported source encoding |
Source file (or an included header) has unsupported encoding. ccache currently
requires UTF-8-encoded source code for MSVC when `msvc_utf8` is true.

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

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


== How ccache works

Ccache detects when you're compiling the same code and reuses previously stored
output. It works by creating a unique hash (the "`input hash`") from various
information that affects the compilation. When the same hash is encountered
again, ccache can supply all the correct compiler outputs from the cache.

For hashing, ccache uses https://blake3.io[BLAKE3], a fast cryptographic hash
algorithm. For data integrity, cached data is protected with
https://xxhash.com[XXH3] checksums to detect corruption.

Ccache has two strategies for gathering the information used to create cache
lookup keys:

* *Preprocessor mode*: Ccache runs the preprocessor on the source code and
  hashes the result.
* *Direct mode*: Ccache hashes the source code and include files directly.

Direct mode is generally faster because it avoids the overhead of running the
preprocessor.

When direct mode doesn't find a cached result (cache miss), ccache falls back to
preprocessor mode unless *depend mode* is enabled. In depend mode, ccache never
runs the preprocessor, even on cache misses. See _<<The depend mode>>_ for
details.


=== 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. To mitigate this problem, ccache records
whether directories specified with `-I` and similar exist at the time of
compilation, which handles most cases. Still, when ccache checks if a result can
be taken from the cache, it currently can't check with 100% accuracy 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, for example:
** a `-Wp,++*++` compiler option other than `-Wp,-MD,<path>`, `-Wp,-MMD,<path>`,
   `-Wp,-D<macro[=defn]>` or `-Wp,-U<macro>`
** most uses of `-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 list generated by MSVC with `/showIncludes`, or the dependency file
generated by other compilers 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. This is also the case for MSVC with `/showIncludes`.
* 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.
* The compiler is not generating dependencies using `-MD` or `-MMD` (for MSVC,
  `/showIncludes` is added automatically if not specified by the user).


== Handling of newly created source files

If modification time (mtime) or status change time (ctime) of the source file or
one of the include files is equal to (or newer than) the time that ccache was
invoked, ccache disables caching completely. This is done as a safety measure to
avoid a race condition (see below). In practice, this is only a problem when
using file systems with very low timestamp granularity. You can set
<<config_sloppiness,*sloppiness*>> to *include_file_ctime,include_file_mtime* to
opt out of the safety measure.

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

1. A source code file is read by ccache and added to the input hash.
2. The source code file is modified.
3. The compiler is executed and reads the modified source code.
4. Ccache stores the compiler output in the cache associated with the incorrect
   key (based on the unmodified source code).


== 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%,10%,60%"]
|==============================================================================
| *Filename* | *Debug level* | *Description*

| `<objectfile>.<timestamp>.ccache-input-c` | 2 |
Binary input hashed by both the direct mode and the preprocessor mode.

| `<objectfile>.<timestamp>.ccache-input-d` | 2 |
Binary input only hashed by the direct mode.

| `<objectfile>.<timestamp>.ccache-input-p` | 2 |
Binary input only hashed by the preprocessor mode.

| `<objectfile>.<timestamp>.ccache-input-text` | 2 |
Human-readable combined diffable text version of the three files above.

| `<objectfile>.<timestamp>.ccache-log` | 1 |
Log for this object file.

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

The timestamp format is
`<year><month><day>_<hour><minute><second>_<microsecond>`.


If you only need the log file, set <<config_debug,*debug_level*>> (environment
variable `CCACHE_DEBUGLEVEL`) to 1.

If <<config_debug_dir,*debug_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. Enable `debug` (`CCACHE_DEBUG`).
2. Build.
3. Clean and build again.
4. Compare the `<objectfile>.<timestamp>.ccache-input-text` files for the two
   builds. This together with the `<objectfile>.<timestamp>.ccache-log` files
   should give you some clues about what is happening.


== Compiling in different directories

When compiling the same source code in different directories, ccache often can't
share cached results if absolute paths become part of the input hash. These
paths appear in:

* preprocessed source code (when using `-g` for debug info or absolute paths in
  `-I` options)
* command-line arguments (absolute paths to source files or include directories)
* macro expansions (`+__FILE__+` macros and preprocessor warnings)

To enable cache sharing across different build directories for debug builds
(using `-g`), normalize debug paths e.g. with GCC option
`-fdebug-prefix-map=$PWD=.` (map current directory to relative) or
`-fdebug-compilation-dir=.` (set compilation directory). Alternatively, set
<<config_hash_dir,*hash_dir*>> to `false` (disables directory hashing).

For builds with absolute paths, set <<config_base_dir,*base_dir*>> to a common
parent directory. Ccache will convert absolute paths under this directory to
relative paths before hashing.

For example, if projects are in `/home/user/projects`, set *base_dir* to
`/home/user/projects` or just `/home/user`.


== Precompiled headers

Ccache has limited support for precompiled headers (PCH) with GCC and Clang.

Required configuration:

1. Set <<config_sloppiness,*sloppiness*>> to `pch_defines,time_macros`

   * This is needed because ccache can't detect time macro usage or `#define`
     changes when using PCH.
   * Consider adding `include_file_mtime,include_file_ctime` for newly created
     files (see _<<Handling of newly created source files>>_).

2. One of:

   * *GCC/Clang*: Use `-include header` (don't use `#include` in source; the
     filename must be sufficient to find the header).
   * *Clang only*: Use `-include-pch` with the generated PCH file.
   * *GCC only*: Add `-fpch-preprocess` when compiling.

3. For Clang, add `-fno-pch-timestamp` to disable timestamp checks.

Troubleshooting:

* Without proper setup, ccache falls back to the non-precompiled header (if
  available).
* If no fallback exists, you'll see "`Preprocessing failed`" in statistics.
* Files using `#pragma once` instead of include guards may cause issues.


== C++ modules

Ccache does currently not support standard C++20 modules.

There is however limited support for "`Clang modules`" (the `-fmodules` option)
with these requirements:

* Set <<config_sloppiness,*sloppiness*>> to `modules`.
* Enable both <<The direct mode,*direct mode*>> and <<The depend mode,*depend
  mode*>>.

This is how it works:

* Ccache hashes `module.modulemap` files but ignores Clang's binary module
  cache.
* This works because identical `module.modulemap` files should produce
  compatible cached results.

NOTE: The preprocessor mode doesn't provide enough information for module
support <<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 local cache

A group of developers can increase the cache hit rate by sharing a local cache
directory. To share a local 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.

An alternative to putting the main cache directory on NFS is to set up a
<<config_remote_storage,remote storage>> file cache.


== 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 space-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>>_ above.


== Troubleshooting

=== General

To understand what ccache is doing you can enable debug logging by setting
<<config_debug,*debug*>> to `true` or using the environment variable
`CCACHE_DEBUG=1`. See _<<Cache debugging>>_ for details.

You can also use `ccache -s` to view cache hit rates, miss reasons and other
performance metrics.


=== Performance

Ccache is designed to work well with minimal configuration, but you can optimize
performance further:

* Place the cache directory on fast storage (SSD preferred).
* Ensure sufficient RAM so cached files stay in the filesystem cache.
* Consider using <<config_remote_storage,remote storage>> for sharing the cache.

To monitor cache effectiveness, compare `ccache -s` output before and after
builds to identify issues.

If "`preprocessed cache hits`" increases instead of "`direct cache hits`",
ccache is falling back to the slower preprocessor mode. Causes include:

* source code changes that don't affect preprocessor output
* changes to include-related options (`-I`, `-include`, `-D`) that don't change
  the actual preprocessed code
* using unsupported preprocessor options like `-Xpreprocessor` or `-Wp,*`
  (except `-Wp,-MD,<path>`, `-Wp,-MMD,<path>`, `-Wp,-D<define>`)
* first compilation after changing <<config_base_dir,*base_dir*>>
* include files with timestamps newer than ccache startup (see
 _<<Handling of newly created source files>>_)
* code containing `+__TIME__+`, `+__DATE__+`, or `+__TIMESTAMP__+` forces
  preprocessor mode (if these macros aren't actually used or you don't need
  current values, set <<config_sloppiness,*sloppiness*>> to `time_macros`)
* input file path changes (affects `+__FILE__+` macro expansion)

If identical code doesn't produce cache hits these are some possible causes:

* time/date macros or generated code with timestamps/build counters
* cache cleanup due to size limits
* environment differences affecting compilation

Common error counters:

* "`Multiple source files`": The compiler was called with multiple source files
  at once (not supported by ccache).
* "`Unsupported compiler option`": Enable debug logging to see which option was
  rejected.
* "`Preprocessing failed`": Could indicate precompiled header issues (see
  _<<Precompiled headers>>_).
* "`Could not use precompiled header`": See _<<Precompiled headers>>_ for setup
  requirements.
* "`Could not use modules`": See _<<C++ modules>>_ for configuration needs and
  check which compiler option was rejected.


=== 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.
