oracular (1) ccache.1.gz

Provided by: ccache_4.10.2-1_amd64 bug

NAME

       ccache - a fast C/C++ compiler cache

SYNOPSIS

       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 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. 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. If you discover an undocumented case
       where ccache changes the output of your compiler, please let us know.

RUN MODES

       There are two different ways to use ccache to cache a compilation:

        1. Prefix your compilation command with ccache. This method is most convenient if you just want to try
           out ccache or wish to use it for some specific projects. Example:

               ccache gcc -c example.c

        2. Let ccache masquerade as the compiler. This method is most useful when you wish to use ccache for all
           your compilations. To do this on a Debian system, it’s easiest to just prepend /usr/lib/ccache to
           your PATH. /usr/lib/ccache contains symlinks for all compilers currently installed as Debian
           packages.

           Alternatively, you can create any symlinks you like yourself like this:

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

           On platforms that don’t support symbolic links you can simply copy ccache to the compiler name
           instead for a similar effect:

               cp /usr/bin/ccache /usr/local/bin/gcc
               cp /usr/bin/ccache /usr/local/bin/g++

           And so forth. This will work as long as the directory with symbolic links or ccache copies comes
           before the directory with the compiler (typically /usr/bin) in PATH.

               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.

COMMAND LINE OPTIONS

       These command line options only apply when you invoke ccache as “ccache”. When ccache masquerades as a
       compiler (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 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 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 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.

       --recompress-threads THREADS
           Use up to THREADS threads when recompressing the cache. The default is to use one thread per CPU.

       -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 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.

       -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 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.

       --trim-recompress-threads THREADS
           Recompress using up to THREADS threads with --trim-recompress. The default is to use one thread per
           CPU.

   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
           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 ignore_options.

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. Command line settings in KEY=VALUE form. Example:

               ccache debug=true compiler_check="%compiler% --version" gcc -c example.c

        2. Environment variables.

        3. The cache-specific configuration file (see below).

        4. The system (read-only) configuration file <sysconfdir>/ccache.conf (typically /etc/ccache.conf or
           /usr/local/etc/ccache.conf).

        5. Compile-time defaults.

       As a special case, if the environment variable CCACHE_CONFIGPATH is set it specifies the configuration
       file, and the system configuration file won’t be read.

   Location of the 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 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 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 cache_dir configuration option for how the cache directory location is determined.

   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 = 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.

       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 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.

       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. 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 counterproductive.

           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 Alice’s command line:

               # 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 instead of using base_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/.config/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 Location of the configuration file.

       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.

       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 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.

       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.

           msvc
               Microsoft Visual C++ (MSVC).

           nvcc
               NVCC (CUDA) compiler.

           other
               Any compiler other than the known types.

       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 file_clone option) or hard linking (the hard_link
           option) is enabled.

       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 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 Zstandard documentation <http://zstd.net> for more information.

       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.

       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.

       debug_dir (CCACHE_DEBUGDIR)
           Specifies where to write per-object debug files if the 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.

       debug_level (CCACHE_DEBUGLEVEL)
           Specifies the amount of information that is written when the debug mode is enabled. See CACHE
           DEBUGGING for more information. The default is 2.

       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.

       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.

       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.

       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.

       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 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 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
           hard_link is enabled).

       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.

       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 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.

           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.

       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.

       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.

       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 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.

       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.

       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
               & ~

       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.

       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.

       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.

       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.

       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.

       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.

       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.

       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.

       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 stats option is set to
           false.

           If you are using this because your ccache directory is read-only, you need to set temporary_dir since
           ccache will fail to create temporary files otherwise. You may also want to set stats to false make
           ccache not even try to update stats files.

       read_only_direct (CCACHE_READONLY_DIRECT or CCACHE_NOREADONLY_DIRECT, see Boolean values above)
           Just like 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 read_only regarding using a read-only
           ccache directory.

       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.

       remote_only (CCACHE_REMOTE_ONLY or CCACHE_NOREMOTE_ONLY, see Boolean values above)
           If true, ccache will only use remote storage. The default is false. Note that cache statistics
           counters will still be kept in the local cache directory unless stats is false. See also Storage
           interaction.

       remote_storage (CCACHE_REMOTE_STORAGE)
           This option specifies one or several storage backends (separated by space) to query after checking
           the local cache (unless remote_only is true). See REMOTE STORAGE BACKENDS for documentation of syntax
           and available backends.

           Examples:

           •   file:/shared/nfs/directoryfile:///shared/nfs/one|read-only file:///shared/nfs/twofile:///Z:/example/windows/folderhttp://example.com/cacheredis://example.com

                   Note
                   In previous ccache versions this option was called secondary_storage
                   (CCACHE_SECONDARY_STORAGE), which can still be used as an alias.

       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.

       run_second_cpp (CCACHE_CPP2 or CCACHE_NOCPP2, see Boolean values above)
           If true, ccache will first run the preprocessor to preprocess the source code (see 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).

           This option is ignored with MSVC, as there is no way to make it compile without preprocessing first.

       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.

           gcno_cwd
               By default, ccache will include the current working directory in the hash when producing a .gcno
               file (when compiling with -ftest-coverage). This is because GCC 9+ includes the current working
               directory in the .gcno file. The gcno_cwd sloppiness makes ccache not hash the current working
               directory so that you can get cache hits when compiling in different directories, with the
               tradeoff of potentially getting an incorrect directory in the .gcno file. gcno_cwd also disables
               hashing of the current working directory if -fprofile-abs-path is used.

                   Note
                   gcno_cwd sloppiness will not have any effect when compiling with --coverage since that
                   implies -fprofile-arcs which always forces CWD to be included in the input hash.

           incbin
               By default, ccache will ignore all files containing an .incbin directive. While this is the
               correct behaviour as ccache does not detect incbin changes, this restriction can make some
               projects difficult to cache. This sloppiness will pretend the .incbin directive doesn’t exist and
               simply allow caching.

           include_file_ctime
               By default, ccache will disable caching if a source code file has a status change time (ctime)
               after the start of the ccache invocation. This sloppiness disables that check. See also [Handling
               of newly created source files].

           include_file_mtime
               By default, ccache will disable caching if a source code file has a modification time (mtime)
               after the start of the ccache invocation. This sloppiness disables that check. See also [Handling
               of newly created source 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.

           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 for more information.

           pch_defines
               Be sloppy about #define directives when precompiling a header file. See PRECOMPILED HEADERS for
               more information.

           random_seed
               Ignore the -frandom-seed option and its arguments when computing the input hash. This is useful
               if your build system generates different seeds between builds and you are OK with reusing cached
               results.

           system_headers
               Only check non-system headers in direct mode. This can be useful if e.g. your system headers tend
               to change but you know that the changes don’t matter. Notes:

               •   This sloppiness is only supported for GCC-like compilers, not MSVC.

               •   System headers are still taken into account for preprocessed lookup.

               •   You can get stale cache hits if the system headers do change in incompatible ways.

               •   See also the ignore_headers_in_manifest setting.

           time_macros
               Ignore __DATE__, __TIME__ and __TIMESTAMP__ being present in the source code.

           See the discussion under TROUBLESHOOTING for more information.

       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] will be disabled as well.

       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.

       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.

       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 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 remote_storage option lets you configure ccache to use one or several remote storage backends. By
       default, the local cache directory located in cache_dir will be queried first and remote storage second,
       but 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.

       A remote storage backend is specified with a URL, optionally followed by a pipe (|) and a pipe-separated
       list of attributes. An attribute is key=value or just key as a short form of key=true. Attribute values
       must be percent-encoded <https://en.wikipedia.org/wiki/Percent-encoding> if they contain percent, pipe or
       space characters.

   Attributes for all backends
       These optional attributes are available for all remote storage backends:

       •   read-only: If true, only read from this backend, don’t write. The default is false.

       •   shards: A comma-separated list of names for sharding (partitioning) the cache entries using
           Rendezvous hashing <https://en.wikipedia.org/wiki/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:

           •   redis://cache-*.example.com|shards=a(3),b(1),c(1.5) will put 55% (3/5.5) of the cache on
               redis://cache-a.example.com, 18% (1/5.5) on redis://cache-b.example.com and 27% (1.5/5.5) on
               redis://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.

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

       ┌──────────────┬────────────────┬──────────────────────────────┐
       │              │                │                              │
       │Local storageRemote storageWhat 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 attribute read-only=true.
       [2] Unless local storage is set to share its cache hits with the reshare option.

       If remote_only is true:

       ┌──────────────┬────────────────┬───────────────────────────┐
       │              │                │                           │
       │Local storageRemote storageWhat 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/directoryfile:///shared/nfs/directory|umask=002|update-mtime=truefile:///Z:/example/windows/folderfile://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.

   HTTP storage backend
       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 How to set up HTTP storage <https://ccache.dev/howto/http-storage.html> for hints on how to set
           up an HTTP server for use with ccache.

       Examples:

       •   http://localhosthttp://someusername:p4ssw0rd@example.com/cache/http://localhost:8080|layout=bazel|connect-timeout=50

       Optional attributes:

       •   bearer-token: Bearer token used to authorize the HTTP requests.

       •   connect-timeout: Timeout (in ms) for network connection. The default is 100.

       •   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.

       •   operation-timeout: Timeout (in ms) for HTTP requests. The default is 10000.

   Redis storage backend
       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 Redis <https://redis.io> (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 configure LRU eviction
           <https://redis.io/topics/lru-cache>.

           Tip
           See How to set up Redis <https://ccache.dev/howto/redis-storage.html> 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 attribute described in REMOTE STORAGE
           BACKENDS.

       Examples:

       •   redis://localhostredis://p4ssw0rd@cache.example.com:6379/0|connect-timeout=50redis+unix:/run/redis.sockredis+unix:///run/redis.sockredis+unix://p4ssw0rd@localhost/run/redis.sock?db=0

       Optional attributes:

       •   connect-timeout: Timeout (in ms) for network connection. The default is 100.

       •   operation-timeout: Timeout (in ms) for Redis commands. The default is 10000.

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 written to the local cache, ccache will trigger an automatic
       cleanup if max_size or max_files is exceeded. The cleanup removes cache entries in 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.

   Manual cleanup
       You can run ccache -c/--cleanup to force cleanup of the whole cache. This will recalculate the cache size
       information and also make sure that the cache size does not exceed max_size and max_files.

CACHE COMPRESSION

       Ccache will by default compress all data it puts into the cache using the compression algorithm Zstandard
       <http://zstd.net> (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 compression and 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 size:      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
           file_clone or hard_link) or unknown files (for instance files created by older ccache versions).

       •   The compression ratio is affected by 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 --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 direct/preprocessed modes and hit rate for local and remote storage.

       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:

       ┌───────────────────────────────────┬───────────────────────────────────────┐
       │                                   │                                       │
       │CounterDescription                           │
       ├───────────────────────────────────┼───────────────────────────────────────┤
       │                                   │                                       │
       │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 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 │
       │                                   │ 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   │
       │                                   │ extra_files_to_hash                   │
       │                                   │ (CCACHE_EXTRAFILES).                  │
       ├───────────────────────────────────┼───────────────────────────────────────┤
       │                                   │                                       │
       │Forced recache                     │ CCACHE_RECACHE was used to overwrite  │
       │                                   │ an existing result.                   │
       ├───────────────────────────────────┼───────────────────────────────────────┤
       │                                   │                                       │
       │Input file modified during         │ An input file was modified during     │
       │compilation                        │ 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 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 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
           compiler_check)

       •   the name of the compiler

       •   the current directory (if hash_dir is enabled)

       •   contents of files specified by 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:

       •   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:

       •   depend_mode is false.

       •   run_second_cpp 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 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 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:

       ┌───────────────────────────────────────────┬─────────────┬──────────────────────────────┐
       │                                           │             │                              │
       │FilenameDebug levelDescription                  │
       ├───────────────────────────────────────────┼─────────────┼──────────────────────────────┤
       │                                           │             │                              │
       │<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 debug_level (environment variable CCACHE_DEBUGLEVEL) to 1.

       If 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

       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 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 precompiled headers with GCC and Clang. However, you have to do some things to
       make it work properly:

       •   You must set 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 #defines 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 sloppiness. See HANDLING OF
           NEWLY CREATED SOURCE 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 use Clang, you must compile with -fno-pch-timestamp.

       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 “Preprocessing
       failed” (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 sloppiness to modules to allow
       caching.

       You must use both direct mode and depend mode. When using 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 hard_link is false (which is the default).

       •   Make sure that all users are in the same group.

       •   Set the configuration option 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 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 sloppiness to
       system_headers to ignore system headers.

       An alternative to putting the main cache directory on NFS is to set up a 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 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 prefix_command to a colon-separated list of commands.

       Unless you set 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 prefix_command is used, ccache will not invoke the other wrapper when
       running the preprocessor, which increases performance. You can use 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
       A general tip for getting information about what ccache is doing is to enable debug logging by setting
       the configuration option debug (or the environment variable CCACHE_DEBUG); see 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 the counter for preprocessed cache hits has been incremented instead of the one for direct cache
           hits, 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,* (except -Wp,-MD,<path>, -Wp,-MMD,<path>, and
               -Wp,-D<define>) is used.

           •   This was the first compilation with a new value of the base directory.

           •   A modification or status change time of one of the include files is too new . See HANDLING OF
               NEWLY CREATED SOURCE 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 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 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
               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 a cache hit counter was not 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 “Preprocessing failed” has been incremented, one possible reason is that precompiled headers are
           being used. See PRECOMPILED HEADERS for how to remedy this.

       •   If “Could not use precompiled header” has been incremented, see PRECOMPILED HEADERS.

       •   If “Could not use modules” has been incremented, see 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
       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.