ccache - a fast C/C++ compiler cache


   ccache [options]
   ccache compiler [compiler options]
   compiler [compiler options]                   (via symbolic link)


   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. Supported languages are C, C++, Objective-C and

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

   *   Keeps statistics on hits/misses.

   *   Automatic cache size management.

   *   Can cache compilations that generate warnings.

   *   Easy installation.

   *   Low overhead.

   *   Optionally uses hard links where possible to avoid copies.

   *   Optionally compresses files in the cache to reduce disk space.

   *   Only knows how to cache the compilation of a single
       C/C++/Objective-C/Objective-C++ file. Other types of compilations
       (multi-file compilation, linking, etc) will silently fall back to
       running the real compiler.

   *   Only works with GCC and compilers that behave similar enough.

   *   Some compiler flags are not supported. If such a flag is detected,
       ccache will silently fall back to running the real compiler.


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

   To use the second method 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:

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

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

       The technique of letting ccache masquerade as the compiler works
       well, but currently doesn't interact well with other tools that do

       Do not use a hard link, use a symbolic link. A hard link will cause
       "interesting" problems.


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

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

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

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

   -h, --help
       Print an options summary page.

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

   -o, --set-config=KEY=VALUE
       Set configuration KEY to VALUE. See CONFIGURATION for more

   -p, --print-config
       Print current configuration options and from where they originate
       (environment variable, configuration file or compile-time default).

   -s, --show-stats
       Print the current statistics summary for the cache.

   -V, --version
       Print version and copyright information.

   -z, --zero-stats
       Zero the cache statistics (but not the configuration 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.


   ccache's default behavior can be overridden by configuration file
   settings, 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 priority of configuration
   settings is as follows (where 1 is highest):

    1. Environment variables.

    2. The cache-specific configuration file <ccachedir>/ccache.conf
       (typically $HOME/.ccache/ccache.conf).

    3. The system-wide configuration file <sysconfdir>/ccache.conf
       (typically /etc/ccache.conf or /usr/local/etc/ccache.conf).

    4. Compile-time defaults.

   As a special case, if the environment variable CCACHE_CONFIGPATH is
   set, ccache reads configuration from the specified path instead of the
   default paths.

   Configuration file syntax
   Configuration files are in a simple "key = value" format, one setting
   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 settings 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" regardless of
   the value (even if set to the empty string), and 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 settings
   Below is a list of available configuration settings. The corresponding
   environment variable name is indicated in parentheses after each
   configuration setting key.

   base_dir (CCACHE_BASEDIR)
       This setting should be an absolute path to a directory. ccache then
       rewrites absolute paths into relative paths before computing the
       hash that identifies the compilation, but only for paths under the
       specified directory. If set to the empty string (which is the
       default), no rewriting is done. See also the discussion under
       COMPILING IN DIFFERENT DIRECTORIES. If using GCC or newer versions
       of Clang, you might want to look into the
       -fdebug-prefix-map=old=new option for relocating debug info to a
       common prefix (mapping prefix with old=new).

   cache_dir (CCACHE_DIR)
       This setting specifies where ccache will keep its cached compiler
       outputs. It will only take effect if set in the system-wide
       configuration file or as an environment variable. The default is

   cache_dir_levels (CCACHE_NLEVELS)
       This setting allows you to choose the number of directory levels in
       the cache directory. The default is 2. The minimum is 1 and the
       maximum is 8.

   compiler (CCACHE_CC)
       This setting 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. This setting can be used to select
       another strategy. Possible values are:

           Hash the content of the compiler binary. This makes ccache very
           slightly slower compared to the mtime setting, but makes it
           cope better with compiler upgrades during a build bootstrapping

           Hash the compiler's mtime and size, which is fast. This is the

           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 the none setting if you
           know what you are doing.

           Use value as the string to calculate hash from. This can be the
           compiler revision number you retrieved earlier and set here via
           environment variable.

       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 setting if possible. See

   compression (CCACHE_COMPRESS or CCACHE_NOCOMPRESS, see Boolean values
       If true, ccache will compress object files and other compiler
       output it puts in the cache. However, this setting has no effect on
       how files are retrieved from the cache; compressed and uncompressed
       results will still be usable regardless of this setting. The
       default is false.

   compression_level (CCACHE_COMPRESSLEVEL)
       This setting determines the level at which ccache will compress
       object files. It only has effect if compression is enabled. The
       value defaults to 6, and must be no lower than 1 (fastest, worst
       compression) and no higher than 9 (slowest, best compression).

   cpp_extension (CCACHE_EXTENSION)
       This setting 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.

   direct_mode (CCACHE_DIRECT or CCACHE_NODIRECT, see Boolean values
       If true, the direct mode will be used. The default is true. See THE

   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.

   extra_files_to_hash (CCACHE_EXTRAFILES)
       This setting 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.

   hard_link (CCACHE_HARDLINK or CCACHE_NOHARDLINK, see Boolean values
       If true, ccache will attempt to use hard links from the cache
       directory when creating the compiler output rather than using a
       file copy. Using hard links may be slightly faster in some
       situations, but can confuse programs like "make" that rely on
       modification times. Another thing to keep in mind is that if the
       resulting object file is modified in any way, this corrupts the
       cached object file as well. Hard links are never made for
       compressed cache files. This means that you should not enable
       compression if you want to use hard links. The default is false.

   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.

           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 setting 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 setting 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.

   keep_comments_cpp (CCACHE_COMMENTS or CCACHE_NOCOMMENTS, see Boolean
   values above)
       If true, ccache will not discard the comments before hashing
       preprocessor output. This can be used to check documentation with

   limit_multiple (CCACHE_LIMIT_MULTIPLE)
       Sets the limit when cleaning up. Files are deleted (in LRU order)
       until the levels are below the limit. The default is 0.8 (= 80%).

   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

   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).

   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".

   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.

   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

   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
       If true, ccache will attempt to use existing cached object files,
       but it will not to try to add anything new to the cache. If you are
       using this because your ccache directory is read-only, then you
       need to set temporary_dir as otherwise ccache will fail to create
       temporary files.

   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

   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.

   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 setting 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 options. Available options are:

           Ignore __FILE__ being present in the source.

           ccache normally examines a file's contents to determine whether
           it matches the cached version. With this option set, ccache
           will consider a file as matching its cached version if the
           sizes, mtimes and ctimes match.

           By default, ccache also will not cache a file if it includes a
           header whose ctime is too new. This option disables that check.

           By default, ccache will not cache a file if it includes a
           header whose mtime is too new. This option disables that check.

           By default, ccache will also include all system headers in the
           manifest. With this option set, ccache will only include system
           headers in the hash but not add the system header files to the
           list of include files.

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

           Ignore __DATE__ and __TIME__ 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.

   temporary_dir (CCACHE_TEMPDIR)
       This setting specifies where ccache will put temporary files. The
       default is <cache_dir>/tmp.

           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 setting specifies the umask for ccache and all child processes
       (such as the compiler). This is mostly useful when you wish to
       share your cache with other users. Note that this also affects the
       file permissions set on the object files created from your

   unify (CCACHE_UNIFY or CCACHE_NOUNIFY, see Boolean values above)
       If true, ccache will use a C/C++ unifier when hashing the
       preprocessor output if the -g option is not used. The unifier is
       slower than a normal hash, so setting this environment variable
       loses a little bit of speed, but it means that ccache can take
       advantage of not recompiling when the changes to the source code
       consist of reformatting only. Note that enabling the unifier
       changes the hash, so cached compilations produced when the unifier
       is enabled cannot be reused when the unifier is disabled, and vice
       versa. Enabling the unifier may result in incorrect line number
       information in compiler warning messages and expansions of the
       __LINE__ macro. Also note that enabling the unifier implies turning
       off the direct mode.


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


   ccache can optionally compress all files it puts into the cache using
   the compression library zlib. While this may involve a tiny performance
   slowdown, it increases the number of files that fit in the cache. You
   can turn on compression with the compression configuration setting and
   you can also tweak the compression level with compression_level.


   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 MD4, a very fast cryptographic hash
   algorithm, for the hashing. (MD4 is nowadays too weak to be useful in
   cryptographic contexts, but it should be safe enough to be used to
   identify recompilations.) On a cache hit, ccache is able to supply all
   of the correct compiler outputs (including all warnings, dependency
   file, etc) from the cache.

   ccache has two ways of doing the detection:

   *   the direct mode, where ccache hashes the source code and include
       files directly

   *   the preprocessor mode, where ccache runs the preprocessor on the
       source code and hashes the result

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

   Common hashed information
   For both modes, the following information is 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 the compiler_check

   *   the name of the compiler

   *   the current directory (if the hash_dir setting is enabled)

   *   contents of files specified by the extra_files_to_hash setting (if

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

   *   the input source file

   *   the command line 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

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

   *   the configuration setting direct_mode is false

   *   a modification time of one of the include files is too new (needed
       to avoid a race condition)

   *   the unifier is enabled (the configuration setting unify is true)

   *   a compiler option not supported by the direct mode is used:

       *   a -Wp,X compiler option other than -Wp,-MD,path, -Wp,-MMD,path
           and -Wp,-D_define_

       *   -Xpreprocessor

   *   the string "__TIME__" is present in the source code

   The preprocessor mode
   In the preprocessor mode, the hash is formed of the common information

   *   the preprocessor output from running the compiler with -E

   *   the command line options except options that affect include files
       (-I, -include, -D, etc; the theory is that these 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.


   Some information included in the hash that identifies a unique
   compilation may 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) may be

   *   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. To mitigate this problem,
   you can specify a "base directory" in the configuration setting
   base_dir to an absolute path to the directory. ccache will then rewrite
   absolute paths that are under the base directory (i.e., paths that have
   the base directory as a prefix) to relative paths when constructing the
   hash. A typical path to use as the base directory is your home
   directory or another directory that is a parent of your build
   directories. (Don't use / as the base directory since that will make
   ccache also rewrite paths to system header files, which doesn't gain

   The drawbacks of using a base directory are:

   *   If you specify an absolute path to the source code file, __FILE__
       macros will be expanded to a relative path instead.

   *   If you specify an absolute path to the source code file and compile
       with -g, the source code path stored in the object file may point
       to the wrong directory, which may prevent debuggers like GDB from
       finding the source code. Sometimes, a work-around is to change the
       directory explicitly with the "cd" command in GDB.


   ccache has support for GCC's precompiled headers. 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__ or __DATE__ 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 must either:

       *   use the -include compiler option to include the precompiled
           header (i.e., don't use #include in the source code to include
           the header); or

       *   (for the Clang compiler) use the -include-pch compiler option
           to include the PCH file generated from the precompiled header;

       *   add the -fpch-preprocess compiler option when compiling.

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


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

   *   Use the same cache directory.

   *   Make sure that the configuration setting hard_link is false (which
       is the default).

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

   *   Set the configuration setting 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 command "find $CCACHE_DIR -type d | xargs chmod
       g+s" might be useful for this.

   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.


   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.


   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 the configuration setting
   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 the prefix_command_cpp configuration setting
   if you also want to invoke the other wrapper when doing preprocessing
   (normally by adding -E).


   *   The direct mode fails to pick up new header files in some rare
       scenarios. See THE DIRECT MODE above.


   A general tip for getting information about what ccache is doing is to
   enable debug logging by setting log_file. The log contains executed
   commands, important decisions that ccache makes, read and written
   files, etc. Another way of keeping track of what is happening is to
   check the output of ccache -s.

   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

   *   If "cache hit (preprocessed)" has been incremented instead of
       "cache hit (direct)", ccache has fallen back to preprocessor mode,
       which is generally slower. Some possible reasons are:

       *   The source code has been modified in such a way that the
           preprocessor output is not affected.

       *   Compiler arguments that are hashed in the direct mode but not
           in the preprocessor mode have changed (-I, -include, -D, etc)
           and they didn't affect the preprocessor output.

       *   The compiler option -Xpreprocessor or -Wp,X (except
           -Wp,-MD,path, -Wp,-MMD,path, and -Wp,-D_define_) is used.

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

       *   A modification time of one of the include files is too new
           (created the same second as the compilation is being done).
           This check is made to avoid a race condition. To fix this,
           create the include file earlier in the build process, if
           possible, or set sloppiness to include_file_mtime if you are
           willing to take the risk. (The race condition consists of these
           events: the preprocessor is run; an include file is modified by
           someone; the new include file is hashed by ccache; the real
           compiler is run on the preprocessor's output, which contains
           data from the old header file; the wrong object file is stored
           in the cache.)

       *   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 __FILE__ preprocessor macro is (potentially) being used and
           the file path has changed. If "__FILE__" is present in the
           source code, ccache hashes the current input file path in order
           to be able to produce the correct object file if the __FILE__
           macro affects the output. If you know that __FILE__ isn't used
           in practise, or don't care if ccache produces objects where
           __FILE__ is expanded to the wrong path, you can set sloppiness
           to file_macro.

   *   If "cache miss" has been incremented even though the same code has
       been compiled and cached before, ccache has either detected that
       something has changed anyway or a cleanup has been performed
       (either explicitly or implicitly when a cache limit has been
       reached). Some perhaps unobvious things that may result in a cache
       miss are usage of __TIME__ or __DATE__ 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 option was rejected.

   *   If "preprocessor error" has been incremented, one possible reason
       is that precompiled headers are being used. See PRECOMPILED HEADERS
       for how to remedy this.

   *   If "can't use precompiled header" has been incremented, see

   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.


   Credits, mailing list information, bug reporting instructions, source
   code, etc, can be found on ccache's web site:


   ccache was originally written by Andrew Tridgell and is currently
   developed and maintained by Joel Rosdahl. See AUTHORS.txt or
   AUTHORS.html and for a list of


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