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CCACHE(1)			 ccache Manual			     CCACHE(1)

NAME
       ccache - a fast C/C++ compiler cache

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

DESCRIPTION
       ccache is a compiler cache. It speeds up recompilation by caching the
       result of previous compilations and detecting when the same compilation
       is being done again. Supported languages are C, C++, Objective-C and
       Objective-C++.

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

   Features
       ·   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.

   Limitations
       ·   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.

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

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

       To use the symlinks method, do something like this:

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

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

	   Warning
	   The technique of letting ccache masquerade as the compiler works
	   well, but currently doesn’t interact well with other tools that do
	   the same thing. See USING CCACHE WITH OTHER COMPILER WRAPPERS.

	   Warning
	   Do not use a hard link, use a symbolic link. A hard link will cause
	   “interesting” problems.

OPTIONS
       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
	   information.

       -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).

EXTRA OPTIONS
       When run as a compiler, ccache usually just takes the same command line
       options as the compiler you are using. The only exception to this is
       the option --ccache-skip. That option can be used to tell ccache to
       avoid interpreting the next option in any way and to pass it along to
       the compiler as-is. Note: --ccache-skip currently only tells ccache not
       to interpret the next option as a special compiler option — the option
       will still be included in the direct mode hash.

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

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

CONFIGURATION
       ccache’s default behavior can be overridden by 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. Boolean options are indicated with
       “[boolean]”

       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.

       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
	   $HOME/.ccache.

       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:

	   content
	       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
	       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 the none setting if you
	       know what you are doing.

	   string:value
	       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
	       USING CCACHE WITH OTHER COMPILER WRAPPERS.

       compression (CCACHE_COMPRESS) [boolean]
	   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) [boolean]
	   If true, the direct mode will be used. The default is true. See THE
	   DIRECT MODE.

       disable (CCACHE_DISABLE) [boolean]
	   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 idetifies the build. The list separator is
	   semicolon on Windows systems and colon on other systems.

       hard_link (CCACHE_HARDLINK) [boolean]
	   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) [boolean]
	   If true, ccache will include the current working directory in the
	   hash that is used to distinguish two compilations. This prevents a
	   problem with the storage of the current working directory in the
	   debug info of a object file, which can lead ccache to give a cached
	   object file that has the working directory in the debug info set
	   incorrectly. This option is off by default as the incorrect setting
	   of this debug info rarely causes problems. If you strike problems
	   with GDB not using the correct directory then enable this option.

       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.

       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
	   USING CCACHE WITH OTHER COMPILER WRAPPERS.

       read_only (CCACHE_READONLY) [boolean]
	   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.

       read_only_direct (CCACHE_READONLY_DIRECT) [boolean]
	   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) [boolean]
	   If true, ccache will not use any previously stored result. New
	   results will still be cached, possibly overwriting any pre-existing
	   results.

       run_second_cpp (CCACHE_CPP2) [boolean]
	   If true, ccache will not use the optimisation of avoiding the
	   second call to the preprocessor by compiling the preprocessed
	   output that was used for finding the hash in the case of a cache
	   miss. This is primarily a debugging option, although it is possible
	   that some unusual compilers will have problems with compiling the
	   preprocessed output, in which case this option could allow ccache
	   to be used anyway.

       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:

	   file_macro
	       Ignore __FILE__ being present in the source.

	   file_stat_matches
	       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.

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

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

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

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

	   See the discussion under TROUBLESHOOTING for more information.

       stats (CCACHE_STATS) [boolean]
	   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.

	       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 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
	   compilations.

       unify (CCACHE_UNIFY) [boolean]
	   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.

CACHE SIZE MANAGEMENT
       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).

CACHE COMPRESSION
       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.

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 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
	   setting)

       ·   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
	   any)

   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
       cases.

       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 and
	       -Wp,-MMD,path

	   ·   -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
       and:

       ·   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.

COMPILING IN DIFFERENT DIRECTORIES
       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
	   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. 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
       anything.)

       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.

PRECOMPILED HEADERS
       ccache has support for GCC’s precompiled headers. However, you have to
       do some things to make it work properly:

       ·   You must set 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;
	       or

	   ·   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
	   available).

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

       ·   Use the same cache directory.

       ·   Make sure that the configuration 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.

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.

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 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 increase
       performance.

BUGS
       ·   ccache doesn’t handle the GNU Assembler’s .incbin directive
	   correctly. This directive can be embedded in the source code inside
	   an asm statement in order to include a file verbatim in the object
	   file. If the included file is modified, ccache doesn’t pick up the
	   change since the inclusion isn’t done by the preprocessor. A
	   workaround of this problem is to set extra_files_to_hash to the
	   path of the included file.

       ·   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 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.

   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
       preferrable.

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

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

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

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

	   ·   The compiler option -Xpreprocessor or -Wp,X (except
	       -Wp,-MD,path and Wp,-MMD,path) 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 outside comments and string literals. 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 outside
	       comments and string literals, 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 outside comments and string literals, 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
	   PRECOMPILED HEADERS.

   Errors when compiling with ccache
       If compilation doesn’t work with ccache, but it works without it, one
       possible reason is that the compiler can’t compile preprocessed output
       correctly. A workaround that may work is to enable run_second_cpp*.
       This will make cache misses slower, though, so it is better to find and
       fix the root cause.

   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: http://ccache.samba.org.

AUTHOR
       ccache was originally written by Andrew Tridgell and is currently
       developed and maintained by Joel Rosdahl. See AUTHORS.txt or
       AUTHORS.html and http://ccache.samba.org/credits.html for a list of
       contributors.

ccache 3.2.4			  10/08/2015			     CCACHE(1)
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