Provided by: ccache_3.4.2-1_amd64 bug

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
       CAVEATS. 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 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 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.

           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” (even if set to the
       empty string), the following case-insensitive negative values are considered an error
       (rather than surprising the user): 0, false, disable and no, 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. 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.

           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 or CCACHE_NOCOMPRESS, see Boolean values above)
           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 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.

       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 above)
           If true, ccache will attempt to use hard links from the cache directory when creating
           the compiler output rather than using a file copy. 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.

               Warning
               Do not enable this option unless you are aware of the consequences. Using hard
               links may be slightly faster in some situations, but there are several pitfalls
               since the resulting object file will share i-node with the cached object file:

            1. If the resulting object file is modified in any way, the cached object file will
               be modified as well. For instance, if you run strip object.o or echo >object.o,
               you will corrupt the cache.

            2. Programs that rely on modification times (like “make”) can be confused since
               ccache updates the cached files' modification times as part of the automatic cache
               size management. This will affect object files in the build tree as well, which
               can retrigger the linking step 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 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 -Wdocumentation.

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

       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). 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.

       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.

       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 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 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.

       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).

       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 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.

           no_system_headers
               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.

           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 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.

               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 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.

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 -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).

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

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

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

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

        1. Count all files in the subdirectory and compute their aggregated size.

        2. Remove files in LRU (least recently used) order until the size is at most
           limit_multiple * max_size / 16 and the number of files is at most limit_multiple *
           max_files / 16, where limit_multiple, max_size and max_files are configuration
           settings.

        3. Set the size and file number counters to match the files that were kept.

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

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

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.

CACHE STATISTICS

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

       ┌───────────────────────────────┬──────────────────────────────────┐
       │NameDescription                      │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │autoconf compile/link          │ Uncachable compilation or        │
       │                               │ linking by an autoconf test.     │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │bad compiler arguments         │ Malformed compiler argument,     │
       │                               │ e.g. missing a value for an      │
       │                               │ option that requires an argument │
       │                               │ or failure to read a file        │
       │                               │ specified by an option argument. │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │cache file missing             │ A file was unexpectedly missing  │
       │                               │ from the cache. This only        │
       │                               │ happens in rare situations, e.g. │
       │                               │ if one ccache instance is about  │
       │                               │ to get a file from the cache     │
       │                               │ while another instance removed   │
       │                               │ the file as part of cache        │
       │                               │ cleanup.                         │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │cache hit (direct)             │ A result was successfully found  │
       │                               │ using the direct mode.           │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │cache hit (preprocessed)       │ A result was successfully found  │
       │                               │ using the preprocessor mode.     │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │cache miss                     │ No result was found.             │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │cache size                     │ Current size of the cache.       │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │called for link                │ The compiler was called for      │
       │                               │ linking, not compiling.          │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │called for preprocessing       │ The compiler was called for      │
       │                               │ preprocessing, not compiling.    │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │can’t use precompiled header   │ Preconditions for using          │
       │                               │ precompiled headers were not     │
       │                               │ fulfilled.                       │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │ccache internal error          │ Unexpected failure, e.g. due to  │
       │                               │ problems reading/writing the     │
       │                               │ cache.                           │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │cleanups performed             │ Number of cleanups performed,    │
       │                               │ either implicitly due to the     │
       │                               │ cache size limit being reached   │
       │                               │ or due to explicit ccache        │
       │                               │ -c/--cleanup calls.              │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │compile failed                 │ The compilation failed. No       │
       │                               │ result stored in the cache.      │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │compiler check failed          │ A compiler check program         │
       │                               │ specified by compiler_check      │
       │                               │ (CCACHE_COMPILERCHECK) failed.   │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │compiler produced empty output │ The compiler’s output file       │
       │                               │ (typically an object file) was   │
       │                               │ empty after compilation.         │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │compiler produced no output    │ The compiler’s output file       │
       │                               │ (typically an object file) was   │
       │                               │ missing after compilation.       │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │compiler produced stdout       │ The compiler wrote data to       │
       │                               │ standard output. This is         │
       │                               │ something that compilers         │
       │                               │ normally never do, so ccache is  │
       │                               │ not designed to store such       │
       │                               │ output in the cache.             │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │couldn’t find the compiler     │ The compiler to execute could    │
       │                               │ not be found.                    │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │error hashing extra file       │ Failure reading a file specified │
       │                               │ by extra_files_to_hash           │
       │                               │ (CCACHE_EXTRAFILES).             │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │files in cache                 │ Current number of files in the   │
       │                               │ cache.                           │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │multiple source files          │ The compiler was called to       │
       │                               │ compile multiple source files in │
       │                               │ one go. This is not supported by │
       │                               │ ccache.                          │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │no input file                  │ No input file was specified to   │
       │                               │ the compiler.                    │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │output to a non-regular file   │ The output path specified with   │
       │                               │ -o is not a file (e.g. a         │
       │                               │ directory or a device node).     │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │output to stdout               │ The compiler was instructed to   │
       │                               │ write its output to standard     │
       │                               │ output using -o -. This is not   │
       │                               │ supported by ccache.             │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │preprocessor error             │ Preprocessing the source code    │
       │                               │ using the compiler’s -E option   │
       │                               │ failed.                          │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │unsupported code directive     │ Code like the assembler .incbin  │
       │                               │ directive was found. This is not │
       │                               │ supported by ccache.             │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │unsupported compiler option    │ A compiler option not supported  │
       │                               │ by ccache was found.             │
       ├───────────────────────────────┼──────────────────────────────────┤
       │                               │                                  │
       │unsupported source language    │ A source language e.g. specified │
       │                               │ with -x was unsupported by       │
       │                               │ ccache.                          │
       └───────────────────────────────┴──────────────────────────────────┘

HOW CCACHE WORKS

       The basic idea is to detect when you are compiling exactly the same code a second time and
       reuse the previously produced output. The detection is done by hashing different kinds of
       information that should be unique for the compilation and then using the hash sum to
       identify the cached output. ccache uses 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)

       ·   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 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 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 -fdebug-prefix-map=old=new option 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 to 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 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 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.

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 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).

CAVEATS

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

       ·   When run via ccache, warning messages produced by GCC 4.9 and newer will only be
           colored when the environment variable GCC_COLORS is set. An alternative to setting
           GCC_COLORS is to pass -fdiagnostics-color explicitly when compiling (but then color
           codes will also be present when redirecting stderr to a file).

       ·   If ccache guesses that the compiler may emit colored warnings, then a compilation with
           stderr referring to a TTY will be considered different from a compilation with a
           redirected stderr, thus not sharing cache entries. This happens for clang by default
           and for GCC when GCC_COLORS is set as mentioned above. If you want to share cache
           hits, you can pass -f[no-]diagnostics-color (GCC) or -f[no-]color-diagnostics (clang)
           explicitly when compiling (but then color codes will be either on or off for both the
           TTY and the redirected case).

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 preferable.

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

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

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

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

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

           ·   This was the first compilation with a new value of the 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 PRECOMPILED HEADERS.

   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.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 https://ccache.samba.org/credits.html
       for a list of contributors.