Provided by: ccache_3.1.6-1_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 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 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, it’s not needed 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.

       -F, --max-files=N
           Set the maximum number of files allowed in the cache. The value is stored inside the
           cache directory and applies to all future compilations. Due to the way the value is
           stored the actual value used is always rounded down to the nearest multiple of 16.

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

       -M, --max-size=SIZE
           Set the maximum size of the files stored in the cache. You can specify a value in
           gigabytes, megabytes or kilobytes by appending a G, M or K to the value. The default
           is gigabytes. The actual value stored is rounded down to the nearest multiple of 16
           kilobytes.

       -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 configured limits).

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.

       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.

ENVIRONMENT VARIABLES

       ccache uses a number of environment variables to control operation. In most cases you
       won’t need any of these as the defaults will be fine.

       CCACHE_BASEDIR
           If you set the environment variable CCACHE_BASEDIR to an absolute path to a directory,
           ccache rewrites absolute paths into relative paths before computing the hash that
           identifies the compilation, but only for paths under the specified directory. See the
           discussion under COMPILING IN DIFFERENT DIRECTORIES.

       CCACHE_CC
           You can optionally set CCACHE_CC to force the name of the compiler to use. If you
           don’t do this then ccache works it out from the command line.

       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. The
           CCACHE_COMPILERCHECK environment variable 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.

           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 CCACHE_PREFIX if possible. See
               USING CCACHE WITH OTHER COMPILER WRAPPERS.

       CCACHE_COMPRESS
           If you set the environment variable CCACHE_COMPRESS then 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.

       CCACHE_CPP2
           If you set the environment variable CCACHE_CPP2 then 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 the intermediate filename extensions used in this
           optimisation, in which case this option could allow ccache to be used anyway.

       CCACHE_DETECT_SHEBANG
           The CCACHE_DETECT_SHEBANG environment variable only has meaning on Windows. It
           instructs ccache to open the executable file to detect the #!/bin/sh string, in which
           case ccache will search for sh.exe in PATH and use that to launch the executable.

       CCACHE_DIR
           The CCACHE_DIR environment variable specifies where ccache will keep its cached
           compiler output. The default is $HOME/.ccache.

       CCACHE_DISABLE
           If you set the environment variable CCACHE_DISABLE then ccache will just call the real
           compiler, bypassing the cache completely.

       CCACHE_EXTENSION
           ccache tries to automatically determine the extension to use for intermediate
           preprocessor files based on the type of file being compiled. Unfortunately this
           sometimes doesn’t work, for example when using the “aCC” compiler on HP-UX. On systems
           like this you can use the CCACHE_EXTENSION option to override the default. On HP-UX
           set this environment variable to i if you use the “aCC” compiler.

       CCACHE_EXTRAFILES
           If you set the environment variable CCACHE_EXTRAFILES to a list of paths then ccache
           will include the contents of those files when calculating the hash sum. The list
           separator is semicolon in Windows systems and colon on other systems.

       CCACHE_HARDLINK
           If you set the environment variable CCACHE_HARDLINK then 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 set the CCACHE_COMPRESS variable if you want to use hard
           links.

       CCACHE_HASHDIR
           This tells ccache to hash the current working directory when calculating 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.

       CCACHE_LOGFILE
           If you set the CCACHE_LOGFILE environment variable then ccache will write information
           on what it is doing to the specified file. This is useful for tracking down problems.

       CCACHE_NLEVELS
           The environment variable CCACHE_NLEVELS allows you to choose the number of levels of
           hash in the cache directory. The default is 2. The minimum is 1 and the maximum is 8.

       CCACHE_NODIRECT
           If you set the environment variable CCACHE_NODIRECT then ccache will not use the
           direct mode.

       CCACHE_NOSTATS
           If you set the environment variable CCACHE_NOSTATS then ccache will not update the
           statistics files on each compilation.

       CCACHE_PATH
           You can optionally set CCACHE_PATH to a colon-separated path where ccache will look
           for the real compilers. If you don’t do this then ccache will look for the first
           executable matching the compiler name in the normal PATH that isn’t a symbolic link to
           ccache itself.

       CCACHE_PREFIX
           This option adds a prefix to the command line that ccache runs when invoking the
           compiler. Also see the section below on using ccache with “distcc”.

       CCACHE_READONLY
           The CCACHE_READONLY environment variable tells ccache to attempt to use existing
           cached object files, but not to try to add anything new to the cache. If you are using
           this because your CCACHE_DIR is read-only, then you may find that you also need to set
           CCACHE_TEMPDIR as otherwise ccache will fail to create temporary files.

       CCACHE_RECACHE
           This forces ccache to not use any cached results, even if it finds them. New results
           are still cached, but existing cache entries are ignored.

       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. The CCACHE_SLOPPINESS environment variable 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.

           include_file_mtime
               Don’t check the modification time of include files in the direct mode.

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

           See the discussion under TROUBLESHOOTING for more information.

       CCACHE_TEMPDIR
           The CCACHE_TEMPDIR environment variable specifies where ccache will put temporary
           files. The default is $CCACHE_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.)

       CCACHE_UMASK
           This sets 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.

       CCACHE_UNIFY
           If you set the environment variable CCACHE_UNIFY then 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 using CCACHE_UNIFY changes
           the hash, so cached compilations with CCACHE_UNIFY set cannot be used when
           CCACHE_UNIFY is not set and vice versa. The reason the unifier is off by default is
           that it can give incorrect line number information in compiler warning messages. Also
           note that enabling the unifier implies turning off the direct mode.

CACHE SIZE MANAGEMENT

       By default ccache has a one 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 involves a negligible performance slowdown, it significantly
       increases the number of files that fit in the cache. You can turn on compression by
       setting the CCACHE_COMPRESS environment variable.

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 CCACHE_COMPILERCHECK)

       ·   the name of the compiler

       ·   the current directory (if CCACHE_HASHDIR is set)

       ·   contents of files specified by CCACHE_EXTRAFILES (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.

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

       ·   the environment variable CCACHE_NODIRECT is set

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

       ·   the unifier is enabled (the environment variable CCACHE_UNIFY is set)

       ·   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 outside comments and string literals 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” by setting the CCACHE_BASEDIR variable 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 CCACHE_BASEDIR 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 CCACHE_SLOPPINESS to time_macros. The reason is that ccache can’t tell
           whether __TIME__ or __DATE__ is used when using a precompiled header.

       ·   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

           ·   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 CCACHE_DIR environment variable setting.

       ·   Unset the CCACHE_HARDLINK environment variable.

       ·   Make sure everyone sets the CCACHE_UMASK environment variable 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 the developers have CCACHE_BASEDIR set appropriately,
       as discussed in the 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 CCACHE_TEMPDIR 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 using the CCACHE_PREFIX option. You just need to set the environment variable
       CCACHE_PREFIX to the name of the wrapper (e.g. distcc) and ccache will prefix the command
       line with the specified command when running the compiler.

       Unless you set CCACHE_COMPILERCHECK 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 CCACHE_PREFIX is not used, ccache will needlessly invoke
       the other wrapper when running the preprocessor.

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 CCACHE_EXTRAFILES to the path of the included file.

TROUBLESHOOTING

   General
       A general tip for getting information about what ccache is doing is to enable debug
       logging by setting CCACHE_LOGFILE. 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 CCACHE_BASEDIR.

           ·   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 CCACHE_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 CCACHE_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 CCACHE_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 CCACHE_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 set CCACHE_CPP2. 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 CCACHE_PREFIX command or compiler wrapper. If
       this happens, you can either find out which object file is broken by reading the debug log
       and then delete the bad object file from the cache, or you can simply 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.