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ccache(1)                        ccache Manual                       ccache(1)




NAME

       ccache - a fast C/C++ compiler cache


SYNOPSIS

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


DESCRIPTION

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

       ccache has been carefully written to always produce exactly the same
       compiler output that you would get without the cache. The only way you
       should be able to tell that you are using ccache is the speed.
       Currently known exceptions to this goal are listed under BUGS. If you
       ever discover an undocumented case where ccache changes the output of
       your compiler, please let us know.

   Features
       o   Keeps statistics on hits/misses.

       o   Automatic cache size management.

       o   Can cache compilations that generate warnings.

       o   Easy installation.

       o   Low overhead.

       o   Optionally uses hard links where possible to avoid copies.

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

   Limitations
       o   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.

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

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


RUN MODES

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

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

       To use the symlinks method, do something like this:

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

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

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

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


OPTIONS

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

       -c, --cleanup
           Clean up the cache by removing old cached files until the specified
           file number and cache size limits are not exceeded. This also
           recalculates the cache file count and size totals. Normally, 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:

               o    %compiler% -v

               o    %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:

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

       o   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:

       o   the extension used by the compiler for a file with preprocessor
           output (normally .i for C code and .ii for C++ code)

       o   the compiler's size and modification time (or other
           compiler-specific information specified by CCACHE_COMPILERCHECK)

       o   the name of the compiler

       o   the current directory (if CCACHE_HASHDIR is set)

       o   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:

       o   the input source file

       o   the command line options

       Based on the hash, a data structure called "manifest" is looked up in
       the cache. The manifest contains:

       o   references to cached compilation results (object file, dependency
           file, etc) that were produced by previous compilations that matched
           the hash

       o   paths to the include files that were read at the time the
           compilation results were stored in the cache

       o   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:

       o   the environment variable CCACHE_NODIRECT is set

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

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

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

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

           o    -Xpreprocessor

       o   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:

       o   the preprocessor output from running the compiler with -E

       o   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)

       o   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:

       o   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.

       o   Paths specified by compiler options (such as -I, -MF, etc) may be
           absolute.

       o   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:

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

       o   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:

       o   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.

       o   You must either:

           o   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

           o   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:

       o   Use the same CCACHE_DIR environment variable setting.

       o   Unset the CCACHE_HARDLINK environment variable.

       o   Make sure everyone sets the CCACHE_UMASK environment variable to
           002. This ensures that cached files are accessible to everyone in
           the group.

       o   Make sure that all users have write permission in the entire cache
           directory (and that you trust all users of the shared cache).

       o   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:

       o   Having the cache on NFS may slow down compilation. Make sure to do
           some benchmarking to see if it's worth it.

       o   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:

       o   Compiler upgrades will not be detected properly.

       o   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

       o   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:

       o   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:

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

           o   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.

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

           o   This was the first compilation with a new value of
               CCACHE_BASEDIR.

           o   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.)

           o   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.

           o   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.

           o   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.

       o   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.

       o   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.

       o   If "unsupported compiler option" has been incremented, enable debug
           logging and check which option was rejected.

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

       o   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, the easiest way of fixing it is this:

        1. Build so that the bad object file ends up in the build tree.

        2. Remove the bad object file from the build tree.

        3. Rebuild with CCACHE_RECACHE set.

       An alternative is to clear the whole cache with ccache -C if you don't
       mind losing other cached results.

       There are no reported issues about ccache producing broken object files
       reproducibly. That doesn't mean it can't happen, so if you find a
       repeatable case, please report it.


MORE INFORMATION

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


AUTHOR

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



ccache 3.1.7                      01/08/2012                         ccache(1)

ccache 3.1.7 - Generated Fri Jan 13 15:24:06 CST 2012
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