ccache [options] ccache compiler [compiler options] compiler [compiler options] (via symbolic link)
ccache has been carefully written to always produce exactly the same compiler output that you would get without the cache. The only way you should be able to tell that you are using ccache is the speed. Currently known exceptions to this goal are listed under CAVEATS. If you ever discover an undocumented case where ccache changes the output of your compiler, please let us know.
•Keeps statistics on hits/misses.
•Automatic cache size management.
•Can cache compilations that generate warnings.
•Optionally compresses files in the cache to reduce disk space.
•Only knows how to cache the compilation of a single C/C++/Objective-C/Objective-C++ file. Other types of compilations (multi-file compilation, linking, etc) will silently fall back to running the real compiler.
•Only works with GCC and compilers that behave similar enough.
•Some compiler flags are not supported. If such a flag is detected, ccache will silently fall back to running the real compiler.
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.
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.
Do not use a hard link, use a symbolic link. A hard link will cause “interesting” problems.
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.
Clear the entire cache, removing all cached files, but keeping the configuration file.
Dump manifest file at PATH in text format. This is only useful when debugging ccache and its behavior.
Print the value of configuration option KEY. See CONFIGURATION for more information.
Print the hash (in format <MD4>-<size>) of the file at PATH. This is only useful when debugging ccache and its behavior.
Print an options summary page.
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.
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.
Print statistics counter IDs and corresponding values machine-parsable (tab-separated) format.
Set configuration option KEY to VALUE. See CONFIGURATION for more information.
Print current configuration options and from where they originate (environment variable, configuration file or compile-time default) in human-readable format.
Print a summary of configuration and statistics counters in human-readable format.
Print version and copyright information.
Zero the cache statistics (but not the configuration options).
--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.
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).
As a special case, if the environment variable CCACHE_CONFIGPATH is set, ccache reads configuration from the specified path instead of the default paths.
# Set maximum cache size to 10 GB: max_size = 10G
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.
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.
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_COMPILER or (deprecated) 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.
By default, ccache includes the modification time (“mtime”) and size of the compiler in the hash to ensure that results retrieved from the cache are accurate. This setting can be used to select another strategy. Possible values are:
Hash the content of the compiler binary. This makes ccache very slightly slower compared to the mtime setting, but makes it cope better with compiler upgrades during a build bootstrapping process.
Hash the compiler’s mtime and size, which is fast. This is the default.
Don’t hash anything. This may be good for situations where you can safely use the cached results even though the compiler’s mtime or size has changed (e.g. if the compiler is built as part of your build system and the compiler’s source has not changed, or if the compiler only has changes that don’t affect code generation). You should only use the none setting if you know what you are doing.
Use value as the string to calculate hash from. This can be the compiler revision number you retrieved earlier and set here via environment variable.
a command string
Hash the standard output and standard error output of the specified command. The string will be split on whitespace to find out the command and arguments to run. No other interpretation of the command string will be done, except that the special word %compiler% will be replaced with the path to the compiler. Several commands can be specified with semicolon as separator. Examples:
%compiler% -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.
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).
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.
debug (CCACHE_DEBUG or CCACHE_NODEBUG, see Boolean values above)
If true, enable the debug mode. The debug mode creates per-object debug files that are helpful when debugging unexpected cache misses. Note however that ccache performance will be reduced slightly. See debugging for more information. The default is false.
depend_mode (CCACHE_DEPEND or CCACHE_NODEPEND, see Boolean values above)
If true, the depend mode will be used. The default is false. See THE DEPEND MODE.
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.
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.
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.
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.
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.
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.
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.
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.
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.
pch_external_checksum (CCACHE_PCH_EXTSUM or CCACHE_NOPCH_EXTSUM, see Boolean values above)
When this option is set, and ccache finds a precompiled header file, ccache will look for a file with the extension “.sum” added (e.g. “pre.h.gch.sum”), and if found, it will hash this file instead of the precompiled header itself to work around the performance penalty of hashing very large files.
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.
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 add new results to the cache. Statistics counters will still be updated, though, unless the stats option is set to false.
If you are using this because your ccache directory is read-only, you need to set temporary_dir since ccache will fail to create temporary files otherwise. You may also want to set stats = false to make ccache not even try to update stats 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).
By default, ccache tries to give as few false cache hits as possible. However, in certain situations it’s possible that you know things that ccache can’t take for granted. This setting makes it possible to tell ccache to relax some checks in order to increase the hit rate. The value should be a comma-separated string with options. Available options are:
Ignore the Clang compiler option -index-store-path and its argument when computing the manifest hash. This is useful if you use Xcode, which uses an index store path derived from the local project path. Note that the index store won’t be updated correctly on cache hits if you enable this option.
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.
Ignore ctimes when file_stat_matches is enabled. This can be useful when backdating files' mtimes in a controlled way.
By default, ccache will not cache a file if it includes a header whose ctime is too new. This option disables that check.
By default, ccache will not cache a file if it includes a header whose mtime is too new. This option disables that check.
ccache includes the environment variables LANG, LC_ALL, LC_CTYPE and LC_MESSAGES in the hash by default since they may affect localization of compiler warning messages. Set this option to tell ccache not to do that.
Be sloppy about #defines when precompiling a header file. See PRECOMPILED HEADERS for more information.
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.
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.
This setting specifies where ccache will put temporary files. The default is <cache_dir>/tmp.
In previous versions of ccache, CCACHE_TEMPDIR had to be on the same filesystem as the CCACHE_DIR path, but this requirement has been relaxed.)
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.
Cleanup can be triggered in two different ways: automatic and manual.
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.
ccache has two ways of gathering information used to look up results in the cache:
•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.
If no previous result is detected (i.e., there is a cache miss) using the direct mode, ccache will fall back to the preprocessor mode unless the depend mode is enabled. In the depend mode, ccache never runs the preprocessor, not even on cache misses. Read more in THE DEPEND MODE below.
•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 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_
•the string __TIME__ is present in the source code
•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.
•The ccache overhead of a cache miss will be much smaller.
•Not running the preprocessor at all can be good if compilation is performed remotely, for instance when using distcc or similar; ccache then won’t make potentially costly preprocessor calls on the local machine.
•The cache hit rate will likely be lower since any change to compiler options or source code will make the hash different. Compare this with the default setup where ccache will fall back to the preprocessor mode, which is tolerant to some types of changes of compiler options and source code changes.
•If -MD is used, the manifest entries will include system header files as well, thus slowing down cache hits slightly, just as using -MD slows down make.
•If -MMD is used, the manifest entries will not include system header files, which means ccache will ignore changes in them.
The depend mode will be disabled if any of the following holds:
•the configuration setting depend_mode is false
•the configuration setting run_second_cpp is false
•the compiler is not generating dependencies using -MD or -MMD
When the debug mode is enabled, ccache will create up to five additional files next to the object file:
In the direct mode, ccache uses the MD4 hash of the ccache-input-c + ccache-input-d data (where + means concatenation), while the ccache-input-c + ccache-input-p data is used in the preprocessor mode.
The ccache-input-text file is a combined text version of the three binary input files. It has three sections (“COMMON”, “DIRECT MODE” and “PREPROCESSOR MODE”), which is turn contain annotations that say what kind of data comes next.
To debug why you don’t get an expected cache hit for an object file, you can do something like this:
1.Build with debug mode enabled.
2.Save the <objectfile>.ccache-* files.
3.Build again with debug mode enabled.
4.Compare <objectfile>.ccache-input-text for the two builds. This together with the <objectfile>.ccache-log files should give you some clues about what is happening.
•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.
•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; the filename itself must be sufficient to find the header, i.e. -I paths are not searched); or
•(for the Clang compiler) use the -include-pch compiler option to include the PCH file generated from the precompiled header; or
•(for the GCC compiler) 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).
•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.
•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.
It is recommended to use the same operating system version when using a shared cache. If operating system versions are different then system include files will likely be different and there will be few or no cache hits between the systems. One way of improving cache hit rate in that case is to set sloppiness to system_headers to ignore system headers.
Unless you set compiler_check to a suitable command (see the description of that configuration option), it is not recommended to use the form ccache anotherwrapper compiler args as the compilation command. It’s also not recommended to use the masquerading technique for the other compiler wrapper. The reason is that by default, ccache will in both cases hash the mtime and size of the other wrapper instead of the real compiler, which means that:
•Compiler upgrades will not be detected properly.
•The cached results will not be shared between compilations with and without the other wrapper.
Another minor thing is that if prefix_command is used, ccache will not invoke the other wrapper when running the preprocessor, which increases performance. You can use the prefix_command_cpp configuration setting if you also want to invoke the other wrapper when doing preprocessing (normally by adding -E).
•The direct mode fails to pick up new header files in some rare scenarios. See THE DIRECT MODE above.
•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).
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_ctime, 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 input file path has changed. ccache includes the input file path in the direct mode hash to be able to take relative include files into account and to produce a correct object file if the source code includes a __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.
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.