1CCACHE(1) CCACHE(1)
2
6 ccache - a fast C/C++ compiler cache
7
9 ccache [ccache options]
10 ccache [KEY=VALUE ...] compiler [compiler options]
11 compiler [compiler options]
12 The first form takes options described in COMMAND LINE OPTIONS below.
14 The second form invokes the compiler, optionally using configuration
15 options as KEY=VALUE arguments. In the third form, ccache is
16 masquerading as the compiler as described in RUN MODES.
17
19 Ccache is a compiler cache. It speeds up recompilation by caching the
20 result of previous compilations and detecting when the same compilation
21 is being done again.
22 Ccache has been carefully written to always produce exactly the same
24 compiler output that you would get without the cache. The only way you
25 should be able to tell that you are using ccache is the speed.
26 Currently known exceptions to this goal are listed under CAVEATS. If
27 you discover an undocumented case where ccache changes the output of
28 your compiler, please let us know.
29
31 There are two different ways to use ccache to cache a compilation:
32 1. Prefix your compilation command with ccache. This method is most
34 convenient if you just want to try out ccache or wish to use it for
35 some specific projects. Example:
36 ccache gcc -c example.c
38 2. Let ccache masquerade as the compiler. This method is most useful
40 when you wish to use ccache for all your compilations. To do this,
41 create a symbolic link to ccache named as the compiler. For
42 example, here is set up ccache to masquerade as gcc and g++:
43 cp ccache /usr/local/bin/
45 ln -s ccache /usr/local/bin/gcc
46 ln -s ccache /usr/local/bin/g++
47 On platforms that don’t support symbolic links you can simply copy
49 ccache to the compiler name instead for a similar effect:
50 cp ccache /usr/local/bin/gcc
52 cp ccache /usr/local/bin/g++
53 And so forth. This will work as long as the directory with symbolic
55 links or ccache copies comes before the directory with the compiler
56 (typically /usr/bin) in PATH.
57 Warning
59 The technique of letting ccache masquerade as the compiler
60 works well, but currently doesn’t interact well with other
61 tools that do the same thing. See USING CCACHE WITH OTHER
62 COMPILER WRAPPERS.
63
65 These command line options only apply when you invoke ccache as
66 “ccache”. When ccache masquerades as a compiler (as described in the
67 previous section), the normal compiler options apply and you should
68 refer to the compiler’s documentation.
69 Common options
71 -c, --cleanup
72 Clean up the cache by removing not recently used cached files until
73 the specified file number and cache size limits are not exceeded.
74 This also recalculates the cache file count and size totals.
75 Normally, there is no need to initiate cleanup manually as ccache
76 keeps the cache below the specified limits at runtime and keeps
77 statistics up to date on each compilation. Forcing a cleanup is
78 mostly useful if you have modified the cache contents manually or
79 believe that the cache size statistics may be inaccurate.
80 -C, --clear
82 Clear the entire cache, removing all cached files, but keeping the
83 configuration file.
84 --config-path PATH
86 Let the command line options operate on configuration file PATH
87 instead of the default. Using this option has the same effect as
88 setting (overriding) the environment variable CCACHE_CONFIGPATH
89 temporarily.
90 -d, --dir PATH
92 Let the command line options operate on cache directory PATH
93 instead of the default. For example, to show statistics for a cache
94 directory at /shared/ccache you can run ccache -d /shared/ccache
95 -s. Using this option has the same effect as setting the
96 environment variable CCACHE_DIR temporarily.
97 --evict-namespace NAMESPACE
99 Remove files created in the given namespace from the cache.
100 --evict-older-than AGE
102 Remove files used less recently than AGE from the cache. AGE should
103 be an unsigned integer with a d (days) or s (seconds) suffix. If
104 combined with --evict-namespace, only remove files within that
105 namespace.
106 -h, --help
108 Print a summary of command line options.
109 -F NUM, --max-files NUM
111 Set the maximum number of files allowed in the cache to NUM. Use 0
112 for no limit. The value is stored in a configuration file in the
113 cache directory and applies to all future compilations.
114 -M SIZE, --max-size SIZE
116 Set the maximum size of the files stored in the cache. SIZE should
117 be a number followed by an optional suffix: kB, MB, GB, TB
118 (decimal), KiB, MiB, GiB or TiB (binary). The default suffix is
119 GiB. Use 0 for no limit. The value is stored in a configuration
120 file in the cache directory and applies to all future compilations.
121 -X LEVEL, --recompress LEVEL
123 Recompress the cache to level LEVEL using the Zstandard algorithm.
124 The level can be an integer, with the same semantics as the
125 compression_level configuration option, or the special value
126 uncompressed for no compression. See [Cache compression] for more
127 information. This can potentially take a long time since all files
128 in the cache need to be visited. Only files that are currently
129 compressed with a different level than LEVEL will be recompressed.
130 --recompress-threads THREADS
132 Use up to THREADS threads when recompressing the cache. The default
133 is to use one thread per CPU.
134 -o KEY=VALUE, --set-config KEY=VALUE
136 Set configuration option KEY to VALUE. See CONFIGURATION for more
137 information.
138 -x, --show-compression
140 Print cache compression statistics. See CACHE COMPRESSION for more
141 information. This can potentially take a long time since all files
142 in the cache need to be visited.
143 -p, --show-config
145 Print current configuration options and from where they originate
146 (environment variable, configuration file or compile-time default)
147 in human-readable format.
148 --show-log-stats
150 Print statistics counters from the stats log in human-readable
151 format. See stats_log. Use -v/--verbose once or twice for more
152 details.
153 -s, --show-stats
155 Print a summary of configuration and statistics counters in
156 human-readable format. Use -v/--verbose once or twice for more
157 details.
158 -v, --verbose
160 Increase verbosity. The option can be given multiple times.
161 -V, --version
163 Print version and copyright information.
164 -z, --zero-stats
166 Zero the cache statistics (but not the configuration options).
167 Options for remote file-based storage
169 --trim-dir PATH
170 Remove not recently used files from directory PATH until it is at
171 most the size specified by --trim-max-size.
172 Warning
174 Don’t use this option to trim the local cache. To trim the
175 local cache directory to a certain size, use
176 CCACHE_MAXSIZE=SIZE ccache -c.
177 --trim-max-size SIZE
179 Specify the maximum size for --trim-dir. SIZE should be a number
180 followed by an optional suffix: kB, MB, GB, TB (decimal), KiB, MiB,
181 GiB or TiB (binary). The default suffix is GiB. Use 0 for no limit.
182 --trim-method METHOD
184 Specify the method to trim a directory with --trim-dir. Possible
185 values are:
186 atime
188 LRU (least recently used) using the file access timestamp. This
189 is the default.
190 mtime
192 LRU (least recently used) using the file modification
193 timestamp.
194 --trim-recompress LEVEL
196 Recompress to level LEVEL using the Zstandard algorithm when using
197 --trim-dir. The level can be an integer, with the same semantics as
198 the compression_level configuration option, or the special value
199 uncompressed for no compression. See [Cache compression] for more
200 information. This can potentially take a long time since all files
201 in the cache need to be visited. Only files that are currently
202 compressed with a different level than LEVEL will be recompressed.
203 --trim-recompress-threads THREADS
205 Recompress using up to THREADS threads with --trim-recompress. The
206 default is to use one thread per CPU.
207 Options for scripting or debugging
209 --checksum-file PATH
210 Print the checksum (128 bit XXH3) of the file at PATH (- for
211 standard input).
212 --extract-result PATH
214 Extract data stored in the result file at PATH (- for standard
215 input). The data will be written to ccache-result.* files in to the
216 current working directory. This option is only useful when
217 debugging ccache and its behavior.
218 -k KEY, --get-config KEY
220 Print the value of configuration option KEY. See CONFIGURATION for
221 more information.
222 --hash-file PATH
224 Print the hash (160 bit BLAKE3) of the file at PATH (- for standard
225 input). This is only useful when debugging ccache and its behavior.
226 --inspect PATH
228 Print the content of a result or manifest file at PATH (- for
229 standard input) to standard output in human-readable format. File
230 content embedded in a result file will however not be printed; use
231 --extract-result to extract the file content. This option is only
232 useful when debugging ccache and its behavior.
233 --print-stats
235 Print statistics counter IDs and corresponding values in
236 machine-parsable (tab-separated) format.
237 Extra options
239 When run as a compiler, ccache usually just takes the same command line
240 options as the compiler you are using. The only exception to this is
241 the option --ccache-skip. That option can be used to tell ccache to
242 avoid interpreting the next option in any way and to pass it along to
243 the compiler as-is.
244 Note
246 --ccache-skip currently only tells ccache not to interpret the next
247 option as a special compiler option — the option will still be
248 included in the direct mode hash.
249 The reason this can be important is that ccache does need to parse the
251 command line and determine what is an input filename and what is a
252 compiler option, as it needs the input filename to determine the name
253 of the resulting object file (among other things). The heuristic ccache
254 uses when parsing the command line is that any argument that exists as
255 a file is treated as an input file name. By using --ccache-skip you can
256 force an option to not be treated as an input file name and instead be
257 passed along to the compiler as a command line option.
258 Another case where --ccache-skip can be useful is if ccache interprets
260 an option specially but shouldn’t, since the option has another meaning
261 for your compiler than what ccache thinks.
262
264 Ccache’s default behavior can be overridden by options in configuration
265 files, which in turn can be overridden by environment variables with
266 names starting with CCACHE_. Ccache normally reads configuration from
267 two files: first a system-level configuration file and secondly a
268 cache-specific configuration file. The priorities of configuration
269 options are as follows (where 1 is highest):
270 1. Command line settings in KEY=VALUE form. Example:
272 ccache debug=true compiler_check="%compiler% --version" gcc -c example.c
274 2. Environment variables.
276 3. The cache-specific configuration file (see below).
278 4. The system (read-only) configuration file <sysconfdir>/ccache.conf
280 (typically /etc/ccache.conf or /usr/local/etc/ccache.conf).
281 5. Compile-time defaults.
283 As a special case, if the environment variable CCACHE_CONFIGPATH is set
285 it specifies the configuration file, and the system configuration file
286 won’t be read.
287 Location of the configuration file
289 The location of the cache-specific configuration file is determined
290 like this on non-Windows systems:
291 1. If CCACHE_CONFIGPATH is set, use that path.
293 2. Otherwise, if the environment variable CCACHE_DIR is set then use
295 $CCACHE_DIR/ccache.conf.
296 3. Otherwise, if cache_dir is set in the system configuration file
298 then use <cache_dir>/ccache.conf.
299 4. Otherwise, if there is a legacy $HOME/.ccache directory then use
301 $HOME/.ccache/ccache.conf.
302 5. Otherwise, if XDG_CONFIG_HOME is set then use
304 $XDG_CONFIG_HOME/ccache/ccache.conf.
305 6. Otherwise, use $HOME/Library/Preferences/ccache/ccache.conf (macOS)
307 or $HOME/.config/ccache/ccache.conf (other systems).
308 On Windows, this is the method used to find the configuration file:
310 1. If CCACHE_CONFIGPATH is set, use that path.
312 2. Otherwise, if the environment variable CCACHE_DIR is set then use
314 %CCACHE_DIR%/ccache.conf.
315 3. Otherwise, if cache_dir is set in the system configuration file
317 then use <cache_dir>\ccache.conf. The system-wide configuration on
318 Windows is %ALLUSERSPROFILE%\ccache\ccache.conf by default. The
319 ALLUSERSPROFILE environment variable is usually C:\ProgramData.
320 4. Otherwise, if there is a legacy %USERPROFILE%\.ccache directory
322 then use %USERPROFILE%\.ccache\ccache.conf.
323 5. Otherwise, use %LOCALAPPDATA%\ccache\ccache.conf if it exists.
325 6. Otherwise, use %APPDATA%\ccache\ccache.conf.
327 See also the cache_dir configuration option for how the cache directory
329 location is determined.
330 Configuration file syntax
332 Configuration files are in a simple “key = value” format, one option
333 per line. Lines starting with a hash sign are comments. Blank lines are
334 ignored, as is whitespace surrounding keys and values. Example:
335 # Set maximum cache size to 10 GB:
337 max_size = 10G
338 Boolean values
340 Some configuration options are boolean values (i.e. truth values). In a
341 configuration file, such values must be set to the string true or
342 false. For the corresponding environment variables, the semantics are a
343 bit different:
344 • A set environment variable means “true” (even if set to the empty
346 string).
347 • The following case-insensitive negative values are considered an
349 error (instead of surprising the user): 0, false, disable and no.
350 • An unset environment variable means “false”.
352 Each boolean environment variable also has a negated form starting with
354 CCACHE_NO. For example, CCACHE_COMPRESS can be set to force compression
355 and CCACHE_NOCOMPRESS can be set to force no compression.
356 Configuration options
358 Below is a list of available configuration options. The corresponding
359 environment variable name is indicated in parentheses after each
360 configuration option key.
361 absolute_paths_in_stderr (CCACHE_ABSSTDERR)
363 This option specifies whether ccache should rewrite relative paths
364 in the compiler’s standard error output to absolute paths. This can
365 be useful if you use base_dir with a build system (e.g. CMake with
366 the "Unix Makefiles" generator) that executes the compiler in a
367 different working directory, which makes relative paths in compiler
368 errors or warnings incorrect. The default is false.
369 base_dir (CCACHE_BASEDIR)
371 This option should be an absolute path to a directory. If set,
372 ccache will rewrite absolute paths into paths relative to the
373 current working directory, but only absolute paths that begin with
374 base_dir. Cache results can then be shared for compilations in
375 different directories even if the project uses absolute paths in
376 the compiler command line. See also the discussion under COMPILING
377 IN DIFFERENT DIRECTORIES. If set to the empty string (which is the
378 default), no rewriting is done.
379 A typical path to use as base_dir is your home directory or another
381 directory that is a parent of your project directories. Don’t use /
382 as the base directory since that will make ccache also rewrite
383 paths to system header files, which typically is contraproductive.
384 For example, say that Alice’s current working directory is
386 /home/alice/project1/build and that she compiles like this:
387 ccache gcc -I/usr/include/example -I/home/alice/project2/include -c /home/alice/project1/src/example.c
389 Here is what ccache will actually execute for different base_dir
391 values:
392 # Current working directory: /home/alice/project1/build
394 # With base_dir = /:
396 gcc -I../../../../usr/include/example -I../../project2/include -c ../src/example.c
397 # With base_dir = /home or /home/alice:
399 gcc -I/usr/include/example -I../../project2/include -c ../src/example.c
400 # With base_dir = /home/alice/project1 or /home/alice/project1/src:
402 gcc -I/usr/include/example -I/home/alice/project2/include -c ../src/example.c
403 If Bob has put project1 and project2 in /home/bob/stuff and both
405 users have set base_dir to /home or /home/$USER, then Bob will get
406 a cache hit (if they share ccache directory) since the actual
407 command line will be identical to that of Alice:
408 # Current working directory: /home/bob/stuff/project1/build
410 # With base_dir = /home or /home/bob:
412 gcc -I/usr/include/example -I../../project2/include -c ../src/example.c
413 Without base_dir there will be a cache miss since the absolute
415 paths will differ. With base_dir set to / there will be a cache
416 miss since the relative path to /usr/include/example will be
417 different. With base_dir set to /home/bob/stuff/project1 there will
418 a cache miss since the path to project2 will be a different
419 absolute path.
420 Warning
422 Rewriting absolute paths to relative is kind of a brittle hack.
423 It works OK in many cases, but there might be cases where
424 things break. One known issue is that absolute paths are not
425 reproduced in dependency files, which can mess up dependency
426 detection in tools like Make and Ninja. If possible, use
427 relative paths in the first place instead instead of using
428 base_dir.
429 cache_dir (CCACHE_DIR)
431 This option specifies where ccache will keep its cached compiler
432 outputs.
433 On non-Windows systems, the default is $HOME/.ccache if such a
435 directory exists, otherwise $XDG_CACHE_HOME/ccache if
436 XDG_CACHE_HOME is set, otherwise $HOME/Library/Caches/ccache
437 (macOS) or $HOME/.config/ccache (other systems).
438 On Windows, the default is %USERPROFILE%\.ccache if such a
440 directory exists, otherwise %LOCALAPPDATA%\ccache.
441 Warning
443 Previous ccache versions defaulted to storing the cache in
444 %APPDATA%\ccache on Windows. This can result in large network
445 file transfers of the cache in domain environments and similar
446 problems. Please check this directory for cache directories and
447 either delete them or the whole directory, or move them to the
448 %LOCALAPPDATA%\ccache directory.
449 See also Location of the configuration file.
451 compiler (CCACHE_COMPILER or (deprecated) CCACHE_CC)
453 This option can be used to force the name of the compiler to use.
454 If set to the empty string (which is the default), ccache works it
455 out from the command line.
456 compiler_check (CCACHE_COMPILERCHECK)
458 By default, ccache includes the modification time (“mtime”) and
459 size of the compiler in the hash to ensure that results retrieved
460 from the cache are accurate. If compiler plugins are used, these
461 plugins will also be added to the hash. This option can be used to
462 select another strategy. Possible values are:
463 content
465 Hash the content of the compiler binary. This makes ccache very
466 slightly slower compared to mtime, but makes it cope better
467 with compiler upgrades during a build bootstrapping process.
468 mtime
470 Hash the compiler’s mtime and size, which is fast. This is the
471 default.
472 none
474 Don’t hash anything. This may be good for situations where you
475 can safely use the cached results even though the compiler’s
476 mtime or size has changed (e.g. if the compiler is built as
477 part of your build system and the compiler’s source has not
478 changed, or if the compiler only has changes that don’t affect
479 code generation). You should only use none if you know what you
480 are doing.
481 string:value
483 Hash value. This can for instance be a compiler revision number
484 or another string that the build system generates to identify
485 the compiler.
486 a command string
488 Hash the standard output and standard error output of the
489 specified command. The string will be split on whitespace to
490 find out the command and arguments to run. No other
491 interpretation of the command string will be done, except that
492 the special word %compiler% will be replaced with the path to
493 the compiler. Several commands can be specified with semicolon
494 as separator. Examples:
495 %compiler% -v
497 %compiler% -dumpmachine; %compiler% -dumpversion
499 You should make sure that the specified command is as fast as
501 possible since it will be run once for each ccache invocation.
502 Identifying the compiler using a command is useful if you want
504 to avoid cache misses when the compiler has been rebuilt but
505 not changed.
506 Another case is when the compiler (as seen by ccache) actually
508 isn’t the real compiler but another compiler wrapper — in that
509 case, the default mtime method will hash the mtime and size of
510 the other compiler wrapper, which means that ccache won’t be
511 able to detect a compiler upgrade. Using a suitable command to
512 identify the compiler is thus safer, but it’s also slower, so
513 you should consider continue using the mtime method in
514 combination with the prefix_command option if possible. See
515 USING CCACHE WITH OTHER COMPILER WRAPPERS.
516 compiler_type (CCACHE_COMPILERTYPE)
518 Ccache normally guesses the compiler type based on the compiler
519 name. The compiler_type option lets you force a compiler type. This
520 can be useful if the compiler has a non-standard name but is
521 actually one of the known compiler types. Possible values are:
522 auto
524 Guess one of the types below based on the compiler name
525 (following symlinks). This is the default.
526 clang
528 Clang-based compiler.
529 clang-cl
531 clang-cl.
532 gcc
534 GCC-based compiler.
535 icl
537 Intel compiler on Windows.
538 msvc
540 Microsoft Visual C++ (MSVC).
541 nvcc
543 NVCC (CUDA) compiler.
544 other
546 Any compiler other than the known types.
547 compression (CCACHE_COMPRESS or CCACHE_NOCOMPRESS, see Boolean values
549 above)
550 If true, ccache will compress data it puts in the cache. However,
551 this option has no effect on how files are retrieved from the
552 cache; compressed and uncompressed results will still be usable
553 regardless of this option. The default is true.
554 Compression is done using the Zstandard algorithm. The algorithm is
556 fast enough that there should be little reason to turn off
557 compression to gain performance. One exception is if the cache is
558 located on a compressed file system, in which case the compression
559 performed by ccache of course is redundant.
560 Compression will be disabled if file cloning (the file_clone
562 option) or hard linking (the hard_link option) is enabled.
563 compression_level (CCACHE_COMPRESSLEVEL)
565 This option determines the level at which ccache will compress
566 object files using the real-time compression algorithm Zstandard.
567 It only has effect if compression is enabled (which it is by
568 default). Zstandard is extremely fast for decompression and very
569 fast for compression for lower compression levels. The default is
570 0.
571 Semantics of compression_level:
573 > 0
575 A positive value corresponds to normal Zstandard compression
576 levels. Lower levels (e.g. 1) mean faster compression but worse
577 compression ratio. Higher levels (e.g. 19) mean slower
578 compression but better compression ratio. The maximum possible
579 value depends on the libzstd version, but at least up to 19 is
580 available for all versions. Decompression speed is essentially
581 the same for all levels. As a rule of thumb, use level 5 or
582 lower since higher levels may slow down compilations
583 noticeably. Higher levels are however useful when recompressing
584 the cache with command line option -X/--recompress.
585 < 0
587 A negative value corresponds to Zstandard’s “ultra-fast”
588 compression levels, which are even faster than level 1 but with
589 less good compression ratios. For instance, level -3
590 corresponds to --fast=3 for the zstd command line tool. In
591 practice, there is little use for levels lower than -5 or so.
592 0 (default)
594 The value 0 means that ccache will choose a suitable level,
595 currently 1.
596 See the Zstandard documentation <http://zstd.net> for more
598 information.
599 cpp_extension (CCACHE_EXTENSION)
601 This option can be used to force a certain extension for the
602 intermediate preprocessed file. The default is to automatically
603 determine the extension to use for intermediate preprocessor files
604 based on the type of file being compiled, but that sometimes
605 doesn’t work. For example, when using the “aCC” compiler on HP-UX,
606 set the cpp extension to i.
607 debug (CCACHE_DEBUG or CCACHE_NODEBUG, see Boolean values above)
609 If true, enable the debug mode. The debug mode creates per-object
610 debug files that are helpful when debugging unexpected cache
611 misses. Note however that ccache performance will be reduced
612 slightly. See CACHE DEBUGGING for more information. The default is
613 false.
614 debug_dir (CCACHE_DEBUGDIR)
616 Specifies where to write per-object debug files if the debug mode
617 is enabled. If set to the empty string, the files will be written
618 next to the object file. If set to a directory, the debug files
619 will be written with full absolute paths in that directory,
620 creating it if needed. The default is the empty string.
621 For example, if debug_dir is set to /example, the current working
623 directory is /home/user and the object file is build/output.o then
624 the debug log will be written to
625 /example/home/user/build/output.o.ccache-log. See also CACHE
626 DEBUGGING.
627 depend_mode (CCACHE_DEPEND or CCACHE_NODEPEND, see Boolean values
629 above)
630 If true, the depend mode will be used. The default is false. See
631 The depend mode.
632 direct_mode (CCACHE_DIRECT or CCACHE_NODIRECT, see Boolean values
634 above)
635 If true, the direct mode will be used. The default is true. See The
636 direct mode.
637 disable (CCACHE_DISABLE or CCACHE_NODISABLE, see Boolean values above)
639 When true, ccache will just call the real compiler, bypassing the
640 cache completely. The default is false.
641 It is also possible to disable ccache for a specific source code
643 file by adding the string ccache:disable in a comment in the first
644 4096 bytes of the file.
645 extra_files_to_hash (CCACHE_EXTRAFILES)
647 This option is a list of paths to files that ccache will include in
648 the the hash sum that identifies the build. The list separator is
649 semicolon on Windows systems and colon on other systems.
650 file_clone (CCACHE_FILECLONE or CCACHE_NOFILECLONE, see Boolean values
652 above)
653 If true, ccache will attempt to use file cloning (also known as
654 “copy on write”, “CoW” or “reflinks”) to store and fetch cached
655 compiler results. file_clone has priority over hard_link. The
656 default is false.
657 Files stored by cloning cannot be compressed, so the cache size
659 will likely be significantly larger if this option is enabled.
660 However, performance may be improved depending on the use case.
661 Unlike the hard_link option, file_clone is completely safe to use,
663 but not all file systems support the feature. For such file
664 systems, ccache will fall back to use plain copying (or hard links
665 if hard_link is enabled).
666 hard_link (CCACHE_HARDLINK or CCACHE_NOHARDLINK, see Boolean values
668 above)
669 If true, ccache will attempt to use hard links to store and fetch
670 cached object files. The default is false.
671 Files stored via hard links cannot be compressed, so the cache size
673 will likely be significantly larger if this option is enabled.
674 However, performance may be improved depending on the use case.
675 Warning
677 Do not enable this option unless you are aware of these
678 caveats:
679 • If the resulting file is modified, the file in the cache will
681 also be modified since they share content, which corrupts the
682 cache entry. As of version 4.0, ccache makes stored and fetched
683 object files read-only as a safety measure. Furthermore, a
684 simple integrity check is made for cached object files by
685 verifying that their sizes are correct. This means that
686 mistakes like strip file.o or echo >file.o will be detected
687 even if the object file is made writable, but a modification
688 that doesn’t change the file size will not.
689 • Programs that don’t expect that files from two different
691 identical compilations are hard links to each other can fail.
692 • Programs that rely on modification times (like make) can be
694 confused if several users (or one user with several build
695 trees) use the same cache directory. The reason for this is
696 that the object files share i-nodes and therefore modification
697 times. If file.o is in build tree A (hard-linked from the
698 cache) and file.o then is produced by ccache in build tree B by
699 hard-linking from the cache, the modification timestamp will be
700 updated for file.o in build tree A as well. This can retrigger
701 relinking in build tree A even though nothing really has
702 changed.
703 hash_dir (CCACHE_HASHDIR or CCACHE_NOHASHDIR, see Boolean values above)
705 If true (which is the default), ccache will include the current
706 working directory (CWD) in the hash that is used to distinguish two
707 compilations when generating debug info (compiler option -g with
708 variations). Exception: The CWD will not be included in the hash if
709 base_dir is set (and matches the CWD) and the compiler option
710 -fdebug-prefix-map is used. See also the discussion under COMPILING
711 IN DIFFERENT DIRECTORIES.
712 The reason for including the CWD in the hash by default is to
714 prevent a problem with the storage of the current working directory
715 in the debug info of an object file, which can lead ccache to
716 return a cached object file that has the working directory in the
717 debug info set incorrectly.
718 You can disable this option to get cache hits when compiling the
720 same source code in different directories if you don’t mind that
721 CWD in the debug info might be incorrect.
722 ignore_headers_in_manifest (CCACHE_IGNOREHEADERS)
724 This option is a list of paths to files (or directories with
725 headers) that ccache will not include in the manifest list that
726 makes up the direct mode. Note that this can cause stale cache hits
727 if those headers do indeed change. The list separator is semicolon
728 on Windows systems and colon on other systems.
729 ignore_options (CCACHE_IGNOREOPTIONS)
731 This option is a space-delimited list of compiler options that
732 ccache will exclude from the hash. Excluding a compiler option from
733 the hash can be useful when you know it doesn’t affect the result
734 (but ccache doesn’t know that), or when it does and you don’t care.
735 If a compiler option in the list is suffixed with an asterisk (*)
736 it will be matched as a prefix. For example, -fmessage-length=*
737 will match both -fmessage-length=20 and -fmessage-length=70.
738 inode_cache (CCACHE_INODECACHE or CCACHE_NOINODECACHE, see Boolean
740 values above)
741 If true, ccache will cache source file hashes based on device,
742 inode and timestamps. This reduces the time spent on hashing
743 include files since the result can be reused between compilations.
744 The default is true. The feature requires temporary_dir to be
745 located on a local filesystem of a supported type.
746 Note
748 The inode cache feature is currently not available on Windows.
749 keep_comments_cpp (CCACHE_COMMENTS or CCACHE_NOCOMMENTS, see Boolean
751 values above)
752 If true, ccache will not discard the comments before hashing
753 preprocessor output. The default is false. This can be used to
754 check documentation with -Wdocumentation.
755 log_file (CCACHE_LOGFILE)
757 If set to a file path, ccache will write information on what it is
758 doing to the specified file. This is useful for tracking down
759 problems.
760 If set to syslog, ccache will log using syslog() instead of to a
762 file. If you use rsyslogd, you can add something like this to
763 /etc/rsyslog.conf or a file in /etc/rsyslog.d:
764 # log ccache to file
766 :programname, isequal, "ccache" /var/log/ccache
767 # remove from syslog
768 & ~
769 max_files (CCACHE_MAXFILES)
771 This option specifies the maximum number of files to keep in the
772 cache. Use 0 for no limit (which is the default). See also CACHE
773 SIZE MANAGEMENT.
774 max_size (CCACHE_MAXSIZE)
776 This option specifies the maximum size of the cache. Use 0 for no
777 limit. The default value is 5G. Available suffixes: k, M, G, T
778 (decimal) and Ki, Mi, Gi, Ti (binary). The default suffix is G. See
779 also CACHE SIZE MANAGEMENT.
780 msvc_dep_prefix (CCACHE_MSVC_DEP_PREFIX)
782 This option specifies the prefix of included files output for MSVC
783 compiler. The default prefix is “Note: including file:”. If you use
784 a localized compiler, this should be set accordingly.
785 namespace (CCACHE_NAMESPACE)
787 If set, the namespace string will be added to the hashed data for
788 each compilation. This will make the associated cache entries
789 logically separate from cache entries with other namespaces, but
790 they will still share the same storage space. Cache entries can
791 also be selectively removed from the local cache with the command
792 line option --evict-namespace, potentially in combination with
793 --evict-older-than.
794 For instance, if you use the same local cache for several disparate
796 projects, you can use a unique namespace string for each one. This
797 allows you to remove cache entries that belong to a certain project
798 if you stop working with that project.
799 path (CCACHE_PATH)
801 If set, ccache will search directories in this list when looking
802 for the real compiler. The list separator is semicolon on Windows
803 systems and colon on other systems. If not set, ccache will look
804 for the first executable matching the compiler name in the normal
805 PATH that isn’t a symbolic link to ccache itself.
806 pch_external_checksum (CCACHE_PCH_EXTSUM or CCACHE_NOPCH_EXTSUM, see
808 Boolean values above)
809 When this option is set, and ccache finds a precompiled header
810 file, ccache will look for a file with the extension “.sum” added
811 (e.g. “pre.h.gch.sum”), and if found, it will hash this file
812 instead of the precompiled header itself to work around the
813 performance penalty of hashing very large files.
814 prefix_command (CCACHE_PREFIX)
816 This option adds a list of prefixes (separated by space) to the
817 command line that ccache uses when invoking the compiler. See also
818 USING CCACHE WITH OTHER COMPILER WRAPPERS.
819 prefix_command_cpp (CCACHE_PREFIX_CPP)
821 This option adds a list of prefixes (separated by space) to the
822 command line that ccache uses when invoking the preprocessor.
823 read_only (CCACHE_READONLY or CCACHE_NOREADONLY, see Boolean values
825 above)
826 If true, ccache will attempt to use existing cached results, but it
827 will not add new results to any cache backend. Statistics counters
828 will still be updated, though, unless the stats option is set to
829 false.
830 If you are using this because your ccache directory is read-only,
832 you need to set temporary_dir since ccache will fail to create
833 temporary files otherwise. You may also want to set stats to false
834 make ccache not even try to update stats files.
835 read_only_direct (CCACHE_READONLY_DIRECT or CCACHE_NOREADONLY_DIRECT,
837 see Boolean values above)
838 Just like read_only except that ccache will only try to retrieve
839 results from the cache using the direct mode, not the preprocessor
840 mode. See documentation for read_only regarding using a read-only
841 ccache directory.
842 recache (CCACHE_RECACHE or CCACHE_NORECACHE, see Boolean values above)
844 If true, ccache will not use any previously stored result. New
845 results will still be cached, possibly overwriting any pre-existing
846 results.
847 remote_only (CCACHE_REMOTE_ONLY or CCACHE_NOREMOTE_ONLY, see Boolean
849 values above)
850 If true, ccache will only use remote storage. The default is false.
851 Note that cache statistics counters will still be kept in the local
852 cache directory unless stats is false. See also Storage
853 interaction.
854 remote_storage (CCACHE_REMOTE_STORAGE)
856 This option specifies one or several storage backends (separated by
857 space) to query after checking the local cache (unless remote_only
858 is true). See REMOTE STORAGE BACKENDS for documentation of syntax
859 and available backends.
860 Examples:
862 • file:/shared/nfs/directory
864 • file:///shared/nfs/one|read-only file:///shared/nfs/two
866 • file:///Z:/example/windows/folder
868 • http://example.com/cache
870 • redis://example.com
872 Note
874 In previous ccache versions this option was called
875 secondary_storage (CCACHE_SECONDARY_STORAGE), which can
876 still be used as an alias.
877 reshare (CCACHE_RESHARE or CCACHE_NORESHARE, see Boolean values above)
879 If true, ccache will write results to remote storage even for local
880 storage cache hits. The default is false.
881 run_second_cpp (CCACHE_CPP2 or CCACHE_NOCPP2, see Boolean values above)
883 If true, ccache will first run the preprocessor to preprocess the
884 source code (see The preprocessor mode) and then on a cache miss
885 run the compiler on the source code to get hold of the object file.
886 This is the default.
887 If false, ccache will first run preprocessor to preprocess the
889 source code and then on a cache miss run the compiler on the
890 preprocessed source code instead of the original source code. This
891 makes cache misses slightly faster since the source code only has
892 to be preprocessed once. The downside is that some compilers won’t
893 produce the same result (for instance diagnostics warnings) when
894 compiling preprocessed source code.
895 A solution to the above mentioned downside is to set run_second_cpp
897 to false and pass -fdirectives-only (for GCC) or -frewrite-includes
898 (for Clang) to the compiler. This will cause the compiler to leave
899 the macros and other preprocessor information, and only process the
900 #include directives. When run in this way, the preprocessor
901 arguments will be passed to the compiler since it still has to do
902 some preprocessing (like macros).
903 This option is ignored with MSVC, as there is no way to make it
905 compile without preprocessing first.
906 sloppiness (CCACHE_SLOPPINESS)
908 By default, ccache tries to give as few false cache hits as
909 possible. However, in certain situations it’s possible that you
910 know things that ccache can’t take for granted. This option makes
911 it possible to tell ccache to relax some checks in order to
912 increase the hit rate. The value should be a comma-separated string
913 with one or several of the following values:
914 clang_index_store
916 Ignore the Clang compiler option -index-store-path and its
917 argument when computing the manifest hash. This is useful if
918 you use Xcode, which uses an index store path derived from the
919 local project path. Note that the index store won’t be updated
920 correctly on cache hits if you enable this sloppiness.
921 file_stat_matches
923 Ccache normally examines a file’s contents to determine whether
924 it matches the cached version. With this sloppiness set, ccache
925 will consider a file as matching its cached version if the
926 mtimes and ctimes match.
927 file_stat_matches_ctime
929 Ignore ctimes when file_stat_matches is enabled. This can be
930 useful when backdating files' mtimes in a controlled way.
931 gcno_cwd
933 By default, ccache will include the current working directory
934 in the hash when producing a .gcno file (when compiling with
935 -ftest-coverage or --coverage). This is because GCC 9+ includes
936 the current working directory in the .gcno file. The gcno_cwd
937 sloppiness makes ccache not hash the current working directory
938 so that you can get cache hits when compiling in different
939 directories, with the tradeoff of potentially getting an
940 incorrect directory in the .gcno file. gcno_cwd also disables
941 hashing of the current working directory if -fprofile-abs-path
942 is used.
943 include_file_ctime
945 By default, ccache will disable the direct mode if an include
946 file has too new ctime. This sloppiness disables that check.
947 See also [Handling of newly created header files].
948 include_file_mtime
950 By default, ccache will disable the direct mode if an include
951 file has too new mtime. This sloppiness disables that check.
952 See also [Handling of newly created header files].
953 ivfsoverlay
955 Ignore the Clang compiler option -ivfsoverlay and its argument.
956 This is useful if you use Xcode, which uses a virtual file
957 system (VFS) for things like combining Objective-C and Swift
958 code.
959 locale
961 Ccache includes the environment variables LANG, LC_ALL,
962 LC_CTYPE and LC_MESSAGES in the hash by default since they may
963 affect localization of compiler warning messages. Set this
964 sloppiness to tell ccache not to do that.
965 modules
967 By default, ccache will not cache compilations if -fmodules is
968 used since it cannot hash the state of compiler’s internal
969 representation of relevant modules. This sloppiness allows
970 caching in such a case. See C++ MODULES for more information.
971 pch_defines
973 Be sloppy about #define directives when precompiling a header
974 file. See PRECOMPILED HEADERS for more information.
975 random_seed
977 Ignore the -frandom-seed option and its arguments when
978 computing the input hash. This is useful if your build system
979 generates different seeds between builds and you are OK with
980 reusing cached results.
981 system_headers
983 By default, ccache will also include all system headers in the
984 manifest. With this sloppiness set, ccache will only include
985 system headers in the hash but not add the system header files
986 to the list of include files.
987 time_macros
989 Ignore __DATE__, __TIME__ and __TIMESTAMP__ being present in
990 the source code.
991 See the discussion under TROUBLESHOOTING for more information.
993 stats (CCACHE_STATS or CCACHE_NOSTATS, see Boolean values above)
995 If true, ccache will update the statistics counters on each
996 compilation. The default is true. If false, [automatic cleanup]
997 will be disabled as well.
998 stats_log (CCACHE_STATSLOG)
1000 If set to a file path, ccache will write statistics counter updates
1001 to the specified file. This is useful for getting statistics for
1002 individual builds. To show a summary of the current stats log, use
1003 ccache --show-log-stats.
1004 Note
1006 Lines in the stats log starting with a hash sign (#) are
1007 comments.
1008 temporary_dir (CCACHE_TEMPDIR)
1010 This option specifies where ccache will put temporary files. The
1011 default is $XDG_RUNTIME_DIR/ccache-tmp (typically
1012 /run/user/<UID>/ccache-tmp) if XDG_RUNTIME_DIR is set and the
1013 directory exists, otherwise <cache_dir>/tmp.
1014 Note
1016 In previous versions of ccache, CCACHE_TEMPDIR had to be on the
1017 same filesystem as the CCACHE_DIR path, but this requirement
1018 has been relaxed.
1019 umask (CCACHE_UMASK)
1021 This option (an octal integer) specifies the umask for files and
1022 directories in the cache directory. This is mostly useful when you
1023 wish to share your cache with other users.
1024 Disabling ccache
1026 To disable ccache completely for all invocations, set disable = true
1027 (CCACHE_DISABLE=1). You can also disable ccache for a certain source
1028 code file by adding the string ccache:disable in a comment in the first
1029 4096 bytes of the file. In the latter case the Ccache disabled
1030 statistics counter will be increased.
1031
1033 The remote_storage option lets you configure ccache to use one or
1034 several remote storage backends. By default, the local cache directory
1035 located in cache_dir will be queried first and remote storage second,
1036 but remote_only can be set to true to disable local storage. Note that
1037 cache statistics counters will still be kept in the local cache
1038 directory — remote storage backends only store compilation results and
1039 manifests.
1040 A remote storage backend is specified with a URL, optionally followed
1042 by a pipe (|) and a pipe-separated list of attributes. An attribute is
1043 key=value or just key as a short form of key=true. Attribute values
1044 must be percent-encoded
1045 <https://en.wikipedia.org/wiki/Percent-encoding> if they contain
1046 percent, pipe or space characters.
1047 Attributes for all backends
1049 These optional attributes are available for all remote storage
1050 backends:
1051 • read-only: If true, only read from this backend, don’t write. The
1053 default is false.
1054 • shards: A comma-separated list of names for sharding (partitioning)
1056 the cache entries using Rendezvous hashing
1057 <https://en.wikipedia.org/wiki/Rendezvous_hashing>, typically to
1058 spread the cache over a server cluster. When set, the storage URL
1059 must contain an asterisk (*), which will be replaced by one of the
1060 shard names to form a real URL. A shard name can optionally have an
1061 appended weight within parentheses to indicate how much of the key
1062 space should be associated with that shard. A shard with weight w
1063 will contain w/S of the cache, where S is the sum of all shard
1064 weights. A weight could for instance be set to represent the
1065 available memory for a memory cache on a specific server. The
1066 default weight is 1.
1067 Examples:
1069 • redis://cache-*.example.com|shards=a(3),b(1),c(1.5) will put
1071 55% (3/5.5) of the cache on redis://cache-a.example.com, 18%
1072 (1/5.5) on redis://cache-b.example.com and 27% (1.5/5.5) on
1073 redis://cache-c.example.com.
1074 • http://example.com/*|shards=alpha,beta will put 50% of the
1076 cache on http://example.com/alpha and 50% on
1077 http://example.com/beta.
1078 Storage interaction
1080 The table below describes the interaction between local and remote
1081 storage on cache hits and misses if remote_only is false (which is the
1082 default):
1083 ┌──────────────┬────────────────┬───────────────────┐
1085 │ │ │ │
1086 │Local storage │ Remote storage │ What happens │
1087 ├──────────────┼────────────────┼───────────────────┤
1088 │ │ │ │
1089 │miss │ miss │ Compile, write to │
1090 │ │ │ local, write to │
1091 │ │ │ remote[1] │
1092 ├──────────────┼────────────────┼───────────────────┤
1093 │ │ │ │
1094 │miss │ hit │ Read from remote, │
1095 │ │ │ write to local │
1096 ├──────────────┼────────────────┼───────────────────┤
1097 │ │ │ │
1098 │hit │ - │ Read from local, │
1099 │ │ │ don’t write to │
1100 │ │ │ remote[2] │
1101 └──────────────┴────────────────┴───────────────────┘
1102 [1] Unless remote storage has attribute read-only=true.
1104 [2] Unless local storage is set to share its cache hits with the
1105 reshare option.
1106 If remote_only is true:
1108 ┌──────────────┬────────────────┬─────────────────────┐
1110 │ │ │ │
1111 │Local storage │ Remote storage │ What happens │
1112 ├──────────────┼────────────────┼─────────────────────┤
1113 │ │ │ │
1114 │- │ miss │ Compile, write to │
1115 │ │ │ remote, don’t write │
1116 │ │ │ to local │
1117 ├──────────────┼────────────────┼─────────────────────┤
1118 │ │ │ │
1119 │- │ hit │ Read from remote, │
1120 │ │ │ don’t write to │
1121 │ │ │ local │
1122 └──────────────┴────────────────┴─────────────────────┘
1123 File storage backend
1125 URL format: file:DIRECTORY or file://[HOST]DIRECTORY
1126 This backend stores data as separate files in a directory structure
1128 below DIRECTORY, similar (but not identical) to the local cache
1129 storage. A typical use case for this backend would be sharing a cache
1130 on an NFS directory. DIRECTORY must start with a slash. HOST can be the
1131 empty string or localhost. On Windows, HOST can also be the name of a
1132 server hosting a shared folder.
1133 Important
1135 ccache will not perform any cleanup of the storage — that has to be
1136 done by other means, for instance by running ccache --trim-dir
1137 periodically.
1138 Examples:
1140 • file:/shared/nfs/directory
1142 • file:///shared/nfs/directory|umask=002|update-mtime=true
1144 • file:///Z:/example/windows/folder
1146 • file://example.com/shared/ccache%20folder
1148 Optional attributes:
1150 • layout: How to store file under the cache directory. Available
1152 values:
1153 • flat: Store all files directly under the cache directory.
1155 • subdirs: Store files in 256 subdirectories of the cache
1157 directory.
1158 The default is subdirs.
1160 • umask: This attribute (an octal integer) overrides the umask to use
1162 for files and directories in the cache directory.
1163 • update-mtime: If true, update the modification time (mtime) of
1165 cache entries that are read. The default is false.
1166 HTTP storage backend
1168 URL format: http://HOST[:PORT][/PATH]
1169 This backend stores data in an HTTP-compatible server. The required
1171 HTTP methods are GET, PUT and DELETE.
1172 Important
1174 ccache will not perform any cleanup of the storage — that has to be
1175 done by other means, for instance by running ccache --trim-dir
1176 periodically.
1177 Note
1179 HTTPS is not supported.
1180 Tip
1182 See How to set up HTTP storage
1183 <https://ccache.dev/howto/http-storage.html> for hints on how to
1184 set up an HTTP server for use with ccache.
1185 Examples:
1187 • http://localhost
1189 • http://someusername:p4ssw0rd@example.com/cache/
1191 • http://localhost:8080|layout=bazel|connect-timeout=50
1193 Optional attributes:
1195 • bearer-token: Bearer token used to authorize the HTTP requests.
1197 • connect-timeout: Timeout (in ms) for network connection. The
1199 default is 100.
1200 • keep-alive: If true, keep the HTTP connection to the storage server
1202 open to avoid reconnects. The default is true.
1203 • layout: How to map key names to the path part of the URL. Available
1205 values:
1206 • bazel: Store values in a format compatible with the Bazel HTTP
1208 caching protocol. More specifically, the entries will be stored
1209 as 64 hex digits under the /ac/ part of the cache.
1210 Note
1212 You may have to disable verification of action cache values
1213 in the server for this to work since ccache entries are not
1214 valid action result metadata values.
1215 • flat: Append the key directly to the path part of the URL (with
1217 a leading slash if needed).
1218 • subdirs: Append the first two characters of the key to the URL
1220 (with a leading slash if needed), followed by a slash and the
1221 rest of the key. This divides the entries into 256 buckets.
1222 The default is subdirs.
1224 • operation-timeout: Timeout (in ms) for HTTP requests. The default
1226 is 10000.
1227 Redis storage backend
1229 URL formats:
1230 redis://[[USERNAME:]PASSWORD@]HOST[:PORT][/DBNUMBER]
1232 redis+unix:SOCKET_PATH[?db=DBNUMBER]
1233 redis+unix://[[USERNAME:]PASSWORD@localhost]SOCKET_PATH[?db=DBNUMBER]
1234 This backend stores data in a Redis <https://redis.io> (or
1236 Redis-compatible) server. There are implementations for both
1237 memory-based and disk-based storage. PORT defaults to 6379 and DBNUMBER
1238 defaults to 0.
1239 Note
1241 ccache will not perform any cleanup of the Redis storage, but you
1242 can configure LRU eviction <https://redis.io/topics/lru-cache>.
1243 Tip
1245 See How to set up Redis
1246 <https://ccache.dev/howto/redis-storage.html> storage" for hints
1247 on setting up a Redis server for use with ccache.
1248 Tip
1250 You can set up a cluster of Redis servers using the shards
1251 attribute described in REMOTE STORAGE BACKENDS.
1252 Examples:
1254 • redis://localhost
1256 • redis://p4ssw0rd@cache.example.com:6379/0|connect-timeout=50
1258 • redis+unix:/run/redis.sock
1260 • redis+unix:///run/redis.sock
1262 • redis+unix://p4ssw0rd@localhost/run/redis.sock?db=0
1264 Optional attributes:
1266 • connect-timeout: Timeout (in ms) for network connection. The
1268 default is 100.
1269 • operation-timeout: Timeout (in ms) for Redis commands. The default
1271 is 10000.
1272
1274 By default, ccache has a 5 GB limit on the total size of files in the
1275 cache and no limit on the number of files. You can set different limits
1276 using the command line options -M/--max-size and -F/--max-files. Use
1277 the -s/--show-stats option to see the cache size and the currently
1278 configured limits (in addition to other various statistics).
1279 Cleanup can be triggered in two different ways: automatic and manual.
1281 Automatic cleanup
1283 After a new compilation result has been written to the local cache,
1284 ccache will trigger an automatic cleanup if max_size or max_files is
1285 exceeded. The cleanup removes cache entries in LRU (least recently
1286 used) order based on the modification time (mtime) of files in the
1287 cache. For this reason, ccache updates mtime of the cache files read on
1288 a cache hit to mark them as recently used.
1289 Manual cleanup
1291 You can run ccache -c/--cleanup to force cleanup of the whole cache.
1292 This will recalculate the cache size information and also make sure
1293 that the cache size does not exceed max_size and max_files.
1294
1296 Ccache will by default compress all data it puts into the cache using
1297 the compression algorithm Zstandard <http://zstd.net> (zstd) using
1298 compression level 1. The algorithm is fast enough that there should be
1299 little reason to turn off compression to gain performance. One
1300 exception is if the cache is located on a compressed file system, in
1301 which case the compression performed by ccache of course is redundant.
1302 See the documentation for the configuration options compression and
1303 compression_level for more information.
1304 You can use the command line option -x/--show-compression to print
1306 information related to compression. Example:
1307 Total data: 14.8 GB (16.0 GB disk blocks)
1309 Compressed data: 11.3 GB (30.6% of original size)
1310 Original size: 36.9 GB
1311 Compression ratio: 3.267 x (69.4% space savings)
1312 Incompressible data: 3.5 GB
1313 Notes:
1315 • The “disk blocks” size is the cache size when taking disk block
1317 size into account. This value should match the “Cache size” value
1318 from “ccache --show-stats”. The other size numbers refer to actual
1319 content sizes.
1320 • “Compressed data” refers to result and manifest files stored in the
1322 cache.
1323 • “Incompressible data” refers to files that are always stored
1325 uncompressed (triggered by enabling file_clone or hard_link) or
1326 unknown files (for instance files created by older ccache
1327 versions).
1328 • The compression ratio is affected by compression_level.
1330 The cache data can also be recompressed to another compression level
1332 (or made uncompressed) with the command line option -X/--recompress. If
1333 you choose to disable compression by default or to use a low
1334 compression level, you can (re)compress newly cached data with a higher
1335 compression level after the build or at another time when there are
1336 more CPU cycles available, for instance every night. Full recompression
1337 potentially takes a lot of time, but only files that are currently
1338 compressed with a different level than the target level will be
1339 recompressed.
1340
1342 ccache --show-stats shows a summary of statistics, including cache
1343 size, cleanups (number of performed cleanups, either implicitly due to
1344 a cache size limit being reached or due to explicit ccache -c calls),
1345 overall hit rate, hit rate for direct/preprocessed modes and hit rate
1346 for local and remote storage.
1347 The summary also includes counters called “Errors” and “Uncacheable”,
1349 which are sums of more detailed counters. To see those detailed
1350 counters, use the -v/--verbose flag. The verbose mode can show the
1351 following counters:
1352 ┌───────────────────────────┬────────────────────────────┐
1354 │ │ │
1355 │Counter │ Description │
1356 ├───────────────────────────┼────────────────────────────┤
1357 │ │ │
1358 │Autoconf compile/link │ Uncacheable compilation or │
1359 │ │ linking by an Autoconf │
1360 │ │ test. │
1361 ├───────────────────────────┼────────────────────────────┤
1362 │ │ │
1363 │Bad compiler arguments │ Malformed compiler │
1364 │ │ argument, e.g. missing a │
1365 │ │ value for a compiler │
1366 │ │ option that requires an │
1367 │ │ argument or failure to │
1368 │ │ read a file specified by a │
1369 │ │ compiler option argument. │
1370 ├───────────────────────────┼────────────────────────────┤
1371 │ │ │
1372 │Called for linking │ The compiler was called │
1373 │ │ for linking, not │
1374 │ │ compiling. Ccache only │
1375 │ │ supports compilation of a │
1376 │ │ single file, i.e. calling │
1377 │ │ the compiler with the -c │
1378 │ │ option to produce a single │
1379 │ │ object file from a single │
1380 │ │ source file. │
1381 ├───────────────────────────┼────────────────────────────┤
1382 │ │ │
1383 │Called for preprocessing │ The compiler was called │
1384 │ │ for preprocessing, not │
1385 │ │ compiling. │
1386 ├───────────────────────────┼────────────────────────────┤
1387 │ │ │
1388 │Ccache disabled │ Ccache was disabled by a │
1389 │ │ ccache:disable string in │
1390 │ │ the source code file. │
1391 ├───────────────────────────┼────────────────────────────┤
1392 │ │ │
1393 │Could not use modules │ Preconditions for using │
1394 │ │ C++ MODULES were not │
1395 │ │ fulfilled. │
1396 ├───────────────────────────┼────────────────────────────┤
1397 │ │ │
1398 │Could not use precompiled │ Preconditions for using │
1399 │header │ precompiled headers were │
1400 │ │ not fulfilled. │
1401 ├───────────────────────────┼────────────────────────────┤
1402 │ │ │
1403 │Could not write to output │ The output path specified │
1404 │file │ with -o could not be │
1405 │ │ written to. │
1406 ├───────────────────────────┼────────────────────────────┤
1407 │ │ │
1408 │Compilation failed │ The compilation failed. No │
1409 │ │ result stored in the │
1410 │ │ cache. │
1411 ├───────────────────────────┼────────────────────────────┤
1412 │ │ │
1413 │Compiler check failed │ A compiler check program │
1414 │ │ specified by │
1415 │ │ compiler_check │
1416 │ │ (CCACHE_COMPILERCHECK) │
1417 │ │ failed. │
1418 ├───────────────────────────┼────────────────────────────┤
1419 │ │ │
1420 │Compiler output file │ One of the files expected │
1421 │missing │ to be produced by the │
1422 │ │ compiler was missing after │
1423 │ │ compilation. │
1424 ├───────────────────────────┼────────────────────────────┤
1425 │ │ │
1426 │Compiler produced empty │ The compiler’s output file │
1427 │output │ (typically an object file) │
1428 │ │ was empty after │
1429 │ │ compilation. │
1430 ├───────────────────────────┼────────────────────────────┤
1431 │ │ │
1432 │Could not find the │ The compiler to execute │
1433 │compiler │ could not be found. │
1434 ├───────────────────────────┼────────────────────────────┤
1435 │ │ │
1436 │Error hashing extra file │ Failure reading a file │
1437 │ │ specified by │
1438 │ │ extra_files_to_hash │
1439 │ │ (CCACHE_EXTRAFILES). │
1440 ├───────────────────────────┼────────────────────────────┤
1441 │ │ │
1442 │Forced recache │ CCACHE_RECACHE was used to │
1443 │ │ overwrite an existing │
1444 │ │ result. │
1445 ├───────────────────────────┼────────────────────────────┤
1446 │ │ │
1447 │Internal error │ Unexpected failure, e.g. │
1448 │ │ due to problems │
1449 │ │ reading/writing the cache. │
1450 ├───────────────────────────┼────────────────────────────┤
1451 │ │ │
1452 │Missing cache file │ A file was unexpectedly │
1453 │ │ missing from the cache. │
1454 │ │ This only happens in rare │
1455 │ │ situations, e.g. if one │
1456 │ │ ccache instance is about │
1457 │ │ to get a file from the │
1458 │ │ cache while another │
1459 │ │ instance removed the file │
1460 │ │ as part of cache cleanup. │
1461 ├───────────────────────────┼────────────────────────────┤
1462 │ │ │
1463 │Multiple source files │ The compiler was called to │
1464 │ │ compile multiple source │
1465 │ │ files in one go. This is │
1466 │ │ not supported by ccache. │
1467 ├───────────────────────────┼────────────────────────────┤
1468 │ │ │
1469 │No input file │ No input file was │
1470 │ │ specified to the compiler. │
1471 ├───────────────────────────┼────────────────────────────┤
1472 │ │ │
1473 │Output to stdout │ The compiler was │
1474 │ │ instructed to write its │
1475 │ │ output to standard output │
1476 │ │ using -o -. This is not │
1477 │ │ supported by ccache. │
1478 ├───────────────────────────┼────────────────────────────┤
1479 │ │ │
1480 │Preprocessing failed │ Preprocessing the source │
1481 │ │ code using the compiler’s │
1482 │ │ -E option failed. │
1483 ├───────────────────────────┼────────────────────────────┤
1484 │ │ │
1485 │Unsupported code directive │ Code like the assembler │
1486 │ │ .incbin directive was │
1487 │ │ found. This is not │
1488 │ │ supported by ccache. │
1489 ├───────────────────────────┼────────────────────────────┤
1490 │ │ │
1491 │Unsupported compiler │ A compiler option not │
1492 │option │ supported by ccache was │
1493 │ │ found. │
1494 ├───────────────────────────┼────────────────────────────┤
1495 │ │ │
1496 │Unsupported environment │ An environment variable │
1497 │variable │ not supported by ccache │
1498 │ │ was set. │
1499 ├───────────────────────────┼────────────────────────────┤
1500 │ │ │
1501 │Unsupported source │ A source language e.g. │
1502 │language │ specified with -x was │
1503 │ │ unsupported by ccache. │
1504 └───────────────────────────┴────────────────────────────┘
1505
1507 The basic idea is to detect when you are compiling exactly the same
1508 code a second time and reuse the previously produced output. The
1509 detection is done by hashing different kinds of information that should
1510 be unique for the compilation and then using the hash sum to identify
1511 the cached output. Ccache uses BLAKE3, a very fast cryptographic hash
1512 algorithm, for the hashing. On a cache hit, ccache is able to supply
1513 all of the correct compiler outputs (including all warnings, dependency
1514 file, etc) from the cache. Data stored in the cache is checksummed with
1515 XXH3, an extremely fast non-cryptographic algorithm, to detect
1516 corruption.
1517 Ccache has two ways of gathering information used to look up results in
1519 the cache:
1520 • the preprocessor mode, where ccache runs the preprocessor on the
1522 source code and hashes the result
1523 • the direct mode, where ccache hashes the source code and include
1525 files directly
1526 The direct mode is generally faster since running the preprocessor has
1528 some overhead.
1529 If no previous result is detected (i.e., there is a cache miss) using
1531 the direct mode, ccache will fall back to the preprocessor mode unless
1532 the depend mode is enabled. In the depend mode, ccache never runs the
1533 preprocessor, not even on cache misses. Read more in The depend mode
1534 below.
1535 Common hashed information
1537 The following information is always included in the hash:
1538 • the extension used by the compiler for a file with preprocessor
1540 output (normally .i for C code and .ii for C++ code)
1541 • the compiler’s size and modification time (or other
1543 compiler-specific information specified by compiler_check)
1544 • the name of the compiler
1546 • the current directory (if hash_dir is enabled)
1548 • contents of files specified by extra_files_to_hash (if any)
1550 The preprocessor mode
1552 In the preprocessor mode, the hash is formed of the common information
1553 and:
1554 • the preprocessor output from running the compiler with -E
1556 • the command line options except those that affect include files
1558 (-I, -include, -D, etc; the theory is that these command line
1559 options will change the preprocessor output if they have any effect
1560 at all)
1561 • any standard error output generated by the preprocessor
1563 Based on the hash, the cached compilation result can be looked up
1565 directly in the cache.
1566 The direct mode
1568 In the direct mode, the hash is formed of the common information and:
1569 • the input source file
1571 • the compiler options
1573 Based on the hash, a data structure called “manifest” is looked up in
1575 the cache. The manifest contains:
1576 • references to cached compilation results (object file, dependency
1578 file, etc) that were produced by previous compilations that matched
1579 the hash
1580 • paths to the include files that were read at the time the
1582 compilation results were stored in the cache
1583 • hash sums of the include files at the time the compilation results
1585 were stored in the cache
1586 The current contents of the include files are then hashed and compared
1588 to the information in the manifest. If there is a match, ccache knows
1589 the result of the compilation. If there is no match, ccache falls back
1590 to running the preprocessor. The output from the preprocessor is parsed
1591 to find the include files that were read. The paths and hash sums of
1592 those include files are then stored in the manifest along with
1593 information about the produced compilation result.
1594 There is a catch with the direct mode: header files that were used by
1596 the compiler are recorded, but header files that were not used, but
1597 would have been used if they existed, are not. So, when ccache checks
1598 if a result can be taken from the cache, it currently can’t check if
1599 the existence of a new header file should invalidate the result. In
1600 practice, the direct mode is safe to use in the absolute majority of
1601 cases.
1602 The direct mode will be disabled if any of the following holds:
1604 • direct_mode is false
1606 • a modification time of one of the include files is too new (needed
1608 to avoid a race condition)
1609 • a compiler option not supported by the direct mode is used:
1611 • a -Wp,* compiler option other than -Wp,-MD,<path>,
1613 -Wp,-MMD,<path>, -Wp,-D<macro[=defn]> or -Wp,-U<macro>
1614 • -Xpreprocessor
1616 • the string __TIME__ is present in the source code
1618 The depend mode
1620 If the depend mode is enabled, ccache will not use the preprocessor at
1621 all. The hash used to identify results in the cache will be based on
1622 the direct mode hash described above plus information about include
1623 files read from the dependency list generated by MSVC with
1624 /showIncludes, or the dependency file generated by other compilers with
1625 -MD or -MMD.
1626 Advantages:
1628 • The ccache overhead of a cache miss will be much smaller.
1630 • Not running the preprocessor at all can be good if compilation is
1632 performed remotely, for instance when using distcc or similar;
1633 ccache then won’t make potentially costly preprocessor calls on the
1634 local machine.
1635 Disadvantages:
1637 • The cache hit rate will likely be lower since any change to
1639 compiler options or source code will make the hash different.
1640 Compare this with the default setup where ccache will fall back to
1641 the preprocessor mode, which is tolerant to some types of changes
1642 of compiler options and source code changes.
1643 • If -MD is used, the manifest entries will include system header
1645 files as well, thus slowing down cache hits slightly, just as using
1646 -MD slows down make. This is also the case for MSVC with
1647 /showIncludes.
1648 • If -MMD is used, the manifest entries will not include system
1650 header files, which means ccache will ignore changes in them.
1651 The depend mode will be disabled if any of the following holds:
1653 • depend_mode is false.
1655 • run_second_cpp is false.
1657 • The compiler is not generating dependencies using -MD or -MMD (for
1659 MSVC, /showIncludes is added automatically if not specified by the
1660 user).
1661
1663 If modification time (mtime) or status change time (ctime) of one of
1664 the include files is equal to (or newer than) the time compilation is
1665 being done, ccache disables the direct mode (or, in the case of a
1666 precompiled header, disables caching completely). This done as a safety
1667 measure to avoid a race condition (see below).
1668 To be able to use a newly created header files in direct mode (or use a
1670 newly precompiled header), either:
1671 • create the include file earlier in the build process, or
1673 • set sloppiness to include_file_ctime,include_file_mtime if you are
1675 willing to take the risk, for instance if you know that your build
1676 system is robust enough not to trigger the race condition.
1677 For reference, the race condition mentioned above consists of these
1679 events:
1680 1. The preprocessor is run.
1682 2. An include file is modified by someone.
1684 3. The new include file is hashed by ccache.
1686 4. The real compiler is run on the preprocessor’s output, which
1688 contains data from the old header file.
1689 5. The wrong object file is stored in the cache.
1691
1693 To find out what information ccache actually is hashing, you can enable
1694 the debug mode via the configuration option debug or by setting
1695 CCACHE_DEBUG in the environment. This can be useful if you are
1696 investigating why you don’t get cache hits. Note that performance will
1697 be reduced slightly.
1698 When the debug mode is enabled, ccache will create up to five
1700 additional files next to the object file:
1701 ┌───────────────────────────────────────────┬───────────────────────────┐
1703 │ │ │
1704 │Filename │ Description │
1705 ├───────────────────────────────────────────┼───────────────────────────┤
1706 │ │ │
1707 │<objectfile>.<timestamp>.ccache-input-c │ Binary input hashed by │
1708 │ │ both the direct mode and │
1709 │ │ the preprocessor mode. │
1710 ├───────────────────────────────────────────┼───────────────────────────┤
1711 │ │ │
1712 │<objectfile>.<timestamp>.ccache-input-d │ Binary input only hashed │
1713 │ │ by the direct mode. │
1714 ├───────────────────────────────────────────┼───────────────────────────┤
1715 │ │ │
1716 │<objectfile>.<timestamp>.ccache-input-p │ Binary input only hashed │
1717 │ │ by the preprocessor mode. │
1718 ├───────────────────────────────────────────┼───────────────────────────┤
1719 │ │ │
1720 │<objectfile>.<timestamp>.ccache-input-text │ Human-readable combined │
1721 │ │ diffable text version of │
1722 │ │ the three files above. │
1723 ├───────────────────────────────────────────┼───────────────────────────┤
1724 │ │ │
1725 │<objectfile>.<timestamp>.ccache-log │ Log for this object file. │
1726 └───────────────────────────────────────────┴───────────────────────────┘
1727 The timestamp format is
1729 `<year><month><day>_<hour><minute><second>_<microsecond>`.
1730 If debug_dir (environment variable CCACHE_DEBUGDIR) is set, the files
1732 above will be written to that directory with full absolute paths
1733 instead of next to the object file.
1734 In the direct mode, ccache uses the 160 bit BLAKE3 hash of the
1736 “ccache-input-c” + “ccache-input-d” data (where + means concatenation),
1737 while the “ccache-input-c” + “ccache-input-p” data is used in the
1738 preprocessor mode.
1739 The “ccache-input-text” file is a combined text version of the three
1741 binary input files. It has three sections (“COMMON”, “DIRECT MODE” and
1742 “PREPROCESSOR MODE”), which is turn contain annotations that say what
1743 kind of data comes next.
1744 To debug why you don’t get an expected cache hit for an object file,
1746 you can do something like this:
1747 1. Enable debug (CCACHE_DEBUG).
1749 2. Build.
1751 3. Clean and build again.
1753 4. Compare the <objectfile>.<timestamp>.ccache-input-text files for
1755 the two builds. This together with the
1756 <objectfile>.<timestamp>.ccache-log files should give you some
1757 clues about what is happening.
1758
1760 Some information included in the hash that identifies a unique
1761 compilation can contain absolute paths:
1762 • The preprocessed source code may contain absolute paths to include
1764 files if the compiler option -g is used or if absolute paths are
1765 given to -I and similar compiler options.
1766 • Paths specified by compiler options (such as -I, -MF, etc) on the
1768 command line may be absolute.
1769 • The source code file path may be absolute, and that path may
1771 substituted for __FILE__ macros in the source code or included in
1772 warnings emitted to standard error by the preprocessor.
1773 This means that if you compile the same code in different locations,
1775 you can’t share compilation results between the different build
1776 directories since you get cache misses because of the absolute build
1777 directory paths that are part of the hash.
1778 Here’s what can be done to enable cache hits between different build
1780 directories:
1781 • If you build with -g (or similar) to add debug information to the
1783 object file, you must either:
1784 • use the compiler option -fdebug-prefix-map=<old>=<new> for
1786 relocating debug info to a common prefix (e.g.
1787 -fdebug-prefix-map=$PWD=.); or
1788 • set hash_dir = false.
1790 • If you use absolute paths anywhere on the command line (e.g. the
1792 source code file path or an argument to compiler options like -I
1793 and -MF), you must set base_dir to an absolute path to a “base
1794 directory”. Ccache will then rewrite absolute paths under that
1795 directory to relative before computing the hash.
1796
1798 Ccache has support for precompiled headers with GCC and Clang. However,
1799 you have to do some things to make it work properly:
1800 • You must set sloppiness to pch_defines,time_macros. The reason is
1802 that ccache can’t tell whether __TIME__, __DATE__ or __TIMESTAMP__
1803 is used when using a precompiled header. Further, it can’t detect
1804 changes in #defines in the source code because of how preprocessing
1805 works in combination with precompiled headers.
1806 • You may also want to include include_file_mtime,include_file_ctime
1808 in sloppiness. See HANDLING OF NEWLY CREATED HEADER FILES.
1809 • You must either:
1811 • use the compiler option -include to include the precompiled
1813 header (i.e., don’t use #include in the source code to include
1814 the header; the filename itself must be sufficient to find the
1815 header, i.e. -I paths are not searched); or
1816 • (for the Clang compiler) use the compiler option -include-pch
1818 to include the PCH file generated from the precompiled header;
1819 or
1820 • (for the GCC compiler) add the compiler option -fpch-preprocess
1822 when compiling.
1823 • If you use Clang, you must compile with -fno-pch-timestamp.
1825 If you don’t do this, either the non-precompiled version of the header
1827 file will be used (if available) or ccache will fall back to running
1828 the real compiler and increase the statistics counter “Preprocessing
1829 failed” (if the non-precompiled header file is not available).
1830
1832 Ccache has support for Clang’s -fmodules option. In practice ccache
1833 only additionally hashes module.modulemap files; it does not know how
1834 Clang handles its cached binary form of modules so those are ignored.
1835 This should not matter in practice: as long as everything else
1836 (including module.modulemap files) is the same the cached result should
1837 work. Still, you must set sloppiness to modules to allow caching.
1838 You must use both direct mode and depend mode. When using the
1840 preprocessor mode Clang does not provide enough information to allow
1841 hashing of module.modulemap files.
1842
1844 A group of developers can increase the cache hit rate by sharing a
1845 local cache directory. To share a local cache without unpleasant side
1846 effects, the following conditions should to be met:
1847 • Use the same cache directory.
1849 • Make sure that the configuration option hard_link is false (which
1851 is the default).
1852 • Make sure that all users are in the same group.
1854 • Set the configuration option umask to 002. This ensures that cached
1856 files are accessible to everyone in the group.
1857 • Make sure that all users have write permission in the entire cache
1859 directory (and that you trust all users of the shared cache).
1860 • Make sure that the setgid bit is set on all directories in the
1862 cache. This tells the filesystem to inherit group ownership for new
1863 directories. The following command might be useful for this:
1864 find $CCACHE_DIR -type d | xargs chmod g+s
1866 The reason to avoid the hard link mode is that the hard links cause
1868 unwanted side effects, as all links to a cached file share the file’s
1869 modification timestamp. This results in false dependencies to be
1870 triggered by timestamp-based build systems whenever another user links
1871 to an existing file. Typically, users will see that their libraries and
1872 binaries are relinked without reason.
1873 You may also want to make sure that a base directory is set
1875 appropriately, as discussed in a previous section.
1876
1878 It is possible to put the cache directory on an NFS filesystem (or
1879 similar filesystems), but keep in mind that:
1880 • Having the cache on NFS may slow down compilation. Make sure to do
1882 some benchmarking to see if it’s worth it.
1883 • Ccache hasn’t been tested very thoroughly on NFS.
1885 A tip is to set temporary_dir to a directory on the local host to avoid
1887 NFS traffic for temporary files.
1888 It is recommended to use the same operating system version when using a
1890 shared cache. If operating system versions are different then system
1891 include files will likely be different and there will be few or no
1892 cache hits between the systems. One way of improving cache hit rate in
1893 that case is to set sloppiness to system_headers to ignore system
1894 headers.
1895 An alternative to putting the main cache directory on NFS is to set up
1897 a remote storage file cache.
1898
1900 The recommended way of combining ccache with another compiler wrapper
1901 (such as “distcc”) is by letting ccache execute the compiler wrapper.
1902 This is accomplished by defining prefix_command, for example by setting
1903 the environment variable CCACHE_PREFIX to the name of the wrapper (e.g.
1904 distcc). Ccache will then prefix the command line with the specified
1905 command when running the compiler. To specify several prefix commands,
1906 set prefix_command to a colon-separated list of commands.
1907 Unless you set compiler_check to a suitable command (see the
1909 description of that configuration option), it is not recommended to use
1910 the form ccache anotherwrapper compiler args as the compilation
1911 command. It’s also not recommended to use the masquerading technique
1912 for the other compiler wrapper. The reason is that by default, ccache
1913 will in both cases hash the mtime and size of the other wrapper instead
1914 of the real compiler, which means that:
1915 • Compiler upgrades will not be detected properly.
1917 • The cached results will not be shared between compilations with and
1919 without the other wrapper.
1920 Another minor thing is that if prefix_command is used, ccache will not
1922 invoke the other wrapper when running the preprocessor, which increases
1923 performance. You can use prefix_command_cpp if you also want to invoke
1924 the other wrapper when doing preprocessing (normally by adding -E).
1925
1927 • The direct mode fails to pick up new header files in some rare
1928 scenarios. See The direct mode above.
1929
1931 General
1932 A general tip for getting information about what ccache is doing is to
1933 enable debug logging by setting the configuration option debug (or the
1934 environment variable CCACHE_DEBUG); see CACHE DEBUGGING for more
1935 information. Another way of keeping track of what is happening is to
1936 check the output of ccache -s.
1937 Performance
1939 Ccache has been written to perform well out of the box, but sometimes
1940 you may have to do some adjustments of how you use the compiler and
1941 ccache in order to improve performance.
1942 Since ccache works best when I/O is fast, put the cache directory on a
1944 fast storage device if possible. Having lots of free memory so that
1945 files in the cache directory stay in the disk cache is also preferable.
1946 A good way of monitoring how well ccache works is to run ccache -s
1948 before and after your build and then compare the statistics counters.
1949 Here are some common problems and what may be done to increase the hit
1950 rate:
1951 • If the counter for preprocessed cache hits has been incremented
1953 instead of the one for direct cache hits, ccache has fallen back to
1954 preprocessor mode, which is generally slower. Some possible reasons
1955 are:
1956 • The source code has been modified in such a way that the
1958 preprocessor output is not affected.
1959 • Compiler arguments that are hashed in the direct mode but not
1961 in the preprocessor mode have changed (-I, -include, -D, etc)
1962 and they didn’t affect the preprocessor output.
1963 • The compiler option -Xpreprocessor or -Wp,* (except
1965 -Wp,-MD,<path>, -Wp,-MMD,<path>, and -Wp,-D<define>) is used.
1966 • This was the first compilation with a new value of the base
1968 directory.
1969 • A modification or status change time of one of the include
1971 files is too new (created the same second as the compilation is
1972 being done). See HANDLING OF NEWLY CREATED HEADER FILES.
1973 • The __TIME__ preprocessor macro is (potentially) being used.
1975 Ccache turns off direct mode if __TIME__ is present in the
1976 source code. This is done as a safety measure since the string
1977 indicates that a __TIME__ macro may affect the output. (To be
1978 sure, ccache would have to run the preprocessor, but the sole
1979 point of the direct mode is to avoid that.) If you know that
1980 __TIME__ isn’t used in practise, or don’t care if ccache
1981 produces objects where __TIME__ is expanded to something in the
1982 past, you can set sloppiness to time_macros.
1983 • The __DATE__ preprocessor macro is (potentially) being used and
1985 the date has changed. This is similar to how __TIME__ is
1986 handled. If __DATE__ is present in the source code, ccache
1987 hashes the current date in order to be able to produce the
1988 correct object file if the __DATE__ macro affects the output.
1989 If you know that __DATE__ isn’t used in practise, or don’t care
1990 if ccache produces objects where __DATE__ is expanded to
1991 something in the past, you can set sloppiness to time_macros.
1992 • The __TIMESTAMP__ preprocessor macro is (potentially) being
1994 used and the source file’s modification time has changed. This
1995 is similar to how __TIME__ is handled. If __TIMESTAMP__ is
1996 present in the source code, ccache hashes the string
1997 representation of the source file’s modification time in order
1998 to be able to produce the correct object file if the
1999 __TIMESTAMP__ macro affects the output. If you know that
2000 __TIMESTAMP__ isn’t used in practise, or don’t care if ccache
2001 produces objects where __TIMESTAMP__ is expanded to something
2002 in the past, you can set sloppiness to time_macros.
2003 • The input file path has changed. Ccache includes the input file
2005 path in the direct mode hash to be able to take relative
2006 include files into account and to produce a correct object file
2007 if the source code includes a __FILE__ macro.
2008 • If a cache hit counter was not incremented even though the same
2010 code has been compiled and cached before, ccache has either
2011 detected that something has changed anyway or a cleanup has been
2012 performed (either explicitly or implicitly when a cache limit has
2013 been reached). Some perhaps unobvious things that may result in a
2014 cache miss are usage of __TIME__, __DATE__ or __TIMESTAMP__ macros,
2015 or use of automatically generated code that contains a timestamp,
2016 build counter or other volatile information.
2017 • If “Multiple source files” has been incremented, it’s an indication
2019 that the compiler has been invoked on several source code files at
2020 once. Ccache doesn’t support that. Compile the source code files
2021 separately if possible.
2022 • If “Unsupported compiler option” has been incremented, enable debug
2024 logging and check which compiler option was rejected.
2025 • If “Preprocessing failed” has been incremented, one possible reason
2027 is that precompiled headers are being used. See PRECOMPILED HEADERS
2028 for how to remedy this.
2029 • If “Could not use precompiled header” has been incremented, see
2031 PRECOMPILED HEADERS.
2032 • If “Could not use modules” has been incremented, see C++ MODULES.
2034 Corrupt object files
2036 It should be noted that ccache is susceptible to general storage
2037 problems. If a bad object file sneaks into the cache for some reason,
2038 it will of course stay bad. Some possible reasons for erroneous object
2039 files are bad hardware (disk drive, disk controller, memory, etc),
2040 buggy drivers or file systems, a bad prefix_command or compiler
2041 wrapper. If this happens, the easiest way of fixing it is this:
2042 1. Build so that the bad object file ends up in the build tree.
2044 2. Remove the bad object file from the build tree.
2046 3. Rebuild with CCACHE_RECACHE set.
2048 An alternative is to clear the whole cache with ccache -C if you don’t
2050 mind losing other cached results.
2051 There are no reported issues about ccache producing broken object files
2053 reproducibly. That doesn’t mean it can’t happen, so if you find a
2054 repeatable case, please report it.
2055
2057 Credits, mailing list information, bug reporting instructions, source
2058 code, etc, can be found on ccache’s web site: https://ccache.dev.
2059
2061 Ccache was originally written by Andrew Tridgell and is currently
2062 developed and maintained by Joel Rosdahl. See AUTHORS.txt or
2063 AUTHORS.html and https://ccache.dev/credits.html for a list of
2064 contributors.
2065Ccache 4.8.2 2023-07-19 CCACHE(1)