1CCACHE(1) CCACHE(1)
2
6 ccache - a fast C/C++ compiler cache
7
9 ccache [options]
10 ccache compiler [compiler options]
11 compiler [compiler options] (ccache masquerading as the compiler)
12
14 Ccache is a compiler cache. It speeds up recompilation by caching the
15 result of previous compilations and detecting when the same compilation
16 is being done again.
17 Ccache has been carefully written to always produce exactly the same
19 compiler output that you would get without the cache. The only way you
20 should be able to tell that you are using ccache is the speed.
21 Currently known exceptions to this goal are listed under CAVEATS. If
22 you discover an undocumented case where ccache changes the output of
23 your compiler, please let us know.
24
26 There are two different ways to use ccache to cache a compilation:
27 1. Prefix your compilation command with ccache. This method is most
29 convenient if you just want to try out ccache or wish to use it for
30 some specific projects. Example:
31 ccache gcc -c example.c
33 2. Let ccache masquerade as the compiler. This method is most useful
35 when you wish to use ccache for all your compilations. To do this,
36 create a symbolic link to ccache named as the compiler. For
37 example, here is set up ccache to masquerade as gcc and g++:
38 cp ccache /usr/local/bin/
40 ln -s ccache /usr/local/bin/gcc
41 ln -s ccache /usr/local/bin/g++
42 On platforms that don’t support symbolic links you can simply copy
44 ccache to the compiler name instead for a similar effect:
45 cp ccache /usr/local/bin/gcc
47 cp ccache /usr/local/bin/g++
48 And so forth. This will work as long as the directory with symbolic
50 links or ccache copies comes before the directory with the compiler
51 (typically /usr/bin) in PATH.
52 Warning
54 The technique of letting ccache masquerade as the compiler
55 works well, but currently doesn’t interact well with other
56 tools that do the same thing. See USING CCACHE WITH OTHER
57 COMPILER WRAPPERS.
58
60 These command line options only apply when you invoke ccache as
61 “ccache”. When ccache masquerades as a compiler (as described in the
62 previous section), the normal compiler options apply and you should
63 refer to the compiler’s documentation.
64 Common options
66 -c, --cleanup
67 Clean up the cache by removing old cached files until the specified
68 file number and cache size limits are not exceeded. This also
69 recalculates the cache file count and size totals. Normally, there
70 is no need to initiate cleanup manually as ccache keeps the cache
71 below the specified limits at runtime and keeps statistics up to
72 date on each compilation. Forcing a cleanup is mostly useful if you
73 manually modify the cache contents or believe that the cache size
74 statistics may be inaccurate.
75 -C, --clear
77 Clear the entire cache, removing all cached files, but keeping the
78 configuration file.
79 --config-path PATH
81 Let the command line options operate on configuration file PATH
82 instead of the default. Using this option has the same effect as
83 setting (overriding) the environment variable CCACHE_CONFIGPATH
84 temporarily.
85 -d, --dir PATH
87 Let the command line options operate on cache directory PATH
88 instead of the default. For example, to show statistics for a cache
89 directory at /shared/ccache you can run ccache -d /shared/ccache
90 -s. Using this option has the same effect as setting the
91 environment variable CCACHE_DIR temporarily.
92 --evict-namespace NAMESPACE
94 Remove files created in the given namespace from the cache.
95 --evict-older-than AGE
97 Remove files older than AGE from the cache. AGE should be an
98 unsigned integer with a d (days) or s (seconds) suffix. If combined
99 with --evict-namespace, only remove old files within that
100 namespace.
101 -h, --help
103 Print a summary of command line options.
104 -F NUM, --max-files NUM
106 Set the maximum number of files allowed in the cache to NUM. Use 0
107 for no limit. The value is stored in a configuration file in the
108 cache directory and applies to all future compilations.
109 -M SIZE, --max-size SIZE
111 Set the maximum size of the files stored in the cache. SIZE should
112 be a number followed by an optional suffix: k, M, G, T (decimal),
113 Ki, Mi, Gi or Ti (binary). The default suffix is G. Use 0 for no
114 limit. The value is stored in a configuration file in the cache
115 directory and applies to all future compilations.
116 -X LEVEL, --recompress LEVEL
118 Recompress the cache to level LEVEL using the Zstandard algorithm.
119 The level can be an integer, with the same semantics as the
120 compression_level configuration option, or the special value
121 uncompressed for no compression. See CACHE COMPRESSION for more
122 information. This can potentionally take a long time since all
123 files in the cache need to be visited. Only files that are
124 currently compressed with a different level than LEVEL will be
125 recompressed.
126 -o KEY=VALUE, --set-config KEY=VALUE
128 Set configuration option KEY to VALUE. See CONFIGURATION for more
129 information.
130 -x, --show-compression
132 Print cache compression statistics. See CACHE COMPRESSION for more
133 information. This can potentionally take a long time since all
134 files in the cache need to be visited.
135 -p, --show-config
137 Print current configuration options and from where they originate
138 (environment variable, configuration file or compile-time default)
139 in human-readable format.
140 --show-log-stats
142 Print statistics counters from the stats log in human-readable
143 format. See stats_log. Use -v/--verbose once or twice for more
144 details.
145 -s, --show-stats
147 Print a summary of configuration and statistics counters in
148 human-readable format. Use -v/--verbose once or twice for more
149 details.
150 -v, --verbose
152 Increase verbosity. The option can be given multiple times.
153 -V, --version
155 Print version and copyright information.
156 -z, --zero-stats
158 Zero the cache statistics (but not the configuration options).
159 Options for remote file-based storage
161 --trim-dir PATH
162 Remove old files from directory PATH until it is at most the size
163 specified by --trim-max-size.
164 Warning
166 Don’t use this option to trim the local cache. To trim the
167 local cache directory to a certain size, use
168 CCACHE_MAXSIZE=SIZE ccache -c.
169 --trim-max-size SIZE
171 Specify the maximum size for --trim-dir. SIZE should be a number
172 followed by an optional suffix: k, M, G, T (decimal), Ki, Mi, Gi or
173 Ti (binary). The default suffix is G.
174 --trim-method METHOD
176 Specify the method to trim a directory with --trim-dir. Possible
177 values are:
178 atime
180 LRU (least recently used) using the file access timestamp. This
181 is the default.
182 mtime
184 LRU (least recently used) using the file modification
185 timestamp.
186 Options for scripting or debugging
188 --checksum-file PATH
189 Print the checksum (128 bit XXH3) of the file at PATH (- for
190 standard input).
191 --extract-result PATH
193 Extract data stored in the result file at PATH (- for standard
194 input). The data will be written to ccache-result.* files in to the
195 current working directory. This option is only useful when
196 debugging ccache and its behavior.
197 -k KEY, --get-config KEY
199 Print the value of configuration option KEY. See CONFIGURATION for
200 more information.
201 --hash-file PATH
203 Print the hash (160 bit BLAKE3) of the file at PATH (- for standard
204 input). This is only useful when debugging ccache and its behavior.
205 --inspect PATH
207 Print the content of a result or manifest file at PATH (- for
208 standard input) to standard output in human-readable format. File
209 content embedded in a result file will however not be printed; use
210 --extract-result to extract the file content. This option is only
211 useful when debugging ccache and its behavior.
212 --print-stats
214 Print statistics counter IDs and corresponding values in
215 machine-parsable (tab-separated) format.
216 Extra options
218 When run as a compiler, ccache usually just takes the same command line
219 options as the compiler you are using. The only exception to this is
220 the option --ccache-skip. That option can be used to tell ccache to
221 avoid interpreting the next option in any way and to pass it along to
222 the compiler as-is.
223 Note
225 --ccache-skip currently only tells ccache not to interpret the next
226 option as a special compiler option — the option will still be
227 included in the direct mode hash.
228 The reason this can be important is that ccache does need to parse the
230 command line and determine what is an input filename and what is a
231 compiler option, as it needs the input filename to determine the name
232 of the resulting object file (among other things). The heuristic ccache
233 uses when parsing the command line is that any argument that exists as
234 a file is treated as an input file name. By using --ccache-skip you can
235 force an option to not be treated as an input file name and instead be
236 passed along to the compiler as a command line option.
237 Another case where --ccache-skip can be useful is if ccache interprets
239 an option specially but shouldn’t, since the option has another meaning
240 for your compiler than what ccache thinks.
241
243 Ccache’s default behavior can be overridden by options in configuration
244 files, which in turn can be overridden by environment variables with
245 names starting with CCACHE_. Ccache normally reads configuration from
246 two files: first a system-level configuration file and secondly a
247 cache-specific configuration file. The priorities of configuration
248 options are as follows (where 1 is highest):
249 1. Environment variables.
251 2. The cache-specific configuration file (see below).
253 3. The system (read-only) configuration file <sysconfdir>/ccache.conf
255 (typically /etc/ccache.conf or /usr/local/etc/ccache.conf).
256 4. Compile-time defaults.
258 As a special case, if the environment variable CCACHE_CONFIGPATH is set
260 it specifies the configuration file, and the system configuration file
261 won’t be read.
262 Location of the configuration file
264 The location of the cache-specific configuration file is determined
265 like this on non-Windows systems:
266 1. If CCACHE_CONFIGPATH is set, use that path.
268 2. Otherwise, if the environment variable CCACHE_DIR is set then use
270 $CCACHE_DIR/ccache.conf.
271 3. Otherwise, if cache_dir is set in the system configuration file
273 then use <cache_dir>/ccache.conf.
274 4. Otherwise, if there is a legacy $HOME/.ccache directory then use
276 $HOME/.ccache/ccache.conf.
277 5. Otherwise, if XDG_CONFIG_HOME is set then use
279 $XDG_CONFIG_HOME/ccache/ccache.conf.
280 6. Otherwise, use $HOME/Library/Preferences/ccache/ccache.conf (macOS)
282 or $HOME/.config/ccache/ccache.conf (other systems).
283 On Windows, this is the method used to find the configuration file:
285 1. If CCACHE_CONFIGPATH is set, use that path.
287 2. Otherwise, if the environment variable CCACHE_DIR is set then use
289 %CCACHE_DIR%/ccache.conf.
290 3. Otherwise, if cache_dir is set in the system configuration file
292 then use <cache_dir>\ccache.conf. The system-wide configuration on
293 Windows is %ALLUSERSPROFILE%\ccache\ccache.conf by default. The
294 ALLUSERSPROFILE environment variable is usually C:\ProgramData.
295 4. Otherwise, if there is a legacy %USERPROFILE%\.ccache directory
297 then use %USERPROFILE%\.ccache\ccache.conf.
298 5. Otherwise, use %LOCALAPPDATA%\ccache\ccache.conf if it exists.
300 6. Otherwise, use %APPDATA%\ccache\ccache.conf.
302 See also the cache_dir configuration option for how the cache directory
304 location is determined.
305 Configuration file syntax
307 Configuration files are in a simple “key = value” format, one option
308 per line. Lines starting with a hash sign are comments. Blank lines are
309 ignored, as is whitespace surrounding keys and values. Example:
310 # Set maximum cache size to 10 GB:
312 max_size = 10G
313 Boolean values
315 Some configuration options are boolean values (i.e. truth values). In a
316 configuration file, such values must be set to the string true or
317 false. For the corresponding environment variables, the semantics are a
318 bit different:
319 • A set environment variable means “true” (even if set to the empty
321 string).
322 • The following case-insensitive negative values are considered an
324 error (instead of surprising the user): 0, false, disable and no.
325 • An unset environment variable means “false”.
327 Each boolean environment variable also has a negated form starting with
329 CCACHE_NO. For example, CCACHE_COMPRESS can be set to force compression
330 and CCACHE_NOCOMPRESS can be set to force no compression.
331 Configuration options
333 Below is a list of available configuration options. The corresponding
334 environment variable name is indicated in parentheses after each
335 configuration option key.
336 absolute_paths_in_stderr (CCACHE_ABSSTDERR)
338 This option specifies whether ccache should rewrite relative paths
339 in the compiler’s standard error output to absolute paths. This can
340 be useful if you use base_dir with a build system (e.g. CMake with
341 the "Unix Makefiles" generator) that executes the compiler in a
342 different working directory, which makes relative paths in compiler
343 errors or warnings incorrect. The default is false.
344 base_dir (CCACHE_BASEDIR)
346 This option should be an absolute path to a directory. If set,
347 ccache will rewrite absolute paths into paths relative to the
348 current working directory, but only absolute paths that begin with
349 base_dir. Cache results can then be shared for compilations in
350 different directories even if the project uses absolute paths in
351 the compiler command line. See also the discussion under COMPILING
352 IN DIFFERENT DIRECTORIES. If set to the empty string (which is the
353 default), no rewriting is done.
354 A typical path to use as base_dir is your home directory or another
356 directory that is a parent of your project directories. Don’t use /
357 as the base directory since that will make ccache also rewrite
358 paths to system header files, which typically is contraproductive.
359 For example, say that Alice’s current working directory is
361 /home/alice/project1/build and that she compiles like this:
362 ccache gcc -I/usr/include/example -I/home/alice/project2/include -c /home/alice/project1/src/example.c
364 Here is what ccache will actually execute for different base_dir
366 values:
367 # Current working directory: /home/alice/project1/build
369 # With base_dir = /:
371 gcc -I../../../../usr/include/example -I../../project2/include -c ../src/example.c
372 # With base_dir = /home or /home/alice:
374 gcc -I/usr/include/example -I../../project2/include -c ../src/example.c
375 # With base_dir = /home/alice/project1 or /home/alice/project1/src:
377 gcc -I/usr/include/example -I/home/alice/project2/include -c ../src/example.c
378 If Bob has put project1 and project2 in /home/bob/stuff and both
380 users have set base_dir to /home or /home/$USER, then Bob will get
381 a cache hit (if they share ccache directory) since the actual
382 command line will be identical to that of Alice:
383 # Current working directory: /home/bob/stuff/project1/build
385 # With base_dir = /home or /home/bob:
387 gcc -I/usr/include/example -I../../project2/include -c ../src/example.c
388 Without base_dir there will be a cache miss since the absolute
390 paths will differ. With base_dir set to / there will be a cache
391 miss since the relative path to /usr/include/example will be
392 different. With base_dir set to /home/bob/stuff/project1 there will
393 a cache miss since the path to project2 will be a different
394 absolute path.
395 Warning
397 Rewriting absolute paths to relative is kind of a brittle hack.
398 It works OK in many cases, but there might be cases where
399 things break. One known issue is that absolute paths are not
400 reproduced in dependency files, which can mess up dependency
401 detection in tools like Make and Ninja. If possible, use
402 relative paths in the first place instead instead of using
403 base_dir.
404 cache_dir (CCACHE_DIR)
406 This option specifies where ccache will keep its cached compiler
407 outputs.
408 On non-Windows systems, the default is $HOME/.ccache if such a
410 directory exists, otherwise $XDG_CACHE_HOME/ccache if
411 XDG_CACHE_HOME is set, otherwise $HOME/Library/Caches/ccache
412 (macOS) or $HOME/.config/ccache (other systems).
413 On Windows, the default is %USERPROFILE%\.ccache if such a
415 directory exists, otherwise %LOCALAPPDATA%\ccache.
416 Warning
418 Previous ccache versions defaulted to storing the cache in
419 %APPDATA%\ccache on Windows. This can result in large network
420 file transfers of the cache in domain environments and similar
421 problems. Please check this directory for cache directories and
422 either delete them or the whole directory, or move them to the
423 %LOCALAPPDATA%\ccache directory.
424 See also Location of the configuration file.
426 compiler (CCACHE_COMPILER or (deprecated) CCACHE_CC)
428 This option can be used to force the name of the compiler to use.
429 If set to the empty string (which is the default), ccache works it
430 out from the command line.
431 compiler_check (CCACHE_COMPILERCHECK)
433 By default, ccache includes the modification time (“mtime”) and
434 size of the compiler in the hash to ensure that results retrieved
435 from the cache are accurate. If compiler plugins are used, these
436 plugins will also be added to the hash. This option can be used to
437 select another strategy. Possible values are:
438 content
440 Hash the content of the compiler binary. This makes ccache very
441 slightly slower compared to mtime, but makes it cope better
442 with compiler upgrades during a build bootstrapping process.
443 mtime
445 Hash the compiler’s mtime and size, which is fast. This is the
446 default.
447 none
449 Don’t hash anything. This may be good for situations where you
450 can safely use the cached results even though the compiler’s
451 mtime or size has changed (e.g. if the compiler is built as
452 part of your build system and the compiler’s source has not
453 changed, or if the compiler only has changes that don’t affect
454 code generation). You should only use none if you know what you
455 are doing.
456 string:value
458 Hash value. This can for instance be a compiler revision number
459 or another string that the build system generates to identify
460 the compiler.
461 a command string
463 Hash the standard output and standard error output of the
464 specified command. The string will be split on whitespace to
465 find out the command and arguments to run. No other
466 interpretation of the command string will be done, except that
467 the special word %compiler% will be replaced with the path to
468 the compiler. Several commands can be specified with semicolon
469 as separator. Examples:
470 %compiler% -v
472 %compiler% -dumpmachine; %compiler% -dumpversion
474 You should make sure that the specified command is as fast as
476 possible since it will be run once for each ccache invocation.
477 Identifying the compiler using a command is useful if you want
479 to avoid cache misses when the compiler has been rebuilt but
480 not changed.
481 Another case is when the compiler (as seen by ccache) actually
483 isn’t the real compiler but another compiler wrapper — in that
484 case, the default mtime method will hash the mtime and size of
485 the other compiler wrapper, which means that ccache won’t be
486 able to detect a compiler upgrade. Using a suitable command to
487 identify the compiler is thus safer, but it’s also slower, so
488 you should consider continue using the mtime method in
489 combination with the prefix_command option if possible. See
490 USING CCACHE WITH OTHER COMPILER WRAPPERS.
491 compiler_type (CCACHE_COMPILERTYPE)
493 Ccache normally guesses the compiler type based on the compiler
494 name. The compiler_type option lets you force a compiler type. This
495 can be useful if the compiler has a non-standard name but is
496 actually one of the known compiler types. Possible values are:
497 auto
499 Guess one of the types below based on the compiler name
500 (following symlinks). This is the default.
501 clang
503 Clang-based compiler.
504 clang-cl
506 clang-cl.
507 gcc
509 GCC-based compiler.
510 icl
512 Intel compiler on Windows.
513 msvc
515 Microsoft Visual C++ (MSVC).
516 nvcc
518 NVCC (CUDA) compiler.
519 other
521 Any compiler other than the known types.
522 compression (CCACHE_COMPRESS or CCACHE_NOCOMPRESS, see Boolean values
524 above)
525 If true, ccache will compress data it puts in the cache. However,
526 this option has no effect on how files are retrieved from the
527 cache; compressed and uncompressed results will still be usable
528 regardless of this option. The default is true.
529 Compression is done using the Zstandard algorithm. The algorithm is
531 fast enough that there should be little reason to turn off
532 compression to gain performance. One exception is if the cache is
533 located on a compressed file system, in which case the compression
534 performed by ccache of course is redundant.
535 Compression will be disabled if file cloning (the file_clone
537 option) or hard linking (the hard_link option) is enabled.
538 compression_level (CCACHE_COMPRESSLEVEL)
540 This option determines the level at which ccache will compress
541 object files using the real-time compression algorithm Zstandard.
542 It only has effect if compression is enabled (which it is by
543 default). Zstandard is extremely fast for decompression and very
544 fast for compression for lower compression levels. The default is
545 0.
546 Semantics of compression_level:
548 > 0
550 A positive value corresponds to normal Zstandard compression
551 levels. Lower levels (e.g. 1) mean faster compression but worse
552 compression ratio. Higher levels (e.g. 19) mean slower
553 compression but better compression ratio. The maximum possible
554 value depends on the libzstd version, but at least up to 19 is
555 available for all versions. Decompression speed is essentially
556 the same for all levels. As a rule of thumb, use level 5 or
557 lower since higher levels may slow down compilations
558 noticeably. Higher levels are however useful when recompressing
559 the cache with command line option -X/--recompress.
560 < 0
562 A negative value corresponds to Zstandard’s “ultra-fast”
563 compression levels, which are even faster than level 1 but with
564 less good compression ratios. For instance, level -3
565 corresponds to --fast=3 for the zstd command line tool. In
566 practice, there is little use for levels lower than -5 or so.
567 0 (default)
569 The value 0 means that ccache will choose a suitable level,
570 currently 1.
571 See the Zstandard documentation <http://zstd.net> for more
573 information.
574 cpp_extension (CCACHE_EXTENSION)
576 This option can be used to force a certain extension for the
577 intermediate preprocessed file. The default is to automatically
578 determine the extension to use for intermediate preprocessor files
579 based on the type of file being compiled, but that sometimes
580 doesn’t work. For example, when using the “aCC” compiler on HP-UX,
581 set the cpp extension to i.
582 debug (CCACHE_DEBUG or CCACHE_NODEBUG, see Boolean values above)
584 If true, enable the debug mode. The debug mode creates per-object
585 debug files that are helpful when debugging unexpected cache
586 misses. Note however that ccache performance will be reduced
587 slightly. See CACHE DEBUGGING for more information. The default is
588 false.
589 debug_dir (CCACHE_DEBUGDIR)
591 Specifies where to write per-object debug files if the debug mode
592 is enabled. If set to the empty string, the files will be written
593 next to the object file. If set to a directory, the debug files
594 will be written with full absolute paths in that directory,
595 creating it if needed. The default is the empty string.
596 For example, if debug_dir is set to /example, the current working
598 directory is /home/user and the object file is build/output.o then
599 the debug log will be written to
600 /example/home/user/build/output.o.ccache-log. See also CACHE
601 DEBUGGING.
602 depend_mode (CCACHE_DEPEND or CCACHE_NODEPEND, see Boolean values
604 above)
605 If true, the depend mode will be used. The default is false. See
606 The depend mode.
607 direct_mode (CCACHE_DIRECT or CCACHE_NODIRECT, see Boolean values
609 above)
610 If true, the direct mode will be used. The default is true. See The
611 direct mode.
612 disable (CCACHE_DISABLE or CCACHE_NODISABLE, see Boolean values above)
614 When true, ccache will just call the real compiler, bypassing the
615 cache completely. The default is false.
616 extra_files_to_hash (CCACHE_EXTRAFILES)
618 This option is a list of paths to files that ccache will include in
619 the the hash sum that identifies the build. The list separator is
620 semicolon on Windows systems and colon on other systems.
621 file_clone (CCACHE_FILECLONE or CCACHE_NOFILECLONE, see Boolean values
623 above)
624 If true, ccache will attempt to use file cloning (also known as
625 “copy on write”, “CoW” or “reflinks”) to store and fetch cached
626 compiler results. file_clone has priority over hard_link. The
627 default is false.
628 Files stored by cloning cannot be compressed, so the cache size
630 will likely be significantly larger if this option is enabled.
631 However, performance may be improved depending on the use case.
632 Unlike the hard_link option, file_clone is completely safe to use,
634 but not all file systems support the feature. For such file
635 systems, ccache will fall back to use plain copying (or hard links
636 if hard_link is enabled).
637 hard_link (CCACHE_HARDLINK or CCACHE_NOHARDLINK, see Boolean values
639 above)
640 If true, ccache will attempt to use hard links to store and fetch
641 cached object files. The default is false.
642 Files stored via hard links cannot be compressed, so the cache size
644 will likely be significantly larger if this option is enabled.
645 However, performance may be improved depending on the use case.
646 Warning
648 Do not enable this option unless you are aware of these
649 caveats:
650 • If the resulting file is modified, the file in the cache will
652 also be modified since they share content, which corrupts the
653 cache entry. As of version 4.0, ccache makes stored and fetched
654 object files read-only as a safety measure. Furthermore, a
655 simple integrity check is made for cached object files by
656 verifying that their sizes are correct. This means that
657 mistakes like strip file.o or echo >file.o will be detected
658 even if the object file is made writeable, but a modification
659 that doesn’t change the file size will not.
660 • Programs that don’t expect that files from two different
662 identical compilations are hard links to each other can fail.
663 • Programs that rely on modification times (like make) can be
665 confused if several users (or one user with several build
666 trees) use the same cache directory. The reason for this is
667 that the object files share i-nodes and therefore modification
668 times. If file.o is in build tree A (hard-linked from the
669 cache) and file.o then is produced by ccache in build tree B by
670 hard-linking from the cache, the modification timestamp will be
671 updated for file.o in build tree A as well. This can retrigger
672 relinking in build tree A even though nothing really has
673 changed.
674 hash_dir (CCACHE_HASHDIR or CCACHE_NOHASHDIR, see Boolean values above)
676 If true (which is the default), ccache will include the current
677 working directory (CWD) in the hash that is used to distinguish two
678 compilations when generating debug info (compiler option -g with
679 variations). Exception: The CWD will not be included in the hash if
680 base_dir is set (and matches the CWD) and the compiler option
681 -fdebug-prefix-map is used. See also the discussion under COMPILING
682 IN DIFFERENT DIRECTORIES.
683 The reason for including the CWD in the hash by default is to
685 prevent a problem with the storage of the current working directory
686 in the debug info of an object file, which can lead ccache to
687 return a cached object file that has the working directory in the
688 debug info set incorrectly.
689 You can disable this option to get cache hits when compiling the
691 same source code in different directories if you don’t mind that
692 CWD in the debug info might be incorrect.
693 ignore_headers_in_manifest (CCACHE_IGNOREHEADERS)
695 This option is a list of paths to files (or directories with
696 headers) that ccache will not include in the manifest list that
697 makes up the direct mode. Note that this can cause stale cache hits
698 if those headers do indeed change. The list separator is semicolon
699 on Windows systems and colon on other systems.
700 ignore_options (CCACHE_IGNOREOPTIONS)
702 This option is a space-delimited list of compiler options that
703 ccache will exclude from the hash. Excluding a compiler option from
704 the hash can be useful when you know it doesn’t affect the result
705 (but ccache doesn’t know that), or when it does and you don’t care.
706 If a compiler option in the list is suffixed with an asterisk (*)
707 it will be matched as a prefix. For example, -fmessage-length=*
708 will match both -fmessage-length=20 and -fmessage-length=70.
709 inode_cache (CCACHE_INODECACHE or CCACHE_NOINODECACHE, see Boolean
711 values above)
712 If true, ccache will cache source file hashes based on device,
713 inode and timestamps. This reduces the time spent on hashing
714 include files since the result can be resused between compilations.
715 The default is true. The feature requires temporary_dir to be
716 located on a local filesystem of a supported type.
717 Note
719 The inode cache feature is currently not available on Windows.
720 keep_comments_cpp (CCACHE_COMMENTS or CCACHE_NOCOMMENTS, see Boolean
722 values above)
723 If true, ccache will not discard the comments before hashing
724 preprocessor output. The default is false. This can be used to
725 check documentation with -Wdocumentation.
726 limit_multiple (CCACHE_LIMIT_MULTIPLE)
728 Sets the limit when cleaning up. Files are deleted (in LRU order)
729 until the levels are below the limit. The default is 0.8 (= 80%).
730 See Automatic cleanup for more information.
731 log_file (CCACHE_LOGFILE)
733 If set to a file path, ccache will write information on what it is
734 doing to the specified file. This is useful for tracking down
735 problems.
736 If set to syslog, ccache will log using syslog() instead of to a
738 file. If you use rsyslogd, you can add something like this to
739 /etc/rsyslog.conf or a file in /etc/rsyslog.d:
740 # log ccache to file
742 :programname, isequal, "ccache" /var/log/ccache
743 # remove from syslog
744 & ~
745 max_files (CCACHE_MAXFILES)
747 This option specifies the maximum number of files to keep in the
748 cache. Use 0 for no limit (which is the default). See also CACHE
749 SIZE MANAGEMENT.
750 max_size (CCACHE_MAXSIZE)
752 This option specifies the maximum size of the cache. Use 0 for no
753 limit. The default value is 5G. Available suffixes: k, M, G, T
754 (decimal) and Ki, Mi, Gi, Ti (binary). The default suffix is G. See
755 also CACHE SIZE MANAGEMENT.
756 msvc_dep_prefix (CCACHE_MSVC_DEP_PREFIX)
758 This option specifies the prefix of included files output for MSVC
759 compiler. The default prefix is “Note: including file:”. If you use
760 a localized compiler, this should be set accordingly.
761 namespace (CCACHE_NAMESPACE)
763 If set, the namespace string will be added to the hashed data for
764 each compilation. This will make the associated cache entries
765 logically separate from cache entries with other namespaces, but
766 they will still share the same storage space. Cache entries can
767 also be selectively removed from the local cache with the command
768 line option --evict-namespace, potentially in combination with
769 --evict-older-than.
770 For instance, if you use the same local cache for several disparate
772 projects, you can use a unique namespace string for each one. This
773 allows you to remove cache entries that belong to a certain project
774 if you stop working with that project.
775 path (CCACHE_PATH)
777 If set, ccache will search directories in this list when looking
778 for the real compiler. The list separator is semicolon on Windows
779 systems and colon on other systems. If not set, ccache will look
780 for the first executable matching the compiler name in the normal
781 PATH that isn’t a symbolic link to ccache itself.
782 pch_external_checksum (CCACHE_PCH_EXTSUM or CCACHE_NOPCH_EXTSUM, see
784 Boolean values above)
785 When this option is set, and ccache finds a precompiled header
786 file, ccache will look for a file with the extension “.sum” added
787 (e.g. “pre.h.gch.sum”), and if found, it will hash this file
788 instead of the precompiled header itself to work around the
789 performance penalty of hashing very large files.
790 prefix_command (CCACHE_PREFIX)
792 This option adds a list of prefixes (separated by space) to the
793 command line that ccache uses when invoking the compiler. See also
794 USING CCACHE WITH OTHER COMPILER WRAPPERS.
795 prefix_command_cpp (CCACHE_PREFIX_CPP)
797 This option adds a list of prefixes (separated by space) to the
798 command line that ccache uses when invoking the preprocessor.
799 read_only (CCACHE_READONLY or CCACHE_NOREADONLY, see Boolean values
801 above)
802 If true, ccache will attempt to use existing cached results, but it
803 will not add new results to any cache backend. Statistics counters
804 will still be updated, though, unless the stats option is set to
805 false.
806 If you are using this because your ccache directory is read-only,
808 you need to set temporary_dir since ccache will fail to create
809 temporary files otherwise. You may also want to set stats to false
810 make ccache not even try to update stats files.
811 read_only_direct (CCACHE_READONLY_DIRECT or CCACHE_NOREADONLY_DIRECT,
813 see Boolean values above)
814 Just like read_only except that ccache will only try to retrieve
815 results from the cache using the direct mode, not the preprocessor
816 mode. See documentation for read_only regarding using a read-only
817 ccache directory.
818 recache (CCACHE_RECACHE or CCACHE_NORECACHE, see Boolean values above)
820 If true, ccache will not use any previously stored result. New
821 results will still be cached, possibly overwriting any pre-existing
822 results.
823 remote_only (CCACHE_REMOTE_ONLY or CCACHE_NOREMOTE_ONLY, see Boolean
825 values above)
826 If true, ccache will only use remote storage. The default is false.
827 Note that cache statistics counters will still be kept in the local
828 cache directory unless stats is false. See also Storage
829 interaction.
830 remote_storage (CCACHE_REMOTE_STORAGE)
832 This option specifies one or several storage backends (separated by
833 space) to query after checking the local cache (unless remote_only
834 is true). See [Remote storage backends] for documentation of syntax
835 and available backends.
836 Examples:
838 • file:/shared/nfs/directory
840 • file:///shared/nfs/one|read-only file:///shared/nfs/two
842 • http://example.com/cache
844 • redis://example.com
846 Note
848 In previous ccache versions this option was called
849 secondary_storage (CCACHE_SECONDARY_STORAGE), which can
850 still be used as an alias.
851 reshare (CCACHE_RESHARE or CCACHE_NORESHARE, see Boolean values above)
853 If true, ccache will write results to remote storage even for local
854 storage cache hits. The default is false.
855 run_second_cpp (CCACHE_CPP2 or CCACHE_NOCPP2, see Boolean values above)
857 If true, ccache will first run the preprocessor to preprocess the
858 source code (see The preprocessor mode) and then on a cache miss
859 run the compiler on the source code to get hold of the object file.
860 This is the default.
861 If false, ccache will first run preprocessor to preprocess the
863 source code and then on a cache miss run the compiler on the
864 preprocessed source code instead of the original source code. This
865 makes cache misses slightly faster since the source code only has
866 to be preprocessed once. The downside is that some compilers won’t
867 produce the same result (for instance diagnostics warnings) when
868 compiling preprocessed source code.
869 A solution to the above mentioned downside is to set run_second_cpp
871 to false and pass -fdirectives-only (for GCC) or -frewrite-includes
872 (for Clang) to the compiler. This will cause the compiler to leave
873 the macros and other preprocessor information, and only process the
874 #include directives. When run in this way, the preprocessor
875 arguments will be passed to the compiler since it still has to do
876 some preprocessing (like macros).
877 This option is ignored with MSVC, as there is no way to make it
879 compile without preprocessing first.
880 sloppiness (CCACHE_SLOPPINESS)
882 By default, ccache tries to give as few false cache hits as
883 possible. However, in certain situations it’s possible that you
884 know things that ccache can’t take for granted. This option makes
885 it possible to tell ccache to relax some checks in order to
886 increase the hit rate. The value should be a comma-separated string
887 with one or several of the following values:
888 clang_index_store
890 Ignore the Clang compiler option -index-store-path and its
891 argument when computing the manifest hash. This is useful if
892 you use Xcode, which uses an index store path derived from the
893 local project path. Note that the index store won’t be updated
894 correctly on cache hits if you enable this sloppiness.
895 file_stat_matches
897 Ccache normally examines a file’s contents to determine whether
898 it matches the cached version. With this sloppiness set, ccache
899 will consider a file as matching its cached version if the
900 mtimes and ctimes match.
901 file_stat_matches_ctime
903 Ignore ctimes when file_stat_matches is enabled. This can be
904 useful when backdating files' mtimes in a controlled way.
905 gcno_cwd
907 By default, ccache will include the current working directory
908 in the hash when producing a .gcno file (when compiling with
909 -ftest-coverage or --coverage). This is because GCC 9+ includes
910 the current working directory in the .gcno file. The gcno_cwd
911 sloppiness makes ccache not hash the current working directory
912 so that you can get cache hits when compiling in different
913 directories, with the tradeoff of potentially getting an
914 incorrect directory in the .gcno file. gcno_cwd also disables
915 hashing of the current working directory if -fprofile-abs-path
916 is used.
917 include_file_ctime
919 By default, ccache will disable the direct mode if an include
920 file has too new ctime. This sloppiness disables that check.
921 See also [Handling of newly created header files].
922 include_file_mtime
924 By default, ccache will disable the direct mode if an include
925 file has too new mtime. This sloppiness disables that check.
926 See also [Handling of newly created header files].
927 ivfsoverlay
929 Ignore the Clang compiler option -ivfsoverlay and its argument.
930 This is useful if you use Xcode, which uses a virtual file
931 system (VFS) for things like combining Objective-C and Swift
932 code.
933 locale
935 Ccache includes the environment variables LANG, LC_ALL,
936 LC_CTYPE and LC_MESSAGES in the hash by default since they may
937 affect localization of compiler warning messages. Set this
938 sloppiness to tell ccache not to do that.
939 modules
941 By default, ccache will not cache compilations if -fmodules is
942 used since it cannot hash the state of compiler’s internal
943 representation of relevant modules. This sloppiness allows
944 caching in such a case. See C++ MODULES for more information.
945 pch_defines
947 Be sloppy about #define directives when precompiling a header
948 file. See PRECOMPILED HEADERS for more information.
949 random_seed
951 Ignore the -frandom-seed option and its arguments when
952 computing the input hash. This is useful if your build system
953 generates different seeds between builds and you are OK with
954 reusing cached results.
955 system_headers
957 By default, ccache will also include all system headers in the
958 manifest. With this sloppiness set, ccache will only include
959 system headers in the hash but not add the system header files
960 to the list of include files.
961 time_macros
963 Ignore __DATE__, __TIME__ and __TIMESTAMP__ being present in
964 the source code.
965 See the discussion under TROUBLESHOOTING for more information.
967 stats (CCACHE_STATS or CCACHE_NOSTATS, see Boolean values above)
969 If true, ccache will update the statistics counters on each
970 compilation. The default is true.
971 stats_log (CCACHE_STATSLOG)
973 If set to a file path, ccache will write statistics counter updates
974 to the specified file. This is useful for getting statistics for
975 individual builds. To show a summary of the current stats log, use
976 ccache --show-log-stats.
977 Note
979 Lines in the stats log starting with a hash sign (#) are
980 comments.
981 temporary_dir (CCACHE_TEMPDIR)
983 This option specifies where ccache will put temporary files. The
984 default is $XDG_RUNTIME_DIR/ccache-tmp (typically
985 /run/user/<UID>/ccache-tmp) if XDG_RUNTIME_DIR is set and the
986 directory exists, otherwise <cache_dir>/tmp.
987 Note
989 In previous versions of ccache, CCACHE_TEMPDIR had to be on the
990 same filesystem as the CCACHE_DIR path, but this requirement
991 has been relaxed.
992 umask (CCACHE_UMASK)
994 This option (an octal integer) specifies the umask for files and
995 directories in the cache directory. This is mostly useful when you
996 wish to share your cache with other users.
997
999 The remote_storage option lets you configure ccache to use one or
1000 several remote storage backends. By default, the local cache directory
1001 located in cache_dir will be queried first and remote storage second,
1002 but remote_only can be set to true to disable local storage. Note that
1003 cache statistics counters will still be kept in the local cache
1004 directory — remote storage backends only store compilation results and
1005 manifests.
1006 A remote storage backend is specified with a URL, optionally followed
1008 by a pipe (|) and a pipe-separated list of attributes. An attribute is
1009 key=value or just key as a short form of key=true. Attribute values
1010 must be percent-encoded
1011 <https://en.wikipedia.org/wiki/Percent-encoding> if they contain
1012 percent, pipe or space characters.
1013 Attributes for all backends
1015 These optional attributes are available for all remote storage
1016 backends:
1017 • read-only: If true, only read from this backend, don’t write. The
1019 default is false.
1020 • shards: A comma-separated list of names for sharding (partitioning)
1022 the cache entries using Rendezvous hashing
1023 <https://en.wikipedia.org/wiki/Rendezvous_hashing>, typically to
1024 spread the cache over a server cluster. When set, the storage URL
1025 must contain an asterisk (*), which will be replaced by one of the
1026 shard names to form a real URL. A shard name can optionally have an
1027 appended weight within parentheses to indicate how much of the key
1028 space should be associated with that shard. A shard with weight w
1029 will contain w/S of the cache, where S is the sum of all shard
1030 weights. A weight could for instance be set to represent the
1031 available memory for a memory cache on a specific server. The
1032 default weight is 1.
1033 Examples:
1035 • redis://cache-*.example.com|shards=a(3),b(1),c(1.5) will put
1037 55% (3/5.5) of the cache on redis://cache-a.example.com, 18%
1038 (1/5.5) on redis://cache-b.example.com and 27% (1.5/5.5) on
1039 redis://cache-c.example.com.
1040 • http://example.com/*|shards=alpha,beta will put 50% of the
1042 cache on http://example.com/alpha and 50% on
1043 http://example.com/beta.
1044 Storage interaction
1046 The table below describes the interaction between local and remote
1047 storage on cache hits and misses if remote_only is false (which is the
1048 default):
1049 ┌──────────────┬────────────────┬───────────────────┐
1051 │ │ │ │
1052 │Local storage │ Remote storage │ What happens │
1053 ├──────────────┼────────────────┼───────────────────┤
1054 │ │ │ │
1055 │miss │ miss │ Compile, write to │
1056 │ │ │ local, write to │
1057 │ │ │ remote[1] │
1058 ├──────────────┼────────────────┼───────────────────┤
1059 │ │ │ │
1060 │miss │ hit │ Read from remote, │
1061 │ │ │ write to local │
1062 ├──────────────┼────────────────┼───────────────────┤
1063 │ │ │ │
1064 │hit │ - │ Read from local, │
1065 │ │ │ don’t write to │
1066 │ │ │ remote[2] │
1067 └──────────────┴────────────────┴───────────────────┘
1068 [1] Unless remote storage has attribute read-only=true.
1070 [2] Unless local storage is set to share its cache hits with the
1071 reshare option.
1072 If remote_only is true:
1074 ┌──────────────┬────────────────┬─────────────────────┐
1076 │ │ │ │
1077 │Local storage │ Remote storage │ What happens │
1078 ├──────────────┼────────────────┼─────────────────────┤
1079 │ │ │ │
1080 │- │ miss │ Compile, write to │
1081 │ │ │ remote, don’t write │
1082 │ │ │ to local │
1083 ├──────────────┼────────────────┼─────────────────────┤
1084 │ │ │ │
1085 │- │ hit │ Read from remote, │
1086 │ │ │ don’t write to │
1087 │ │ │ local │
1088 └──────────────┴────────────────┴─────────────────────┘
1089 File storage backend
1091 URL format: file:DIRECTORY or file://[HOST]DIRECTORY
1092 This backend stores data as separate files in a directory structure
1094 below DIRECTORY, similar (but not identical) to the local cache
1095 storage. A typical use case for this backend would be sharing a cache
1096 on an NFS directory. DIRECTORY must start with a slash. HOST can be the
1097 empty string or localhost. On Windows, HOST can also be the name of a
1098 server hosting a shared folder.
1099 Important
1101 ccache will not perform any cleanup of the storage — that has to be
1102 done by other means, for instance by running ccache --trim-dir
1103 periodically.
1104 Examples:
1106 • file:/shared/nfs/directory
1108 • file:///shared/nfs/directory|umask=002|update-mtime=true
1110 • file://example.com/shared/folder
1112 Optional attributes:
1114 • layout: How to store file under the cache directory. Available
1116 values:
1117 • flat: Store all files directly under the cache directory.
1119 • subdirs: Store files in 256 subdirectories of the cache
1121 directory.
1122 The default is subdirs.
1124 • umask: This attribute (an octal integer) overrides the umask to use
1126 for files and directories in the cache directory.
1127 • update-mtime: If true, update the modification time (mtime) of
1129 cache entries that are read. The default is false.
1130 HTTP storage backend
1132 URL format: http://HOST[:PORT][/PATH]
1133 This backend stores data in an HTTP-compatible server. The required
1135 HTTP methods are GET, PUT and DELETE.
1136 Important
1138 ccache will not perform any cleanup of the storage — that has to be
1139 done by other means, for instance by running ccache --trim-dir
1140 periodically.
1141 Note
1143 HTTPS is not supported.
1144 Tip
1146 See How to set up HTTP storage
1147 <https://ccache.dev/howto/http-storage.html> for hints on how to
1148 set up an HTTP server for use with ccache.
1149 Examples:
1151 • http://localhost
1153 • http://someusername:p4ssw0rd@example.com/cache/
1155 • http://localhost:8080|layout=bazel|connect-timeout=50
1157 Optional attributes:
1159 • bearer-token: Bearer token used to authorize the HTTP requests.
1161 • connect-timeout: Timeout (in ms) for network connection. The
1163 default is 100.
1164 • keep-alive: If true, keep the HTTP connection to the storage server
1166 open to avoid reconnects. The default is true.
1167 • layout: How to map key names to the path part of the URL. Available
1169 values:
1170 • bazel: Store values in a format compatible with the Bazel HTTP
1172 caching protocol. More specifically, the entries will be stored
1173 as 64 hex digits under the /ac/ part of the cache.
1174 Note
1176 You may have to disable verification of action cache values
1177 in the server for this to work since ccache entries are not
1178 valid action result metadata values.
1179 • flat: Append the key directly to the path part of the URL (with
1181 a leading slash if needed).
1182 • subdirs: Append the first two characters of the key to the URL
1184 (with a leading slash if needed), followed by a slash and the
1185 rest of the key. This divides the entries into 256 buckets.
1186 The default is subdirs.
1188 • operation-timeout: Timeout (in ms) for HTTP requests. The default
1190 is 10000.
1191 Redis storage backend
1193 URL formats:
1194 redis://[[USERNAME:]PASSWORD@]HOST[:PORT][/DBNUMBER]
1196 redis+unix:SOCKET_PATH[?db=DBNUMBER]
1197 redis+unix://[[USERNAME:]PASSWORD@localhost]SOCKET_PATH[?db=DBNUMBER]
1198 This backend stores data in a Redis <https://redis.io> (or
1200 Redis-compatible) server. There are implementations for both
1201 memory-based and disk-based storage. PORT defaults to 6379 and DBNUMBER
1202 defaults to 0.
1203 Note
1205 ccache will not perform any cleanup of the Redis storage, but you
1206 can configure LRU eviction <https://redis.io/topics/lru-cache>.
1207 Tip
1209 See How to set up Redis
1210 <https://ccache.dev/howto/redis-storage.html> storage" for hints
1211 on setting up a Redis server for use with ccache.
1212 Tip
1214 You can set up a cluster of Redis servers using the shards
1215 attribute described in REMOTE STORAGE BACKENDS.
1216 Examples:
1218 • redis://localhost
1220 • redis://p4ssw0rd@cache.example.com:6379/0|connect-timeout=50
1222 • redis+unix:/run/redis.sock
1224 • redis+unix:///run/redis.sock
1226 • redis+unix://p4ssw0rd@localhost/run/redis.sock?db=0
1228 Optional attributes:
1230 • connect-timeout: Timeout (in ms) for network connection. The
1232 default is 100.
1233 • operation-timeout: Timeout (in ms) for Redis commands. The default
1235 is 10000.
1236
1238 By default, ccache has a 5 GB limit on the total size of files in the
1239 cache and no limit on the number of files. You can set different limits
1240 using the command line options -M/--max-size and -F/--max-files. Use
1241 the -s/--show-stats option to see the cache size and the currently
1242 configured limits (in addition to other various statistics).
1243 Cleanup can be triggered in two different ways: automatic and manual.
1245 Automatic cleanup
1247 Ccache maintains counters for various statistics about the cache,
1248 including the size and number of all cached files. In order to improve
1249 performance and reduce issues with concurrent ccache invocations, there
1250 is one statistics file for each of the sixteen subdirectories in the
1251 cache.
1252 After a new compilation result has been written to the cache, ccache
1254 will update the size and file number statistics for the subdirectory
1255 (one of sixteen) to which the result was written. Then, if the size
1256 counter for said subdirectory is greater than max_size / 16 or the file
1257 number counter is greater than max_files / 16, automatic cleanup is
1258 triggered.
1259 When automatic cleanup is triggered for a subdirectory in the cache,
1261 ccache will:
1262 1. Count all files in the subdirectory and compute their aggregated
1264 size.
1265 2. Remove files in LRU (least recently used) order until the size is
1267 at most limit_multiple * max_size / 16 and the number of files is
1268 at most limit_multiple * max_files / 16, where limit_multiple,
1269 max_size and max_files are configuration options.
1270 3. Set the size and file number counters to match the files that were
1272 kept.
1273 The reason for removing more files than just those needed to not exceed
1275 the max limits is that a cleanup is a fairly slow operation, so it
1276 would not be a good idea to trigger it often, like after each cache
1277 miss.
1278 The LRU cleanup makes use of the file modification time (mtime) of
1280 cache entries; ccache updates mtime of the cache entries read on a
1281 cache hit to mark them as "recently used".
1282 Manual cleanup
1284 You can run ccache -c/--cleanup to force cleanup of the whole cache,
1285 i.e. all of the sixteen subdirectories. This will recalculate the
1286 statistics counters and make sure that the configuration options
1287 max_size and max_files are not exceeded. Note that limit_multiple is
1288 not taken into account for manual cleanup.
1289
1291 Ccache will by default compress all data it puts into the cache using
1292 the compression algorithm Zstandard <http://zstd.net> (zstd) using
1293 compression level 1. The algorithm is fast enough that there should be
1294 little reason to turn off compression to gain performance. One
1295 exception is if the cache is located on a compressed file system, in
1296 which case the compression performed by ccache of course is redundant.
1297 See the documentation for the configuration options compression and
1298 compression_level for more information.
1299 You can use the command line option -x/--show-compression to print
1301 information related to compression. Example:
1302 Total data: 14.8 GB (16.0 GB disk blocks)
1304 Compressed data: 11.3 GB (30.6% of original size)
1305 Original size: 36.9 GB
1306 Compression ratio: 3.267 x (69.4% space savings)
1307 Incompressible data: 3.5 GB
1308 Notes:
1310 • The “disk blocks” size is the cache size when taking disk block
1312 size into account. This value should match the “Cache size” value
1313 from “ccache --show-stats”. The other size numbers refer to actual
1314 content sizes.
1315 • “Compressed data” refers to result and manifest files stored in the
1317 cache.
1318 • “Incompressible data” refers to files that are always stored
1320 uncompressed (triggered by enabling file_clone or hard_link) or
1321 unknown files (for instance files created by older ccache
1322 versions).
1323 • The compression ratio is affected by compression_level.
1325 The cache data can also be recompressed to another compression level
1327 (or made uncompressed) with the command line option -X/--recompress. If
1328 you choose to disable compression by default or to use a low
1329 compression level, you can (re)compress newly cached data with a higher
1330 compression level after the build or at another time when there are
1331 more CPU cycles available, for instance every night. Full recompression
1332 potentially takes a lot of time, but only files that are currently
1333 compressed with a different level than the target level will be
1334 recompressed.
1335
1337 ccache --show-stats shows a summary of statistics, including cache
1338 size, cleanups (number of performed cleanups, either implicitly due to
1339 a cache size limit being reached or due to explicit ccache -c calls),
1340 overall hit rate, hit rate for direct/preprocessed modes and hit rate
1341 for local and remote storage.
1342 The summary also includes counters called “Errors” and “Uncacheable”,
1344 which are sums of more detailed counters. To see those detailed
1345 counters, use the -v/--verbose flag. The verbose mode can show the
1346 following counters:
1347 ┌───────────────────────────┬────────────────────────────┐
1349 │ │ │
1350 │Counter │ Description │
1351 ├───────────────────────────┼────────────────────────────┤
1352 │ │ │
1353 │Autoconf compile/link │ Uncacheable compilation or │
1354 │ │ linking by an Autoconf │
1355 │ │ test. │
1356 ├───────────────────────────┼────────────────────────────┤
1357 │ │ │
1358 │Bad compiler arguments │ Malformed compiler │
1359 │ │ argument, e.g. missing a │
1360 │ │ value for a compiler │
1361 │ │ option that requires an │
1362 │ │ argument or failure to │
1363 │ │ read a file specified by a │
1364 │ │ compiler option argument. │
1365 ├───────────────────────────┼────────────────────────────┤
1366 │ │ │
1367 │Called for linking │ The compiler was called │
1368 │ │ for linking, not │
1369 │ │ compiling. Ccache only │
1370 │ │ supports compilation of a │
1371 │ │ single file, i.e. calling │
1372 │ │ the compiler with the -c │
1373 │ │ option to produce a single │
1374 │ │ object file from a single │
1375 │ │ source file. │
1376 ├───────────────────────────┼────────────────────────────┤
1377 │ │ │
1378 │Called for preprocessing │ The compiler was called │
1379 │ │ for preprocessing, not │
1380 │ │ compiling. │
1381 ├───────────────────────────┼────────────────────────────┤
1382 │ │ │
1383 │Could not use modules │ Preconditions for using │
1384 │ │ C++ MODULES were not │
1385 │ │ fulfilled. │
1386 ├───────────────────────────┼────────────────────────────┤
1387 │ │ │
1388 │Could not use precompiled │ Preconditions for using │
1389 │header │ precompiled headers were │
1390 │ │ not fulfilled. │
1391 ├───────────────────────────┼────────────────────────────┤
1392 │ │ │
1393 │Could not write to output │ The output path specified │
1394 │file │ with -o could not be │
1395 │ │ written to. │
1396 ├───────────────────────────┼────────────────────────────┤
1397 │ │ │
1398 │Compilation failed │ The compilation failed. No │
1399 │ │ result stored in the │
1400 │ │ cache. │
1401 ├───────────────────────────┼────────────────────────────┤
1402 │ │ │
1403 │Compiler check failed │ A compiler check program │
1404 │ │ specified by │
1405 │ │ compiler_check │
1406 │ │ (CCACHE_COMPILERCHECK) │
1407 │ │ failed. │
1408 ├───────────────────────────┼────────────────────────────┤
1409 │ │ │
1410 │Compiler output file │ One of the files expected │
1411 │missing │ to be produced by the │
1412 │ │ compiler was missing after │
1413 │ │ compilation. │
1414 ├───────────────────────────┼────────────────────────────┤
1415 │ │ │
1416 │Compiler produced empty │ The compiler’s output file │
1417 │output │ (typically an object file) │
1418 │ │ was empty after │
1419 │ │ compilation. │
1420 ├───────────────────────────┼────────────────────────────┤
1421 │ │ │
1422 │Could not find the │ The compiler to execute │
1423 │compiler │ could not be found. │
1424 ├───────────────────────────┼────────────────────────────┤
1425 │ │ │
1426 │Error hashing extra file │ Failure reading a file │
1427 │ │ specified by │
1428 │ │ extra_files_to_hash │
1429 │ │ (CCACHE_EXTRAFILES). │
1430 ├───────────────────────────┼────────────────────────────┤
1431 │ │ │
1432 │Forced recache │ CCACHE_RECACHE was used to │
1433 │ │ overwrite an existing │
1434 │ │ result. │
1435 ├───────────────────────────┼────────────────────────────┤
1436 │ │ │
1437 │Internal error │ Unexpected failure, e.g. │
1438 │ │ due to problems │
1439 │ │ reading/writing the cache. │
1440 ├───────────────────────────┼────────────────────────────┤
1441 │ │ │
1442 │Missing cache file │ A file was unexpectedly │
1443 │ │ missing from the cache. │
1444 │ │ This only happens in rare │
1445 │ │ situations, e.g. if one │
1446 │ │ ccache instance is about │
1447 │ │ to get a file from the │
1448 │ │ cache while another │
1449 │ │ instance removed the file │
1450 │ │ as part of cache cleanup. │
1451 ├───────────────────────────┼────────────────────────────┤
1452 │ │ │
1453 │Multiple source files │ The compiler was called to │
1454 │ │ compile multiple source │
1455 │ │ files in one go. This is │
1456 │ │ not supported by ccache. │
1457 ├───────────────────────────┼────────────────────────────┤
1458 │ │ │
1459 │No input file │ No input file was │
1460 │ │ specified to the compiler. │
1461 ├───────────────────────────┼────────────────────────────┤
1462 │ │ │
1463 │Output to stdout │ The compiler was │
1464 │ │ instructed to write its │
1465 │ │ output to standard output │
1466 │ │ using -o -. This is not │
1467 │ │ supported by ccache. │
1468 ├───────────────────────────┼────────────────────────────┤
1469 │ │ │
1470 │Preprocessing failed │ Preprocessing the source │
1471 │ │ code using the compiler’s │
1472 │ │ -E option failed. │
1473 ├───────────────────────────┼────────────────────────────┤
1474 │ │ │
1475 │Unsupported code directive │ Code like the assembler │
1476 │ │ .incbin directive was │
1477 │ │ found. This is not │
1478 │ │ supported by ccache. │
1479 ├───────────────────────────┼────────────────────────────┤
1480 │ │ │
1481 │Unsupported compiler │ A compiler option not │
1482 │option │ supported by ccache was │
1483 │ │ found. │
1484 ├───────────────────────────┼────────────────────────────┤
1485 │ │ │
1486 │Unsupported environment │ An environment variable │
1487 │variable │ not supported by ccache │
1488 │ │ was set. │
1489 ├───────────────────────────┼────────────────────────────┤
1490 │ │ │
1491 │Unsupported source │ A source language e.g. │
1492 │language │ specified with -x was │
1493 │ │ unsupported by ccache. │
1494 └───────────────────────────┴────────────────────────────┘
1495
1497 The basic idea is to detect when you are compiling exactly the same
1498 code a second time and reuse the previously produced output. The
1499 detection is done by hashing different kinds of information that should
1500 be unique for the compilation and then using the hash sum to identify
1501 the cached output. Ccache uses BLAKE3, a very fast cryptographic hash
1502 algorithm, for the hashing. On a cache hit, ccache is able to supply
1503 all of the correct compiler outputs (including all warnings, dependency
1504 file, etc) from the cache. Data stored in the cache is checksummed with
1505 XXH3, an extremely fast non-cryptographic algorithm, to detect
1506 corruption.
1507 Ccache has two ways of gathering information used to look up results in
1509 the cache:
1510 • the preprocessor mode, where ccache runs the preprocessor on the
1512 source code and hashes the result
1513 • the direct mode, where ccache hashes the source code and include
1515 files directly
1516 The direct mode is generally faster since running the preprocessor has
1518 some overhead.
1519 If no previous result is detected (i.e., there is a cache miss) using
1521 the direct mode, ccache will fall back to the preprocessor mode unless
1522 the depend mode is enabled. In the depend mode, ccache never runs the
1523 preprocessor, not even on cache misses. Read more in The depend mode
1524 below.
1525 Common hashed information
1527 The following information is always included in the hash:
1528 • the extension used by the compiler for a file with preprocessor
1530 output (normally .i for C code and .ii for C++ code)
1531 • the compiler’s size and modification time (or other
1533 compiler-specific information specified by compiler_check)
1534 • the name of the compiler
1536 • the current directory (if hash_dir is enabled)
1538 • contents of files specified by extra_files_to_hash (if any)
1540 The preprocessor mode
1542 In the preprocessor mode, the hash is formed of the common information
1543 and:
1544 • the preprocessor output from running the compiler with -E
1546 • the command line options except those that affect include files
1548 (-I, -include, -D, etc; the theory is that these command line
1549 options will change the preprocessor output if they have any effect
1550 at all)
1551 • any standard error output generated by the preprocessor
1553 Based on the hash, the cached compilation result can be looked up
1555 directly in the cache.
1556 The direct mode
1558 In the direct mode, the hash is formed of the common information and:
1559 • the input source file
1561 • the compiler options
1563 Based on the hash, a data structure called “manifest” is looked up in
1565 the cache. The manifest contains:
1566 • references to cached compilation results (object file, dependency
1568 file, etc) that were produced by previous compilations that matched
1569 the hash
1570 • paths to the include files that were read at the time the
1572 compilation results were stored in the cache
1573 • hash sums of the include files at the time the compilation results
1575 were stored in the cache
1576 The current contents of the include files are then hashed and compared
1578 to the information in the manifest. If there is a match, ccache knows
1579 the result of the compilation. If there is no match, ccache falls back
1580 to running the preprocessor. The output from the preprocessor is parsed
1581 to find the include files that were read. The paths and hash sums of
1582 those include files are then stored in the manifest along with
1583 information about the produced compilation result.
1584 There is a catch with the direct mode: header files that were used by
1586 the compiler are recorded, but header files that were not used, but
1587 would have been used if they existed, are not. So, when ccache checks
1588 if a result can be taken from the cache, it currently can’t check if
1589 the existence of a new header file should invalidate the result. In
1590 practice, the direct mode is safe to use in the absolute majority of
1591 cases.
1592 The direct mode will be disabled if any of the following holds:
1594 • direct_mode is false
1596 • a modification time of one of the include files is too new (needed
1598 to avoid a race condition)
1599 • a compiler option not supported by the direct mode is used:
1601 • a -Wp,* compiler option other than -Wp,-MD,<path>,
1603 -Wp,-MMD,<path> and -Wp,-D<define>
1604 • -Xpreprocessor
1606 • the string __TIME__ is present in the source code
1608 The depend mode
1610 If the depend mode is enabled, ccache will not use the preprocessor at
1611 all. The hash used to identify results in the cache will be based on
1612 the direct mode hash described above plus information about include
1613 files read from the dependency list generated by MSVC with
1614 /showIncludes, or the dependency file generated by other compilers with
1615 -MD or -MMD.
1616 Advantages:
1618 • The ccache overhead of a cache miss will be much smaller.
1620 • Not running the preprocessor at all can be good if compilation is
1622 performed remotely, for instance when using distcc or similar;
1623 ccache then won’t make potentially costly preprocessor calls on the
1624 local machine.
1625 Disadvantages:
1627 • The cache hit rate will likely be lower since any change to
1629 compiler options or source code will make the hash different.
1630 Compare this with the default setup where ccache will fall back to
1631 the preprocessor mode, which is tolerant to some types of changes
1632 of compiler options and source code changes.
1633 • If -MD is used, the manifest entries will include system header
1635 files as well, thus slowing down cache hits slightly, just as using
1636 -MD slows down make. This is also the case for MSVC with
1637 /showIncludes.
1638 • If -MMD is used, the manifest entries will not include system
1640 header files, which means ccache will ignore changes in them.
1641 The depend mode will be disabled if any of the following holds:
1643 • depend_mode is false.
1645 • run_second_cpp is false.
1647 • The compiler is not generating dependencies using -MD or -MMD (for
1649 MSVC, /showIncludes is added automatically if not specified by the
1650 user).
1651
1653 If modification time (mtime) or status change time (ctime) of one of
1654 the include files is equal to (or newer than) the time compilation is
1655 being done, ccache disables the direct mode (or, in the case of a
1656 precompiled header, disables caching completely). This done as a safety
1657 measure to avoid a race condition (see below).
1658 To be able to use a newly created header files in direct mode (or use a
1660 newly precompiled header), either:
1661 • create the include file earlier in the build process, or
1663 • set sloppiness to include_file_ctime,include_file_mtime if you are
1665 willing to take the risk, for instance if you know that your build
1666 system is robust enough not to trigger the race condition.
1667 For reference, the race condition mentioned above consists of these
1669 events:
1670 1. The preprocessor is run.
1672 2. An include file is modified by someone.
1674 3. The new include file is hashed by ccache.
1676 4. The real compiler is run on the preprocessor’s output, which
1678 contains data from the old header file.
1679 5. The wrong object file is stored in the cache.
1681
1683 To find out what information ccache actually is hashing, you can enable
1684 the debug mode via the configuration option debug or by setting
1685 CCACHE_DEBUG in the environment. This can be useful if you are
1686 investigating why you don’t get cache hits. Note that performance will
1687 be reduced slightly.
1688 When the debug mode is enabled, ccache will create up to five
1690 additional files next to the object file:
1691 ┌───────────────────────────────────────────┬───────────────────────────┐
1693 │ │ │
1694 │Filename │ Description │
1695 ├───────────────────────────────────────────┼───────────────────────────┤
1696 │ │ │
1697 │<objectfile>.<timestamp>.ccache-input-c │ Binary input hashed by │
1698 │ │ both the direct mode and │
1699 │ │ the preprocessor mode. │
1700 ├───────────────────────────────────────────┼───────────────────────────┤
1701 │ │ │
1702 │<objectfile>.<timestamp>.ccache-input-d │ Binary input only hashed │
1703 │ │ by the direct mode. │
1704 ├───────────────────────────────────────────┼───────────────────────────┤
1705 │ │ │
1706 │<objectfile>.<timestamp>.ccache-input-p │ Binary input only hashed │
1707 │ │ by the preprocessor mode. │
1708 ├───────────────────────────────────────────┼───────────────────────────┤
1709 │ │ │
1710 │<objectfile>.<timestamp>.ccache-input-text │ Human-readable combined │
1711 │ │ diffable text version of │
1712 │ │ the three files above. │
1713 ├───────────────────────────────────────────┼───────────────────────────┤
1714 │ │ │
1715 │<objectfile>.<timestamp>.ccache-log │ Log for this object file. │
1716 └───────────────────────────────────────────┴───────────────────────────┘
1717 The timestamp format is
1719 `<year><month><day>_<hour><minute><second>_<microsecond>`.
1720 If debug_dir (environment variable CCACHE_DEBUGDIR) is set, the files
1722 above will be written to that directory with full absolute paths
1723 instead of next to the object file.
1724 In the direct mode, ccache uses the 160 bit BLAKE3 hash of the
1726 “ccache-input-c” + “ccache-input-d” data (where + means concatenation),
1727 while the “ccache-input-c” + “ccache-input-p” data is used in the
1728 preprocessor mode.
1729 The “ccache-input-text” file is a combined text version of the three
1731 binary input files. It has three sections (“COMMON”, “DIRECT MODE” and
1732 “PREPROCESSOR MODE”), which is turn contain annotations that say what
1733 kind of data comes next.
1734 To debug why you don’t get an expected cache hit for an object file,
1736 you can do something like this:
1737 1. Enable debug (CCACHE_DEBUG).
1739 2. Build.
1741 3. Clean and build again.
1743 4. Compare the <objectfile>.<timestamp>.ccache-input-text files for
1745 the two builds. This together with the
1746 <objectfile>.<timestamp>.ccache-log files should give you some
1747 clues about what is happening.
1748
1750 Some information included in the hash that identifies a unique
1751 compilation can contain absolute paths:
1752 • The preprocessed source code may contain absolute paths to include
1754 files if the compiler option -g is used or if absolute paths are
1755 given to -I and similar compiler options.
1756 • Paths specified by compiler options (such as -I, -MF, etc) on the
1758 command line may be absolute.
1759 • The source code file path may be absolute, and that path may
1761 substituted for __FILE__ macros in the source code or included in
1762 warnings emitted to standard error by the preprocessor.
1763 This means that if you compile the same code in different locations,
1765 you can’t share compilation results between the different build
1766 directories since you get cache misses because of the absolute build
1767 directory paths that are part of the hash.
1768 Here’s what can be done to enable cache hits between different build
1770 directories:
1771 • If you build with -g (or similar) to add debug information to the
1773 object file, you must either:
1774 • use the compiler option -fdebug-prefix-map=<old>=<new> for
1776 relocating debug info to a common prefix (e.g.
1777 -fdebug-prefix-map=$PWD=.); or
1778 • set hash_dir = false.
1780 • If you use absolute paths anywhere on the command line (e.g. the
1782 source code file path or an argument to compiler options like -I
1783 and -MF), you must set base_dir to an absolute path to a “base
1784 directory”. Ccache will then rewrite absolute paths under that
1785 directory to relative before computing the hash.
1786
1788 Ccache has support for precompiled headers with GCC and Clang. However,
1789 you have to do some things to make it work properly:
1790 • You must set sloppiness to pch_defines,time_macros. The reason is
1792 that ccache can’t tell whether __TIME__, __DATE__ or __TIMESTAMP__
1793 is used when using a precompiled header. Further, it can’t detect
1794 changes in #defines in the source code because of how preprocessing
1795 works in combination with precompiled headers.
1796 • You may also want to include include_file_mtime,include_file_ctime
1798 in sloppiness. See HANDLING OF NEWLY CREATED HEADER FILES.
1799 • You must either:
1801 • use the compiler option -include to include the precompiled
1803 header (i.e., don’t use #include in the source code to include
1804 the header; the filename itself must be sufficient to find the
1805 header, i.e. -I paths are not searched); or
1806 • (for the Clang compiler) use the compiler option -include-pch
1808 to include the PCH file generated from the precompiled header;
1809 or
1810 • (for the GCC compiler) add the compiler option -fpch-preprocess
1812 when compiling.
1813 • If you use Clang, you must compile with -fno-pch-timestamp.
1815 If you don’t do this, either the non-precompiled version of the header
1817 file will be used (if available) or ccache will fall back to running
1818 the real compiler and increase the statistics counter “Preprocessing
1819 failed” (if the non-precompiled header file is not available).
1820
1822 Ccache has support for Clang’s -fmodules option. In practice ccache
1823 only additionally hashes module.modulemap files; it does not know how
1824 Clang handles its cached binary form of modules so those are ignored.
1825 This should not matter in practice: as long as everything else
1826 (including module.modulemap files) is the same the cached result should
1827 work. Still, you must set sloppiness to modules to allow caching.
1828 You must use both direct mode and depend mode. When using the
1830 preprocessor mode Clang does not provide enough information to allow
1831 hashing of module.modulemap files.
1832
1834 A group of developers can increase the cache hit rate by sharing a
1835 local cache directory. To share a local cache without unpleasant side
1836 effects, the following conditions should to be met:
1837 • Use the same cache directory.
1839 • Make sure that the configuration option hard_link is false (which
1841 is the default).
1842 • Make sure that all users are in the same group.
1844 • Set the configuration option umask to 002. This ensures that cached
1846 files are accessible to everyone in the group.
1847 • Make sure that all users have write permission in the entire cache
1849 directory (and that you trust all users of the shared cache).
1850 • Make sure that the setgid bit is set on all directories in the
1852 cache. This tells the filesystem to inherit group ownership for new
1853 directories. The following command might be useful for this:
1854 find $CCACHE_DIR -type d | xargs chmod g+s
1856 The reason to avoid the hard link mode is that the hard links cause
1858 unwanted side effects, as all links to a cached file share the file’s
1859 modification timestamp. This results in false dependencies to be
1860 triggered by timestamp-based build systems whenever another user links
1861 to an existing file. Typically, users will see that their libraries and
1862 binaries are relinked without reason.
1863 You may also want to make sure that a base directory is set
1865 appropriately, as discussed in a previous section.
1866
1868 It is possible to put the cache directory on an NFS filesystem (or
1869 similar filesystems), but keep in mind that:
1870 • Having the cache on NFS may slow down compilation. Make sure to do
1872 some benchmarking to see if it’s worth it.
1873 • Ccache hasn’t been tested very thoroughly on NFS.
1875 A tip is to set temporary_dir to a directory on the local host to avoid
1877 NFS traffic for temporary files.
1878 It is recommended to use the same operating system version when using a
1880 shared cache. If operating system versions are different then system
1881 include files will likely be different and there will be few or no
1882 cache hits between the systems. One way of improving cache hit rate in
1883 that case is to set sloppiness to system_headers to ignore system
1884 headers.
1885 An alternative to putting the main cache directory on NFS is to set up
1887 a remote storage file cache.
1888
1890 The recommended way of combining ccache with another compiler wrapper
1891 (such as “distcc”) is by letting ccache execute the compiler wrapper.
1892 This is accomplished by defining prefix_command, for example by setting
1893 the environment variable CCACHE_PREFIX to the name of the wrapper (e.g.
1894 distcc). Ccache will then prefix the command line with the specified
1895 command when running the compiler. To specify several prefix commands,
1896 set prefix_command to a colon-separated list of commands.
1897 Unless you set compiler_check to a suitable command (see the
1899 description of that configuration option), it is not recommended to use
1900 the form ccache anotherwrapper compiler args as the compilation
1901 command. It’s also not recommended to use the masquerading technique
1902 for the other compiler wrapper. The reason is that by default, ccache
1903 will in both cases hash the mtime and size of the other wrapper instead
1904 of the real compiler, which means that:
1905 • Compiler upgrades will not be detected properly.
1907 • The cached results will not be shared between compilations with and
1909 without the other wrapper.
1910 Another minor thing is that if prefix_command is used, ccache will not
1912 invoke the other wrapper when running the preprocessor, which increases
1913 performance. You can use prefix_command_cpp if you also want to invoke
1914 the other wrapper when doing preprocessing (normally by adding -E).
1915
1917 • The direct mode fails to pick up new header files in some rare
1918 scenarios. See The direct mode above.
1919
1921 General
1922 A general tip for getting information about what ccache is doing is to
1923 enable debug logging by setting the configuration option debug (or the
1924 environment variable CCACHE_DEBUG); see CACHE DEBUGGING for more
1925 information. Another way of keeping track of what is happening is to
1926 check the output of ccache -s.
1927 Performance
1929 Ccache has been written to perform well out of the box, but sometimes
1930 you may have to do some adjustments of how you use the compiler and
1931 ccache in order to improve performance.
1932 Since ccache works best when I/O is fast, put the cache directory on a
1934 fast storage device if possible. Having lots of free memory so that
1935 files in the cache directory stay in the disk cache is also preferable.
1936 A good way of monitoring how well ccache works is to run ccache -s
1938 before and after your build and then compare the statistics counters.
1939 Here are some common problems and what may be done to increase the hit
1940 rate:
1941 • If the counter for preprocessed cache hits has been incremented
1943 instead of the one for direct cache hits, ccache has fallen back to
1944 preprocessor mode, which is generally slower. Some possible reasons
1945 are:
1946 • The source code has been modified in such a way that the
1948 preprocessor output is not affected.
1949 • Compiler arguments that are hashed in the direct mode but not
1951 in the preprocessor mode have changed (-I, -include, -D, etc)
1952 and they didn’t affect the preprocessor output.
1953 • The compiler option -Xpreprocessor or -Wp,* (except
1955 -Wp,-MD,<path>, -Wp,-MMD,<path>, and -Wp,-D<define>) is used.
1956 • This was the first compilation with a new value of the base
1958 directory.
1959 • A modification or status change time of one of the include
1961 files is too new (created the same second as the compilation is
1962 being done). See HANDLING OF NEWLY CREATED HEADER FILES.
1963 • The __TIME__ preprocessor macro is (potentially) being used.
1965 Ccache turns off direct mode if __TIME__ is present in the
1966 source code. This is done as a safety measure since the string
1967 indicates that a __TIME__ macro may affect the output. (To be
1968 sure, ccache would have to run the preprocessor, but the sole
1969 point of the direct mode is to avoid that.) If you know that
1970 __TIME__ isn’t used in practise, or don’t care if ccache
1971 produces objects where __TIME__ is expanded to something in the
1972 past, you can set sloppiness to time_macros.
1973 • The __DATE__ preprocessor macro is (potentially) being used and
1975 the date has changed. This is similar to how __TIME__ is
1976 handled. If __DATE__ is present in the source code, ccache
1977 hashes the current date in order to be able to produce the
1978 correct object file if the __DATE__ macro affects the output.
1979 If you know that __DATE__ isn’t used in practise, or don’t care
1980 if ccache produces objects where __DATE__ is expanded to
1981 something in the past, you can set sloppiness to time_macros.
1982 • The __TIMESTAMP__ preprocessor macro is (potentially) being
1984 used and the source file’s modification time has changed. This
1985 is similar to how __TIME__ is handled. If __TIMESTAMP__ is
1986 present in the source code, ccache hashes the string
1987 representation of the source file’s modification time in order
1988 to be able to produce the correct object file if the
1989 __TIMESTAMP__ macro affects the output. If you know that
1990 __TIMESTAMP__ isn’t used in practise, or don’t care if ccache
1991 produces objects where __TIMESTAMP__ is expanded to something
1992 in the past, you can set sloppiness to time_macros.
1993 • The input file path has changed. Ccache includes the input file
1995 path in the direct mode hash to be able to take relative
1996 include files into account and to produce a correct object file
1997 if the source code includes a __FILE__ macro.
1998 • If a cache hit counter was not incremented even though the same
2000 code has been compiled and cached before, ccache has either
2001 detected that something has changed anyway or a cleanup has been
2002 performed (either explicitly or implicitly when a cache limit has
2003 been reached). Some perhaps unobvious things that may result in a
2004 cache miss are usage of __TIME__, __DATE__ or __TIMESTAMP__ macros,
2005 or use of automatically generated code that contains a timestamp,
2006 build counter or other volatile information.
2007 • If “Multiple source files” has been incremented, it’s an indication
2009 that the compiler has been invoked on several source code files at
2010 once. Ccache doesn’t support that. Compile the source code files
2011 separately if possible.
2012 • If “Unsupported compiler option” has been incremented, enable debug
2014 logging and check which compiler option was rejected.
2015 • If “Preprocessing failed” has been incremented, one possible reason
2017 is that precompiled headers are being used. See PRECOMPILED HEADERS
2018 for how to remedy this.
2019 • If “Could not use precompiled header” has been incremented, see
2021 PRECOMPILED HEADERS.
2022 • If “Could not use modules” has been incremented, see C++ MODULES.
2024 Corrupt object files
2026 It should be noted that ccache is susceptible to general storage
2027 problems. If a bad object file sneaks into the cache for some reason,
2028 it will of course stay bad. Some possible reasons for erroneous object
2029 files are bad hardware (disk drive, disk controller, memory, etc),
2030 buggy drivers or file systems, a bad prefix_command or compiler
2031 wrapper. If this happens, the easiest way of fixing it is this:
2032 1. Build so that the bad object file ends up in the build tree.
2034 2. Remove the bad object file from the build tree.
2036 3. Rebuild with CCACHE_RECACHE set.
2038 An alternative is to clear the whole cache with ccache -C if you don’t
2040 mind losing other cached results.
2041 There are no reported issues about ccache producing broken object files
2043 reproducibly. That doesn’t mean it can’t happen, so if you find a
2044 repeatable case, please report it.
2045
2047 Credits, mailing list information, bug reporting instructions, source
2048 code, etc, can be found on ccache’s web site: https://ccache.dev.
2049
2051 Ccache was originally written by Andrew Tridgell and is currently
2052 developed and maintained by Joel Rosdahl. See AUTHORS.txt or
2053 AUTHORS.html and https://ccache.dev/credits.html for a list of
2054 contributors.
2055Ccache 4.7.4 2022-12-10 CCACHE(1)