1CCACHE(1) ccache Manual CCACHE(1)
2
6 ccache - a fast C/C++ compiler cache
7
9 ccache [options]
10 ccache compiler [compiler options]
11 compiler [compiler options] (via symbolic link)
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. Supported languages are C, C++, Objective-C and
17 Objective-C++.
18 ccache has been carefully written to always produce exactly the same
20 compiler output that you would get without the cache. The only way you
21 should be able to tell that you are using ccache is the speed.
22 Currently known exceptions to this goal are listed under BUGS. If you
23 ever discover an undocumented case where ccache changes the output of
24 your compiler, please let us know.
25 Features
27 · Keeps statistics on hits/misses.
28 · Automatic cache size management.
30 · Can cache compilations that generate warnings.
32 · Easy installation.
34 · Low overhead.
36 · Optionally uses hard links where possible to avoid copies.
38 · Optionally compresses files in the cache to reduce disk space.
40 Limitations
42 · Only knows how to cache the compilation of a single
43 C/C++/Objective-C/Objective-C++ file. Other types of compilations
44 (multi-file compilation, linking, etc) will silently fall back to
45 running the real compiler.
46 · Only works with GCC and compilers that behave similar enough.
48 · Some compiler flags are not supported. If such a flag is detected,
50 ccache will silently fall back to running the real compiler.
51
53 There are two ways to use ccache. You can either prefix your
54 compilation commands with ccache or you can let ccache masquerade as
55 the compiler by creating a symbolic link (named as the compiler) to
56 ccache. The first method is most convenient if you just want to try out
57 ccache or wish to use it for some specific projects. The second method
58 is most useful for when you wish to use ccache for all your
59 compilations.
60 To use the first method, just make sure that ccache is in your PATH.
62 To use the symlinks method, do something like this:
64 cp ccache /usr/local/bin/
66 ln -s ccache /usr/local/bin/gcc
67 ln -s ccache /usr/local/bin/g++
68 ln -s ccache /usr/local/bin/cc
69 ln -s ccache /usr/local/bin/c++
70 And so forth. This will work as long as the directory with symlinks
72 comes before the path to the compiler (which is usually in /usr/bin).
73 After installing you may wish to run “which gcc” to make sure that the
74 correct link is being used.
75 Warning
77 The technique of letting ccache masquerade as the compiler works
78 well, but currently doesn’t interact well with other tools that do
79 the same thing. See USING CCACHE WITH OTHER COMPILER WRAPPERS.
80 Warning
82 Do not use a hard link, use a symbolic link. A hard link will cause
83 “interesting” problems.
84
86 These options only apply when you invoke ccache as “ccache”. When
87 invoked as a compiler (via a symlink as described in the previous
88 section), the normal compiler options apply and you should refer to the
89 compiler’s documentation.
90 -c, --cleanup
92 Clean up the cache by removing old cached files until the specified
93 file number and cache size limits are not exceeded. This also
94 recalculates the cache file count and size totals. Normally, it’s
95 not needed to initiate cleanup manually as ccache keeps the cache
96 below the specified limits at runtime and keeps statistics up to
97 date on each compilation. Forcing a cleanup is mostly useful if you
98 manually modify the cache contents or believe that the cache size
99 statistics may be inaccurate.
100 -C, --clear
102 Clear the entire cache, removing all cached files.
103 -F, --max-files=N
105 Set the maximum number of files allowed in the cache. The value is
106 stored inside the cache directory and applies to all future
107 compilations. Due to the way the value is stored the actual value
108 used is always rounded down to the nearest multiple of 16.
109 -h, --help
111 Print an options summary page.
112 -M, --max-size=SIZE
114 Set the maximum size of the files stored in the cache. You can
115 specify a value in gigabytes, megabytes or kilobytes by appending a
116 G, M or K to the value. The default is gigabytes. The actual value
117 stored is rounded down to the nearest multiple of 16 kilobytes.
118 -s, --show-stats
120 Print the current statistics summary for the cache.
121 -V, --version
123 Print version and copyright information.
124 -z, --zero-stats
126 Zero the cache statistics (but not the configured limits).
127
129 When run as a compiler, ccache usually just takes the same command line
130 options as the compiler you are using. The only exception to this is
131 the option --ccache-skip. That option can be used to tell ccache to
132 avoid interpreting the next option in any way and to pass it along to
133 the compiler as-is.
134 The reason this can be important is that ccache does need to parse the
136 command line and determine what is an input filename and what is a
137 compiler option, as it needs the input filename to determine the name
138 of the resulting object file (among other things). The heuristic ccache
139 uses when parsing the command line is that any argument that exists as
140 a file is treated as an input file name. By using --ccache-skip you can
141 force an option to not be treated as an input file name and instead be
142 passed along to the compiler as a command line option.
143 Another case where --ccache-skip can be useful is if ccache interprets
145 an option specially but shouldn’t, since the option has another meaning
146 for your compiler than what ccache thinks.
147
149 ccache uses a number of environment variables to control operation. In
150 most cases you won’t need any of these as the defaults will be fine.
151 CCACHE_BASEDIR
153 If you set the environment variable CCACHE_BASEDIR to an absolute
154 path to a directory, ccache rewrites absolute paths into relative
155 paths before computing the hash that identifies the compilation,
156 but only for paths under the specified directory. See the
157 discussion under COMPILING IN DIFFERENT DIRECTORIES.
158 CCACHE_CC
160 You can optionally set CCACHE_CC to force the name of the compiler
161 to use. If you don’t do this then ccache works it out from the
162 command line.
163 CCACHE_COMPILERCHECK
165 By default, ccache includes the modification time (“mtime”) and
166 size of the compiler in the hash to ensure that results retrieved
167 from the cache are accurate. The CCACHE_COMPILERCHECK environment
168 variable can be used to select another strategy. Possible values
169 are:
170 content
172 Hash the content of the compiler binary. This makes ccache very
173 slightly slower compared to the mtime setting, but makes it
174 cope better with compiler upgrades during a build bootstrapping
175 process.
176 mtime
178 Hash the compiler’s mtime and size, which is fast. This is the
179 default.
180 none
182 Don’t hash anything. This may be good for situations where you
183 can safely use the cached results even though the compiler’s
184 mtime or size has changed (e.g. if the compiler is built as
185 part of your build system and the compiler’s source has not
186 changed, or if the compiler only has changes that don’t affect
187 code generation). You should only use the none setting if you
188 know what you are doing.
189 a command string
191 Hash the standard output and standard error output of the
192 specified command. The string will be split on whitespace to
193 find out the command and arguments to run. No other
194 interpretation of the command string will be done, except that
195 the special word “%compiler%” will be replaced with the path to
196 the compiler. Several commands can be specified with semicolon
197 as separator. Examples:
198 · %compiler% -v
200 · %compiler% -dumpmachine; %compiler% -dumpversion
202 You should make sure that the specified command is as fast as
204 possible since it will be run once for each ccache invocation.
205 Identifying the compiler using a command is useful if you want
207 to avoid cache misses when the compiler has been rebuilt but
208 not changed.
209 Another case is when the compiler (as seen by ccache) actually
211 isn’t the real compiler but another compiler wrapper — in that
212 case, the default mtime method will hash the mtime and size of
213 the other compiler wrapper, which means that ccache won’t be
214 able to detect a compiler upgrade. Using a suitable command to
215 identify the compiler is thus safer, but it’s also slower, so
216 you should consider continue using the mtime method in
217 combination with CCACHE_PREFIX if possible. See USING CCACHE
218 WITH OTHER COMPILER WRAPPERS.
219 CCACHE_COMPRESS
221 If you set the environment variable CCACHE_COMPRESS then ccache
222 will compress object files and other compiler output it puts in the
223 cache. However, this setting has no effect on how files are
224 retrieved from the cache; compressed and uncompressed results will
225 still be usable regardless of this setting.
226 CCACHE_CPP2
228 If you set the environment variable CCACHE_CPP2 then ccache will
229 not use the optimisation of avoiding the second call to the
230 preprocessor by compiling the preprocessed output that was used for
231 finding the hash in the case of a cache miss. This is primarily a
232 debugging option, although it is possible that some unusual
233 compilers will have problems with the intermediate filename
234 extensions used in this optimisation, in which case this option
235 could allow ccache to be used anyway.
236 CCACHE_DETECT_SHEBANG
238 The CCACHE_DETECT_SHEBANG environment variable only has meaning on
239 Windows. It instructs ccache to open the executable file to detect
240 the #!/bin/sh string, in which case ccache will search for sh.exe
241 in PATH and use that to launch the executable.
242 CCACHE_DIR
244 The CCACHE_DIR environment variable specifies where ccache will
245 keep its cached compiler output. The default is $HOME/.ccache.
246 CCACHE_DISABLE
248 If you set the environment variable CCACHE_DISABLE then ccache will
249 just call the real compiler, bypassing the cache completely.
250 CCACHE_EXTENSION
252 ccache tries to automatically determine the extension to use for
253 intermediate preprocessor files based on the type of file being
254 compiled. Unfortunately this sometimes doesn’t work, for example
255 when using the “aCC” compiler on HP-UX. On systems like this you
256 can use the CCACHE_EXTENSION option to override the default. On
257 HP-UX set this environment variable to i if you use the “aCC”
258 compiler.
259 CCACHE_EXTRAFILES
261 If you set the environment variable CCACHE_EXTRAFILES to a list of
262 paths then ccache will include the contents of those files when
263 calculating the hash sum. The list separator is semicolon in
264 Windows systems and colon on other systems.
265 CCACHE_HARDLINK
267 If you set the environment variable CCACHE_HARDLINK then ccache
268 will attempt to use hard links from the cache directory when
269 creating the compiler output rather than using a file copy. Using
270 hard links may be slightly faster in some situations, but can
271 confuse programs like “make” that rely on modification times.
272 Another thing to keep in mind is that if the resulting object file
273 is modified in any way, this corrupts the cached object file as
274 well. Hard links are never made for compressed cache files. This
275 means that you should not set the CCACHE_COMPRESS variable if you
276 want to use hard links.
277 CCACHE_HASHDIR
279 This tells ccache to hash the current working directory when
280 calculating the hash that is used to distinguish two compilations.
281 This prevents a problem with the storage of the current working
282 directory in the debug info of a object file, which can lead ccache
283 to give a cached object file that has the working directory in the
284 debug info set incorrectly. This option is off by default as the
285 incorrect setting of this debug info rarely causes problems. If you
286 strike problems with GDB not using the correct directory then
287 enable this option.
288 CCACHE_LOGFILE
290 If you set the CCACHE_LOGFILE environment variable then ccache will
291 write information on what it is doing to the specified file. This
292 is useful for tracking down problems.
293 CCACHE_NLEVELS
295 The environment variable CCACHE_NLEVELS allows you to choose the
296 number of levels of hash in the cache directory. The default is 2.
297 The minimum is 1 and the maximum is 8.
298 CCACHE_NODIRECT
300 If you set the environment variable CCACHE_NODIRECT then ccache
301 will not use the direct mode.
302 CCACHE_NOSTATS
304 If you set the environment variable CCACHE_NOSTATS then ccache will
305 not update the statistics files on each compilation.
306 CCACHE_PATH
308 You can optionally set CCACHE_PATH to a colon-separated path where
309 ccache will look for the real compilers. If you don’t do this then
310 ccache will look for the first executable matching the compiler
311 name in the normal PATH that isn’t a symbolic link to ccache
312 itself.
313 CCACHE_PREFIX
315 This option adds a prefix to the command line that ccache runs when
316 invoking the compiler. Also see the section below on using ccache
317 with “distcc”.
318 CCACHE_READONLY
320 The CCACHE_READONLY environment variable tells ccache to attempt to
321 use existing cached object files, but not to try to add anything
322 new to the cache. If you are using this because your CCACHE_DIR is
323 read-only, then you may find that you also need to set
324 CCACHE_TEMPDIR as otherwise ccache will fail to create temporary
325 files.
326 CCACHE_RECACHE
328 This forces ccache to not use any cached results, even if it finds
329 them. New results are still cached, but existing cache entries are
330 ignored.
331 CCACHE_SLOPPINESS
333 By default, ccache tries to give as few false cache hits as
334 possible. However, in certain situations it’s possible that you
335 know things that ccache can’t take for granted. The
336 CCACHE_SLOPPINESS environment variable makes it possible to tell
337 ccache to relax some checks in order to increase the hit rate. The
338 value should be a comma-separated string with options. Available
339 options are:
340 file_macro
342 Ignore __FILE__ being present in the source.
343 include_file_mtime
345 Don’t check the modification time of include files in the
346 direct mode.
347 time_macros
349 Ignore __DATE__ and __TIME__ being present in the source code.
350 See the discussion under TROUBLESHOOTING for more information.
352 CCACHE_TEMPDIR
354 The CCACHE_TEMPDIR environment variable specifies where ccache will
355 put temporary files. The default is $CCACHE_DIR/tmp.
356 Note
358 In previous versions of ccache, CCACHE_TEMPDIR had to be on the
359 same filesystem as the CCACHE_DIR path, but this requirement
360 has been relaxed.)
361 CCACHE_UMASK
363 This sets the umask for ccache and all child processes (such as the
364 compiler). This is mostly useful when you wish to share your cache
365 with other users. Note that this also affects the file permissions
366 set on the object files created from your compilations.
367 CCACHE_UNIFY
369 If you set the environment variable CCACHE_UNIFY then ccache will
370 use a C/C++ unifier when hashing the preprocessor output if the -g
371 option is not used. The unifier is slower than a normal hash, so
372 setting this environment variable loses a little bit of speed, but
373 it means that ccache can take advantage of not recompiling when the
374 changes to the source code consist of reformatting only. Note that
375 using CCACHE_UNIFY changes the hash, so cached compilations with
376 CCACHE_UNIFY set cannot be used when CCACHE_UNIFY is not set and
377 vice versa. The reason the unifier is off by default is that it can
378 give incorrect line number information in compiler warning
379 messages. Also note that enabling the unifier implies turning off
380 the direct mode.
381
383 By default ccache has a one gigabyte limit on the total size of files
384 in the cache and no maximum number of files. You can set different
385 limits using the -M/--max-size and -F/--max-files options. Use ccache
386 -s/--show-stats to see the cache size and the currently configured
387 limits (in addition to other various statistics).
388
390 ccache can optionally compress all files it puts into the cache using
391 the compression library zlib. While this involves a negligible
392 performance slowdown, it significantly increases the number of files
393 that fit in the cache. You can turn on compression by setting the
394 CCACHE_COMPRESS environment variable.
395
397 The basic idea is to detect when you are compiling exactly the same
398 code a second time and reuse the previously produced output. The
399 detection is done by hashing different kinds of information that should
400 be unique for the compilation and then using the hash sum to identify
401 the cached output. ccache uses MD4, a very fast cryptographic hash
402 algorithm, for the hashing. (MD4 is nowadays too weak to be useful in
403 cryptographic contexts, but it should be safe enough to be used to
404 identify recompilations.) On a cache hit, ccache is able to supply all
405 of the correct compiler outputs (including all warnings, dependency
406 file, etc) from the cache.
407 ccache has two ways of doing the detection:
409 · the direct mode, where ccache hashes the source code and include
411 files directly
412 · the preprocessor mode, where ccache runs the preprocessor on the
414 source code and hashes the result
415 The direct mode is generally faster since running the preprocessor has
417 some overhead.
418 Common hashed information
420 For both modes, the following information is included in the hash:
421 · the extension used by the compiler for a file with preprocessor
423 output (normally .i for C code and .ii for C++ code)
424 · the compiler’s size and modification time (or other
426 compiler-specific information specified by CCACHE_COMPILERCHECK)
427 · the name of the compiler
429 · the current directory (if CCACHE_HASHDIR is set)
431 · contents of files specified by CCACHE_EXTRAFILES (if any)
433 The direct mode
435 In the direct mode, the hash is formed of the common information and:
436 · the input source file
438 · the command line options
440 Based on the hash, a data structure called “manifest” is looked up in
442 the cache. The manifest contains:
443 · references to cached compilation results (object file, dependency
445 file, etc) that were produced by previous compilations that matched
446 the hash
447 · paths to the include files that were read at the time the
449 compilation results were stored in the cache
450 · hash sums of the include files at the time the compilation results
452 were stored in the cache
453 The current contents of the include files are then hashed and compared
455 to the information in the manifest. If there is a match, ccache knows
456 the result of the compilation. If there is no match, ccache falls back
457 to running the preprocessor. The output from the preprocessor is parsed
458 to find the include files that were read. The paths and hash sums of
459 those include files are then stored in the manifest along with
460 information about the produced compilation result.
461 The direct mode will be disabled if any of the following holds:
463 · the environment variable CCACHE_NODIRECT is set
465 · a modification time of one of the include files is too new (needed
467 to avoid a race condition)
468 · the unifier is enabled (the environment variable CCACHE_UNIFY is
470 set)
471 · a compiler option not supported by the direct mode is used:
473 · a -Wp,X compiler option other than -Wp,-MD,path and
475 -Wp,-MMD,path
476 · -Xpreprocessor
478 · the string “__TIME__” is present outside comments and string
480 literals in the source code
481 The preprocessor mode
483 In the preprocessor mode, the hash is formed of the common information
484 and:
485 · the preprocessor output from running the compiler with -E
487 · the command line options except options that affect include files
489 (-I, -include, -D, etc; the theory is that these options will
490 change the preprocessor output if they have any effect at all)
491 · any standard error output generated by the preprocessor
493 Based on the hash, the cached compilation result can be looked up
495 directly in the cache.
496
498 Some information included in the hash that identifies a unique
499 compilation may contain absolute paths:
500 · The preprocessed source code may contain absolute paths to include
502 files if the compiler option -g is used or if absolute paths are
503 given to -I and similar compiler options.
504 · Paths specified by compiler options (such as -I, -MF, etc) may be
506 absolute.
507 · The source code file path may be absolute, and that path may
509 substituted for __FILE__ macros in the source code or included in
510 warnings emitted to standard error by the preprocessor.
511 This means that if you compile the same code in different locations,
513 you can’t share compilation results between the different build
514 directories since you get cache misses because of the absolute build
515 directory paths that are part of the hash. To mitigate this problem,
516 you can specify a “base directory” by setting the CCACHE_BASEDIR
517 variable to an absolute path to the directory. ccache will then rewrite
518 absolute paths that are under the base directory (i.e., paths that have
519 the base directory as a prefix) to relative paths when constructing the
520 hash. A typical path to use as the base directory is your home
521 directory or another directory that is a parent of your build
522 directories. (Don’t use / as the base directory since that will make
523 ccache also rewrite paths to system header files, which doesn’t gain
524 anything.)
525 The drawbacks of using CCACHE_BASEDIR are:
527 · If you specify an absolute path to the source code file, __FILE__
529 macros will be expanded to a relative path instead.
530 · If you specify an absolute path to the source code file and compile
532 with -g, the source code path stored in the object file may point
533 to the wrong directory, which may prevent debuggers like GDB from
534 finding the source code. Sometimes, a work-around is to change the
535 directory explicitly with the “cd” command in GDB.
536
538 ccache has support for GCC’s precompiled headers. However, you have to
539 do some things to make it work properly:
540 · You must set CCACHE_SLOPPINESS to time_macros. The reason is that
542 ccache can’t tell whether __TIME__ or __DATE__ is used when using a
543 precompiled header.
544 · You must either:
546 · use the -include compiler option to include the precompiled
548 header (i.e., don’t use #include in the source code to include
549 the header); or
550 · add the -fpch-preprocess compiler option when compiling.
552 If you don’t do this, either the non-precompiled version of the
554 header file will be used (if available) or ccache will fall back to
555 running the real compiler and increase the statistics counter
556 “preprocessor error” (if the non-precompiled header file is not
557 available).
558
560 A group of developers can increase the cache hit rate by sharing a
561 cache directory. To share a cache without unpleasant side effects, the
562 following conditions should to be met:
563 · Use the same CCACHE_DIR environment variable setting.
565 · Unset the CCACHE_HARDLINK environment variable.
567 · Make sure everyone sets the CCACHE_UMASK environment variable to
569 002. This ensures that cached files are accessible to everyone in
570 the group.
571 · Make sure that all users have write permission in the entire cache
573 directory (and that you trust all users of the shared cache).
574 · Make sure that the setgid bit is set on all directories in the
576 cache. This tells the filesystem to inherit group ownership for new
577 directories. The command “find $CCACHE_DIR -type d | xargs chmod
578 g+s” might be useful for this.
579 The reason to avoid the hard link mode is that the hard links cause
581 unwanted side effects, as all links to a cached file share the file’s
582 modification timestamp. This results in false dependencies to be
583 triggered by timestamp-based build systems whenever another user links
584 to an existing file. Typically, users will see that their libraries and
585 binaries are relinked without reason.
586 You may also want to make sure that the developers have CCACHE_BASEDIR
588 set appropriately, as discussed in the previous section.
589
591 It is possible to put the cache directory on an NFS filesystem (or
592 similar filesystems), but keep in mind that:
593 · Having the cache on NFS may slow down compilation. Make sure to do
595 some benchmarking to see if it’s worth it.
596 · ccache hasn’t been tested very thoroughly on NFS.
598 A tip is to set CCACHE_TEMPDIR to a directory on the local host to
600 avoid NFS traffic for temporary files.
601
603 The recommended way of combining ccache with another compiler wrapper
604 (such as “distcc”) is by using the CCACHE_PREFIX option. You just need
605 to set the environment variable CCACHE_PREFIX to the name of the
606 wrapper (e.g. distcc) and ccache will prefix the command line with the
607 specified command when running the compiler.
608 Unless you set CCACHE_COMPILERCHECK to a suitable command (see the
610 description of that configuration option), it is not recommended to use
611 the form ccache anotherwrapper compiler args as the compilation
612 command. It’s also not recommended to use the masquerading technique
613 for the other compiler wrapper. The reason is that by default, ccache
614 will in both cases hash the mtime and size of the other wrapper instead
615 of the real compiler, which means that:
616 · Compiler upgrades will not be detected properly.
618 · The cached results will not be shared between compilations with and
620 without the other wrapper.
621 Another minor thing is that if CCACHE_PREFIX is not used, ccache will
623 needlessly invoke the other wrapper when running the preprocessor.
624
626 · ccache doesn’t handle the GNU Assembler’s .incbin directive
627 correctly. This directive can be embedded in the source code inside
628 an asm statement in order to include a file verbatim in the object
629 file. If the included file is modified, ccache doesn’t pick up the
630 change since the inclusion isn’t done by the preprocessor. A
631 workaround of this problem is to set CCACHE_EXTRAFILES to the path
632 of the included file.
633
635 General
636 A general tip for getting information about what ccache is doing is to
637 enable debug logging by setting CCACHE_LOGFILE. The log contains
638 executed commands, important decisions that ccache makes, read and
639 written files, etc. Another way of keeping track of what is happening
640 is to check the output of ccache -s.
641 Performance
643 ccache has been written to perform well out of the box, but sometimes
644 you may have to do some adjustments of how you use the compiler and
645 ccache in order to improve performance.
646 Since ccache works best when I/O is fast, put the cache directory on a
648 fast storage device if possible. Having lots of free memory so that
649 files in the cache directory stay in the disk cache is also
650 preferrable.
651 A good way of monitoring how well ccache works is to run ccache -s
653 before and after your build and then compare the statistics counters.
654 Here are some common problems and what may be done to increase the hit
655 rate:
656 · If “cache hit (preprocessed)” has been incremented instead of
658 “cache hit (direct)”, ccache has fallen back to preprocessor mode,
659 which is generally slower. Some possible reasons are:
660 · The source code has been modified in such a way that the
662 preprocessor output is not affected.
663 · Compiler arguments that are hashed in the direct mode but not
665 in the preprocessor mode have changed (-I, -include, -D, etc)
666 and they didn’t affect the preprocessor output.
667 · The compiler option -Xpreprocessor or -Wp,X (except
669 -Wp,-MD,path and Wp,-MMD,path) is used.
670 · This was the first compilation with a new value of
672 CCACHE_BASEDIR.
673 · A modification time of one of the include files is too new
675 (created the same second as the compilation is being done).
676 This check is made to avoid a race condition. To fix this,
677 create the include file earlier in the build process, if
678 possible, or set CCACHE_SLOPPINESS to include_file_mtime if you
679 are willing to take the risk. (The race condition consists of
680 these events: the preprocessor is run; an include file is
681 modified by someone; the new include file is hashed by ccache;
682 the real compiler is run on the preprocessor’s output, which
683 contains data from the old header file; the wrong object file
684 is stored in the cache.)
685 · The __TIME__ preprocessor macro is (potentially) being used.
687 ccache turns off direct mode if “__TIME__” is present in the
688 source code outside comments and string literals. This is done
689 as a safety measure since the string indicates that a __TIME__
690 macro may affect the output. (To be sure, ccache would have to
691 run the preprocessor, but the sole point of the direct mode is
692 to avoid that.) If you know that __TIME__ isn’t used in
693 practise, or don’t care if ccache produces objects where
694 __TIME__ is expanded to something in the past, you can set
695 CCACHE_SLOPPINESS to time_macros.
696 · The __DATE__ preprocessor macro is (potentially) being used and
698 the date has changed. This is similar to how __TIME__ is
699 handled. If “__DATE__” is present in the source code outside
700 comments and string literals, ccache hashes the current date in
701 order to be able to produce the correct object file if the
702 __DATE__ macro affects the output. If you know that __DATE__
703 isn’t used in practise, or don’t care if ccache produces
704 objects where __DATE__ is expanded to something in the past,
705 you can set CCACHE_SLOPPINESS to time_macros.
706 · The __FILE__ preprocessor macro is (potentially) being used and
708 the file path has changed. If “__FILE__” is present in the
709 source code outside comments and string literals, ccache hashes
710 the current input file path in order to be able to produce the
711 correct object file if the __FILE__ macro affects the output.
712 If you know that __FILE__ isn’t used in practise, or don’t care
713 if ccache produces objects where __FILE__ is expanded to the
714 wrong path, you can set CCACHE_SLOPPINESS to file_macro.
715 · If “cache miss” has been incremented even though the same code has
717 been compiled and cached before, ccache has either detected that
718 something has changed anyway or a cleanup has been performed
719 (either explicitly or implicitly when a cache limit has been
720 reached). Some perhaps unobvious things that may result in a cache
721 miss are usage of __TIME__ or __DATE__ macros, or use of
722 automatically generated code that contains a timestamp, build
723 counter or other volatile information.
724 · If “multiple source files” has been incremented, it’s an indication
726 that the compiler has been invoked on several source code files at
727 once. ccache doesn’t support that. Compile the source code files
728 separately if possible.
729 · If “unsupported compiler option” has been incremented, enable debug
731 logging and check which option was rejected.
732 · If “preprocessor error” has been incremented, one possible reason
734 is that precompiled headers are being used. See PRECOMPILED HEADERS
735 for how to remedy this.
736 · If “can’t use precompiled header” has been incremented, see
738 PRECOMPILED HEADERS.
739 Errors when compiling with ccache
741 If compilation doesn’t work with ccache, but it works without it, one
742 possible reason is that the compiler can’t compile preprocessed output
743 correctly. A workaround that may work is to set CCACHE_CPP2. This will
744 make cache misses slower, though, so it is better to find and fix the
745 root cause.
746 Corrupt object files
748 It should be noted that ccache is susceptible to general storage
749 problems. If a bad object file sneaks into the cache for some reason,
750 it will of course stay bad. Some possible reasons for erroneous object
751 files are bad hardware (disk drive, disk controller, memory, etc),
752 buggy drivers or file systems, a bad CCACHE_PREFIX command or compiler
753 wrapper. If this happens, you can either find out which object file is
754 broken by reading the debug log and then delete the bad object file
755 from the cache, or you can simply clear the whole cache with ccache -C
756 if you don’t mind losing other cached results.
757 There are no reported issues about ccache producing broken object files
759 reproducibly. That doesn’t mean it can’t happen, so if you find a
760 repeatable case, please report it.
761
763 Credits, mailing list information, bug reporting instructions, source
764 code, etc, can be found on ccache’s web site: http://ccache.samba.org.
765
767 ccache was originally written by Andrew Tridgell and is currently
768 developed and maintained by Joel Rosdahl. See AUTHORS.txt or
769 AUTHORS.html and http://ccache.samba.org/credits.html for a list of
770 contributors.
771ccache 3.1.4 01/09/2011 CCACHE(1)