ccache - a fast C/C++ compiler cache
ccache compiler [compiler options]
compiler [compiler options] (via symbolic link)
ccache is a compiler cache. It speeds up recompilation by caching the result of
previous compilations and detecting when the same compilation is being done
again. Supported languages are C, C++, Objective-C and Objective-C++.
ccache has been carefully written to always produce exactly the same compiler
output that you would get without the cache. The only way you should be able
to tell that you are using ccache is the speed. Currently known exceptions to
this goal are listed under CAVEATS. If you ever discover an undocumented case
where ccache changes the output of your compiler, please let us know.
•Keeps statistics on hits/misses.
•Automatic cache size management.
•Can cache compilations that generate
•Optionally uses hard links where
possible to avoid copies.
•Optionally compresses files in the
cache to reduce disk space.
•Only knows how to cache the
compilation of a single C/C++/Objective-C/Objective-C++ file. Other types of
compilations (multi-file compilation, linking, etc) will silently fall back to
running the real compiler.
•Only works with GCC and compilers that
behave similar enough.
•Some compiler flags are not supported.
If such a flag is detected, ccache will silently fall back to running the real
There are two ways to use ccache. You can either prefix your compilation
commands with ccache
or you can let ccache masquerade as the compiler
by creating a symbolic link (named as the compiler) to ccache. The first
method is most convenient if you just want to try out ccache or wish to use it
for some specific projects. The second method is most useful for when you wish
to use ccache for all your compilations.
To use the second method on a Debian system, it's easiest to just prepend
to your PATH
symlinks for all compilers currently installed as Debian packages.
Alternatively, you can create any symlinks you like yourself like this:
ln -s /usr/bin/ccache /usr/local/bin/gcc
ln -s /usr/bin/ccache /usr/local/bin/g++
ln -s /usr/bin/ccache /usr/local/bin/cc
ln -s /usr/bin/ccache /usr/local/bin/c++
And so forth. This will work as long as the directory with symlinks comes before
the path to the compiler (which is usually in /usr/bin). After installing you
may wish to run “which gcc” to make sure that the correct link
is being used.
The technique of letting ccache masquerade as the compiler works well, but
currently doesn’t interact well with other tools that do the same
thing. See USING CCACHE WITH OTHER COMPILER WRAPPERS.
Do not use a hard link, use a symbolic link. A hard link will cause
These options only apply when you invoke ccache as “ccache”. When
invoked as a compiler (via a symlink as described in the previous section),
the normal compiler options apply and you should refer to the
Clean up the cache by removing old cached
files until the specified file number and cache size limits are not exceeded.
This also recalculates the cache file count and size totals. Normally, there
is no need to initiate cleanup manually as ccache keeps the cache below the
specified limits at runtime and keeps statistics up to date on each
compilation. Forcing a cleanup is mostly useful if you manually modify the
cache contents or believe that the cache size statistics may be
Clear the entire cache, removing all cached
files, but keeping the configuration file.
Set the maximum number of files allowed in the
cache. Use 0 for no limit. The value is stored in a configuration file in the
cache directory and applies to all future compilations.
Print an options summary page.
Set the maximum size of the files stored in
the cache. SIZE should be a number followed by an optional suffix: k,
M, G, T (decimal), Ki, Mi, Gi or Ti (binary). The default suffix is G. Use 0
for no limit. The value is stored in a configuration file in the cache
directory and applies to all future compilations.
Set configuration KEY to VALUE.
See CONFIGURATION for more information.
Print current configuration options and from
where they originate (environment variable, configuration file or compile-time
Print the current statistics summary for the
Print version and copyright information.
Zero the cache statistics (but not the
When run as a compiler, ccache usually just takes the same command line options
as the compiler you are using. The only exception to this is the option
. That option can be used to tell ccache to avoid
interpreting the next option in any way and to pass it along to the compiler
currently only tells ccache not to interpret the next
option as a special compiler option — the option will still be included
in the direct mode hash.
The reason this can be important is that ccache does need to parse the command
line and determine what is an input filename and what is a compiler option, as
it needs the input filename to determine the name of the resulting object file
(among other things). The heuristic ccache uses when parsing the command line
is that any argument that exists as a file is treated as an input file name.
By using --ccache-skip
you can force an option to not be treated as an
input file name and instead be passed along to the compiler as a command line
Another case where --ccache-skip
can be useful is if ccache interprets an
option specially but shouldn’t, since the option has another meaning
for your compiler than what ccache thinks.
ccache’s default behavior can be overridden by configuration file
settings, which in turn can be overridden by environment variables with names
starting with CCACHE_
. ccache normally reads configuration from two
files: first a system-level configuration file and secondly a cache-specific
configuration file. The priority of configuration settings is as follows
(where 1 is highest):
2.The cache-specific configuration file
3.The system-wide configuration file
<sysconfdir>/ccache.conf (typically /etc/ccache.conf or
As a special case, if the environment variable CCACHE_CONFIGPATH
ccache reads configuration from the specified path instead of the default
Configuration files are in a simple “key = value” format, one
setting per line. Lines starting with a hash sign are comments. Blank lines
are ignored, as is whitespace surrounding keys and values. Example:
# Set maximum cache size to 10 GB:
max_size = 10G
Some settings are boolean values (i.e. truth values). In a configuration file,
such values must be set to the string true
. For the
corresponding environment variables, the semantics are a bit different: a set
environment variable means “true” regardless of the value (even
if set to the empty string), and an unset environment variable means
“false”. Each boolean environment variable also has a negated
form starting with CCACHE_NO
. For example, CCACHE_COMPRESS
be set to force compression and CCACHE_NOCOMPRESS
can be set to force
Below is a list of available configuration settings. The corresponding
environment variable name is indicated in parentheses after each configuration
This setting should be an absolute path to a
directory. ccache then rewrites absolute paths into relative paths before
computing the hash that identifies the compilation, but only for paths under
the specified directory. If set to the empty string (which is the default), no
rewriting is done. See also the discussion under COMPILING IN DIFFERENT
DIRECTORIES. If using GCC or newer versions of Clang, you might want to look
into the -fdebug-prefix-map=old=new option for relocating debug info to
a common prefix (mapping prefix with old=new).
This setting specifies where ccache will keep
its cached compiler outputs. It will only take effect if set in the
system-wide configuration file or as an environment variable. The default is
This setting allows you to choose the number
of directory levels in the cache directory. The default is 2. The minimum is 1
and the maximum is 8.
This setting can be used to force the name of
the compiler to use. If set to the empty string (which is the default), ccache
works it out from the command line.
By default, ccache includes the modification
time (“mtime”) and size of the compiler in the hash to ensure
that results retrieved from the cache are accurate. This setting can be used
to select another strategy. Possible values are:
Hash the content of the compiler binary. This
makes ccache very slightly slower compared to the mtime setting, but
makes it cope better with compiler upgrades during a build bootstrapping
Hash the compiler’s mtime and size,
which is fast. This is the default.
Don’t hash anything. This may be good
for situations where you can safely use the cached results even though the
compiler’s mtime or size has changed (e.g. if the compiler is built as
part of your build system and the compiler’s source has not changed, or
if the compiler only has changes that don’t affect code generation).
You should only use the none setting if you know what you are
Use value as the string to calculate
hash from. This can be the compiler revision number you retrieved earlier and
set here via environment variable.
a command string
Hash the standard output and standard error
output of the specified command. The string will be split on whitespace to
find out the command and arguments to run. No other interpretation of the
command string will be done, except that the special word %compiler%
will be replaced with the path to the compiler. Several commands can be
specified with semicolon as separator. Examples:
%compiler% -dumpmachine; %compiler% -dumpversion
You should make sure that the specified command is as fast as possible since it
will be run once for each ccache invocation.
Identifying the compiler using a command is useful if you want to avoid cache
misses when the compiler has been rebuilt but not changed.
Another case is when the compiler (as seen by ccache) actually isn’t the
real compiler but another compiler wrapper — in that case, the default
method will hash the mtime and size of the other compiler
wrapper, which means that ccache won’t be able to detect a compiler
upgrade. Using a suitable command to identify the compiler is thus safer, but
it’s also slower, so you should consider continue using the
method in combination with the prefix_command
possible. See USING CCACHE WITH OTHER COMPILER WRAPPERS.
Boolean values above)
If true, ccache will compress object files and
other compiler output it puts in the cache. However, this setting has no
effect on how files are retrieved from the cache; compressed and uncompressed
results will still be usable regardless of this setting. The default is
This setting determines the level at which
ccache will compress object files. It only has effect if compression is
enabled. The value defaults to 6, and must be no lower than 1 (fastest, worst
compression) and no higher than 9 (slowest, best compression).
This setting can be used to force a certain
extension for the intermediate preprocessed file. The default is to
automatically determine the extension to use for intermediate preprocessor
files based on the type of file being compiled, but that sometimes
doesn’t work. For example, when using the “aCC” compiler
on HP-UX, set the cpp extension to i.
, see Boolean
If true, the direct mode will be used. The
default is true. See THE DIRECT MODE.
, see Boolean
When true, ccache will just call the real
compiler, bypassing the cache completely. The default is false.
This setting is a list of paths to files that
ccache will include in the the hash sum that identifies the build. The list
separator is semicolon on Windows systems and colon on other systems.
Boolean values above)
If true, ccache will attempt to use hard links
from the cache directory when creating the compiler output rather than using a
file copy. Using hard links may be slightly faster in some situations, but can
confuse programs like “make” that rely on modification times.
Another thing to keep in mind is that if the resulting object file is modified
in any way, this corrupts the cached object file as well. Hard links are never
made for compressed cache files. This means that you should not enable
compression if you want to use hard links. The default is false.
, see Boolean
If true (which is the default), ccache will
include the current working directory (CWD) in the hash that is used to
distinguish two compilations when generating debug info (compiler option
with variations). Exception: The CWD will not be included in the
hash if base_dir
is set (and matches the CWD) and the compiler option
The reason for including the CWD in the hash by default is to prevent a problem
with the storage of the current working directory in the debug info of an
object file, which can lead ccache to return a cached object file that has the
working directory in the debug info set incorrectly.
You can disable this setting to get cache hits when compiling the same source
code in different directories if you don’t mind that CWD in the debug
info might be incorrect.
This setting is a list of paths to files (or
directories with headers) that ccache will not include in the manifest
list that makes up the direct mode. Note that this can cause stale cache hits
if those headers do indeed change. The list separator is semicolon on Windows
systems and colon on other systems.
see Boolean values above)
If true, ccache will not discard the comments
before hashing preprocessor output. This can be used to check documentation
Sets the limit when cleaning up. Files are
deleted (in LRU order) until the levels are below the limit. The default is
0.8 (= 80%).
If set to a file path, ccache will write
information on what it is doing to the specified file. This is useful for
tracking down problems.
This option specifies the maximum number of
files to keep in the cache. Use 0 for no limit (which is the default).
This option specifies the maximum size of the
cache. Use 0 for no limit. The default value is 5G. Available suffixes: k, M,
G, T (decimal) and Ki, Mi, Gi, Ti (binary). The default suffix is
If set, ccache will search directories in this
list when looking for the real compiler. The list separator is semicolon on
Windows systems and colon on other systems. If not set, ccache will look for
the first executable matching the compiler name in the normal PATH that
isn’t a symbolic link to ccache itself.
This option adds a list of prefixes (separated
by space) to the command line that ccache uses when invoking the compiler. See
also USING CCACHE WITH OTHER COMPILER WRAPPERS.
This option adds a list of prefixes (separated
by space) to the command line that ccache uses when invoking the
Boolean values above)
If true, ccache will attempt to use existing
cached object files, but it will not to try to add anything new to the cache.
If you are using this because your ccache directory is read-only, then you
need to set temporary_dir as otherwise ccache will fail to create
, see Boolean values above)
Just like read_only except that ccache
will only try to retrieve results from the cache using the direct mode, not
the preprocessor mode. See documentation for read_only regarding using
a read-only ccache directory.
, see Boolean
If true, ccache will not use any previously
stored result. New results will still be cached, possibly overwriting any
, see Boolean
If true, ccache will first run the
preprocessor to preprocess the source code (see THE PREPROCESSOR MODE) and
then on a cache miss run the compiler on the source code to get hold of the
object file. This is the default.
If false, ccache will first run preprocessor to preprocess the source code and
then on a cache miss run the compiler on the preprocessed source code
instead of the original source code. This makes cache misses slightly faster
since the source code only has to be preprocessed once. The downside is that
some compilers won’t produce the same result (for instance diagnostics
warnings) when compiling preprocessed source code.
By default, ccache tries to give as few false
cache hits as possible. However, in certain situations it’s possible
that you know things that ccache can’t take for granted. This setting
makes it possible to tell ccache to relax some checks in order to increase the
hit rate. The value should be a comma-separated string with options. Available
Ignore __FILE__ being present in the
ccache normally examines a file’s
contents to determine whether it matches the cached version. With this option
set, ccache will consider a file as matching its cached version if the sizes,
mtimes and ctimes match.
By default, ccache also will not cache a file
if it includes a header whose ctime is too new. This option disables that
By default, ccache will not cache a file if it
includes a header whose mtime is too new. This option disables that
By default, ccache will also include all
system headers in the manifest. With this option set, ccache will only include
system headers in the hash but not add the system header files to the list of
Be sloppy about #defines when precompiling a
header file. See PRECOMPILED HEADERS for more information.
Ignore __DATE__ and __TIME__
being present in the source code.
See the discussion under TROUBLESHOOTING for more information.
, see Boolean values
If true, ccache will update the statistics
counters on each compilation. The default is true.
This setting specifies where ccache will put
temporary files. The default is <cache_dir>/tmp
In previous versions of ccache, CCACHE_TEMPDIR
had to be on the same
filesystem as the CCACHE_DIR
path, but this requirement has been
This setting specifies the umask for ccache
and all child processes (such as the compiler). This is mostly useful when you
wish to share your cache with other users. Note that this also affects the
file permissions set on the object files created from your compilations.
, see Boolean values
If true, ccache will use a C/C++ unifier when
hashing the preprocessor output if the -g option is not used. The
unifier is slower than a normal hash, so setting this environment variable
loses a little bit of speed, but it means that ccache can take advantage of
not recompiling when the changes to the source code consist of reformatting
only. Note that enabling the unifier changes the hash, so cached compilations
produced when the unifier is enabled cannot be reused when the unifier is
disabled, and vice versa. Enabling the unifier may result in incorrect line
number information in compiler warning messages and expansions of the
__LINE__ macro. Also note that enabling the unifier implies turning off
the direct mode.
By default, ccache has a five gigabyte limit on the total size of files in the
cache and no maximum number of files. You can set different limits using the
to see the cache size and the currently
configured limits (in addition to other various statistics).
ccache can optionally compress all files it puts into the cache using the
compression library zlib. While this may involve a tiny performance slowdown,
it increases the number of files that fit in the cache. You can turn on
compression with the compression
configuration setting and you can also
tweak the compression level with compression_level
can show the following statistics:
||Uncachable compilation or linking by an autoconf test.
|bad compiler arguments
||Malformed compiler argument, e.g. missing a value for an option that
requires an argument or failure to read a file specified by an option
|cache file missing
||A file was unexpectedly missing from the cache. This only happens in
rare situations, e.g. if one ccache instance is about to get a file from
the cache while another instance removed the file as part of cache
|cache hit (direct)
||A result was successfully found using the direct mode.
|cache hit (preprocessed)
||A result was successfully found using the preprocessor mode.
||No result was found.
||Current size of the cache.
|called for link
||The compiler was called for linking, not compiling.
|called for preprocessing
||The compiler was called for preprocessing, not compiling.
|can’t use precompiled header
||Preconditions for using precompiled headers were not fulfilled.
|ccache internal error
||Unexpected failure, e.g. due to problems reading/writing the cache.
||Number of cleanups performed, either implicitly due to the cache size
limit being reached or due to explicit ccache -c/--cleanup
||The compilation failed. No result stored in the cache.
|compiler check failed
||A compiler check program specified by compiler_check
|compiler produced empty output
||The compiler’s output file (typically an object file) was empty
|compiler produced no output
||The compiler’s output file (typically an object file) was missing
|compiler produced stdout
||The compiler wrote data to standard output. This is something that
compilers normally never do, so ccache is not designed to store such
output in the cache.
|couldn’t find the compiler
||The compiler to execute could not be found.
|error hashing extra file
||Failure reading a file specified by extra_files_to_hash
|files in cache
||Current number of files in the cache.
|multiple source files
||The compiler was called to compile multiple source files in one go. This
is not supported by ccache.
|no input file
||No input file was specified to the compiler.
|output to a non-regular file
||The output path specified with -o is not a file (e.g. a directory
or a device node).
|output to stdout
||The compiler was instructed to write its output to standard output using
-o -. This is not supported by ccache.
||Preprocessing the source code using the compiler’s -E
|unsupported code directive
||Code like the assembler “.incbin” directive was found.
This is not supported by ccache.
|unsupported compiler option
||A compiler option not supported by ccache was found.
|unsupported source language
||A source language e.g. specified with -x was unsupported by
The basic idea is to detect when you are compiling exactly the same code a
second time and reuse the previously produced output. The detection is done by
hashing different kinds of information that should be unique for the
compilation and then using the hash sum to identify the cached output. ccache
uses MD4, a very fast cryptographic hash algorithm, for the hashing. (MD4 is
nowadays too weak to be useful in cryptographic contexts, but it should be
safe enough to be used to identify recompilations.) On a cache hit, ccache is
able to supply all of the correct compiler outputs (including all warnings,
dependency file, etc) from the cache.
ccache has two ways of doing the detection:
•the direct mode, where ccache
hashes the source code and include files directly
•the preprocessor mode, where
ccache runs the preprocessor on the source code and hashes the result
The direct mode is generally faster since running the preprocessor has some
For both modes, the following information is included in the hash:
•the extension used by the compiler for
a file with preprocessor output (normally .i for C code and .ii
for C++ code)
•the compiler’s size and
modification time (or other compiler-specific information specified by the
•the name of the compiler
•the current directory (if the
hash_dir setting is enabled)
•contents of files specified by the
extra_files_to_hash setting (if any)
In the direct mode, the hash is formed of the common information and:
•the input source file
•the command line options
Based on the hash, a data structure called “manifest” is looked up
in the cache. The manifest contains:
•references to cached compilation
results (object file, dependency file, etc) that were produced by previous
compilations that matched the hash
•paths to the include files that were
read at the time the compilation results were stored in the cache
•hash sums of the include files at the
time the compilation results were stored in the cache
The current contents of the include files are then hashed and compared to the
information in the manifest. If there is a match, ccache knows the result of
the compilation. If there is no match, ccache falls back to running the
preprocessor. The output from the preprocessor is parsed to find the include
files that were read. The paths and hash sums of those include files are then
stored in the manifest along with information about the produced compilation
There is a catch with the direct mode: header files that were used by the
compiler are recorded, but header files that were not
used, but would
have been used if they existed, are not. So, when ccache checks if a result
can be taken from the cache, it currently can’t check if the existence
of a new header file should invalidate the result. In practice, the direct
mode is safe to use in the absolute majority of cases.
The direct mode will be disabled if any of the following holds:
•the configuration setting
direct_mode is false
•a modification time of one of the
include files is too new (needed to avoid a race condition)
•the unifier is enabled (the
configuration setting unify is true)
•a compiler option not supported by the
direct mode is used:
•a -Wp,X compiler
option other than -Wp,-MD,path,
-Wp,-MMD,path and -Wp,-D_define_
•the string “__TIME__” is
present in the source code
In the preprocessor mode, the hash is formed of the common information and:
•the preprocessor output from running
the compiler with -E
•the command line options except
options that affect include files ( -I, -include, -D,
etc; the theory is that these options will change the preprocessor output if
they have any effect at all)
•any standard error output generated by
Based on the hash, the cached compilation result can be looked up directly in
Some information included in the hash that identifies a unique compilation may
contain absolute paths:
•The preprocessed source code may
contain absolute paths to include files if the compiler option -g is
used or if absolute paths are given to -I and similar compiler
•Paths specified by compiler options
(such as -I, -MF, etc) may be absolute.
•The source code file path may be
absolute, and that path may substituted for __FILE__ macros in the
source code or included in warnings emitted to standard error by the
This means that if you compile the same code in different locations, you
can’t share compilation results between the different build directories
since you get cache misses because of the absolute build directory paths that
are part of the hash. To mitigate this problem, you can specify a “base
directory” in the configuration setting base_dir
to an absolute
path to the directory. ccache will then rewrite absolute paths that are under
the base directory (i.e., paths that have the base directory as a prefix) to
relative paths when constructing the hash. A typical path to use as the base
directory is your home directory or another directory that is a parent of your
build directories. (Don’t use / as the base directory since that will
make ccache also rewrite paths to system header files, which doesn’t
The drawbacks of using a base directory are:
•If you specify an absolute path to the
source code file, __FILE__ macros will be expanded to a relative path
•If you specify an absolute path to the
source code file and compile with -g, the source code path stored in
the object file may point to the wrong directory, which may prevent debuggers
like GDB from finding the source code. Sometimes, a work-around is to change
the directory explicitly with the “cd” command in GDB.
ccache has support for GCC’s precompiled headers. However, you have to do
some things to make it work properly:
•You must set sloppiness to
pch_defines,time_macros. The reason is that ccache can’t tell
whether __TIME__ or __DATE__ is used when using a precompiled
header. Further, it can’t detect changes in #defines in the source code
because of how preprocessing works in combination with precompiled
•You must either:
•use the -include compiler
option to include the precompiled header (i.e., don’t use
#include in the source code to include the header); or
•(for the Clang compiler) use the
-include-pch compiler option to include the PCH file generated from the
precompiled header; or
•add the -fpch-preprocess
compiler option when compiling.
If you don’t do this, either the non-precompiled version of the header
file will be used (if available) or ccache will fall back to running the real
compiler and increase the statistics counter “preprocessor
error” (if the non-precompiled header file is not available).
A group of developers can increase the cache hit rate by sharing a cache
directory. To share a cache without unpleasant side effects, the following
conditions should to be met:
•Use the same cache directory.
•Make sure that the configuration
setting hard_link is false (which is the default).
•Make sure that all users are in the
•Set the configuration setting
umask to 002. This ensures that cached files are accessible to everyone
in the group.
•Make sure that all users have write
permission in the entire cache directory (and that you trust all users of the
•Make sure that the setgid bit is set
on all directories in the cache. This tells the filesystem to inherit group
ownership for new directories. The following command might be useful for this:
find $CCACHE_DIR -type d | xargs chmod g+s
The reason to avoid the hard link mode is that the hard links cause unwanted
side effects, as all links to a cached file share the file’s
modification timestamp. This results in false dependencies to be triggered by
timestamp-based build systems whenever another user links to an existing file.
Typically, users will see that their libraries and binaries are relinked
You may also want to make sure that a base directory is set appropriately, as
discussed in a previous section.
It is possible to put the cache directory on an NFS filesystem (or similar
filesystems), but keep in mind that:
•Having the cache on NFS may slow down
compilation. Make sure to do some benchmarking to see if it’s worth
•ccache hasn’t been tested very
thoroughly on NFS.
A tip is to set temporary_dir
to a directory on the local host to avoid
NFS traffic for temporary files.
The recommended way of combining ccache with another compiler wrapper (such as
“distcc”) is by letting ccache execute the compiler wrapper.
This is accomplished by defining the configuration setting
, for example by setting the environment variable
to the name of the wrapper (e.g. distcc
will then prefix the command line with the specified command when running the
compiler. To specify several prefix commands, set prefix_command
colon-separated list of commands.
Unless you set compiler_check
to a suitable command (see the description
of that configuration option), it is not recommended to use the form ccache
anotherwrapper compiler args
as the compilation command. It’s also
not recommended to use the masquerading technique for the other compiler
wrapper. The reason is that by default, ccache will in both cases hash the
mtime and size of the other wrapper instead of the real compiler, which means
•Compiler upgrades will not be detected
•The cached results will not be shared
between compilations with and without the other wrapper.
Another minor thing is that if prefix_command
is used, ccache will not
invoke the other wrapper when running the preprocessor, which increases
performance. You can use the prefix_command_cpp
if you also want to invoke the other wrapper when doing preprocessing
(normally by adding -E
•The direct mode fails to pick up new
header files in some rare scenarios. See THE DIRECT MODE above.
A general tip for getting information about what ccache is doing is to enable
debug logging by setting log_file
. The log contains executed commands,
important decisions that ccache makes, read and written files, etc. Another
way of keeping track of what is happening is to check the output of ccache
ccache has been written to perform well out of the box, but sometimes you may
have to do some adjustments of how you use the compiler and ccache in order to
Since ccache works best when I/O is fast, put the cache directory on a fast
storage device if possible. Having lots of free memory so that files in the
cache directory stay in the disk cache is also preferable.
A good way of monitoring how well ccache works is to run ccache -s
and after your build and then compare the statistics counters. Here are some
common problems and what may be done to increase the hit rate:
•If “cache hit
(preprocessed)” has been incremented instead of “cache hit
(direct)”, ccache has fallen back to preprocessor mode, which is
generally slower. Some possible reasons are:
•The source code has been modified in
such a way that the preprocessor output is not affected.
•Compiler arguments that are hashed in
the direct mode but not in the preprocessor mode have changed ( -I,
-include, -D, etc) and they didn’t affect the
•The compiler option
-Xpreprocessor or -Wp,X (except
-Wp,-MD, path, -Wp,-MMD,path, and
-Wp,-D_define_) is used.
•This was the first compilation with a
new value of the base directory setting.
•A modification time of one of the
include files is too new (created the same second as the compilation is being
done). This check is made to avoid a race condition. To fix this, create the
include file earlier in the build process, if possible, or set
sloppiness to include_file_mtime if you are willing to take the
risk. (The race condition consists of these events: the preprocessor is run;
an include file is modified by someone; the new include file is hashed by
ccache; the real compiler is run on the preprocessor’s output, which
contains data from the old header file; the wrong object file is stored in the
•The __TIME__ preprocessor macro
is (potentially) being used. ccache turns off direct mode if
“__TIME__” is present in the source code. This is done as a
safety measure since the string indicates that a __TIME__ macro
may affect the output. (To be sure, ccache would have to run the
preprocessor, but the sole point of the direct mode is to avoid that.) If you
know that __TIME__ isn’t used in practise, or don’t care
if ccache produces objects where __TIME__ is expanded to something in
the past, you can set sloppiness to time_macros.
•The __DATE__ preprocessor macro
is (potentially) being used and the date has changed. This is similar to how
__TIME__ is handled. If “__DATE__” is present in the
source code, ccache hashes the current date in order to be able to produce the
correct object file if the __DATE__ macro affects the output. If you
know that __DATE__ isn’t used in practise, or don’t care
if ccache produces objects where __DATE__ is expanded to something in
the past, you can set sloppiness to time_macros.
•The __FILE__ preprocessor macro
is (potentially) being used and the file path has changed. If
“__FILE__” is present in the source code, ccache hashes the
current input file path in order to be able to produce the correct object file
if the __FILE__ macro affects the output. If you know that
__FILE__ isn’t used in practise, or don’t care if ccache
produces objects where __FILE__ is expanded to the wrong path, you can
set sloppiness to file_macro.
•If “cache miss” has been
incremented even though the same code has been compiled and cached before,
ccache has either detected that something has changed anyway or a cleanup has
been performed (either explicitly or implicitly when a cache limit has been
reached). Some perhaps unobvious things that may result in a cache miss are
usage of __TIME__ or __DATE__ macros, or use of automatically
generated code that contains a timestamp, build counter or other volatile
•If “multiple source
files” has been incremented, it’s an indication that the
compiler has been invoked on several source code files at once. ccache
doesn’t support that. Compile the source code files separately if
•If “unsupported compiler
option” has been incremented, enable debug logging and check which
option was rejected.
•If “preprocessor error”
has been incremented, one possible reason is that precompiled headers are
being used. See PRECOMPILED HEADERS for how to remedy this.
•If “can’t use
precompiled header” has been incremented, see PRECOMPILED
It should be noted that ccache is susceptible to general storage problems. If a
bad object file sneaks into the cache for some reason, it will of course stay
bad. Some possible reasons for erroneous object files are bad hardware (disk
drive, disk controller, memory, etc), buggy drivers or file systems, a bad
or compiler wrapper. If this happens, the easiest way of
fixing it is this:
1.Build so that the bad object file ends up
in the build tree.
2.Remove the bad object file from the build
3.Rebuild with CCACHE_RECACHE
An alternative is to clear the whole cache with ccache -C
don’t mind losing other cached results.
There are no reported issues about ccache producing broken object files
reproducibly. That doesn’t mean it can’t happen, so if you find
a repeatable case, please report it.
Credits, mailing list information, bug reporting instructions, source code, etc,
can be found on ccache’s web site: https://ccache.samba.org.
ccache was originally written by Andrew Tridgell and is currently developed and
maintained by Joel Rosdahl. See AUTHORS.txt or AUTHORS.html and
https://ccache.samba.org/credits.html for a list of contributors.