Provided by: ccache_4.10.2-1build1_amd64 
NAME
ccache - a fast C/C++ compiler cache
SYNOPSIS
ccache [ccache options]
ccache [KEY=VALUE ...] compiler [compiler options]
compiler [compiler options]
The first form takes options described in COMMAND LINE OPTIONS below. The second form invokes the
compiler, optionally using configuration options as KEY=VALUE arguments. In the third form, ccache is
masquerading as the compiler as described in RUN MODES.
DESCRIPTION
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.
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 discover an undocumented case
where ccache changes the output of your compiler, please let us know.
RUN MODES
There are two different ways to use ccache to cache a compilation:
1. Prefix your compilation command with ccache. This method is most convenient if you just want to try
out ccache or wish to use it for some specific projects. Example:
ccache gcc -c example.c
2. Let ccache masquerade as the compiler. This method is most useful when you wish to use ccache for all
your compilations. To do this on a Debian system, it’s easiest to just prepend /usr/lib/ccache to
your PATH. /usr/lib/ccache contains symlinks for all compilers currently installed as Debian
packages.
Alternatively, you can create any symlinks you like yourself like this:
cp /usr/bin/ccache /usr/local/bin/
ln -s ccache /usr/local/bin/gcc
ln -s ccache /usr/local/bin/g++
On platforms that don’t support symbolic links you can simply copy ccache to the compiler name
instead for a similar effect:
cp /usr/bin/ccache /usr/local/bin/gcc
cp /usr/bin/ccache /usr/local/bin/g++
And so forth. This will work as long as the directory with symbolic links or ccache copies comes
before the directory with the compiler (typically /usr/bin) in PATH.
Warning
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.
COMMAND LINE OPTIONS
These command line options only apply when you invoke ccache as “ccache”. When ccache masquerades as a
compiler (as described in the previous section), the normal compiler options apply and you should refer
to the compiler’s documentation.
Common options
-c, --cleanup
Clean up the cache by removing not recently used 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 have modified the cache contents manually or believe that the cache size statistics may
be inaccurate.
-C, --clear
Clear the entire cache, removing all cached files, but keeping the configuration file.
--config-path PATH
Let the command line options operate on configuration file PATH instead of the default. Using this
option has the same effect as setting (overriding) the environment variable CCACHE_CONFIGPATH
temporarily.
-d, --dir PATH
Let the command line options operate on cache directory PATH instead of the default. For example, to
show statistics for a cache directory at /shared/ccache you can run ccache -d /shared/ccache -s.
Using this option has the same effect as setting the environment variable CCACHE_DIR temporarily.
--evict-namespace NAMESPACE
Remove files created in the given namespace from the cache.
--evict-older-than AGE
Remove files used less recently than AGE from the cache. AGE should be an unsigned integer with a d
(days) or s (seconds) suffix. If combined with --evict-namespace, only remove files within that
namespace.
-h, --help
Print a summary of command line options.
-F NUM, --max-files NUM
Set the maximum number of files allowed in the cache to NUM. Use 0 for no limit. The value is stored
in a configuration file in the cache directory and applies to all future compilations.
-M SIZE, --max-size SIZE
Set the maximum size of the files stored in the cache. SIZE should be a number followed by an
optional suffix: kB, MB, GB, TB (decimal), KiB, MiB, GiB or TiB (binary). The default suffix is GiB.
Use 0 for no limit. The value is stored in a configuration file in the cache directory and applies to
all future compilations.
-X LEVEL, --recompress LEVEL
Recompress the cache to level LEVEL using the Zstandard algorithm. The level can be an integer, with
the same semantics as the compression_level configuration option, or the special value uncompressed
for no compression. See [Cache compression] for more information. This can potentially take a long
time since all files in the cache need to be visited. Only files that are currently compressed with a
different level than LEVEL will be recompressed.
--recompress-threads THREADS
Use up to THREADS threads when recompressing the cache. The default is to use one thread per CPU.
-o KEY=VALUE, --set-config KEY=VALUE
Set configuration option KEY to VALUE in the configuration file. See CONFIGURATION for more
information.
-x, --show-compression
Print cache compression statistics. See CACHE COMPRESSION for more information. This can potentially
take a long time since all files in the cache need to be visited.
-p, --show-config
Print current configuration options and from where they originate (environment variable,
configuration file or compile-time default) in human-readable format.
--show-log-stats
Print statistics counters from the stats log in human-readable format. See stats_log. Use
-v/--verbose once or twice for more details.
-s, --show-stats
Print a summary of configuration and statistics counters in human-readable format. Use -v/--verbose
once or twice for more details.
-v, --verbose
Increase verbosity. The option can be given multiple times.
-V, --version
Print version and copyright information.
-z, --zero-stats
Zero the cache statistics (but not the configuration options).
Options for remote file-based storage
--trim-dir PATH
Remove not recently used files from directory PATH until it is at most the size specified by
--trim-max-size.
Warning
Don’t use this option to trim the local cache. To trim the local cache directory to a certain
size, use CCACHE_MAXSIZE=SIZE ccache -c.
--trim-max-size SIZE
Specify the maximum size for --trim-dir. SIZE should be a number followed by an optional suffix: kB,
MB, GB, TB (decimal), KiB, MiB, GiB or TiB (binary). The default suffix is GiB. Use 0 for no limit.
--trim-method METHOD
Specify the method to trim a directory with --trim-dir. Possible values are:
atime
LRU (least recently used) using the file access timestamp. This is the default.
mtime
LRU (least recently used) using the file modification timestamp.
--trim-recompress LEVEL
Recompress to level LEVEL using the Zstandard algorithm when using --trim-dir. The level can be an
integer, with the same semantics as the compression_level configuration option, or the special value
uncompressed for no compression. See [Cache compression] for more information. This can potentially
take a long time since all files in the cache need to be visited. Only files that are currently
compressed with a different level than LEVEL will be recompressed.
--trim-recompress-threads THREADS
Recompress using up to THREADS threads with --trim-recompress. The default is to use one thread per
CPU.
Options for scripting or debugging
--checksum-file PATH
Print the checksum (128 bit XXH3) of the file at PATH (- for standard input).
--extract-result PATH
Extract data stored in the result file at PATH (- for standard input). The data will be written to
ccache-result.* files in to the current working directory. This option is only useful when debugging
ccache and its behavior.
--format FORMAT
Specify format for --print-log-stats and --print-stats. Possible values are:
tab
Tab separated. This is the default.
json
JSON formatted.
-k KEY, --get-config KEY
Print the value of configuration option KEY. See CONFIGURATION for more information.
--hash-file PATH
Print the hash (160 bit BLAKE3) of the file at PATH (- for standard input). This is only useful when
debugging ccache and its behavior.
--inspect PATH
Print the content of a result or manifest file at PATH (- for standard input) to standard output in
human-readable format. File content embedded in a result file will however not be printed; use
--extract-result to extract the file content. This option is only useful when debugging ccache and
its behavior.
--print-log-stats
Print statistics counters from the stats log in machine-parsable (tab-separated or JSON) format. See
stats_log and --format.
--print-stats
Print statistics counter IDs and corresponding values in machine-parsable (tab-separated or JSON)
format. See --format.
--print-version
Print version and don’t do anything else.
Extra options
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 --ccache-skip. 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 as-is.
Note
--ccache-skip currently only tells ccache not to interpret the next option as a special compiler
option — the option will still be included in the direct mode hash.
The reason this can be important is that ccache does need to parse the command line and determine what is
an input filename and what is a compiler option, as it needs the input filename to determine the name of
the resulting object file (among other things). The heuristic ccache uses when parsing the command line
is that any argument that exists as a file is treated as an input file name. By using --ccache-skip you
can force an option to not be treated as an input file name and instead be passed along to the compiler
as a command line option.
Another case where --ccache-skip can be useful is if ccache interprets an option specially but shouldn’t,
since the option has another meaning for your compiler than what ccache thinks.
See also ignore_options.
CONFIGURATION
Ccache’s default behavior can be overridden by options in configuration files, 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 priorities of configuration options are as follows (where 1 is highest):
1. Command line settings in KEY=VALUE form. Example:
ccache debug=true compiler_check="%compiler% --version" gcc -c example.c
2. Environment variables.
3. The cache-specific configuration file (see below).
4. The system (read-only) configuration file <sysconfdir>/ccache.conf (typically /etc/ccache.conf or
/usr/local/etc/ccache.conf).
5. Compile-time defaults.
As a special case, if the environment variable CCACHE_CONFIGPATH is set it specifies the configuration
file, and the system configuration file won’t be read.
Location of the configuration file
The location of the cache-specific configuration file is determined like this on non-Windows systems:
1. If CCACHE_CONFIGPATH is set, use that path.
2. Otherwise, if the environment variable CCACHE_DIR is set then use $CCACHE_DIR/ccache.conf.
3. Otherwise, if cache_dir is set in the system configuration file then use <cache_dir>/ccache.conf.
4. Otherwise, if there is a legacy $HOME/.ccache directory then use $HOME/.ccache/ccache.conf.
5. Otherwise, if XDG_CONFIG_HOME is set then use $XDG_CONFIG_HOME/ccache/ccache.conf.
6. Otherwise, use $HOME/Library/Preferences/ccache/ccache.conf (macOS) or
$HOME/.config/ccache/ccache.conf (other systems).
On Windows, this is the method used to find the configuration file:
1. If CCACHE_CONFIGPATH is set, use that path.
2. Otherwise, if the environment variable CCACHE_DIR is set then use %CCACHE_DIR%/ccache.conf.
3. Otherwise, if cache_dir is set in the system configuration file then use <cache_dir>\ccache.conf. The
system-wide configuration on Windows is %ALLUSERSPROFILE%\ccache\ccache.conf by default. The
ALLUSERSPROFILE environment variable is usually C:\ProgramData.
4. Otherwise, if there is a legacy %USERPROFILE%\.ccache directory then use
%USERPROFILE%\.ccache\ccache.conf.
5. Otherwise, use %LOCALAPPDATA%\ccache\ccache.conf if it exists.
6. Otherwise, use %APPDATA%\ccache\ccache.conf.
See also the cache_dir configuration option for how the cache directory location is determined.
Configuration value syntax
All configuration values support expansion of environment variables. The syntax is similar to POSIX shell
syntax: $VAR or ${VAR}. Both variants will expand to the value of the environment variable VAR.
Two consecutive dollar signs ($$) will expand to a single dollar sign ($).
Configuration file syntax
Configuration files are in a simple “key = value” format, one option 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
Boolean values
Some configuration options are boolean values (i.e. truth values). In a configuration file, such values
must be set to the string true or false. For the corresponding environment variables, the semantics are a
bit different:
• A set environment variable means “true” (even if set to the empty string).
• The following case-insensitive negative values are considered an error (instead of surprising the
user): 0, false, disable and no.
• An unset environment variable means “false”.
Each boolean environment variable also has a negated form starting with CCACHE_NO. For example,
CCACHE_COMPRESS can be set to force compression and CCACHE_NOCOMPRESS can be set to force no compression.
Configuration options
Below is a list of available configuration options. The corresponding environment variable name is
indicated in parentheses after each configuration option key.
absolute_paths_in_stderr (CCACHE_ABSSTDERR)
This option specifies whether ccache should rewrite relative paths in the compiler’s standard error
output to absolute paths. This can be useful if you use base_dir with a build system (e.g. CMake with
the "Unix Makefiles" generator) that executes the compiler in a different working directory, which
makes relative paths in compiler errors or warnings incorrect. The default is false.
base_dir (CCACHE_BASEDIR)
This option should be an absolute path to a directory. If set, ccache will rewrite absolute paths
into paths relative to the current working directory, but only absolute paths that begin with
base_dir. Cache results can then be shared for compilations in different directories even if the
project uses absolute paths in the compiler command line. See also the discussion under COMPILING IN
DIFFERENT DIRECTORIES. If set to the empty string (which is the default), no rewriting is done.
A typical path to use as base_dir is your home directory or another directory that is a parent of
your project directories. Don’t use / as the base directory since that will make ccache also rewrite
paths to system header files, which typically is counterproductive.
For example, say that Alice’s current working directory is /home/alice/project1/build and that she
compiles like this:
ccache gcc -I/usr/include/example -I/home/alice/project2/include -c /home/alice/project1/src/example.c
Here is what ccache will actually execute for different base_dir values:
# Current working directory: /home/alice/project1/build
# With base_dir = /:
gcc -I../../../../usr/include/example -I../../project2/include -c ../src/example.c
# With base_dir = /home or /home/alice:
gcc -I/usr/include/example -I../../project2/include -c ../src/example.c
# With base_dir = /home/alice/project1 or /home/alice/project1/src:
gcc -I/usr/include/example -I/home/alice/project2/include -c ../src/example.c
If Bob has put project1 and project2 in /home/bob/stuff and both users have set base_dir to /home or
/home/$USER, then Bob will get a cache hit (if they share ccache directory) since the actual command
line will be identical to Alice’s command line:
# Current working directory: /home/bob/stuff/project1/build
# With base_dir = /home or /home/bob:
gcc -I/usr/include/example -I../../project2/include -c ../src/example.c
Without base_dir there will be a cache miss since the absolute paths will differ. With base_dir set
to / there will be a cache miss since the relative path to /usr/include/example will be different.
With base_dir set to /home/bob/stuff/project1 there will a cache miss since the path to project2 will
be a different absolute path.
Warning
Rewriting absolute paths to relative is kind of a brittle hack. It works OK in many cases, but
there might be cases where things break. One known issue is that absolute paths are not
reproduced in dependency files, which can mess up dependency detection in tools like Make and
Ninja. If possible, use relative paths in the first place instead instead of using base_dir.
cache_dir (CCACHE_DIR)
This option specifies where ccache will keep its cached compiler outputs.
On non-Windows systems, the default is $HOME/.ccache if such a directory exists, otherwise
$XDG_CACHE_HOME/ccache if XDG_CACHE_HOME is set, otherwise $HOME/Library/Caches/ccache (macOS) or
$HOME/.config/ccache (other systems).
On Windows, the default is %USERPROFILE%\.ccache if such a directory exists, otherwise
%LOCALAPPDATA%\ccache.
Warning
Previous ccache versions defaulted to storing the cache in %APPDATA%\ccache on Windows. This can
result in large network file transfers of the cache in domain environments and similar problems.
Please check this directory for cache directories and either delete them or the whole directory,
or move them to the %LOCALAPPDATA%\ccache directory.
See also Location of the configuration file.
compiler (CCACHE_COMPILER or (deprecated) CCACHE_CC)
This option 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.
compiler_check (CCACHE_COMPILERCHECK)
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. If compiler plugins are used, these
plugins will also be added to the hash. This option can be used to select another strategy. Possible
values are:
content
Hash the content of the compiler binary. This makes ccache very slightly slower compared to
mtime, but makes it cope better with compiler upgrades during a build bootstrapping process.
mtime
Hash the compiler’s mtime and size, which is fast. This is the default.
none
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 none if you know what you are doing.
string:value
Hash value. This can for instance be a compiler revision number or another string that the build
system generates to identify the compiler.
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% -v
%compiler% -dumpmachine; %compiler% -dumpversion
You should make sure that the specified command is as fast as possible since it will be run once
for each ccache invocation.
Identifying the compiler using a command is useful if you want to avoid cache misses when the
compiler has been rebuilt but not changed.
Another case is when the compiler (as seen by ccache) actually isn’t the real compiler but
another compiler wrapper — in that case, the default mtime method will hash the mtime and size of
the other compiler wrapper, which means that ccache won’t be able to detect a compiler upgrade.
Using a suitable command to identify the compiler is thus safer, but it’s also slower, so you
should consider continue using the mtime method in combination with the prefix_command option if
possible. See USING CCACHE WITH OTHER COMPILER WRAPPERS.
compiler_type (CCACHE_COMPILERTYPE)
Ccache normally guesses the compiler type based on the compiler name. The compiler_type option lets
you force a compiler type. This can be useful if the compiler has a non-standard name but is actually
one of the known compiler types. Possible values are:
auto
Guess one of the types below based on the compiler name (following symlinks). This is the
default.
clang
Clang-based compiler.
clang-cl
clang-cl.
gcc
GCC-based compiler.
icl
Intel compiler on Windows.
msvc
Microsoft Visual C++ (MSVC).
nvcc
NVCC (CUDA) compiler.
other
Any compiler other than the known types.
compression (CCACHE_COMPRESS or CCACHE_NOCOMPRESS, see Boolean values above)
If true, ccache will compress data it puts in the cache. However, this option has no effect on how
files are retrieved from the cache; compressed and uncompressed results will still be usable
regardless of this option. The default is true.
Compression is done using the Zstandard algorithm. The algorithm is fast enough that there should be
little reason to turn off compression to gain performance. One exception is if the cache is located
on a compressed file system, in which case the compression performed by ccache of course is
redundant.
Compression will be disabled if file cloning (the file_clone option) or hard linking (the hard_link
option) is enabled.
compression_level (CCACHE_COMPRESSLEVEL)
This option determines the level at which ccache will compress object files using the real-time
compression algorithm Zstandard. It only has effect if compression is enabled (which it is by
default). Zstandard is extremely fast for decompression and very fast for compression for lower
compression levels. The default is 0.
Semantics of compression_level:
> 0
A positive value corresponds to normal Zstandard compression levels. Lower levels (e.g. 1) mean
faster compression but worse compression ratio. Higher levels (e.g. 19) mean slower compression
but better compression ratio. The maximum possible value depends on the libzstd version, but at
least up to 19 is available for all versions. Decompression speed is essentially the same for all
levels. As a rule of thumb, use level 5 or lower since higher levels may slow down compilations
noticeably. Higher levels are however useful when recompressing the cache with command line
option -X/--recompress.
< 0
A negative value corresponds to Zstandard’s “ultra-fast” compression levels, which are even
faster than level 1 but with less good compression ratios. For instance, level -3 corresponds to
--fast=3 for the zstd command line tool. In practice, there is little use for levels lower than
-5 or so.
0 (default)
The value 0 means that ccache will choose a suitable level, currently 1.
See the Zstandard documentation <http://zstd.net> for more information.
cpp_extension (CCACHE_EXTENSION)
This option can be used to force a certain extension for the intermediate preprocessed file. The
default is to automatically determine the extension to use for intermediate preprocessor files based
on the type of file being compiled, but that sometimes doesn’t work. For example, when using the
“aCC” compiler on HP-UX, set the cpp extension to i.
debug (CCACHE_DEBUG or CCACHE_NODEBUG, see Boolean values above)
If true, enable the debug mode. The debug mode creates per-object debug files that are helpful when
debugging unexpected cache misses. Note however that ccache performance will be reduced slightly. See
CACHE DEBUGGING for more information. The default is false.
debug_dir (CCACHE_DEBUGDIR)
Specifies where to write per-object debug files if the debug mode is enabled. If set to the empty
string, the files will be written next to the object file. If set to a directory, the debug files
will be written with full absolute paths in that directory, creating it if needed. The default is the
empty string.
For example, if debug_dir is set to /example, the current working directory is /home/user and the
object file is build/output.o then the debug log will be written to
/example/home/user/build/output.o.ccache-log. See also CACHE DEBUGGING.
debug_level (CCACHE_DEBUGLEVEL)
Specifies the amount of information that is written when the debug mode is enabled. See CACHE
DEBUGGING for more information. The default is 2.
depend_mode (CCACHE_DEPEND or CCACHE_NODEPEND, see Boolean values above)
If true, the depend mode will be used. The default is false. See The depend mode.
direct_mode (CCACHE_DIRECT or CCACHE_NODIRECT, see Boolean values above)
If true, the direct mode will be used. The default is true. See The direct mode.
disable (CCACHE_DISABLE or CCACHE_NODISABLE, see Boolean values above)
When true, ccache will just call the real compiler, bypassing the cache completely. The default is
false.
It is also possible to disable ccache for a specific source code file by adding the string
ccache:disable in a comment in the first 4096 bytes of the file.
extra_files_to_hash (CCACHE_EXTRAFILES)
This option 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.
file_clone (CCACHE_FILECLONE or CCACHE_NOFILECLONE, see Boolean values above)
If true, ccache will attempt to use file cloning (also known as “copy on write”, “CoW” or “reflinks”)
to store and fetch cached compiler results. file_clone has priority over hard_link. The default is
false.
Files stored by cloning cannot be compressed, so the cache size will likely be significantly larger
if this option is enabled. However, performance may be improved depending on the use case.
Unlike the hard_link option, file_clone is completely safe to use, but not all file systems support
the feature. For such file systems, ccache will fall back to use plain copying (or hard links if
hard_link is enabled).
hard_link (CCACHE_HARDLINK or CCACHE_NOHARDLINK, see Boolean values above)
If true, ccache will attempt to use hard links to store and fetch cached object files. The default is
false.
Files stored via hard links cannot be compressed, so the cache size will likely be significantly
larger if this option is enabled. However, performance may be improved depending on the use case.
Warning
Do not enable this option unless you are aware of these caveats:
• If the resulting file is modified, the file in the cache will also be modified since they share
content, which corrupts the cache entry. As of version 4.0, ccache makes stored and fetched
object files read-only as a safety measure. Furthermore, a simple integrity check is made for
cached object files by verifying that their sizes are correct. This means that mistakes like
strip file.o or echo >file.o will be detected even if the object file is made writable, but a
modification that doesn’t change the file size will not.
• Programs that don’t expect that files from two different identical compilations are hard links to
each other can fail.
• Programs that rely on modification times (like make) can be confused if several users (or one
user with several build trees) use the same cache directory. The reason for this is that the
object files share i-nodes and therefore modification times. If file.o is in build tree A
(hard-linked from the cache) and file.o then is produced by ccache in build tree B by
hard-linking from the cache, the modification timestamp will be updated for file.o in build tree
A as well. This can retrigger relinking in build tree A even though nothing really has changed.
hash_dir (CCACHE_HASHDIR or CCACHE_NOHASHDIR, see Boolean values above)
If true (which is the default), ccache will include the current working directory (CWD) in the hash
that is used to distinguish two compilations when generating debug info (compiler option -g with
variations). Exception: The CWD will not be included in the hash if base_dir is set (and matches the
CWD) and the compiler option -fdebug-prefix-map is used. See also the discussion under COMPILING IN
DIFFERENT DIRECTORIES.
The reason for including the CWD in the hash by default is to prevent a problem with the storage of
the current working directory in the debug info of an object file, which can lead ccache to return a
cached object file that has the working directory in the debug info set incorrectly.
You can disable this option 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.
ignore_headers_in_manifest (CCACHE_IGNOREHEADERS)
This option 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.
ignore_options (CCACHE_IGNOREOPTIONS)
This option is a space-delimited list of compiler options that ccache will ignore. Entries in the
list can optionally end with an asterisk (*) to matching any option suffix. For example,
-fmessage-length=* will match both -fmessage-length=20 and -fmessage-length=70. A matching compiler
option will neither be interpreted specially nor be part of the input hash. Ignoring a compiler
option from the hash can be useful when you know it doesn’t affect the result (and ccache doesn’t
know that), or when it does and you don’t care. See also Extra options.
inode_cache (CCACHE_INODECACHE or CCACHE_NOINODECACHE, see Boolean values above)
If true, ccache will cache source file hashes based on device, inode and timestamps. This reduces the
time spent on hashing include files since the result can be reused between compilations. The default
is true. The feature requires temporary_dir to be located on a local filesystem of a supported type.
Note
Support for the inode cache feature on Windows is experimental. On Windows the default is false.
keep_comments_cpp (CCACHE_COMMENTS or CCACHE_NOCOMMENTS, see Boolean values above)
If true, ccache will not discard the comments before hashing preprocessor output. The default is
false. This can be used to check documentation with -Wdocumentation.
log_file (CCACHE_LOGFILE)
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.
If set to syslog, ccache will log using syslog() instead of to a file. If you use rsyslogd, you can
add something like this to /etc/rsyslog.conf or a file in /etc/rsyslog.d:
# log ccache to file
:programname, isequal, "ccache" /var/log/ccache
# remove from syslog
& ~
max_files (CCACHE_MAXFILES)
This option specifies the maximum number of files to keep in the cache. Use 0 for no limit (which is
the default). See also CACHE SIZE MANAGEMENT.
max_size (CCACHE_MAXSIZE)
This option specifies the maximum size of the cache. Use 0 for no limit. The default value is 5G.
Available suffixes: k, M, G, T (decimal) and Ki, Mi, Gi, Ti (binary). The default suffix is G. See
also CACHE SIZE MANAGEMENT.
msvc_dep_prefix (CCACHE_MSVC_DEP_PREFIX)
This option specifies the prefix of included files output for MSVC compiler. The default prefix is
“Note: including file:”. If you use a localized compiler, this should be set accordingly.
namespace (CCACHE_NAMESPACE)
If set, the namespace string will be added to the hashed data for each compilation. This will make
the associated cache entries logically separate from cache entries with other namespaces, but they
will still share the same storage space. Cache entries can also be selectively removed from the local
cache with the command line option --evict-namespace, potentially in combination with
--evict-older-than.
For instance, if you use the same local cache for several disparate projects, you can use a unique
namespace string for each one. This allows you to remove cache entries that belong to a certain
project if you stop working with that project.
path (CCACHE_PATH)
If set, ccache will search directories in this list when looking for the real compiler. The list
separator is semicolon on Windows systems and colon on other systems. If not set, ccache will look
for the first executable matching the compiler name in the normal PATH that isn’t a symbolic link to
ccache itself.
pch_external_checksum (CCACHE_PCH_EXTSUM or CCACHE_NOPCH_EXTSUM, see Boolean values above)
When this option is set, and ccache finds a precompiled header file, ccache will look for a file with
the extension “.sum” added (e.g. “pre.h.gch.sum”), and if found, it will hash this file instead of
the precompiled header itself to work around the performance penalty of hashing very large files.
prefix_command (CCACHE_PREFIX)
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.
prefix_command_cpp (CCACHE_PREFIX_CPP)
This option adds a list of prefixes (separated by space) to the command line that ccache uses when
invoking the preprocessor.
read_only (CCACHE_READONLY or CCACHE_NOREADONLY, see Boolean values above)
If true, ccache will attempt to use existing cached results, but it will not add new results to any
cache backend. Statistics counters will still be updated, though, unless the stats option is set to
false.
If you are using this because your ccache directory is read-only, you need to set temporary_dir since
ccache will fail to create temporary files otherwise. You may also want to set stats to false make
ccache not even try to update stats files.
read_only_direct (CCACHE_READONLY_DIRECT or CCACHE_NOREADONLY_DIRECT, see Boolean values above)
Just like read_only except that ccache will only try to retrieve results from the cache using the
direct mode, not the preprocessor mode. See documentation for read_only regarding using a read-only
ccache directory.
recache (CCACHE_RECACHE or CCACHE_NORECACHE, see Boolean values above)
If true, ccache will not use any previously stored result. New results will still be cached, possibly
overwriting any pre-existing results.
remote_only (CCACHE_REMOTE_ONLY or CCACHE_NOREMOTE_ONLY, see Boolean values above)
If true, ccache will only use remote storage. The default is false. Note that cache statistics
counters will still be kept in the local cache directory unless stats is false. See also Storage
interaction.
remote_storage (CCACHE_REMOTE_STORAGE)
This option specifies one or several storage backends (separated by space) to query after checking
the local cache (unless remote_only is true). See REMOTE STORAGE BACKENDS for documentation of syntax
and available backends.
Examples:
• file:/shared/nfs/directory
• file:///shared/nfs/one|read-only file:///shared/nfs/two
• file:///Z:/example/windows/folder
• http://example.com/cache
• redis://example.com
Note
In previous ccache versions this option was called secondary_storage
(CCACHE_SECONDARY_STORAGE), which can still be used as an alias.
reshare (CCACHE_RESHARE or CCACHE_NORESHARE, see Boolean values above)
If true, ccache will write results to remote storage even for local storage cache hits. The default
is false.
run_second_cpp (CCACHE_CPP2 or CCACHE_NOCPP2, see Boolean values above)
If true, ccache will first run the preprocessor to preprocess the source code (see The preprocessor
mode) and then on a cache miss run the compiler on the source code to get hold of the object file.
This is the default.
If false, ccache will first run preprocessor to preprocess the source code and then on a cache miss
run the compiler on the preprocessed source code instead of the original source code. This makes
cache misses slightly faster since the source code only has to be preprocessed once. The downside is
that some compilers won’t produce the same result (for instance diagnostics warnings) when compiling
preprocessed source code.
A solution to the above mentioned downside is to set run_second_cpp to false and pass
-fdirectives-only (for GCC) or -frewrite-includes (for Clang) to the compiler. This will cause the
compiler to leave the macros and other preprocessor information, and only process the #include
directives. When run in this way, the preprocessor arguments will be passed to the compiler since it
still has to do some preprocessing (like macros).
This option is ignored with MSVC, as there is no way to make it compile without preprocessing first.
sloppiness (CCACHE_SLOPPINESS)
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 option 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 one or several of the following values:
clang_index_store
Ignore the Clang compiler option -index-store-path and its argument when computing the manifest
hash. This is useful if you use Xcode, which uses an index store path derived from the local
project path. Note that the index store won’t be updated correctly on cache hits if you enable
this sloppiness.
file_stat_matches
Ccache normally examines a file’s contents to determine whether it matches the cached version.
With this sloppiness set, ccache will consider a file as matching its cached version if the
mtimes and ctimes match.
file_stat_matches_ctime
Ignore ctimes when file_stat_matches is enabled. This can be useful when backdating files' mtimes
in a controlled way.
gcno_cwd
By default, ccache will include the current working directory in the hash when producing a .gcno
file (when compiling with -ftest-coverage). This is because GCC 9+ includes the current working
directory in the .gcno file. The gcno_cwd sloppiness makes ccache not hash the current working
directory so that you can get cache hits when compiling in different directories, with the
tradeoff of potentially getting an incorrect directory in the .gcno file. gcno_cwd also disables
hashing of the current working directory if -fprofile-abs-path is used.
Note
gcno_cwd sloppiness will not have any effect when compiling with --coverage since that
implies -fprofile-arcs which always forces CWD to be included in the input hash.
incbin
By default, ccache will ignore all files containing an .incbin directive. While this is the
correct behaviour as ccache does not detect incbin changes, this restriction can make some
projects difficult to cache. This sloppiness will pretend the .incbin directive doesn’t exist and
simply allow caching.
include_file_ctime
By default, ccache will disable caching if a source code file has a status change time (ctime)
after the start of the ccache invocation. This sloppiness disables that check. See also [Handling
of newly created source files].
include_file_mtime
By default, ccache will disable caching if a source code file has a modification time (mtime)
after the start of the ccache invocation. This sloppiness disables that check. See also [Handling
of newly created source files].
ivfsoverlay
Ignore the Clang compiler option -ivfsoverlay and its argument. This is useful if you use Xcode,
which uses a virtual file system (VFS) for things like combining Objective-C and Swift code.
locale
Ccache includes the environment variables LANG, LC_ALL, LC_CTYPE and LC_MESSAGES in the hash by
default since they may affect localization of compiler warning messages. Set this sloppiness to
tell ccache not to do that.
modules
By default, ccache will not cache compilations if -fmodules is used since it cannot hash the
state of compiler’s internal representation of relevant modules. This sloppiness allows caching
in such a case. See C++ MODULES for more information.
pch_defines
Be sloppy about #define directives when precompiling a header file. See PRECOMPILED HEADERS for
more information.
random_seed
Ignore the -frandom-seed option and its arguments when computing the input hash. This is useful
if your build system generates different seeds between builds and you are OK with reusing cached
results.
system_headers
Only check non-system headers in direct mode. This can be useful if e.g. your system headers tend
to change but you know that the changes don’t matter. Notes:
• This sloppiness is only supported for GCC-like compilers, not MSVC.
• System headers are still taken into account for preprocessed lookup.
• You can get stale cache hits if the system headers do change in incompatible ways.
• See also the ignore_headers_in_manifest setting.
time_macros
Ignore __DATE__, __TIME__ and __TIMESTAMP__ being present in the source code.
See the discussion under TROUBLESHOOTING for more information.
stats (CCACHE_STATS or CCACHE_NOSTATS, see Boolean values above)
If true, ccache will update the statistics counters on each compilation. The default is true. If
false, [automatic cleanup] will be disabled as well.
stats_log (CCACHE_STATSLOG)
If set to a file path, ccache will write statistics counter updates to the specified file. This is
useful for getting statistics for individual builds. To show a summary of the current stats log, use
ccache --show-log-stats.
Note
Lines in the stats log starting with a hash sign (#) are comments.
temporary_dir (CCACHE_TEMPDIR)
This option specifies where ccache will put temporary files. The default is
$XDG_RUNTIME_DIR/ccache-tmp (typically /run/user/<UID>/ccache-tmp) if XDG_RUNTIME_DIR is set and the
directory exists, otherwise <cache_dir>/tmp.
Note
In previous versions of ccache, CCACHE_TEMPDIR had to be on the same filesystem as the CCACHE_DIR
path, but this requirement has been relaxed.
umask (CCACHE_UMASK)
This option (an octal integer) specifies the umask for files and directories in the cache directory.
This is mostly useful when you wish to share your cache with other users.
Disabling ccache
To disable ccache completely for all invocations, set disable = true (CCACHE_DISABLE=1). You can also
disable ccache for a certain source code file by adding the string ccache:disable in a comment in the
first 4096 bytes of the file. In the latter case the Ccache disabled statistics counter will be
increased.
REMOTE STORAGE BACKENDS
The remote_storage option lets you configure ccache to use one or several remote storage backends. By
default, the local cache directory located in cache_dir will be queried first and remote storage second,
but remote_only can be set to true to disable local storage. Note that cache statistics counters will
still be kept in the local cache directory — remote storage backends only store compilation results and
manifests.
A remote storage backend is specified with a URL, optionally followed by a pipe (|) and a pipe-separated
list of attributes. An attribute is key=value or just key as a short form of key=true. Attribute values
must be percent-encoded <https://en.wikipedia.org/wiki/Percent-encoding> if they contain percent, pipe or
space characters.
Attributes for all backends
These optional attributes are available for all remote storage backends:
• read-only: If true, only read from this backend, don’t write. The default is false.
• shards: A comma-separated list of names for sharding (partitioning) the cache entries using
Rendezvous hashing <https://en.wikipedia.org/wiki/Rendezvous_hashing>, typically to spread the cache
over a server cluster. When set, the storage URL must contain an asterisk (*), which will be replaced
by one of the shard names to form a real URL. A shard name can optionally have an appended weight
within parentheses to indicate how much of the key space should be associated with that shard. A
shard with weight w will contain w/S of the cache, where S is the sum of all shard weights. A weight
could for instance be set to represent the available memory for a memory cache on a specific server.
The default weight is 1.
Examples:
• redis://cache-*.example.com|shards=a(3),b(1),c(1.5) will put 55% (3/5.5) of the cache on
redis://cache-a.example.com, 18% (1/5.5) on redis://cache-b.example.com and 27% (1.5/5.5) on
redis://cache-c.example.com.
• http://example.com/*|shards=alpha,beta will put 50% of the cache on http://example.com/alpha and
50% on http://example.com/beta.
Storage interaction
The table below describes the interaction between local and remote storage on cache hits and misses if
remote_only is false (which is the default):
┌───────────────┬────────────────┬──────────────────────────────┐
│ │ │ │
│ Local storage │ Remote storage │ What happens │
├───────────────┼────────────────┼──────────────────────────────┤
│ │ │ │
│ miss │ miss │ Compile, write to local, │
│ │ │ write to remote[1] │
├───────────────┼────────────────┼──────────────────────────────┤
│ │ │ │
│ miss │ hit │ Read from remote, write to │
│ │ │ local │
├───────────────┼────────────────┼──────────────────────────────┤
│ │ │ │
│ hit │ - │ Read from local, don’t write │
│ │ │ to remote[2] │
└───────────────┴────────────────┴──────────────────────────────┘
[1] Unless remote storage has attribute read-only=true.
[2] Unless local storage is set to share its cache hits with the reshare option.
If remote_only is true:
┌───────────────┬────────────────┬───────────────────────────┐
│ │ │ │
│ Local storage │ Remote storage │ What happens │
├───────────────┼────────────────┼───────────────────────────┤
│ │ │ │
│ - │ miss │ Compile, write to remote, │
│ │ │ don’t write to local │
├───────────────┼────────────────┼───────────────────────────┤
│ │ │ │
│ - │ hit │ Read from remote, don’t │
│ │ │ write to local │
└───────────────┴────────────────┴───────────────────────────┘
File storage backend
URL format: file:DIRECTORY or file://[HOST]DIRECTORY
This backend stores data as separate files in a directory structure below DIRECTORY, similar (but not
identical) to the local cache storage. A typical use case for this backend would be sharing a cache on an
NFS directory. DIRECTORY must start with a slash. HOST can be the empty string or localhost. On Windows,
HOST can also be the name of a server hosting a shared folder.
Important
ccache will not perform any cleanup of the storage — that has to be done by other means, for instance
by running ccache --trim-dir periodically.
Examples:
• file:/shared/nfs/directory
• file:///shared/nfs/directory|umask=002|update-mtime=true
• file:///Z:/example/windows/folder
• file://example.com/shared/ccache%20folder
Optional attributes:
• layout: How to store file under the cache directory. Available values:
• flat: Store all files directly under the cache directory.
• subdirs: Store files in 256 subdirectories of the cache directory.
The default is subdirs.
• umask: This attribute (an octal integer) overrides the umask to use for files and directories in the
cache directory.
• update-mtime: If true, update the modification time (mtime) of cache entries that are read. The
default is false.
HTTP storage backend
URL format: http://HOST[:PORT][/PATH]
This backend stores data in an HTTP-compatible server. The required HTTP methods are GET, PUT and DELETE.
Important
ccache will not perform any cleanup of the storage — that has to be done by other means, for instance
by running ccache --trim-dir periodically.
Note
HTTPS is not supported.
Tip
See How to set up HTTP storage <https://ccache.dev/howto/http-storage.html> for hints on how to set
up an HTTP server for use with ccache.
Examples:
• http://localhost
• http://someusername:p4ssw0rd@example.com/cache/
• http://localhost:8080|layout=bazel|connect-timeout=50
Optional attributes:
• bearer-token: Bearer token used to authorize the HTTP requests.
• connect-timeout: Timeout (in ms) for network connection. The default is 100.
• keep-alive: If true, keep the HTTP connection to the storage server open to avoid reconnects. The
default is true.
• layout: How to map key names to the path part of the URL. Available values:
• bazel: Store values in a format compatible with the Bazel HTTP caching protocol. More
specifically, the entries will be stored as 64 hex digits under the /ac/ part of the cache.
Note
You may have to disable verification of action cache values in the server for this to work
since ccache entries are not valid action result metadata values.
• flat: Append the key directly to the path part of the URL (with a leading slash if needed).
• subdirs: Append the first two characters of the key to the URL (with a leading slash if needed),
followed by a slash and the rest of the key. This divides the entries into 256 buckets.
The default is subdirs.
• operation-timeout: Timeout (in ms) for HTTP requests. The default is 10000.
Redis storage backend
URL formats:
redis://[[USERNAME:]PASSWORD@]HOST[:PORT][/DBNUMBER]
redis+unix:SOCKET_PATH[?db=DBNUMBER]
redis+unix://[[USERNAME:]PASSWORD@localhost]SOCKET_PATH[?db=DBNUMBER]
This backend stores data in a Redis <https://redis.io> (or Redis-compatible) server. There are
implementations for both memory-based and disk-based storage. PORT defaults to 6379 and DBNUMBER defaults
to 0.
Note
ccache will not perform any cleanup of the Redis storage, but you can configure LRU eviction
<https://redis.io/topics/lru-cache>.
Tip
See How to set up Redis <https://ccache.dev/howto/redis-storage.html> storage" for hints on setting
up a Redis server for use with ccache.
Tip
You can set up a cluster of Redis servers using the shards attribute described in REMOTE STORAGE
BACKENDS.
Examples:
• redis://localhost
• redis://p4ssw0rd@cache.example.com:6379/0|connect-timeout=50
• redis+unix:/run/redis.sock
• redis+unix:///run/redis.sock
• redis+unix://p4ssw0rd@localhost/run/redis.sock?db=0
Optional attributes:
• connect-timeout: Timeout (in ms) for network connection. The default is 100.
• operation-timeout: Timeout (in ms) for Redis commands. The default is 10000.
CACHE SIZE MANAGEMENT
By default, ccache has a 5 GB limit on the total size of files in the cache and no limit on the number of
files. You can set different limits using the command line options -M/--max-size and -F/--max-files. Use
the -s/--show-stats option to see the cache size and the currently configured limits (in addition to
other various statistics).
Cleanup can be triggered in two different ways: automatic and manual.
Automatic cleanup
After a new compilation result has been written to the local cache, ccache will trigger an automatic
cleanup if max_size or max_files is exceeded. The cleanup removes cache entries in LRU (least recently
used) order based on the modification time (mtime) of files in the cache. For this reason, ccache updates
mtime of the cache files read on a cache hit to mark them as recently used.
Manual cleanup
You can run ccache -c/--cleanup to force cleanup of the whole cache. This will recalculate the cache size
information and also make sure that the cache size does not exceed max_size and max_files.
CACHE COMPRESSION
Ccache will by default compress all data it puts into the cache using the compression algorithm Zstandard
<http://zstd.net> (zstd) using compression level 1. The algorithm is fast enough that there should be
little reason to turn off compression to gain performance. One exception is if the cache is located on a
compressed file system, in which case the compression performed by ccache of course is redundant. See the
documentation for the configuration options compression and compression_level for more information.
You can use the command line option -x/--show-compression to print information related to compression.
Example:
Total data: 14.8 GB (16.0 GB disk blocks)
Compressed data: 11.3 GB (30.6% of original size)
Original size: 36.9 GB
Compression ratio: 3.267 x (69.4% space savings)
Incompressible data: 3.5 GB
Notes:
• The “disk blocks” size is the cache size when taking disk block size into account. This value should
match the “Cache size” value from “ccache --show-stats”. The other size numbers refer to actual
content sizes.
• “Compressed data” refers to result and manifest files stored in the cache.
• “Incompressible data” refers to files that are always stored uncompressed (triggered by enabling
file_clone or hard_link) or unknown files (for instance files created by older ccache versions).
• The compression ratio is affected by compression_level.
The cache data can also be recompressed to another compression level (or made uncompressed) with the
command line option -X/--recompress. If you choose to disable compression by default or to use a low
compression level, you can (re)compress newly cached data with a higher compression level after the build
or at another time when there are more CPU cycles available, for instance every night. Full recompression
potentially takes a lot of time, but only files that are currently compressed with a different level than
the target level will be recompressed.
CACHE STATISTICS
ccache --show-stats shows a summary of statistics, including cache size, cleanups (number of performed
cleanups, either implicitly due to a cache size limit being reached or due to explicit ccache -c calls),
overall hit rate, hit rate for direct/preprocessed modes and hit rate for local and remote storage.
The summary also includes counters called “Errors” and “Uncacheable”, which are sums of more detailed
counters. To see those detailed counters, use the -v/--verbose flag. The verbose mode can show the
following counters:
┌────────────────────────────────────┬───────────────────────────────────────┐
│ │ │
│ Counter │ Description │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Autoconf compile/link │ Uncacheable compilation or linking by │
│ │ an Autoconf test. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Bad compiler arguments │ Malformed compiler argument, e.g. │
│ │ missing a value for a compiler option │
│ │ that requires an argument or failure │
│ │ to read a file specified by a │
│ │ compiler option argument. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Called for linking │ The compiler was called for linking, │
│ │ not compiling. Ccache only supports │
│ │ compilation of a single file, i.e. │
│ │ calling the compiler with the -c │
│ │ option to produce a single object │
│ │ file from a single source file. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Called for preprocessing │ The compiler was called for │
│ │ preprocessing, not compiling. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Ccache disabled │ Ccache was disabled by a │
│ │ ccache:disable string in the source │
│ │ code file. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Could not use modules │ Preconditions for using C++ MODULES │
│ │ were not fulfilled. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Could not use precompiled header │ Preconditions for using precompiled │
│ │ headers were not fulfilled. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Could not read or parse input file │ An input file could not be read or │
│ │ parsed (see the debug log for │
│ │ details). │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Could not write to output file │ The output path specified with -o │
│ │ could not be written to. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Compilation failed │ The compilation failed. No result │
│ │ stored in the cache. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Compiler check failed │ A compiler check program specified by │
│ │ compiler_check (CCACHE_COMPILERCHECK) │
│ │ failed. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Compiler output file missing │ One of the files expected to be │
│ │ produced by the compiler was missing │
│ │ after compilation. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Compiler produced empty output │ The compiler’s output file (typically │
│ │ an object file) was empty after │
│ │ compilation. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Could not 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 │
│ │ (CCACHE_EXTRAFILES). │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Forced recache │ CCACHE_RECACHE was used to overwrite │
│ │ an existing result. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Input file modified during │ An input file was modified during │
│ compilation │ compilation. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Internal error │ Unexpected failure, e.g. due to │
│ │ problems reading/writing the cache. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Missing cache file │ 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 │
│ │ cleanup. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ 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 stdout │ The compiler was instructed to write │
│ │ its output to standard output using │
│ │ -o -. This is not supported by │
│ │ ccache. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Preprocessing failed │ Preprocessing the source code using │
│ │ the compiler’s -E option failed. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ 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 environment variable │ An environment variable not supported │
│ │ by ccache was set. │
├────────────────────────────────────┼───────────────────────────────────────┤
│ │ │
│ Unsupported source language │ A source language e.g. specified with │
│ │ -x was unsupported by ccache. │
└────────────────────────────────────┴───────────────────────────────────────┘
HOW CCACHE WORKS
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
BLAKE3, a very fast cryptographic hash algorithm, for the hashing. On a cache hit, ccache is able to
supply all of the correct compiler outputs (including all warnings, dependency file, etc) from the cache.
Data stored in the cache is checksummed with XXH3, an extremely fast non-cryptographic algorithm, to
detect corruption.
Ccache has two ways of gathering information used to look up results in the cache:
• the preprocessor mode, where ccache runs the preprocessor on the source code and hashes the result
• the direct mode, where ccache hashes the source code and include files directly
The direct mode is generally faster since running the preprocessor has some overhead.
If no previous result is detected (i.e., there is a cache miss) using the direct mode, ccache will fall
back to the preprocessor mode unless the depend mode is enabled. In the depend mode, ccache never runs
the preprocessor, not even on cache misses. Read more in The depend mode below.
Common hashed information
The following information is always 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
compiler_check)
• the name of the compiler
• the current directory (if hash_dir is enabled)
• contents of files specified by extra_files_to_hash (if any)
The preprocessor mode
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 those that affect include files (-I, -include, -D, etc; the theory is
that these command line options will change the preprocessor output if they have any effect at all)
• any standard error output generated by the preprocessor
Based on the hash, the cached compilation result can be looked up directly in the cache.
The direct mode
In the direct mode, the hash is formed of the common information and:
• the input source file
• the compiler options
Based on the hash, a data structure called “manifest” is looked up in the cache. The manifest contains:
• references to cached compilation results (object file, dependency file, etc) that were produced by
previous compilations that matched the hash
• paths to the include files that were read at the time the compilation results were stored in the
cache
• hash sums of the include files at the time the compilation results were stored in the cache
The current contents of the include files are then hashed and compared to the information in the
manifest. If there is a match, ccache knows the result of the compilation. If there is no match, ccache
falls back to running the preprocessor. The output from the preprocessor is parsed to find the include
files that were read. The paths and hash sums of those include files are then stored in the manifest
along with information about the produced compilation result.
There is a catch with the direct mode: header files that were used by the compiler are recorded, but
header files that were not used, but would have been used if they existed, are not. So, when ccache
checks if a result can be taken from the cache, it currently can’t check if the existence of a new header
file should invalidate the result. In practice, the direct mode is safe to use in the absolute majority
of cases.
The direct mode will be disabled if any of the following holds:
• direct_mode is false
• a modification time of one of the include files is too new (needed to avoid a race condition)
• a compiler option not supported by the direct mode is used, for example:
• a -Wp,* compiler option other than -Wp,-MD,<path>, -Wp,-MMD,<path>, -Wp,-D<macro[=defn]> or
-Wp,-U<macro>
• most uses of -Xpreprocessor
• the string __TIME__ is present in the source code
The depend mode
If the depend mode is enabled, ccache will not use the preprocessor at all. The hash used to identify
results in the cache will be based on the direct mode hash described above plus information about include
files read from the dependency list generated by MSVC with /showIncludes, or the dependency file
generated by other compilers with -MD or -MMD.
Advantages:
• The ccache overhead of a cache miss will be much smaller.
• Not running the preprocessor at all can be good if compilation is performed remotely, for instance
when using distcc or similar; ccache then won’t make potentially costly preprocessor calls on the
local machine.
Disadvantages:
• The cache hit rate will likely be lower since any change to compiler options or source code will make
the hash different. Compare this with the default setup where ccache will fall back to the
preprocessor mode, which is tolerant to some types of changes of compiler options and source code
changes.
• If -MD is used, the manifest entries will include system header files as well, thus slowing down
cache hits slightly, just as using -MD slows down make. This is also the case for MSVC with
/showIncludes.
• If -MMD is used, the manifest entries will not include system header files, which means ccache will
ignore changes in them.
The depend mode will be disabled if any of the following holds:
• depend_mode is false.
• run_second_cpp is false.
• The compiler is not generating dependencies using -MD or -MMD (for MSVC, /showIncludes is added
automatically if not specified by the user).
HANDLING OF NEWLY CREATED SOURCE FILES
If modification time (mtime) or status change time (ctime) of the source file or one of the include files
is equal to (or newer than) the time that ccache was invoked, ccache disables caching completely. This is
done as a safety measure to avoid a race condition (see below). In practice, this is only a problem when
using file systems with very low timestamp granularity. You can set sloppiness to
include_file_ctime,include_file_mtime to opt out of the safety measure.
For reference, the race condition mentioned above consists of these events:
1. A source code file is read by ccache and added to the input hash.
2. The source code file is modified.
3. The compiler is executed and reads the modified source code.
4. Ccache stores the compiler output in the cache associated with the incorrect key (based on the
unmodified source code).
CACHE DEBUGGING
To find out what information ccache actually is hashing, you can enable the debug mode via the
configuration option debug or by setting CCACHE_DEBUG in the environment. This can be useful if you are
investigating why you don’t get cache hits. Note that performance will be reduced slightly.
When the debug mode is enabled, ccache will create up to five additional files next to the object file:
┌────────────────────────────────────────────┬─────────────┬──────────────────────────────┐
│ │ │ │
│ Filename │ Debug level │ Description │
├────────────────────────────────────────────┼─────────────┼──────────────────────────────┤
│ │ │ │
│ <objectfile>.<timestamp>.ccache-input-c │ 2 │ Binary input hashed by both │
│ │ │ the direct mode and the │
│ │ │ preprocessor mode. │
├────────────────────────────────────────────┼─────────────┼──────────────────────────────┤
│ │ │ │
│ <objectfile>.<timestamp>.ccache-input-d │ 2 │ Binary input only hashed by │
│ │ │ the direct mode. │
├────────────────────────────────────────────┼─────────────┼──────────────────────────────┤
│ │ │ │
│ <objectfile>.<timestamp>.ccache-input-p │ 2 │ Binary input only hashed by │
│ │ │ the preprocessor mode. │
├────────────────────────────────────────────┼─────────────┼──────────────────────────────┤
│ │ │ │
│ <objectfile>.<timestamp>.ccache-input-text │ 2 │ Human-readable combined │
│ │ │ diffable text version of the │
│ │ │ three files above. │
├────────────────────────────────────────────┼─────────────┼──────────────────────────────┤
│ │ │ │
│ <objectfile>.<timestamp>.ccache-log │ 1 │ Log for this object file. │
└────────────────────────────────────────────┴─────────────┴──────────────────────────────┘
The timestamp format is <year><month><day>_<hour><minute><second>_<microsecond>.
If you only need the log file, set debug_level (environment variable CCACHE_DEBUGLEVEL) to 1.
If debug_dir (environment variable CCACHE_DEBUGDIR) is set, the files above will be written to that
directory with full absolute paths instead of next to the object file.
In the direct mode, ccache uses the 160 bit BLAKE3 hash of the “ccache-input-c” + “ccache-input-d” data
(where + means concatenation), while the “ccache-input-c” + “ccache-input-p” data is used in the
preprocessor mode.
The “ccache-input-text” file is a combined text version of the three binary input files. It has three
sections (“COMMON”, “DIRECT MODE” and “PREPROCESSOR MODE”), which is turn contain annotations that say
what kind of data comes next.
To debug why you don’t get an expected cache hit for an object file, you can do something like this:
1. Enable debug (CCACHE_DEBUG).
2. Build.
3. Clean and build again.
4. Compare the <objectfile>.<timestamp>.ccache-input-text files for the two builds. This together with
the <objectfile>.<timestamp>.ccache-log files should give you some clues about what is happening.
COMPILING IN DIFFERENT DIRECTORIES
Some information included in the hash that identifies a unique compilation can 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 options.
• Paths specified by compiler options (such as -I, -MF, etc) on the command line may be absolute.
• The source code file path may be absolute, and that path may substituted for __FILE__ macros in the
source code or included in warnings emitted to standard error by the preprocessor.
This means that if you compile the same code in different locations, you can’t share compilation results
between the different build directories since you get cache misses because of the absolute build
directory paths that are part of the hash.
Here’s what can be done to enable cache hits between different build directories:
• If you build with -g (or similar) to add debug information to the object file, you must either:
• use the compiler option -fdebug-prefix-map=<old>=<new> for relocating debug info to a common
prefix (e.g. -fdebug-prefix-map=$PWD=.); or
• set hash_dir = false.
• If you use absolute paths anywhere on the command line (e.g. the source code file path or an argument
to compiler options like -I and -MF), you must set base_dir to an absolute path to a “base
directory”. Ccache will then rewrite absolute paths under that directory to relative before computing
the hash.
PRECOMPILED HEADERS
Ccache has support for precompiled headers with GCC and Clang. 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__, __DATE__ or __TIMESTAMP__ is used when using a precompiled header. Further, it can’t detect
changes in #defines in the source code because of how preprocessing works in combination with
precompiled headers.
• You may also want to include include_file_mtime,include_file_ctime in sloppiness. See HANDLING OF
NEWLY CREATED SOURCE FILES.
• You must either:
• use the compiler option -include to include the precompiled header (i.e., don’t use #include in
the source code to include the header; the filename itself must be sufficient to find the header,
i.e. -I paths are not searched); or
• (for the Clang compiler) use the compiler option -include-pch to include the PCH file generated
from the precompiled header; or
• (for the GCC compiler) add the compiler option -fpch-preprocess when compiling.
• If you use Clang, you must compile with -fno-pch-timestamp.
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 “Preprocessing
failed” (if the non-precompiled header file is not available).
C++ MODULES
Ccache has support for Clang’s -fmodules option. In practice ccache only additionally hashes
module.modulemap files; it does not know how Clang handles its cached binary form of modules so those are
ignored. This should not matter in practice: as long as everything else (including module.modulemap
files) is the same the cached result should work. Still, you must set sloppiness to modules to allow
caching.
You must use both direct mode and depend mode. When using the preprocessor mode Clang does not provide
enough information to allow hashing of module.modulemap files.
SHARING A LOCAL CACHE
A group of developers can increase the cache hit rate by sharing a local cache directory. To share a
local cache without unpleasant side effects, the following conditions should to be met:
• Use the same cache directory.
• Make sure that the configuration option hard_link is false (which is the default).
• Make sure that all users are in the same group.
• Set the configuration option umask to 002. This ensures that cached files are accessible to everyone
in the group.
• Make sure that all users have write permission in the entire cache directory (and that you trust all
users of the shared cache).
• Make sure that the setgid bit is set on all directories in the cache. This tells the filesystem to
inherit group ownership for new directories. The following command might be useful for this:
find $CCACHE_DIR -type d | xargs chmod g+s
The reason to avoid the hard link mode is that the hard links cause unwanted side effects, as all links
to a cached file share the file’s modification timestamp. This results in false dependencies to be
triggered by timestamp-based build systems whenever another user links to an existing file. Typically,
users will see that their libraries and binaries are relinked without reason.
You may also want to make sure that a base directory is set appropriately, as discussed in a previous
section.
SHARING A CACHE ON NFS
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 it.
• Ccache hasn’t been tested very thoroughly on NFS.
A tip is to set temporary_dir to a directory on the local host to avoid NFS traffic for temporary files.
It is recommended to use the same operating system version when using a shared cache. If operating system
versions are different then system include files will likely be different and there will be few or no
cache hits between the systems. One way of improving cache hit rate in that case is to set sloppiness to
system_headers to ignore system headers.
An alternative to putting the main cache directory on NFS is to set up a remote storage file cache.
USING CCACHE WITH OTHER COMPILER WRAPPERS
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 prefix_command, for example by
setting the environment variable CCACHE_PREFIX to the name of the wrapper (e.g. distcc). Ccache will then
prefix the command line with the specified command when running the compiler. To specify several prefix
commands, set prefix_command to a 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 that:
• Compiler upgrades will not be detected properly.
• The cached results will not be shared between compilations with and without the other wrapper.
Another minor thing is that if prefix_command is used, ccache will not invoke the other wrapper when
running the preprocessor, which increases performance. You can use prefix_command_cpp if you also want to
invoke the other wrapper when doing preprocessing (normally by adding -E).
CAVEATS
• The direct mode fails to pick up new header files in some rare scenarios. See The direct mode above.
TROUBLESHOOTING
General
A general tip for getting information about what ccache is doing is to enable debug logging by setting
the configuration option debug (or the environment variable CCACHE_DEBUG); see CACHE DEBUGGING for more
information. Another way of keeping track of what is happening is to check the output of ccache -s.
Performance
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 improve performance.
Since ccache works best when I/O is fast, put the cache directory on a fast storage device if possible.
Having lots of free memory so that files in the cache directory stay in the disk cache is also
preferable.
A good way of monitoring how well ccache works is to run ccache -s before and after your build and then
compare the statistics counters. Here are some common problems and what may be done to increase the hit
rate:
• If the counter for preprocessed cache hits has been incremented instead of the one for direct cache
hits, ccache has fallen back to preprocessor mode, which is generally slower. Some possible reasons
are:
• The source code has been modified in such a way that the preprocessor output is not affected.
• Compiler arguments that are hashed in the direct mode but not in the preprocessor mode have
changed (-I, -include, -D, etc) and they didn’t affect the preprocessor output.
• The compiler option -Xpreprocessor or -Wp,* (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.
• A modification or status change time of one of the include files is too new . See HANDLING OF
NEWLY CREATED SOURCE FILES.
• 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 __TIMESTAMP__ preprocessor macro is (potentially) being used and the source file’s
modification time has changed. This is similar to how __TIME__ is handled. If __TIMESTAMP__ is
present in the source code, ccache hashes the string representation of the source file’s
modification time in order to be able to produce the correct object file if the __TIMESTAMP__
macro affects the output. If you know that __TIMESTAMP__ isn’t used in practise, or don’t care if
ccache produces objects where __TIMESTAMP__ is expanded to something in the past, you can set
sloppiness to time_macros.
• The input file path has changed. Ccache includes the input file path in the direct mode hash to
be able to take relative include files into account and to produce a correct object file if the
source code includes a __FILE__ macro.
• If a cache hit counter was not 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__, __DATE__ or __TIMESTAMP__ macros, or use of
automatically generated code that contains a timestamp, build counter or other volatile information.
• If “Multiple source files” has been incremented, it’s an indication that the compiler has been
invoked on several source code files at once. Ccache doesn’t support that. Compile the source code
files separately if possible.
• If “Unsupported compiler option” has been incremented, enable debug logging and check which compiler
option was rejected.
• If “Preprocessing failed” has been incremented, one possible reason is that precompiled headers are
being used. See PRECOMPILED HEADERS for how to remedy this.
• If “Could not use precompiled header” has been incremented, see PRECOMPILED HEADERS.
• If “Could not use modules” has been incremented, see C++ MODULES.
Corrupt object files
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
prefix_command 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 tree.
3. Rebuild with CCACHE_RECACHE set.
An alternative is to clear the whole cache with ccache -C if you don’t mind losing other cached results.
There are no reported issues about ccache producing broken object files reproducibly. That doesn’t mean
it can’t happen, so if you find a repeatable case, please report it.
MORE INFORMATION
Credits, mailing list information, bug reporting instructions, source code, etc, can be found on ccache’s
web site: https://ccache.dev.
AUTHOR
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.dev/credits.html for a list of contributors.
Ccache 4.10.2 2024-11-13 CCACHE(1)