Provided by: ccache_4.5.1-1_amd64 
NAME
ccache - a fast C/C++ compiler cache
SYNOPSIS
ccache [options]
ccache compiler [compiler options]
compiler [compiler options] (via symbolic link)
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 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 first method, just make sure that ccache is in your PATH.
To use the second method 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:
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.
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.
Warning
Use a symbolic links for masquerading, not hard links.
COMMAND LINE OPTIONS
These command line 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 compiler’s documentation.
Common options
-c, --cleanup
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 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 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 older 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 old
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: 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.
-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 potentionally 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.
-o KEY=VALUE, --set-config KEY=VALUE
Set configuration option KEY to VALUE. See CONFIGURATION for more information.
-x, --show-compression
Print cache compression statistics. See CACHE COMPRESSION for more information. This
can potentionally 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 secondary storage
--trim-dir PATH
Remove old 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 primary cache. To trim the primary 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: k, M, G, T (decimal), Ki, Mi, Gi or Ti (binary). The default suffix
is G.
--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.
Options for scripting or debugging
--checksum-file PATH
Print the checksum (128 bit XXH3) of the file at PATH (- for standard input).
--dump-manifest PATH
Dump manifest file at PATH (- for standard input) in text format to standard output.
This is only useful when debugging ccache and its behavior.
--dump-result PATH
Dump result file at PATH (- for standard input) in text format to standard output.
This is only useful when debugging ccache and its behavior.
--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 is only
useful when debugging ccache and its behavior.
-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.
--print-stats
Print statistics counter IDs and corresponding values in machine-parsable
(tab-separated) format.
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.
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. Environment variables.
2. The primary (cache-specific) configuration file (see below).
3. The secondary (system-wide read-only) configuration file <sysconfdir>/ccache.conf
(typically /etc/ccache.conf or /usr/local/etc/ccache.conf).
4. Compile-time defaults.
As a special case, if the the environment variable CCACHE_CONFIGPATH is set it specifies
the primary configuration file and the secondary (system-wide) configuration file won’t be
read.
Location of the primary configuration file
The location of the primary (cache-specific) configuration is determined like this:
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 secondary (system-wide) 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 %APPDATA%/ccache/ccache.conf (Windows),
$HOME/Library/Preferences/ccache/ccache.conf (macOS) or
$HOME/.config/ccache/ccache.conf (other systems).
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
contraproductive.
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 that of Alice:
# 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.
cache_dir (CCACHE_DIR)
This option specifies where ccache will keep its cached compiler outputs. The default
is $XDG_CACHE_HOME/ccache if XDG_CACHE_HOME is set, otherwise $HOME/.cache/ccache.
Exception: If the legacy directory $HOME/.ccache exists then that directory is the
default.
See also Location of the primary configuration file.
If you want to use another CCACHE_DIR value temporarily for one ccache invocation you
can use the -d/--dir command line option instead.
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. 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.
gcc
GCC-based compiler.
nvcc
NVCC (CUDA) compiler.
other
Any compiler other than the known types.
pump
distcc’s “pump” script.
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.
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.
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 guard.
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 writeable, 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 exclude
from the hash. Excluding a compiler option from the hash can be useful when you know
it doesn’t affect the result (but ccache doesn’t know that), or when it does and you
don’t care. If a compiler option in the list is suffixed with an asterisk (*) it will
be matched as a prefix. For example, -fmessage-length=* will match both
-fmessage-length=20 and -fmessage-length=70.
inode_cache (CCACHE_INODECACHE or CCACHE_NOINODECACHE, see Boolean values above)
If true, enables caching of source file hashes based on device, inode and timestamps.
This will reduce the time spent on hashing included files as the result can be resused
between compilations.
The feature is still experimental and thus off by default. It is currently not
available on Windows.
The feature requires temporary_dir to be located on a local filesystem.
keep_comments_cpp (CCACHE_COMMENTS or CCACHE_NOCOMMENTS, see Boolean values above)
If true, ccache will not discard the comments before hashing preprocessor output. This
can be used to check documentation with -Wdocumentation.
limit_multiple (CCACHE_LIMIT_MULTIPLE)
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%). See Automatic cleanup for more
information.
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.
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 primary cache with the command line
option --evict-namespace, potentially in combination with --evict-older-than. . For
instance, if you use the same primary 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 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.
reshare (CCACHE_RESHARE or CCACHE_NORESHARE, see Boolean values above)
If true, ccache will write results to secondary storage even for primary 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).
secondary_storage (CCACHE_SECONDARY_STORAGE)
This option specifies one or several storage backends (separated by space) to query
after the primary cache storage. See SECONDARY STORAGE BACKENDS for documentation of
syntax and available backends.
Examples:
• file:/shared/nfs/directory
• file:///shared/nfs/one|read-only file:///shared/nfs/two
• http://example.com/cache
• redis://example.com
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.
include_file_ctime
By default, ccache will not cache a file if it includes a header whose ctime is
too new. This sloppiness disables that check. See also HANDLING OF NEWLY CREATED
HEADER FILES.
include_file_mtime
By default, ccache will not cache a file if it includes a header whose mtime is
too new. This sloppiness disables that check. See also HANDLING OF NEWLY CREATED
HEADER 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.
pch_defines
Be sloppy about #define directives when precompiling a header file. See
PRECOMPILED HEADERS for more information.
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.
system_headers
By default, ccache will also include all system headers in the manifest. With this
sloppiness set, ccache will only include system headers in the hash but not add
the system header files to the list of include files.
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.
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
/run/user/<UID>/ccache-tmp if /run/user/<UID> 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.
SECONDARY STORAGE BACKENDS
The secondary_storage option lets you configure ccache to use one or several other storage
backends in addition to the primary cache storage located in cache_dir. Note that cache
statistics counters will still be kept in the primary cache directory — secondary storage
backends only store cache results and manifests.
A secondary 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 secondary 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.
• share-hits: If true, write hits for this backend to primary storage. The default is
true.
Storage interaction
The table below describes the interaction between primary and secondary storage on cache
hits and misses:
┌────────────────┬───────────────────┬──────────────────────────┐
│ │ │ │
│Primary storage │ Secondary storage │ What happens │
├────────────────┼───────────────────┼──────────────────────────┤
│ │ │ │
│miss │ miss │ Compile, write to │
│ │ │ primary, write to │
│ │ │ secondary[1] │
├────────────────┼───────────────────┼──────────────────────────┤
│ │ │ │
│miss │ hit │ Read from secondary, │
│ │ │ write to primary[2] │
├────────────────┼───────────────────┼──────────────────────────┤
│ │ │ │
│hit │ - │ Read from primary, don’t │
│ │ │ write to secondary[3] │
└────────────────┴───────────────────┴──────────────────────────┘
[1] Unless secondary storage has attribute read-only=true.
[2] Unless secondary storage has attribute share-hits=false.
[3] Unless primary storage is set to share its cache hits with the reshare option.
File storage backend
URL format: file:DIRECTORY or file://DIRECTORY
This backend stores data as separate files in a directory structure below DIRECTORY (an
absolute path), similar (but not identical) to the primary cache storage. A typical use
case for this backend would be sharing a cache on an NFS directory.
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
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:
• 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 false.
Note
Connection keep-alive is disabled by default because with the current HTTP
implementation uploads to the remote end might fail in case the server closes the
connection due to a keep-alive timeout. If the general case with short compilation
times should be accelerated or the server is configured with a long-enough
timeout, then connection keep-alive could be enabled.
• 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 format: redis://[[USERNAME:]PASSWORD@]HOST[:PORT][/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
SECONDARY STORAGE BACKENDS.
Examples:
• redis://localhost
• redis://p4ssw0rd@cache.example.com:6379/0|connect-timeout=50
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
Ccache maintains counters for various statistics about the cache, including the size and
number of all cached files. In order to improve performance and reduce issues with
concurrent ccache invocations, there is one statistics file for each of the sixteen
subdirectories in the cache.
After a new compilation result has been written to the cache, ccache will update the size
and file number statistics for the subdirectory (one of sixteen) to which the result was
written. Then, if the size counter for said subdirectory is greater than max_size / 16 or
the file number counter is greater than max_files / 16, automatic cleanup is triggered.
When automatic cleanup is triggered for a subdirectory in the cache, ccache will:
1. Count all files in the subdirectory and compute their aggregated size.
2. Remove files in LRU (least recently used) order until the size is at most
limit_multiple * max_size / 16 and the number of files is at most limit_multiple *
max_files / 16, where limit_multiple, max_size and max_files are configuration
options.
3. Set the size and file number counters to match the files that were kept.
The reason for removing more files than just those needed to not exceed the max limits is
that a cleanup is a fairly slow operation, so it would not be a good idea to trigger it
often, like after each cache miss.
Manual cleanup
You can run ccache -c/--cleanup to force cleanup of the whole cache, i.e. all of the
sixteen subdirectories. This will recalculate the statistics counters and make sure that
the configuration options max_size and max_files are not exceeded. Note that
limit_multiple is not taken into account for manual cleanup.
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 primary and secondary 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. │
├─────────────────────────────────┼──────────────────────────────────┤
│ │ │
│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 write to output file │ The output path specified with │
│ │ -o is not a file (e.g. a │
│ │ directory or a device node). │
├─────────────────────────────────┼──────────────────────────────────┤
│ │ │
│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 produced empty output │ The compiler’s output file │
│ │ (typically an object file) was │
│ │ empty after compilation. │
├─────────────────────────────────┼──────────────────────────────────┤
│ │ │
│Compiler produced no output │ The compiler’s output file │
│ │ (typically an object file) was │
│ │ missing after compilation. │
├─────────────────────────────────┼──────────────────────────────────┤
│ │ │
│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. │
├─────────────────────────────────┼──────────────────────────────────┤
│ │ │
│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. │
├─────────────────────────────────┼──────────────────────────────────┤
│ │ │
│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 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:
• a -Wp,* compiler option other than -Wp,-MD,<path>, -Wp,-MMD,<path> and
-Wp,-D<define>
• -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 file generated by the
compiler 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.
• 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.
HANDLING OF NEWLY CREATED HEADER FILES
If modification time (mtime) or status change time (ctime) of one of the include files is
the same second as the time compilation is being done, ccache disables the direct mode
(or, in the case of a precompiled header, disables caching completely). This done as a
safety measure to avoid a race condition (see below).
To be able to use a newly created header files in direct mode (or use a newly precompiled
header), either:
• create the include file earlier in the build process, or
• set sloppiness to include_file_ctime,include_file_mtime if you are willing to take the
risk, for instance if you know that your build system is robust enough not to trigger
the race condition.
For reference, the race condition mentioned above consists of these events:
1. The preprocessor is run.
2. An include file is modified by someone.
3. The new include file is hashed by ccache.
4. The real compiler is run on the preprocessor’s output, which contains data from the
old header file.
5. The wrong object file is stored in the cache.
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 │ Description │
├───────────────────────────────┼──────────────────────────────────┤
│ │ │
│<objectfile>.ccache-input-c │ Binary input hashed by both the │
│ │ direct mode and the preprocessor │
│ │ mode. │
├───────────────────────────────┼──────────────────────────────────┤
│ │ │
│<objectfile>.ccache-input-d │ Binary input only hashed by the │
│ │ direct mode. │
├───────────────────────────────┼──────────────────────────────────┤
│ │ │
│<objectfile>.ccache-input-p │ Binary input only hashed by the │
│ │ preprocessor mode. │
├───────────────────────────────┼──────────────────────────────────┤
│ │ │
│<objectfile>.ccache-input-text │ Human-readable combined diffable │
│ │ text version of the three files │
│ │ above. │
├───────────────────────────────┼──────────────────────────────────┤
│ │ │
│<objectfile>.ccache-log │ Log for this object file. │
└───────────────────────────────┴──────────────────────────────────┘
If config_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. Build with debug mode enabled.
2. Save the <objectfile>.ccache-* files.
3. Build again with debug mode enabled.
4. Compare <objectfile>.ccache-input-text for the two builds. This together with the
<objectfile>.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 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__, __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 HEADER 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 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 CACHE
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 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 secondary 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
(created the same second as the compilation is being done). See HANDLING OF NEWLY
CREATED HEADER 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.