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CCACHE(1) CCACHE(1)

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 com-
piler as described in RUN MODES.

DESCRIPTION
Ccache is a compiler cache. It speeds up recompilation by caching the re-
sult of previous compilations and detecting when the same compilation is
being done again.

Ccache has been carefully written to always produce exactly the same com-
piler 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 ex-
ceptions to this goal are listed under CAVEATS. If you discover an undocu-
mented 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 conve-
nient 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, create a
symbolic link to ccache named as the compiler. For example, here is set
up ccache to masquerade as gcc and g++:

cp 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 ccache /usr/local/bin/gcc
cp 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 sec-
tion), the normal compiler options apply and you should refer to the com-
piler'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 con-
figuration 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 (over-
riding) 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 op-
tion 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 di-
rectory 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 direc-
tory 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 compres-
sion_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 in-
formation. 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 (en-
vironment variable, configuration file or compile-time default) in hu-
man-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-read-
able 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 fol-
lowed 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 uncom-
pressed for no compression. See [Cache compression] for more informa-
tion. 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 de-
fault 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 in-
put). 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 embed-
ded 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 ma-
chine-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 op-
tions as the compiler you are using. The only exception to this is the op-
tion --ccache-skip. That option can be used to tell ccache to avoid inter-
preting 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 op-
tion 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 com-
mand 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 result-
ing object file (among other things). The heuristic ccache uses when pars-
ing 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 con-
figuration 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_CON-
FIG_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 lo-
cation 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 ig-
nored, 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 dif-
ferent:

* 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 envi-
ronment 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 re-
sults 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 di-
rectory 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/al-
ice/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 val-
ues:

# 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 out-
puts.

On non-Windows systems, the default is $HOME/.ccache if such a direc-
tory 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 %APP-
DATA%\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 %LOCALAPP-
DATA%\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 de-
fault.

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 com-
piler.

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 com-
mand 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 pos-
sible 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 con-
sider continue using the mtime method in combination with the pre-
fix_command option if possible. See USING CCACHE WITH OTHER COM-
PILER 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; com-
pressed 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 com-
pressed 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 lev-
els. Lower levels (e.g. 1) mean faster compression but worse com-
pression 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 ver-
sions. 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/--recom-
press.

< 0
A negative value corresponds to Zstandard's "ultra-fast" compres-
sion 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, cur-
rently 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 intermedi-
ate 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 DE-
BUGGING 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 di-
rectory is /home/user and the object file is build/output.o then the
debug log will be written to /example/home/user/build/out-
put.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 di-
rect 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 re-
sults. 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, per-
formance 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 en-
abled).

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 en-
try. As of version 4.0, ccache makes stored and fetched object
files read-only as a safety measure. Furthermore, a simple in-
tegrity 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 con-
fused 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 compila-
tions 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 di-
rect 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 com-
piler 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 preproces-
sor output. The default is false. This can be used to check documenta-
tion with -Wdocumentation.

log_file (CCACHE_LOGFILE)
If set to a file path, ccache will write information on what it is do-
ing 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/rsys-
log.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 MANAGE-
MENT.

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 (deci-
mal) 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 com-
piler. The default prefix is "Note: including file:". If you use a lo-
calized 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 sep-
arate from cache entries with other namespaces, but they will still
share the same storage space. Cache entries can also be selectively re-
moved from the local cache with the command line option --evict-name-
space, 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 al-
lows 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 re-
sults from the cache using the direct mode, not the preprocessor mode.
See documentation for read_only regarding using a read-only ccache di-
rectory.

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 sec-
ondary_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 pre-
processed once. The downside is that some compilers won't produce the
same result (for instance diagnostics warnings) when compiling pre-
processed 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 #in-
clude 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 work-
ing 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 trade-
off of potentially getting an incorrect directory in the .gcno
file. gcno_cwd also disables hashing of the current working direc-
tory 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 di-
rective. While this is the correct behaviour as ccache does not de-
tect incbin changes, this restriction can make some projects diffi-
cult 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 invoca-
tion. 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 local-
ization 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 representa-
tion 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 dif-
ferent seeds between builds and you are OK with reusing cached re-
sults.

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 compila-
tion. The default is true. If false, [automatic cleanup] will be dis-
abled 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 individ-
ual 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 de-
fault 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 direc-
tories 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 per-
cent-encoded <https://en.wikipedia.org/wiki/Percent-encoding> if they con-
tain 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 de-
fault 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 as-
sociated 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 in-
stance 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 re-
dis://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.

[1] Unless remote storage has attribute read-only=true.
[2] Unless local storage is set to share its cache hits with the reshare
option.

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 typi-
cal 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 local-
host. 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 periodi-
cally.

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 periodi-
cally.

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-compati-
ble) 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 per-
formed by ccache of course is redundant. See the documentation for the con-
figuration options compression and compression_level for more information.

You can use the command line option -x/--show-compression to print informa-
tion 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 uncom-
pressed (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 af-
ter 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 re-
mote storage.

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 hash-
ing. 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-crypto-
graphic 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 de-
pend mode is enabled. In the depend mode, ccache never runs the preproces-
sor, 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 re-
sult 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 pro-
duced 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 exam-
ple:

* 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 per-
formed 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 in-
correct 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.

The timestamp format is <year><month><day>_<hour><minute><second>_<mi-
crosecond>.

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-in-
put-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 "PREPROCES-
SOR 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 com-
mand line may be absolute.

* The source code file path may be absolute, and that path may substi-
tuted 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 direc-
tories:

* If you build with -g (or similar) to add debug information to the ob-
ject file, you must either:

* use the compiler option -fdebug-prefix-map=<old>=<new> for relocat-
ing 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 be-
fore 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 com-
bination 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 han-
dles its cached binary form of modules so those are ignored. This should
not matter in practice: as long as everything else (including module.mod-
ulemap 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 mod-
ule.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 di-
rectory (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 directo-
ries. 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 un-
wanted side effects, as all links to a cached file share the file's modifi-
cation 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 re-
linked 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 in-
clude 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 ac-
complished by defining prefix_command, for example by setting the environ-
ment 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_com-
mand 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 rec-
ommended 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 in-
voke the other wrapper when running the preprocessor, which increases per-
formance. 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 scenar-
ios. See The direct mode above.

TROUBLESHOOTING
General
A general tip for getting information about what ccache is doing is to en-
able debug logging by setting the configuration option debug (or the envi-
ronment 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 preproces-
sor mode, which is generally slower. Some possible reasons are:

* The source code has been modified in such a way that the preproces-
sor 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 direc-
tory.

* 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 prac-
tise, 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 sloppi-
ness to time_macros.

* The __TIMESTAMP__ preprocessor macro is (potentially) being used
and the source file's modification time has changed. This is simi-
lar 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 some-
thing 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 sepa-
rately 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 PRECOM-
PILED 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 re-
producibly. 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 2024-07-22 CCACHE(1)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=ccache&sektion=1&manpath=FreeBSD+15.1-RELEASE+and+Ports>

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