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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 that speeds up recompilation by storing the re-
sults of previous compilations and reusing them when the same compilation
is performed again.

Ccache is designed to produce exactly the same compiler output as a normal
compilation. The only difference you should notice is faster build times.
Any known exceptions to this behavior are documented in the CAVEATS sec-
tion. If you find a case where ccache produces different output than ex-
pected, please report it to us.

RUN MODES
There are two ways to use ccache to cache compilations:

1. Prefix method: Add ccache before your compilation command. This is the
simplest approach when you want to try ccache or use it for specific
projects. Example:

ccache gcc -c example.c

2. Masquerade method: Make ccache appear as the compiler by creating a
symbolic link to ccache with the compiler's name. This is useful when
you want to use ccache for all compilations. To set up ccache to mas-
querade as gcc and g++:

cp ccache /usr/local/bin/
ln -s ccache /usr/local/bin/gcc
ln -s ccache /usr/local/bin/g++

On systems that don't support symbolic links, you can copy ccache in-
stead:

cp ccache /usr/local/bin/gcc
cp ccache /usr/local/bin/g++

This works as long as the directory containing the symbolic links or
ccache copies appears before the real compiler directory (typically
/usr/bin) in your PATH.

Warning

The masquerade method works well but may conflict with other tools
that use the same technique. See USING CCACHE WITH OTHER COMPILER
WRAPPERS for more information.

COMMAND LINE OPTIONS
These command line options apply only when you invoke ccache directly as
"ccache". When ccache masquerades as a compiler (as described in the previ-
ous section), you should use the normal compiler options and refer to your
compiler's documentation.

Common options
-c, --cleanup
Clean up the cache by removing not recently used cached files until the
specified file number and cache size limits are not exceeded. This also
recalculates the cache file count and size totals. Normally, there is
no need to initiate cleanup manually as ccache keeps the cache below
the specified limits at runtime and keeps statistics up to date on each
compilation. Forcing a cleanup is mostly useful if you have modified
the cache contents manually or believe that the cache size statistics
may be inaccurate.

-C, --clear
Clear the entire cache, removing all cached files, but keeping the 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.

-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. See Cache
statistics for more information.

--stop-storage-helpers
Storage helpers normally stop after a period of client inactivity, but
this option can be used to stop any running helpers immediately.

--threads THREADS
Use up to THREADS threads for threaded operations. The default is to
use one thread per CPU.

-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 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 differ-
ent level than LEVEL will be recompressed.

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
You can customize ccache's behavior using configuration files and environ-
ment variables. Configuration options are processed in the following order
of priority (highest first):

1. Command line settings in KEY=VALUE form. Example:

ccache debug=true compiler_check="%compiler% --version" gcc -c example.c

2. Environment variables starting with CCACHE_.

3. A directory-specific configuration file.

4. The cache-specific configuration file.

5. The system-wide configuration file <sysconfdir>/ccache.conf (typically
/etc/ccache.conf or /usr/local/etc/ccache.conf).

6. Compile-time defaults.

Exception: If the environment variable CCACHE_CONFIGPATH is set, it speci-
fies the only configuration file that will be read (environment variables
and command line settings still apply).

Cache-specific 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.

Directory-specific configuration file
Ccache searches for a ccache.conf file (separate from the cache-specific
configuration file described above) in the current working directory or any
parent directory. The found file must be owned by the effective user and
must not be world-writable; otherwise ccache will abort with an error. The
search stops when reaching:

* a directory listed in a "ceiling directory" (by default the user's home
directory)

* a directory that contains a "ceiling marker" (by default .git)

* a directory owned by another user

* a file system boundary (mount point)

By default, the file cannot set "unsafe options", i.e. those that affect
which commands to execute, which files to write (except cache entries) and
which remote storage to use. It is possible to allow unsafe options by
adding an entry to safe_dirs.

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 = 10GB

Multi-line values
Values can span multiple lines using indentation-based continuation. Lines
that start with whitespace (spaces or tabs) are treated as continuation
lines and are joined to the previous value with a single space. Comments
and blank lines within a multi-line value are skipped. For example:

ignore_options =
-Wall
-Wextra
# This is a comment within the value
-pedantic
# This ends the multi-line value
compiler = gcc

This is equivalent to:

ignore_options = -Wall -Wextra -pedantic
compiler = gcc

Multi-line values are particularly useful for options that accept multiple
items.

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.

Options that define a list of paths have their entries separated by ; on
Windows and : on other systems.

absolute_paths_in_stderr (CCACHE_ABSSTDERR)
This option specifies whether ccache should rewrite relative paths in
the compiler's textual output (standard error and standard 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 specifies a list of absolute paths. When set, ccache con-
verts absolute paths to relative paths before hashing, but only for
paths that start with one of the specified base directories.

This enables cache sharing between compilations in different directo-
ries, even when the project uses absolute paths. See COMPILING IN DIF-
FERENT DIRECTORIES_ for more details. When empty (the default), no path
rewriting occurs.

Avoid using / as the base directory as this will also rewrite system
header paths, which is usually counterproductive.

Example scenario: Alice works in /home/alice/project1/build and com-
piles 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 set-
tings:

# 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 stores the same projects in /home/bob/stuff and both users set
base_dir to /home or /home/$USER, they will share cache hits because
the rewritten command lines will be identical:

# 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 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/.cache/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 Cache-specific configuration file.

ceiling_dirs (CCACHE_CEILING_DIRS)
This option specifies absolute directories where ccache will stop
searching for a directory-specific configuration file. You might want
to set this if accessing files in parent directories is too slow, e.g.
on network file systems. The default is to stop looking when reaching
the user's home directory.

ceiling_markers (CCACHE_CEILING_MARKERS)
This option specifies one or more file or directory names (separated by
; on Windows, : on other systems). When searching for a directory-spe-
cific configuration file, ccache will stop searching parent directories
when it reaches a directory containing any of the specified names. The
default is .git.

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 ; as the 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.

icx
Intel LLVM-based compiler.

icx-cl
Intel LLVM-based MSVC-compatible compiler.

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 <https://facebook.github.io/zstd/> 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
hash sum that identifies the build.

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 the compiler op-
tions -fdebug-prefix-map or -fdebug-compilation-dir are used appropri-
ately. See also the discussion under COMPILING IN DIFFERENT DIRECTO-
RIES.

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.

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.

libexec_dirs (CCACHE_LIBEXEC_DIRS)
If set, ccache will look for helper programs in these directories in-
stead of the default.

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 5GiB. Available suffixes: kB, MB, GB, TB
(decimal) and KiB, MiB, GiB, TiB (binary). The default suffix is GiB.
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.

msvc_utf8 (CCACHE_MSVC_UTF8)
This option adds /utf-8 to the msvc command line when executing the
preprocessor to ensure that filenames are not garbled for non-ascii
characters. This implicitly enables /validate-charset and treats the
source code as utf-8 which may cause compilation errors if comments in
your code have characters in the [128, 255] range for a given Windows
system codepage which results in an invalid utf-8 sequence. The default
is true.

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. If not set, ccache will look for the first exe-
cutable matching the compiler name in the normal PATH that isn't a sym-
bolic 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
whitespace) to query after checking the local cache (unless remote_only
is true). See REMOTE STORAGE BACKENDS for documentation of syntax and
available backends.

Examples:

remote_storage = file:/shared/nfs/directory+`
remote_storage =
file:///shared/nfs/one read-only
file:///shared/nfs/two
remote_storage = file:///Z:/example/windows/folder
remote_storage = http://example.com/cache
remote_storage = redis://example.com

Note

In previous ccache versions this option was called secondary_stor-
age (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.

response_file_format (CCACHE_RESPONSE_FILE_FORMAT)
Ccache normally guesses the response file format based on the compiler
type. The response_file_format option lets you force the response file
quoting behavior. This can be useful if the compiler supports both
POSIX and Windows response file quoting. Possible values are:

auto
Guess one of the formats below based on the compiler type. This is
the default.

posix
POSIX quoting behavior.

windows
Windows quoting behavior.

safe_dirs (CCACHE_SAFE_DIRS)
List of trusted directories for which directory-specific configuration
files are allowed to set "unsafe options" (see Directory-specific con-
figuration file). The default is empty.

Allowed list values: * matches any directory. An absolute path ending
with /* matches any subdirectory (at any depth) under that path. An ab-
solute path not ending in /* matches only that directory.

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. Avail-
able values (separated by whitespace or comma):

clang_index_store
Use case: Xcode projects with varying index store paths.
Effect: Ignores the -index-store-path and -index-unit-output-path
options when hashing.
Trade-off: Index won't update correctly on cache hits.

file_stat_matches
Use case: When file content checking is too slow.
Effect: Uses file timestamps instead of content for cache valida-
tion.
Trade-off: May miss content changes with identical timestamps.

file_stat_matches_ctime
Use case: When controlling file timestamps manually.
Effect: Ignores status change time when file_stat_matches is en-
abled.
Trade-off: May miss some file system changes.

gcno_cwd
Use case: Code coverage builds in different directories.
Effect: Ignores current directory when creating .gcno files
(-ftest-coverage). Also disables hashing of the current working di-
rectory if -fprofile-abs-path is used.
Trade-off: Directory information in coverage files may be incor-
rect.

Note

No effect with --coverage (it implies -fprofile-arcs).

incbin
Use case: Projects using assembly .incbin directives.
Effect: Allows caching files with .incbin directives.
Trade-off: Won't detect changes to included binary files.

include_file_ctime
Use case: Build systems that modify file timestamps frequently.
Effect: Ignores file status change time when checking for recent
modifications.
Trade-off: May miss recent changes to source files.

include_file_mtime
Use case: Build systems that modify file timestamps frequently.
Effect: Ignores file modification time when checking for recent
changes.
Trade-off: May miss recent modifications to source files.

ivfsoverlay
Use case: Xcode projects mixing Objective-C and Swift.
Effect: Ignores -ivfsoverlay virtual filesystem option.
Trade-off: May not detect VFS-related changes.

locale
Use case: Builds in environments with varying locale settings.
Effect: Ignores locale environment variables (LANG, LC_ALL,
LC_CTYPE, LC_MESSAGES).
Trade-off: Compiler warning messages may vary between cached and
fresh builds.

modules
Use case: Clang builds using -fmodules.
Effect: Allows caching when C++ modules are used.
Trade-off: May not detect changes in module internal state.

See C++ MODULES for details.

pch_defines
Use case: Projects with precompiled headers.
Effect: Relaxes checking of #define directives in precompiled head-
ers.
Trade-off: May not detect some macro definition changes.

See PRECOMPILED HEADERS for details.

random_seed
Use case: Builds with varying -frandom-seed values.
Effect: Ignores random seed values in compilation hash.
Trade-off: Builds may not be fully reproducible.

system_headers
Use case: Systems with frequently changing system headers.
Effect: Only tracks non-system headers in direct mode.
Trade-off: Won't detect system header changes that affect compila-
tion.
Limitations: Only supported for GCC-like compilers (not MSVC). Sys-
tem headers are still checked in preprocessor mode.

See also ignore_headers_in_manifest.

time_macros
Use case: Code using time macros but values don't matter.
Effect: Ignores __DATE__, __TIME__, and __TIMESTAMP__ in source.
Trade-off: Time values in output will be from cached compilation.

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 disabled
as well.

stats_log (CCACHE_STATSLOG)
If set to a file path, ccache will write statistics counter updates to
the specified file. This is useful for getting statistics for 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.

Storage helper process
ccache can spawn a long-lived local storage helper process (installed sepa-
rately) to handle communication with the remote storage. Storage helpers
keep remote connections up for efficiency and terminate after a while on
inactivity.

The storage helper program is called ccache-storage-<scheme>, where
<scheme> is the scheme part of a URL, e.g. https. The program can be placed
in these locations (in priority order):

1. In a location specified by the helper property (see below).

2. In a libexec directory.

3. Next to the ccache executable.

4. In $PATH.

If the program is not found, ccache falls back to one of the builtin back-
ends if available, otherwise ccache exits with an error.

Configuration syntax
A remote storage backend is specified by a URL and can be followed by a
whitespace-separated list of properties (key=value) and custom attributes
(@key=value). A missing =value is shorthand for setting the value to true.
Values must be percent-encoded
<https://en.wikipedia.org/wiki/Percent-encoding> if they contain %, | or
whitespace characters. For compatibility with older ccache versions, |
characters are treated as spaces.

Properties
* data-timeout: Timeout for send/receive data transfer. Resets whenever
data is received or can be sent. Default: 10s.

* helper: Override which storage helper to spawn (either a filename or a
full path). This is mainly useful when developing or testing new
helpers.

* idle-timeout: Timeout for the helper process to wait before exiting af-
ter client inactivity (0s: stay up indefinitely). Default: 10m.

* read-only: If true, only read from this backend, don't write. The de-
fault is false.

* request-timeout: Timeout for the whole request. Default: 1m.

* 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:

* http://cache-*.example.com shards=a(3),b(1),c(1.5) will put 55%
(3/5.5) of the cache on http://cache-a.example.com, 18% (1/5.5) on
http://cache-b.example.com and 27% (1.5/5.5) on http://cache-c.ex-
ample.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.

Timeouts can be specified with an ms (milliseconds), s (seconds), m (min-
utes), h (hours) or d (days) suffix.

Custom attributes
Custom attributes on the form @key=value are specific to each storage
helper implementation. See the backend's documentation for which attributes
are available. Examples:

* file:///example/directory @umask=002 @update-mtime=true

* https://example.com/path @bearer-token=deadbeef

[1] Unless remote storage has property 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.

CRSH storage backend
URL format: crsh:IPC_ENDPOINT

This backend connects to an already running ccache storage helper (CRSH)
process listening on IPC_ENDPOINT (Unix socket path or Windows named pipe).
The use case for this is mainly when developing or debugging a storage
helper, or in special setups where the helper program is started and man-
aged externally.

Examples:

* crsh:/path/to/your.sock (Unix socket)

* crsh:name-of-your-pipe (Windows named pipe)

HTTP storage backend
Warning

Built-in support for the HTTP protocol will likely be removed in a fu-
ture ccache release.

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

Optional attributes:

* @bearer-token: Bearer token used to authorize the HTTP requests.

* @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.

* @header: Add the key=value pair to the HTTP headers of the request. For
example: @header=Content-Type=application/octet-stream adds "Con-
tent-Type: application/octet-stream" to the HTTP headers of the re-
quest.

Redis storage backend
Warning

Built-in support for the Redis protocol will likely be removed in a fu-
ture ccache release.

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 property de-
scribed in REMOTE STORAGE BACKENDS.

Examples:

* redis://localhost

* redis://p4ssw0rd@cache.example.com:6379/0

* redis+unix:/run/redis.sock

* redis+unix:///run/redis.sock

* redis+unix://p4ssw0rd@localhost/run/redis.sock?db=0

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 stored in the local cache, ccache
will trigger an automatic cleanup if max_size or max_files is exceeded. The
cleanup removes cache entries in approximate LRU (least recently used) or-
der 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.

For performance reasons only entries in a subset of the cache are consid-
ered when automatic cleanup is triggered, so the oldest entries aren't al-
ways removed first but the overall behavior approximates LRU over time.

Manual cleanup
Run ccache --cleanup to force cleanup of the whole cache. This will recal-
culate the cache size information and make sure that the cache size does
not exceed max_size and max_files.

Note that there is no guarantee that only the oldest entries are evicted,
as discussed in Automatic cleanup above. To evict based on age use ccache
--evict-older-than AGE.

CACHE COMPRESSION
By default, ccache compresses all cached data using the Zstandard
<http://zstd.net> (zstd) algorithm at compression level 1. This algorithm
is fast enough that compression rarely impacts performance. You might want
to disable compression only if your cache is stored on an already-com-
pressed filesystem, where ccache's compression would be redundant.

For configuration details, see compression and compression_level.

Use ccache --show-compression to display compression information. Example
output:

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:

* Disk blocks: The actual cache size accounting for filesystem block
size. This should match the "Cache size" from ccache --show-stats.

* Compressed data: Result and manifest files stored in the cache.

* Incompressible data: Files stored without compression (due to
file_clone or hard_link settings) or legacy files from older ccache
versions.

* Compression ratio: Affected by compression_level.

You can recompress cached data to different compression levels using ccache
--recompress. Only files with different compression levels than the target
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.

The statistics counters are not used by ccache itself during builds. This
means that you can safely reset them at any time using ccache --zero-stats
without affecting the build process. For example, you might reset them be-
fore a build so that ccache --show-stats will only summarize the results
from that specific build. Alternatively, you can set stats_log before
starting the build and then run ccache --show-log-stats afterward to view
build-specific statistics. This approach allows the statistics counters to
continue tracking the entire lifetime of the cache while still giving you
detailed information for individual builds. Another advantage of stats_log
is that it collects statistics without interference from other concurrent
builds that access the same cache.

HOW CCACHE WORKS
Ccache detects when you're compiling the same code and reuses previously
stored output. It works by creating a unique hash (the "input hash") from
various information that affects the compilation. When the same hash is en-
countered again, ccache can supply all the correct compiler outputs from
the cache.

For hashing, ccache uses BLAKE3 <https://blake3.io>, a fast cryptographic
hash algorithm. For data integrity, cached data is protected with XXH3
<https://xxhash.com> checksums to detect corruption.

Ccache has two strategies for gathering the information used to create
cache lookup keys:

* Preprocessor mode: Ccache runs the preprocessor on the source code and
hashes the result.

* Direct mode: Ccache hashes the source code and include files directly.

Direct mode is generally faster because it avoids the overhead of running
the preprocessor.

When direct mode doesn't find a cached result (cache miss), ccache falls
back to preprocessor mode unless depend mode is enabled. In depend mode,
ccache never runs the preprocessor, even on cache misses. See The depend
mode for details.

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. To mitigate this problem, ccache
records whether directories specified with -I and similar exist at the time
of compilation, which handles most cases. Still, when ccache checks if a
result can be taken from the cache, it currently can't check with 100% ac-
curacy 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.

* 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
When compiling the same source code in different directories, ccache often
can't share cached results if absolute paths become part of the input hash.
These paths appear in:

* preprocessed source code (when using -g for debug info or absolute
paths in -I options)

* command-line arguments (absolute paths to source files or include di-
rectories)

* macro expansions (__FILE__ macros and preprocessor warnings)

To enable cache sharing across different build directories for debug builds
(using -g), normalize debug paths e.g. with GCC option -fdebug-pre-
fix-map=$PWD=. (map current directory to relative) or -fdebug-compila-
tion-dir=. (set compilation directory). Alternatively, set hash_dir to
false (disables directory hashing).

For builds with absolute paths, set base_dir to a common parent directory.
Ccache will convert absolute paths under this directory to relative paths
before hashing.

For example, if projects are in /home/user/projects, set base_dir to
/home/user/projects or just /home/user.

PRECOMPILED HEADERS
Ccache has limited support for precompiled headers (PCH) with GCC and
Clang.

Required configuration:

1. Set sloppiness to pch_defines,time_macros

* This is needed because ccache can't detect time macro usage or #de-
fine changes when using PCH.

* Consider adding include_file_mtime,include_file_ctime for newly
created files (see HANDLING OF NEWLY CREATED SOURCE FILES).

2. One of:

* GCC/Clang: Use -include header (don't use #include in source; the
filename must be sufficient to find the header).

* Clang only: Use -include-pch with the generated PCH file.

* GCC only: Add -fpch-preprocess when compiling.

3. For Clang, add -fno-pch-timestamp to disable timestamp checks.

Troubleshooting:

* Without proper setup, ccache falls back to the non-precompiled header
(if available).

* If no fallback exists, you'll see "Preprocessing failed" in statistics.

* Files using #pragma once instead of include guards may cause issues.

C++ MODULES
Ccache does currently not support standard C++20 modules.

There is however limited support for "Clang modules" (the -fmodules option)
with these requirements:

* Set sloppiness to modules.

* Enable both direct mode and depend mode.

This is how it works:

* Ccache hashes module.modulemap files but ignores Clang's binary module
cache.

* This works because identical module.modulemap files should produce com-
patible cached results.

Note

The preprocessor mode doesn't provide enough information for module
support depend mode. When using the preprocessor mode Clang does not
provide enough information to allow hashing of module.modulemap files.

SHARING A LOCAL CACHE
A group of developers can increase the cache hit rate by sharing a local
cache directory. To share a local cache without unpleasant side effects,
the following conditions should to be met:

* Use the same cache directory.

* Make sure that the configuration option hard_link is false (which is
the default).

* Make sure that all users are in the same group.

* Set the configuration option umask to 002. This ensures that cached
files are accessible to everyone in the group.

* Make sure that all users have write permission in the entire cache 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 space-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
To understand what ccache is doing you can enable debug logging by setting
debug to true or using the environment variable CCACHE_DEBUG=1. See CACHE
DEBUGGING for details.

You can also use ccache -s to view cache hit rates, miss reasons and other
performance metrics.

Performance
Ccache is designed to work well with minimal configuration, but you can op-
timize performance further:

* Place the cache directory on fast storage (SSD preferred).

* Ensure sufficient RAM so cached files stay in the filesystem cache.

* Consider using remote storage for sharing the cache.

To monitor cache effectiveness, compare ccache -s output before and after
builds to identify issues.

If "preprocessed cache hits" increases instead of "direct cache hits",
ccache is falling back to the slower preprocessor mode. Causes include:

* source code changes that don't affect preprocessor output

* changes to include-related options (-I, -include, -D) that don't change
the actual preprocessed code

* using unsupported preprocessor options like -Xpreprocessor or -Wp,*
(except -Wp,-MD,<path>, -Wp,-MMD,<path>, -Wp,-D<define>)

* first compilation after changing base_dir

* include files with timestamps newer than ccache startup (see HANDLING
OF NEWLY CREATED SOURCE FILES)

* code containing __TIME__, __DATE__, or __TIMESTAMP__ forces preproces-
sor mode (if these macros aren't actually used or you don't need cur-
rent values, set sloppiness to time_macros)

* input file path changes (affects __FILE__ macro expansion)

If identical code doesn't produce cache hits these are some possible
causes:

* time/date macros or generated code with timestamps/build counters

* cache cleanup due to size limits

* environment differences affecting compilation

Common error counters:

* "Multiple source files": The compiler was called with multiple source
files at once (not supported by ccache).

* "Unsupported compiler option": Enable debug logging to see which option
was rejected.

* "Preprocessing failed": Could indicate precompiled header issues (see
PRECOMPILED HEADERS).

* "Could not use precompiled header": See PRECOMPILED HEADERS for setup
requirements.

* "Could not use modules": See C++ MODULES for configuration needs and
check which compiler option was rejected.

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 4.13.6 2026-05-04 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+Ports+15.1.quarterly>

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