![]() |
![]()
| ![]() |
![]()
NAMEccache - a fast C/C++ compiler cache SYNOPSISccache [ccache options] ccache [KEY=VALUE ...] compiler [compiler options] compiler [compiler options] The first form takes options described in COMMAND LINE OPTIONS below. The second form invokes the compiler, optionally using configuration options as KEY=VALUE arguments. In the third form, ccache is masquerading as the compiler as described in RUN MODES. DESCRIPTIONCcache is a compiler cache. It speeds up recompilation by caching the result of previous compilations and detecting when the same compilation is being done again. Ccache has been carefully written to always produce exactly the same compiler output that you would get without the cache. The only way you should be able to tell that you are using ccache is the speed. Currently known exceptions to this goal are listed under CAVEATS. If you discover an undocumented case where ccache changes the output of your compiler, please let us know. RUN MODESThere are two different ways to use ccache to cache a compilation: 1.Prefix your compilation command with ccache.
This method is most convenient if you just want to try out ccache or wish to
use it for some specific projects. Example:
ccache gcc -c example.c 2.Let ccache masquerade as the compiler. This method is
most useful when you wish to use ccache for all your compilations. To do this,
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,
COMMAND LINE OPTIONSThese command line options only apply when you invoke ccache as “ccache”. When ccache masquerades as a compiler (as described in the previous section), the normal compiler options apply and you should refer to the compiler’s documentation. Common options-c, --cleanup Clean up the cache by removing not recently used cached
files until the specified file number and cache size limits are not exceeded.
This also recalculates the cache file count and size totals. Normally, there
is no need to initiate cleanup manually as ccache keeps the cache below the
specified limits at runtime and keeps statistics up to date on each
compilation. Forcing a cleanup is mostly useful if you have modified the cache
contents manually or believe that the cache size statistics may be
inaccurate.
-C, --clear Clear the entire cache, removing all cached files, but
keeping the configuration file.
--config-path PATH Let the command line options operate on configuration
file PATH instead of the default. Using this option has the same effect
as setting (overriding) the environment variable CCACHE_CONFIGPATH
temporarily.
-d, --dir PATH Let the command line options operate on cache directory
PATH instead of the default. For example, to show statistics for a
cache directory at /shared/ccache you can run ccache -d
/shared/ccache -s. Using this option has the same effect as setting the
environment variable CCACHE_DIR temporarily.
--evict-namespace NAMESPACE Remove files created in the given namespace from
the cache.
--evict-older-than AGE Remove files used less recently than AGE from the
cache. AGE should be an unsigned integer with a d (days) or
s (seconds) suffix. If combined with --evict-namespace, only
remove files within that namespace.
-h, --help Print a summary of command line options.
-F NUM, --max-files NUM Set the maximum number of files allowed in the cache to
NUM. Use 0 for no limit. The value is stored in a configuration file in
the cache directory and applies to all future compilations.
-M SIZE, --max-size SIZE Set the maximum size of the files stored in the cache.
SIZE should be a number followed by an optional suffix: kB, MB, GB, TB
(decimal), KiB, MiB, GiB or TiB (binary). The default suffix is GiB. Use 0 for
no limit. The value is stored in a configuration file in the cache directory
and applies to all future compilations.
-X LEVEL, --recompress LEVEL Recompress the cache to level LEVEL using the
Zstandard algorithm. The level can be an integer, with the same semantics as
the compression_level configuration option, or the special value
uncompressed for no compression. See [Cache compression]
for more information. This can potentially take a long time since all files in
the cache need to be visited. Only files that are currently compressed with a
different level than LEVEL will be recompressed.
--recompress-threads THREADS Use up to THREADS threads when recompressing the
cache. The default is to use one thread per CPU.
-o KEY=VALUE, --set-config KEY=VALUE Set configuration option KEY to VALUE in
the configuration file. See CONFIGURATION for more information.
-x, --show-compression Print cache compression statistics. See CACHE
COMPRESSION for more information. This can potentially take a long time
since all files in the cache need to be visited.
-p, --show-config Print current configuration options and from where they
originate (environment variable, configuration file or compile-time default)
in human-readable format.
--show-log-stats Print statistics counters from the stats log in
human-readable format. See stats_log. Use -v/--verbose
once or twice for more details.
-s, --show-stats Print a summary of configuration and statistics counters
in human-readable format. Use -v/--verbose once or twice for
more details.
-v, --verbose Increase verbosity. The option can be given multiple
times.
-V, --version Print version and copyright information.
-z, --zero-stats Zero the cache statistics (but not the configuration
options).
Options for remote file-based storage--trim-dir PATH Remove not recently used files from directory PATH
until it is at most the size specified by --trim-max-size.
Warning Don’t use this option to trim the local cache. To trim the
local cache
--trim-max-size SIZE Specify the maximum size for --trim-dir.
SIZE should be a number followed by an optional suffix: kB, MB, GB, TB
(decimal), KiB, MiB, GiB or TiB (binary). The default suffix is GiB. Use 0 for
no limit.
--trim-method METHOD Specify the method to trim a directory with
--trim-dir. Possible values are:
atime LRU (least recently used) using the file access
timestamp. This is the default.
mtime LRU (least recently used) using the file modification
timestamp.
--trim-recompress LEVEL Recompress to level LEVEL using the Zstandard
algorithm when using --trim-dir. The level can be an integer, with the
same semantics as the compression_level configuration option, or the
special value uncompressed for no compression. See [Cache
compression] for more information. This can potentially take a long
time since all files in the cache need to be visited. Only files that are
currently compressed with a different level than LEVEL will be
recompressed.
--trim-recompress-threads THREADS Recompress using up to THREADS threads with
--trim-recompress. The default is to use one thread per CPU.
Options for scripting or debugging--checksum-file PATH Print the checksum (128 bit XXH3) of the file at
PATH (- for standard input).
--extract-result PATH Extract data stored in the result file at PATH
(- for standard input). The data will be written to
ccache-result.* files in to the current working directory. This option
is only useful when debugging ccache and its behavior.
--format FORMAT Specify format for --print-log-stats and
--print-stats. Possible values are:
tab Tab separated. This is the default.
json JSON formatted.
-k KEY, --get-config KEY Print the value of configuration option KEY. See
CONFIGURATION for more information.
--hash-file PATH Print the hash (160 bit BLAKE3) of the file at
PATH (- for standard input). This is only useful when debugging
ccache and its behavior.
--inspect PATH Print the content of a result or manifest file at
PATH (- for standard input) to standard output in human-readable
format. File content embedded in a result file will however not be printed;
use --extract-result to extract the file content. This option is only
useful when debugging ccache and its behavior.
--print-log-stats Print statistics counters from the stats log in
machine-parsable (tab-separated or JSON) format. See stats_log and
--format.
--print-stats Print statistics counter IDs and corresponding values in
machine-parsable (tab-separated or JSON) format. See --format.
--print-version Print version and don’t do anything else.
Extra optionsWhen run as a compiler, ccache usually just takes the same command line options as the compiler you are using. The only exception to this is the option --ccache-skip. That option can be used to tell ccache to avoid interpreting the next option in any way and to pass it along to the compiler as-is. Note --ccache-skip currently only tells ccache not to interpret
the next
The reason this can be important is that ccache does need to parse the command line and determine what is an input filename and what is a compiler option, as it needs the input filename to determine the name of the resulting object file (among other things). The heuristic ccache uses when parsing the command line is that any argument that exists as a file is treated as an input file name. By using --ccache-skip you can force an option to not be treated as an input file name and instead be passed along to the compiler as a command line option. Another case where --ccache-skip can be useful is if ccache interprets an option specially but shouldn’t, since the option has another meaning for your compiler than what ccache thinks. See also ignore_options. CONFIGURATIONCcache’s default behavior can be overridden by options in configuration files, which in turn can be overridden by environment variables with names starting with CCACHE_. Ccache normally reads configuration from two files: first a system-level configuration file and secondly a cache-specific configuration file. The priorities of configuration options are as follows (where 1 is highest): 1.Command line settings in KEY=VALUE form.
Example:
ccache debug=true compiler_check="%compiler% --version" gcc -c example.c 2.Environment variables.
3.The cache-specific configuration file (see
below).
4.The system (read-only) configuration file
<sysconfdir>/ccache.conf (typically /etc/ccache.conf or
/usr/local/etc/ccache.conf).
5.Compile-time defaults.
As a special case, if the environment variable CCACHE_CONFIGPATH is set it specifies the configuration file, and the system configuration file won’t be read. Location of the configuration fileThe location of the cache-specific configuration file is determined like this on non-Windows systems: 1.If CCACHE_CONFIGPATH is set, use that
path.
2.Otherwise, if the environment variable
CCACHE_DIR is set then use $CCACHE_DIR/ccache.conf.
3.Otherwise, if cache_dir is set in the system
configuration file then use <cache_dir>/ccache.conf.
4.Otherwise, if there is a legacy $HOME/.ccache
directory then use $HOME/.ccache/ccache.conf.
5.Otherwise, if XDG_CONFIG_HOME is set then use
$XDG_CONFIG_HOME/ccache/ccache.conf.
6.Otherwise, use
$HOME/Library/Preferences/ccache/ccache.conf (macOS) or
$HOME/.config/ccache/ccache.conf (other systems).
On Windows, this is the method used to find the configuration file: 1.If CCACHE_CONFIGPATH is set, use that
path.
2.Otherwise, if the environment variable
CCACHE_DIR is set then use %CCACHE_DIR%/ccache.conf.
3.Otherwise, if cache_dir is set in the system
configuration file then use <cache_dir>\ccache.conf. The
system-wide configuration on Windows is
%ALLUSERSPROFILE%\ccache\ccache.conf by default. The
ALLUSERSPROFILE environment variable is usually
C:\ProgramData.
4.Otherwise, if there is a legacy
%USERPROFILE%\.ccache directory then use
%USERPROFILE%\.ccache\ccache.conf.
5.Otherwise, use
%LOCALAPPDATA%\ccache\ccache.conf if it exists.
6.Otherwise, use
%APPDATA%\ccache\ccache.conf.
See also the cache_dir configuration option for how the cache directory location is determined. Configuration value syntaxAll 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 syntaxConfiguration files are in a simple “key = value” format, one option per line. Lines starting with a hash sign are comments. Blank lines are ignored, as is whitespace surrounding keys and values. Example: # Set maximum cache size to 10 GB: max_size = 10G Boolean valuesSome configuration options are boolean values (i.e. truth values). In a configuration file, such values must be set to the string true or false. For the corresponding environment variables, the semantics are a bit different: •A set environment variable means
“true” (even if set to the empty string).
•The following case-insensitive negative values
are considered an error (instead of surprising the user): 0,
false, disable and no.
•An unset environment variable means
“false”.
Each boolean environment variable also has a negated form starting with CCACHE_NO. For example, CCACHE_COMPRESS can be set to force compression and CCACHE_NOCOMPRESS can be set to force no compression. Configuration optionsBelow is a list of available configuration options. The corresponding environment variable name is indicated in parentheses after each configuration option key. absolute_paths_in_stderr (CCACHE_ABSSTDERR) This option specifies whether ccache should rewrite
relative paths in the compiler’s standard error output to absolute
paths. This can be useful if you use base_dir with a build system (e.g.
CMake with the "Unix Makefiles" generator) that executes the
compiler in a different working directory, which makes relative paths in
compiler errors or warnings incorrect. The default is false.
base_dir (CCACHE_BASEDIR) This option should be an absolute path to a directory. If
set, ccache will rewrite absolute paths into paths relative to the current
working directory, but only absolute paths that begin with base_dir.
Cache results can then be shared for compilations in different directories
even if the project uses absolute paths in the compiler command line. See also
the discussion under COMPILING IN DIFFERENT DIRECTORIES. If set to the
empty string (which is the default), no rewriting is done.
A typical path to use as base_dir is your home directory or another directory that is a parent of your project directories. Don’t use / as the base directory since that will make ccache also rewrite paths to system header files, which typically is counterproductive. For example, say that Alice’s current working directory is /home/alice/project1/build and that she compiles like this: ccache gcc -I/usr/include/example -I/home/alice/project2/include -c /home/alice/project1/src/example.c Here is what ccache will actually execute for different base_dir values: # Current working directory: /home/alice/project1/build # With base_dir = /: gcc -I../../../../usr/include/example -I../../project2/include -c ../src/example.c # With base_dir = /home or /home/alice: gcc -I/usr/include/example -I../../project2/include -c ../src/example.c # With base_dir = /home/alice/project1 or /home/alice/project1/src: gcc -I/usr/include/example -I/home/alice/project2/include -c ../src/example.c If Bob has put project1 and project2 in /home/bob/stuff and both users have set base_dir to /home or /home/$USER, then Bob will get a cache hit (if they share ccache directory) since the actual command line will be identical to Alice’s command line: # Current working directory: /home/bob/stuff/project1/build # With base_dir = /home or /home/bob: gcc -I/usr/include/example -I../../project2/include -c ../src/example.c Without base_dir there will be a cache miss since the absolute paths will differ. With base_dir set to / there will be a cache miss since the relative path to /usr/include/example will be different. With base_dir set to /home/bob/stuff/project1 there will a cache miss since the path to project2 will be a different absolute path. Warning Rewriting absolute paths to relative is kind of a brittle hack. It
cache_dir (CCACHE_DIR) This option specifies where ccache will keep its cached
compiler outputs.
On non-Windows systems, the default is $HOME/.ccache if such a directory exists, otherwise $XDG_CACHE_HOME/ccache if XDG_CACHE_HOME is set, otherwise $HOME/Library/Caches/ccache (macOS) or $HOME/.config/ccache (other systems). On Windows, the default is %USERPROFILE%\.ccache if such a directory exists, otherwise %LOCALAPPDATA%\ccache. Warning Previous ccache versions defaulted to storing the cache in
See also Location of the configuration file. compiler (CCACHE_COMPILER or (deprecated) CCACHE_CC) This option can be used to force the name of the compiler
to use. If set to the empty string (which is the default), ccache works it out
from the command line.
compiler_check (CCACHE_COMPILERCHECK) By default, ccache includes the modification time
(“mtime”) and size of the compiler in the hash to ensure that
results retrieved from the cache are accurate. If compiler plugins are used,
these plugins will also be added to the hash. This option can be used to
select another strategy. Possible values are:
content Hash the content of the compiler binary. This makes
ccache very slightly slower compared to mtime, but makes it cope better
with compiler upgrades during a build bootstrapping process.
mtime Hash the compiler’s mtime and size, which is fast.
This is the default.
none Don’t hash anything. This may be good for
situations where you can safely use the cached results even though the
compiler’s mtime or size has changed (e.g. if the compiler is built as
part of your build system and the compiler’s source has not changed, or
if the compiler only has changes that don’t affect code generation).
You should only use none if you know what you are doing.
string:value Hash value. This can for instance be a compiler
revision number or another string that the build system generates to identify
the compiler.
a command string Hash the standard output and standard error output of the
specified command. The string will be split on whitespace to find out the
command and arguments to run. No other interpretation of the command string
will be done, except that the special word %compiler% will be replaced
with the path to the compiler. Several commands can be specified with
semicolon as separator. Examples:
%compiler% -v %compiler% -dumpmachine; %compiler% -dumpversion You should make sure that the specified command is as fast as possible since it will be run once for each ccache invocation. Identifying the compiler using a command is useful if you want to avoid cache misses when the compiler has been rebuilt but not changed. Another case is when the compiler (as seen by ccache) actually isn’t the real compiler but another compiler wrapper — in that case, the default mtime method will hash the mtime and size of the other compiler wrapper, which means that ccache won’t be able to detect a compiler upgrade. Using a suitable command to identify the compiler is thus safer, but it’s also slower, so you should consider continue using the mtime method in combination with the prefix_command option if possible. See USING CCACHE WITH OTHER COMPILER WRAPPERS. compiler_type (CCACHE_COMPILERTYPE) Ccache normally guesses the compiler type based on the
compiler name. The compiler_type option lets you force a compiler type.
This can be useful if the compiler has a non-standard name but is actually one
of the known compiler types. Possible values are:
auto Guess one of the types below based on the compiler name
(following symlinks). This is the default.
clang Clang-based compiler.
clang-cl clang-cl.
gcc GCC-based compiler.
icl Intel compiler on Windows.
msvc Microsoft Visual C++ (MSVC).
nvcc NVCC (CUDA) compiler.
other Any compiler other than the known types.
compression (CCACHE_COMPRESS or CCACHE_NOCOMPRESS, see Boolean values above) If true, ccache will compress data it puts in the cache.
However, this option has no effect on how files are retrieved from the cache;
compressed and uncompressed results will still be usable regardless of this
option. The default is true.
Compression is done using the Zstandard algorithm. The algorithm is fast enough that there should be little reason to turn off compression to gain performance. One exception is if the cache is located on a compressed file system, in which case the compression performed by ccache of course is redundant. Compression will be disabled if file cloning (the file_clone option) or hard linking (the hard_link option) is enabled. compression_level (CCACHE_COMPRESSLEVEL) This option determines the level at which ccache will
compress object files using the real-time compression algorithm Zstandard. It
only has effect if compression is enabled (which it is by default).
Zstandard is extremely fast for decompression and very fast for compression
for lower compression levels. The default is 0.
Semantics of compression_level: > 0 A positive value corresponds to normal Zstandard
compression levels. Lower levels (e.g. 1) mean faster compression but
worse compression ratio. Higher levels (e.g. 19) mean slower
compression but better compression ratio. The maximum possible value depends
on the libzstd version, but at least up to 19 is available for all versions.
Decompression speed is essentially the same for all levels. As a rule of
thumb, use level 5 or lower since higher levels may slow down compilations
noticeably. Higher levels are however useful when recompressing the cache with
command line option -X/--recompress.
< 0 A negative value corresponds to Zstandard’s
“ultra-fast” compression levels, which are even faster than
level 1 but with less good compression ratios. For instance, level -3
corresponds to --fast=3 for the zstd command line tool. In
practice, there is little use for levels lower than -5 or so.
0 (default) The value 0 means that ccache will choose a
suitable level, currently 1.
See the Zstandard documentation <http://zstd.net> for more information. cpp_extension (CCACHE_EXTENSION) This option can be used to force a certain extension for
the intermediate preprocessed file. The default is to automatically determine
the extension to use for intermediate preprocessor files based on the type of
file being compiled, but that sometimes doesn’t work. For example, when
using the “aCC” compiler on HP-UX, set the cpp extension to
i.
debug (CCACHE_DEBUG or CCACHE_NODEBUG, see Boolean values above) If true, enable the debug mode. The debug mode creates
per-object debug files that are helpful when debugging unexpected cache
misses. Note however that ccache performance will be reduced slightly. See
CACHE DEBUGGING for more information. The default is false.
debug_dir (CCACHE_DEBUGDIR) Specifies where to write per-object debug files if the
debug mode is enabled. If set to the empty string, the files will be written
next to the object file. If set to a directory, the debug files will be
written with full absolute paths in that directory, creating it if needed. The
default is the empty string.
For example, if debug_dir is set to /example, the current working directory is /home/user and the object file is build/output.o then the debug log will be written to /example/home/user/build/output.o.ccache-log. See also CACHE DEBUGGING. debug_level (CCACHE_DEBUGLEVEL) Specifies the amount of information that is written when
the debug mode is enabled. See CACHE DEBUGGING for more information.
The default is 2.
depend_mode (CCACHE_DEPEND or CCACHE_NODEPEND, see Boolean values above) If true, the depend mode will be used. The default is
false. See The depend mode.
direct_mode (CCACHE_DIRECT or CCACHE_NODIRECT, see Boolean values above) If true, the direct mode will be used. The default is
true. See The direct mode.
disable (CCACHE_DISABLE or CCACHE_NODISABLE, see Boolean values above) When true, ccache will just call the real compiler,
bypassing the cache completely. The default is false.
It is also possible to disable ccache for a specific source code file by adding the string ccache:disable in a comment in the first 4096 bytes of the file. extra_files_to_hash (CCACHE_EXTRAFILES) This option is a list of paths to files that ccache will
include in the the hash sum that identifies the build. The list separator is
semicolon on Windows systems and colon on other systems.
file_clone (CCACHE_FILECLONE or CCACHE_NOFILECLONE, see Boolean values above) If true, ccache will attempt to use file cloning (also
known as “copy on write”, “CoW” or
“reflinks”) to store and fetch cached compiler results.
file_clone has priority over hard_link. The default is false.
Files stored by cloning cannot be compressed, so the cache size will likely be significantly larger if this option is enabled. However, performance may be improved depending on the use case. Unlike the hard_link option, file_clone is completely safe to use, but not all file systems support the feature. For such file systems, ccache will fall back to use plain copying (or hard links if hard_link is enabled). hard_link (CCACHE_HARDLINK or CCACHE_NOHARDLINK, see Boolean values above) If true, ccache will attempt to use hard links to store
and fetch cached object files. The default is false.
Files stored via hard links cannot be compressed, so the cache size will likely be significantly larger if this option is enabled. However, performance may be improved depending on the use case. Warning Do not enable this option unless you are aware of these caveats: •If the resulting file is modified, the file in
the cache will also be modified since they share content, which corrupts the
cache entry. As of version 4.0, ccache makes stored and fetched object files
read-only as a safety measure. Furthermore, a simple integrity check is made
for cached object files by verifying that their sizes are correct. This means
that mistakes like strip file.o or echo >file.o will be
detected even if the object file is made writable, but a modification that
doesn’t change the file size will not.
•Programs that don’t expect that files from
two different identical compilations are hard links to each other can
fail.
•Programs that rely on modification times (like
make) can be confused if several users (or one user with several build
trees) use the same cache directory. The reason for this is that the object
files share i-nodes and therefore modification times. If file.o is in
build tree A (hard-linked from the cache) and file.o then is
produced by ccache in build tree B by hard-linking from the cache, the
modification timestamp will be updated for file.o in build tree
A as well. This can retrigger relinking in build tree A even
though nothing really has changed.
hash_dir (CCACHE_HASHDIR or CCACHE_NOHASHDIR, see Boolean values above) If true (which is the default), ccache will include the
current working directory (CWD) in the hash that is used to distinguish two
compilations when generating debug info (compiler option -g with
variations). Exception: The CWD will not be included in the hash if
base_dir is set (and matches the CWD) and the compiler option
-fdebug-prefix-map is used. See also the discussion under COMPILING
IN DIFFERENT DIRECTORIES.
The reason for including the CWD in the hash by default is to prevent a problem with the storage of the current working directory in the debug info of an object file, which can lead ccache to return a cached object file that has the working directory in the debug info set incorrectly. You can disable this option to get cache hits when compiling the same source code in different directories if you don’t mind that CWD in the debug info might be incorrect. ignore_headers_in_manifest (CCACHE_IGNOREHEADERS) This option is a list of paths to files (or directories
with headers) that ccache will not include in the manifest list that
makes up the direct mode. Note that this can cause stale cache hits if those
headers do indeed change. The list separator is semicolon on Windows systems
and colon on other systems.
ignore_options (CCACHE_IGNOREOPTIONS) This option is a space-delimited list of compiler options
that ccache will ignore. Entries in the list can optionally end with an
asterisk (*) to matching any option suffix. For example,
-fmessage-length=* will match both -fmessage-length=20 and
-fmessage-length=70. A matching compiler option will neither be
interpreted specially nor be part of the input hash. Ignoring a compiler
option from the hash can be useful when you know it doesn’t affect the
result (and ccache doesn’t know that), or when it does and you
don’t care. See also Extra options.
inode_cache (CCACHE_INODECACHE or CCACHE_NOINODECACHE, see Boolean values above) If true, ccache will cache source file hashes based on
device, inode and timestamps. This reduces the time spent on hashing include
files since the result can be reused between compilations. The default is
true. The feature requires temporary_dir to be located on a local
filesystem of a supported type.
Note Support for the inode cache feature on Windows is experimental. On
Windows
keep_comments_cpp (CCACHE_COMMENTS or CCACHE_NOCOMMENTS, see Boolean values above) If true, ccache will not discard the comments before
hashing preprocessor output. The default is false. This can be used to check
documentation with -Wdocumentation.
log_file (CCACHE_LOGFILE) If set to a file path, ccache will write information on
what it is doing to the specified file. This is useful for tracking down
problems.
If set to syslog, ccache will log using syslog() instead of to a file. If you use rsyslogd, you can add something like this to /etc/rsyslog.conf or a file in /etc/rsyslog.d: # log ccache to file :programname, isequal, "ccache" /var/log/ccache # remove from syslog & ~ max_files (CCACHE_MAXFILES) This option specifies the maximum number of files to keep
in the cache. Use 0 for no limit (which is the default). See also CACHE
SIZE MANAGEMENT.
max_size (CCACHE_MAXSIZE) This option specifies the maximum size of the cache. Use
0 for no limit. The default value is 5G. Available suffixes: k, M, G, T
(decimal) and Ki, Mi, Gi, Ti (binary). The default suffix is G. See also
CACHE SIZE MANAGEMENT.
msvc_dep_prefix (CCACHE_MSVC_DEP_PREFIX) This option specifies the prefix of included files output
for MSVC compiler. The default prefix is “Note: including
file:”. If you use a localized compiler, this should be set
accordingly.
namespace (CCACHE_NAMESPACE) If set, the namespace string will be added to the hashed
data for each compilation. This will make the associated cache entries
logically separate from cache entries with other namespaces, but they will
still share the same storage space. Cache entries can also be selectively
removed from the local cache with the command line option
--evict-namespace, potentially in combination with
--evict-older-than.
For instance, if you use the same local cache for several disparate projects, you can use a unique namespace string for each one. This allows you to remove cache entries that belong to a certain project if you stop working with that project. path (CCACHE_PATH) If set, ccache will search directories in this list when
looking for the real compiler. The list separator is semicolon on Windows
systems and colon on other systems. If not set, ccache will look for the first
executable matching the compiler name in the normal PATH that
isn’t a symbolic link to ccache itself.
pch_external_checksum (CCACHE_PCH_EXTSUM or CCACHE_NOPCH_EXTSUM, see Boolean values above) When this option is set, and ccache finds a precompiled
header file, ccache will look for a file with the extension
“.sum” added (e.g. “pre.h.gch.sum”), and if found,
it will hash this file instead of the precompiled header itself to work around
the performance penalty of hashing very large files.
prefix_command (CCACHE_PREFIX) This option adds a list of prefixes (separated by space)
to the command line that ccache uses when invoking the compiler. See also
USING CCACHE WITH OTHER COMPILER WRAPPERS.
prefix_command_cpp (CCACHE_PREFIX_CPP) This option adds a list of prefixes (separated by space)
to the command line that ccache uses when invoking the preprocessor.
read_only (CCACHE_READONLY or CCACHE_NOREADONLY, see Boolean values above) If true, ccache will attempt to use existing cached
results, but it will not add new results to any cache backend. Statistics
counters will still be updated, though, unless the stats option is set
to false.
If you are using this because your ccache directory is read-only, you need to set temporary_dir since ccache will fail to create temporary files otherwise. You may also want to set stats to false make ccache not even try to update stats files. read_only_direct (CCACHE_READONLY_DIRECT or CCACHE_NOREADONLY_DIRECT, see Boolean values above) Just like read_only except that ccache will only
try to retrieve results from the cache using the direct mode, not the
preprocessor mode. See documentation for read_only regarding using a
read-only ccache directory.
recache (CCACHE_RECACHE or CCACHE_NORECACHE, see Boolean values above) If true, ccache will not use any previously stored
result. New results will still be cached, possibly overwriting any
pre-existing results.
remote_only (CCACHE_REMOTE_ONLY or CCACHE_NOREMOTE_ONLY, see Boolean values above) If true, ccache will only use remote storage. The default
is false. Note that cache statistics counters will still be kept in the local
cache directory unless stats is false. See also Storage
interaction.
remote_storage (CCACHE_REMOTE_STORAGE) This option specifies one or several storage backends
(separated by space) to query after checking the local cache (unless
remote_only is true). See REMOTE STORAGE BACKENDS for
documentation of syntax and available backends.
Examples: •file:/shared/nfs/directory
•file:///shared/nfs/one|read-only
file:///shared/nfs/two
•file:///Z:/example/windows/folder
•http://example.com/cache
•redis://example.com
Note In previous ccache versions this option was called
secondary_storage
reshare (CCACHE_RESHARE or CCACHE_NORESHARE, see Boolean values above) If true, ccache will write results to remote storage even
for local storage cache hits. The default is false.
run_second_cpp (CCACHE_CPP2 or CCACHE_NOCPP2, see Boolean values above) If true, ccache will first run the preprocessor to
preprocess the source code (see The preprocessor mode) and then on a
cache miss run the compiler on the source code to get hold of the object file.
This is the default.
If false, ccache will first run preprocessor to preprocess the source code and then on a cache miss run the compiler on the preprocessed source code instead of the original source code. This makes cache misses slightly faster since the source code only has to be preprocessed once. The downside is that some compilers won’t produce the same result (for instance diagnostics warnings) when compiling preprocessed source code. A solution to the above mentioned downside is to set run_second_cpp to false and pass -fdirectives-only (for GCC) or -frewrite-includes (for Clang) to the compiler. This will cause the compiler to leave the macros and other preprocessor information, and only process the #include directives. When run in this way, the preprocessor arguments will be passed to the compiler since it still has to do some preprocessing (like macros). This option is ignored with MSVC, as there is no way to make it compile without preprocessing first. sloppiness (CCACHE_SLOPPINESS) By default, ccache tries to give as few false cache hits
as possible. However, in certain situations it’s possible that you know
things that ccache can’t take for granted. This option makes it
possible to tell ccache to relax some checks in order to increase the hit
rate. The value should be a comma-separated string with one or several of the
following values:
clang_index_store Ignore the Clang compiler option -index-store-path
and its argument when computing the manifest hash. This is useful if you use
Xcode, which uses an index store path derived from the local project path.
Note that the index store won’t be updated correctly on cache hits if
you enable this sloppiness.
file_stat_matches Ccache normally examines a file’s contents to
determine whether it matches the cached version. With this sloppiness set,
ccache will consider a file as matching its cached version if the mtimes and
ctimes match.
file_stat_matches_ctime Ignore ctimes when file_stat_matches is enabled.
This can be useful when backdating files' mtimes in a controlled way.
gcno_cwd By default, ccache will include the current working
directory in the hash when producing a .gcno file (when compiling with
-ftest-coverage). This is because GCC 9+ includes the current working
directory in the .gcno file. The gcno_cwd sloppiness makes
ccache not hash the current working directory so that you can get cache hits
when compiling in different directories, with the tradeoff of potentially
getting an incorrect directory in the .gcno file. gcno_cwd also
disables hashing of the current working directory if -fprofile-abs-path
is used.
Note gcno_cwd sloppiness will not have any effect when compiling
with
incbin By default, ccache will ignore all files containing an
.incbin directive. While this is the correct behaviour as ccache does
not detect incbin changes, this restriction can make some projects difficult
to cache. This sloppiness will pretend the .incbin directive
doesn’t exist and simply allow caching.
include_file_ctime By default, ccache will disable caching if a source code
file has a status change time (ctime) after the start of the ccache
invocation. This sloppiness disables that check. See also [Handling of
newly created source files].
include_file_mtime By default, ccache will disable caching if a source code
file has a modification time (mtime) after the start of the ccache invocation.
This sloppiness disables that check. See also [Handling of newly created
source files].
ivfsoverlay Ignore the Clang compiler option -ivfsoverlay and
its argument. This is useful if you use Xcode, which uses a virtual file
system (VFS) for things like combining Objective-C and Swift code.
locale Ccache includes the environment variables LANG,
LC_ALL, LC_CTYPE and LC_MESSAGES in the hash by default
since they may affect localization of compiler warning messages. Set this
sloppiness to tell ccache not to do that.
modules By default, ccache will not cache compilations if
-fmodules is used since it cannot hash the state of compiler’s
internal representation of relevant modules. This sloppiness allows caching in
such a case. See C++ MODULES for more information.
pch_defines Be sloppy about #define directives when
precompiling a header file. See PRECOMPILED HEADERS for more
information.
random_seed Ignore the -frandom-seed option and its arguments
when computing the input hash. This is useful if your build system generates
different seeds between builds and you are OK with reusing cached
results.
system_headers Only check non-system headers in direct mode. This can be
useful if e.g. your system headers tend to change but you know that the
changes don’t matter. Notes:
•This sloppiness is only supported for GCC-like
compilers, not MSVC.
•System headers are still taken into account for
preprocessed lookup.
•You can get stale cache hits if the system
headers do change in incompatible ways.
•See also the ignore_headers_in_manifest
setting.
time_macros Ignore __DATE__, __TIME__ and
__TIMESTAMP__ being present in the source code.
See the discussion under TROUBLESHOOTING for more information. stats (CCACHE_STATS or CCACHE_NOSTATS, see Boolean values above) If true, ccache will update the statistics counters on
each compilation. The default is true. If false, [automatic cleanup]
will be disabled as well.
stats_log (CCACHE_STATSLOG) If set to a file path, ccache will write statistics
counter updates to the specified file. This is useful for getting statistics
for individual builds. To show a summary of the current stats log, use
ccache --show-log-stats.
Note Lines in the stats log starting with a hash sign (#) are comments. temporary_dir (CCACHE_TEMPDIR) This option specifies where ccache will put temporary
files. The default is $XDG_RUNTIME_DIR/ccache-tmp (typically
/run/user/<UID>/ccache-tmp) if XDG_RUNTIME_DIR is set and
the directory exists, otherwise <cache_dir>/tmp.
Note In previous versions of ccache, CCACHE_TEMPDIR had to be on
the same
umask (CCACHE_UMASK) This option (an octal integer) specifies the umask for
files and directories in the cache directory. This is mostly useful when you
wish to share your cache with other users.
Disabling ccacheTo 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 BACKENDSThe remote_storage option lets you configure ccache to use one or several remote storage backends. By default, the local cache directory located in cache_dir will be queried first and remote storage second, but remote_only can be set to true to disable local storage. Note that cache statistics counters will still be kept in the local cache directory — remote storage backends only store compilation results and manifests. A remote storage backend is specified with a URL, optionally followed by a pipe (|) and a pipe-separated list of attributes. An attribute is key=value or just key as a short form of key=true. Attribute values must be percent-encoded <https://en.wikipedia.org/wiki/Percent-encoding> if they contain percent, pipe or space characters. Attributes for all backendsThese optional attributes are available for all remote storage backends: •read-only: If true, only read from
this backend, don’t write. The default is false.
•shards: A comma-separated list of names
for sharding (partitioning) the cache entries using Rendezvous hashing
<https://en.wikipedia.org/wiki/Rendezvous_hashing>, typically to spread
the cache over a server cluster. When set, the storage URL must contain an
asterisk (*), which will be replaced by one of the shard names to form
a real URL. A shard name can optionally have an appended weight within
parentheses to indicate how much of the key space should be associated with
that shard. A shard with weight w will contain w/S of the
cache, where S is the sum of all shard weights. A weight could for
instance be set to represent the available memory for a memory cache on a
specific server. The default weight is 1.
Examples: •redis://cache-*.example.com|shards=a(3),b(1),c(1.5)
will put 55% (3/5.5) of the cache on redis://cache-a.example.com, 18%
(1/5.5) on redis://cache-b.example.com and 27% (1.5/5.5) on
redis://cache-c.example.com.
•http://example.com/*|shards=alpha,beta
will put 50% of the cache on http://example.com/alpha and 50% on
http://example.com/beta.
Storage interactionThe table below describes the interaction between local and remote storage on cache hits and misses if remote_only is false (which is the default):
[1] Unless remote storage has attribute read-only=true.
If remote_only is true:
File storage backendURL format: file:DIRECTORY or file://[HOST]DIRECTORY This backend stores data as separate files in a directory structure below DIRECTORY, similar (but not identical) to the local cache storage. A typical use case for this backend would be sharing a cache on an NFS directory. DIRECTORY must start with a slash. HOST can be the empty string or localhost. On Windows, HOST can also be the name of a server hosting a shared folder. Important ccache will not perform any cleanup of the storage — that
has to be
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 backendURL 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
Note HTTPS is not supported. Tip See
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
•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 backendURL formats: redis://[[USERNAME:]PASSWORD@]HOST[:PORT][/DBNUMBER]
This backend stores data in a Redis <https://redis.io> (or Redis-compatible) server. There are implementations for both memory-based and disk-based storage. PORT defaults to 6379 and DBNUMBER defaults to 0. Note ccache will not perform any cleanup of the Redis storage, but you
can
Tip See
Tip You can set up a cluster of Redis servers using the shards
attribute
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 MANAGEMENTBy 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 cleanupAfter 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 cleanupYou 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 COMPRESSIONCcache will by default compress all data it puts into the cache using the compression algorithm Zstandard <http://zstd.net> (zstd) using compression level 1. The algorithm is fast enough that there should be little reason to turn off compression to gain performance. One exception is if the cache is located on a compressed file system, in which case the compression performed by ccache of course is redundant. See the documentation for the configuration options compression and compression_level for more information. You can use the command line option -x/--show-compression to print information related to compression. Example: Total data: 14.8 GB (16.0 GB disk blocks) Compressed data: 11.3 GB (30.6% of original size) Notes: •The “disk blocks” size is the cache
size when taking disk block size into account. This value should match the
“Cache size” value from “ccache --show-stats”. The
other size numbers refer to actual content sizes.
•“Compressed data” refers to result
and manifest files stored in the cache.
•“Incompressible data” refers to
files that are always stored uncompressed (triggered by enabling
file_clone or hard_link) or unknown files (for instance files
created by older ccache versions).
•The compression ratio is affected by
compression_level.
The cache data can also be recompressed to another compression level (or made uncompressed) with the command line option -X/--recompress. If you choose to disable compression by default or to use a low compression level, you can (re)compress newly cached data with a higher compression level after the build or at another time when there are more CPU cycles available, for instance every night. Full recompression potentially takes a lot of time, but only files that are currently compressed with a different level than the target level will be recompressed. CACHE STATISTICSccache --show-stats shows a summary of statistics, including cache size, cleanups (number of performed cleanups, either implicitly due to a cache size limit being reached or due to explicit ccache -c calls), overall hit rate, hit rate for direct/preprocessed modes and hit rate for local and remote storage. The summary also includes counters called “Errors” and “Uncacheable”, which are sums of more detailed counters. To see those detailed counters, use the -v/--verbose flag. The verbose mode can show the following counters:
HOW CCACHE WORKSThe basic idea is to detect when you are compiling exactly the same code a second time and reuse the previously produced output. The detection is done by hashing different kinds of information that should be unique for the compilation and then using the hash sum to identify the cached output. Ccache uses BLAKE3, a very fast cryptographic hash algorithm, for the hashing. On a cache hit, ccache is able to supply all of the correct compiler outputs (including all warnings, dependency file, etc) from the cache. Data stored in the cache is checksummed with XXH3, an extremely fast non-cryptographic algorithm, to detect corruption. Ccache has two ways of gathering information used to look up results in the cache: •the preprocessor mode, where ccache runs
the preprocessor on the source code and hashes the result
•the direct mode, where ccache hashes the
source code and include files directly
The direct mode is generally faster since running the preprocessor has some overhead. If no previous result is detected (i.e., there is a cache miss) using the direct mode, ccache will fall back to the preprocessor mode unless the depend mode is enabled. In the depend mode, ccache never runs the preprocessor, not even on cache misses. Read more in The depend mode below. Common hashed informationThe 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 modeIn 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 modeIn the direct mode, the hash is formed of the common information and: •the input source file
•the compiler options
Based on the hash, a data structure called “manifest” is looked up in the cache. The manifest contains: •references to cached compilation results (object
file, dependency file, etc) that were produced by previous compilations that
matched the hash
•paths to the include files that were read at the
time the compilation results were stored in the cache
•hash sums of the include files at the time the
compilation results were stored in the cache
The current contents of the include files are then hashed and compared to the information in the manifest. If there is a match, ccache knows the result of the compilation. If there is no match, ccache falls back to running the preprocessor. The output from the preprocessor is parsed to find the include files that were read. The paths and hash sums of those include files are then stored in the manifest along with information about the produced compilation result. There is a catch with the direct mode: header files that were used by the compiler are recorded, but header files that were not used, but would have been used if they existed, are not. So, when ccache checks if a result can be taken from the cache, it currently can’t check if the existence of a new header file should invalidate the result. In practice, the direct mode is safe to use in the absolute majority of cases. The direct mode will be disabled if any of the following holds: •direct_mode is false
•a modification time of one of the include files
is too new (needed to avoid a race condition)
•a compiler option not supported by the direct
mode is used, for example:
•a -Wp,* compiler option other than
-Wp,-MD,<path>, -Wp,-MMD,<path>,
-Wp,-D<macro[=defn]> or -Wp,-U<macro>
•most uses of -Xpreprocessor
•the string __TIME__ is present in the
source code
The depend modeIf the depend mode is enabled, ccache will not use the preprocessor at all. The hash used to identify results in the cache will be based on the direct mode hash described above plus information about include files read from the dependency list generated by MSVC with /showIncludes, or the dependency file generated by other compilers with -MD or -MMD. Advantages: •The ccache overhead of a cache miss will be much
smaller.
•Not running the preprocessor at all can be good
if compilation is performed remotely, for instance when using distcc or
similar; ccache then won’t make potentially costly preprocessor calls
on the local machine.
Disadvantages: •The cache hit rate will likely be lower since any
change to compiler options or source code will make the hash different.
Compare this with the default setup where ccache will fall back to the
preprocessor mode, which is tolerant to some types of changes of compiler
options and source code changes.
•If -MD is used, the manifest entries will
include system header files as well, thus slowing down cache hits slightly,
just as using -MD slows down make. This is also the case for MSVC with
/showIncludes.
•If -MMD is used, the manifest entries will
not include system header files, which means ccache will ignore changes in
them.
The depend mode will be disabled if any of the following holds: •depend_mode is false.
•run_second_cpp is false.
•The compiler is not generating dependencies using
-MD or -MMD (for MSVC, /showIncludes is added
automatically if not specified by the user).
HANDLING OF NEWLY CREATED SOURCE FILESIf modification time (mtime) or status change time (ctime) of the source file or one of the include files is equal to (or newer than) the time that ccache was invoked, ccache disables caching completely. This is done as a safety measure to avoid a race condition (see below). In practice, this is only a problem when using file systems with very low timestamp granularity. You can set sloppiness to include_file_ctime,include_file_mtime to opt out of the safety measure. For reference, the race condition mentioned above consists of these events: 1.A source code file is read by ccache and added to the
input hash.
2.The source code file is modified.
3.The compiler is executed and reads the modified source
code.
4.Ccache stores the compiler output in the cache
associated with the incorrect key (based on the unmodified source code).
CACHE DEBUGGINGTo find out what information ccache actually is hashing, you can enable the debug mode via the configuration option debug or by setting CCACHE_DEBUG in the environment. This can be useful if you are investigating why you don’t get cache hits. Note that performance will be reduced slightly. When the debug mode is enabled, ccache will create up to five additional files next to the object file:
The timestamp format is <year><month><day>_<hour><minute><second>_<microsecond>. If you only need the log file, set debug_level (environment variable CCACHE_DEBUGLEVEL) to 1. If debug_dir (environment variable CCACHE_DEBUGDIR) is set, the files above will be written to that directory with full absolute paths instead of next to the object file. In the direct mode, ccache uses the 160 bit BLAKE3 hash of the “ccache-input-c” + “ccache-input-d” data (where + means concatenation), while the “ccache-input-c” + “ccache-input-p” data is used in the preprocessor mode. The “ccache-input-text” file is a combined text version of the three binary input files. It has three sections (“COMMON”, “DIRECT MODE” and “PREPROCESSOR MODE”), which is turn contain annotations that say what kind of data comes next. To debug why you don’t get an expected cache hit for an object file, you can do something like this: 1.Enable debug (CCACHE_DEBUG).
2.Build.
3.Clean and build again.
4.Compare the
<objectfile>.<timestamp>.ccache-input-text files for the
two builds. This together with the
<objectfile>.<timestamp>.ccache-log files should give you
some clues about what is happening.
COMPILING IN DIFFERENT DIRECTORIESSome information included in the hash that identifies a unique compilation can contain absolute paths: •The preprocessed source code may contain absolute
paths to include files if the compiler option -g is used or if absolute
paths are given to -I and similar compiler options.
•Paths specified by compiler options (such as
-I, -MF, etc) on the command line may be absolute.
•The source code file path may be absolute, and
that path may substituted for __FILE__ macros in the source code or
included in warnings emitted to standard error by the preprocessor.
This means that if you compile the same code in different locations, you can’t share compilation results between the different build directories since you get cache misses because of the absolute build directory paths that are part of the hash. Here’s what can be done to enable cache hits between different build directories: •If you build with -g (or similar) to add
debug information to the object file, you must either:
•use the compiler option
-fdebug-prefix-map=<old>=<new> for relocating debug info to
a common prefix (e.g. -fdebug-prefix-map=$PWD=.); or
•set hash_dir = false.
•If you use absolute paths anywhere on the command
line (e.g. the source code file path or an argument to compiler options like
-I and -MF), you must set base_dir to an absolute path to
a “base directory”. Ccache will then rewrite absolute paths
under that directory to relative before computing the hash.
PRECOMPILED HEADERSCcache has support for precompiled headers with GCC and Clang. However, you have to do some things to make it work properly: •You must set sloppiness to
pch_defines,time_macros. The reason is that ccache can’t tell
whether __TIME__, __DATE__ or __TIMESTAMP__ is used when
using a precompiled header. Further, it can’t detect changes in
#defines in the source code because of how preprocessing works in
combination with precompiled headers.
•You may also want to include
include_file_mtime,include_file_ctime in sloppiness. See
HANDLING OF NEWLY CREATED SOURCE FILES.
•You must either:
•use the compiler option -include to
include the precompiled header (i.e., don’t use #include in the
source code to include the header; the filename itself must be sufficient to
find the header, i.e. -I paths are not searched); or
•(for the Clang compiler) use the compiler option
-include-pch to include the PCH file generated from the precompiled
header; or
•(for the GCC compiler) add the compiler option
-fpch-preprocess when compiling.
•If you use Clang, you must compile with
-fno-pch-timestamp.
If you don’t do this, either the non-precompiled version of the header file will be used (if available) or ccache will fall back to running the real compiler and increase the statistics counter “Preprocessing failed” (if the non-precompiled header file is not available). C++ MODULESCcache has support for Clang’s -fmodules option. In practice ccache only additionally hashes module.modulemap files; it does not know how Clang handles its cached binary form of modules so those are ignored. This should not matter in practice: as long as everything else (including module.modulemap files) is the same the cached result should work. Still, you must set sloppiness to modules to allow caching. You must use both direct mode and depend mode. When using the preprocessor mode Clang does not provide enough information to allow hashing of module.modulemap files. SHARING A LOCAL CACHEA group of developers can increase the cache hit rate by sharing a local cache directory. To share a local cache without unpleasant side effects, the following conditions should to be met: •Use the same cache directory.
•Make sure that the configuration option
hard_link is false (which is the default).
•Make sure that all users are in the same
group.
•Set the configuration option umask to
002. This ensures that cached files are accessible to everyone in the
group.
•Make sure that all users have write permission in
the entire cache directory (and that you trust all users of the shared
cache).
•Make sure that the setgid bit is set on all
directories in the cache. This tells the filesystem to inherit group ownership
for new directories. The following command might be useful for this:
find $CCACHE_DIR -type d | xargs chmod g+s The reason to avoid the hard link mode is that the hard links cause unwanted side effects, as all links to a cached file share the file’s modification timestamp. This results in false dependencies to be triggered by timestamp-based build systems whenever another user links to an existing file. Typically, users will see that their libraries and binaries are relinked without reason. You may also want to make sure that a base directory is set appropriately, as discussed in a previous section. SHARING A CACHE ON NFSIt is possible to put the cache directory on an NFS filesystem (or similar filesystems), but keep in mind that: •Having the cache on NFS may slow down
compilation. Make sure to do some benchmarking to see if it’s worth
it.
•Ccache hasn’t been tested very thoroughly
on NFS.
A tip is to set temporary_dir to a directory on the local host to avoid NFS traffic for temporary files. It is recommended to use the same operating system version when using a shared cache. If operating system versions are different then system include files will likely be different and there will be few or no cache hits between the systems. One way of improving cache hit rate in that case is to set sloppiness to system_headers to ignore system headers. An alternative to putting the main cache directory on NFS is to set up a remote storage file cache. USING CCACHE WITH OTHER COMPILER WRAPPERSThe recommended way of combining ccache with another compiler wrapper (such as “distcc”) is by letting ccache execute the compiler wrapper. This is accomplished by defining prefix_command, for example by setting the environment variable CCACHE_PREFIX to the name of the wrapper (e.g. distcc). Ccache will then prefix the command line with the specified command when running the compiler. To specify several prefix commands, set prefix_command to a colon-separated list of commands. Unless you set compiler_check to a suitable command (see the description of that configuration option), it is not recommended to use the form ccache anotherwrapper compiler args as the compilation command. It’s also not recommended to use the masquerading technique for the other compiler wrapper. The reason is that by default, ccache will in both cases hash the mtime and size of the other wrapper instead of the real compiler, which means that: •Compiler upgrades will not be detected
properly.
•The cached results will not be shared between
compilations with and without the other wrapper.
Another minor thing is that if prefix_command is used, ccache will not invoke the other wrapper when running the preprocessor, which increases performance. You can use prefix_command_cpp if you also want to invoke the other wrapper when doing preprocessing (normally by adding -E). CAVEATS•The direct mode fails to pick up new header files
in some rare scenarios. See The direct mode above.
TROUBLESHOOTINGGeneralA general tip for getting information about what ccache is doing is to enable debug logging by setting the configuration option debug (or the environment variable CCACHE_DEBUG); see CACHE DEBUGGING for more information. Another way of keeping track of what is happening is to check the output of ccache -s. PerformanceCcache has been written to perform well out of the box, but sometimes you may have to do some adjustments of how you use the compiler and ccache in order to improve performance. Since ccache works best when I/O is fast, put the cache directory on a fast storage device if possible. Having lots of free memory so that files in the cache directory stay in the disk cache is also preferable. A good way of monitoring how well ccache works is to run ccache -s before and after your build and then compare the statistics counters. Here are some common problems and what may be done to increase the hit rate: •If the counter for preprocessed cache hits has
been incremented instead of the one for direct cache hits, ccache has fallen
back to preprocessor mode, which is generally slower. Some possible reasons
are:
•The source code has been modified in such a way
that the preprocessor output is not affected.
•Compiler arguments that are hashed in the direct
mode but not in the preprocessor mode have changed (-I,
-include, -D, etc) and they didn’t affect the
preprocessor output.
•The compiler option -Xpreprocessor or
-Wp,* (except -Wp,-MD,<path>,
-Wp,-MMD,<path>, and -Wp,-D<define>) is used.
•This was the first compilation with a new value
of the base directory.
•A modification or status change time of one of
the include files is too new . See HANDLING OF NEWLY CREATED SOURCE
FILES.
•The __TIME__ preprocessor macro is
(potentially) being used. Ccache turns off direct mode if __TIME__ is
present in the source code. This is done as a safety measure since the string
indicates that a __TIME__ macro may affect the output. (To be
sure, ccache would have to run the preprocessor, but the sole point of the
direct mode is to avoid that.) If you know that __TIME__ isn’t
used in practise, or don’t care if ccache produces objects where
__TIME__ is expanded to something in the past, you can set
sloppiness to time_macros.
•The __DATE__ preprocessor macro is
(potentially) being used and the date has changed. This is similar to how
__TIME__ is handled. If __DATE__ is present in the source code,
ccache hashes the current date in order to be able to produce the correct
object file if the __DATE__ macro affects the output. If you know that
__DATE__ isn’t used in practise, or don’t care if ccache
produces objects where __DATE__ is expanded to something in the past,
you can set sloppiness to time_macros.
•The __TIMESTAMP__ preprocessor macro is
(potentially) being used and the source file’s modification time has
changed. This is similar to how __TIME__ is handled. If
__TIMESTAMP__ is present in the source code, ccache hashes the string
representation of the source file’s modification time in order to be
able to produce the correct object file if the __TIMESTAMP__ macro
affects the output. If you know that __TIMESTAMP__ isn’t used in
practise, or don’t care if ccache produces objects where
__TIMESTAMP__ is expanded to something in the past, you can set
sloppiness to time_macros.
•The input file path has changed. Ccache includes
the input file path in the direct mode hash to be able to take relative
include files into account and to produce a correct object file if the source
code includes a __FILE__ macro.
•If a cache hit counter was not incremented even
though the same code has been compiled and cached before, ccache has either
detected that something has changed anyway or a cleanup has been performed
(either explicitly or implicitly when a cache limit has been reached). Some
perhaps unobvious things that may result in a cache miss are usage of
__TIME__, __DATE__ or __TIMESTAMP__ macros, or use of
automatically generated code that contains a timestamp, build counter or other
volatile information.
•If “Multiple source files” has been
incremented, it’s an indication that the compiler has been invoked on
several source code files at once. Ccache doesn’t support that. Compile
the source code files separately if possible.
•If “Unsupported compiler option”
has been incremented, enable debug logging and check which compiler option was
rejected.
•If “Preprocessing failed” has been
incremented, one possible reason is that precompiled headers are being used.
See PRECOMPILED HEADERS for how to remedy this.
•If “Could not use precompiled
header” has been incremented, see PRECOMPILED HEADERS.
•If “Could not use modules” has been
incremented, see C++ MODULES.
Corrupt object filesIt should be noted that ccache is susceptible to general storage problems. If a bad object file sneaks into the cache for some reason, it will of course stay bad. Some possible reasons for erroneous object files are bad hardware (disk drive, disk controller, memory, etc), buggy drivers or file systems, a bad prefix_command or compiler wrapper. If this happens, the easiest way of fixing it is this: 1.Build so that the bad object file ends up in the build
tree.
2.Remove the bad object file from the build tree.
3.Rebuild with CCACHE_RECACHE set.
An alternative is to clear the whole cache with ccache -C if you don’t mind losing other cached results. There are no reported issues about ccache producing broken object files reproducibly. That doesn’t mean it can’t happen, so if you find a repeatable case, please report it. MORE INFORMATIONCredits, mailing list information, bug reporting instructions, source code, etc, can be found on ccache’s web site: <https://ccache.dev>. AUTHORCcache 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.
|