CCACHE(1)

NAME

ccache - a fast C/C++ compiler cache

SYNOPSIS

ccache [options]
ccache compiler [compiler options]
compiler [compiler options]                   (via symbolic link)

DESCRIPTION

ccache is a compiler cache. It speeds up recompilation by caching the
result of previous compilations and detecting when the same compilation is being done again. Supported languages are C, C++, Objective-C and
Objective-C++.

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 BUGS. If you
ever discover an undocumented case where ccache changes the output of
your compiler, please let us know.
Features: o Keeps statistics on hits/misses.; o Automatic cache size management.; o Can cache compilations that generate warnings.; o Easy installation.; o Low overhead.; o Optionally uses hard links where possible to avoid copies.; o Optionally compresses files in the cache to reduce disk space.
Limitations: o Only knows how to cache the compilation of a single
C/C++/Objective-C/Objective-C++ file. Other types of compilations
(multi-file compilation, linking, etc) will silently fall back to
running the real compiler.; o Only works with GCC and compilers that behave similar enough.; o Some compiler flags are not supported. If such a flag is detected,
ccache will silently fall back to running the real compiler.

RUN MODES

There are two ways to use ccache. You can either prefix your
compilation commands with ccache or you can create a symbolic link (named as your compiler) to ccache. The first method is most convenient if you just want to try out ccache or wish to use it for some specific projects. The second method is most useful for when you wish to use
ccache for all your compilations.

To install for usage by the first method just make sure ccache is in your path.

To install for the second method, do something like this:: cp ccache /usr/local/bin/
ln -s ccache /usr/local/bin/gcc
ln -s ccache /usr/local/bin/g++
ln -s ccache /usr/local/bin/cc
ln -s ccache /usr/local/bin/c++
And so forth. This will work as long as /usr/local/bin comes before the path to the compiler (which is usually in /usr/bin). After installing you may wish to run "which gcc" to make sure that the correct link is being used.: Note
Do not use a hard link, use a symbolic link. A hard link will cause "interesting" problems.

OPTIONS

These options only apply when you invoke ccache as "ccache". When
invoked as a compiler (via a symlink as described in the previous
section), the normal compiler options apply and you should refer to the compiler's documentation.

-c, --cleanup: Clean up the cache by removing old cached files until the specified file number and cache size limits are not exceeded. This also
recalculates the cache file count and size totals. Normally, it's
not needed to initiate cleanup manually as ccache keeps the cache
below the specified limits at runtime and keeps statistics up to
date on each compilation. Forcing a cleanup is mostly useful if you manually modify the cache contents or believe that the cache size
statistics may be inaccurate.
-C, --clear: Clear the entire cache, removing all cached files.
-F, --max-files=N: Set the maximum number of files allowed in the cache. The value is stored inside the cache directory and applies to all future
compilations. Due to the way the value is stored the actual value
used is always rounded down to the nearest multiple of 16.
-h, --help: Print an options summary page.
-M, --max-size=SIZE: Set the maximum size of the files stored in the cache. You can
specify a value in gigabytes, megabytes or kilobytes by appending a G, M or K to the value. The default is gigabytes. The actual value stored is rounded down to the nearest multiple of 16 kilobytes.
-s, --show-stats: Print the current statistics summary for the cache.
-V, --version: Print version and copyright information.
-z, --zero-stats: Zero the cache statistics (but not the configured limits).

EXTRA OPTIONS

When run as a compiler, ccache usually just takes the same command line options as the compiler you are using. The only exception to this is
the option --ccache-skip. That option can be used to tell ccache that the next option is definitely not a input filename, and should be
passed along to the compiler as-is.

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.

ENVIRONMENT VARIABLES

ccache uses a number of environment variables to control operation. In most cases you won't need any of these as the defaults will be fine.

CCACHE_BASEDIR: If you set the environment variable CCACHE_BASEDIR to an absolute path to a directory, ccache rewrites absolute paths into relative
paths before computing the hash that identifies the compilation,
but only for paths under the specified directory. See the
discussion under COMPILING IN DIFFERENT DIRECTORIES.
CCACHE_CC: You can optionally set CCACHE_CC to force the name of the compiler to use. If you don't do this then ccache works it out from the
command line.
CCACHE_COMPRESS: If you set the environment variable CCACHE_COMPRESS then ccache will compress object files and other compiler output it puts in the cache. However, this setting has no effect on how files are
retrieved from the cache; compressed and uncompressed results will still be usable regardless of this setting.
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. The CCACHE_COMPILERCHECK environment variable can be used to select another strategy. Possible values
are:; content
Hash the content of the compiler binary.; mtime
Hash the compiler's mtime and size. This is the default.; none
Don't hash anything.; Setting the variable to content makes ccache very slightly slower, but makes it cope better with compiler upgrades during a build
bootstrapping process. The none setting 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 set the variable to none if you know what you are doing.
CCACHE_CPP2: If you set the environment variable CCACHE_CPP2 then ccache will not use the optimisation of avoiding the second call to the
preprocessor by compiling the preprocessed output that was used for finding the hash in the case of a cache miss. This is primarily a
debugging option, although it is possible that some unusual
compilers will have problems with the intermediate filename
extensions used in this optimisation, in which case this option
could allow ccache to be used anyway.
CCACHE_DIR: The CCACHE_DIR environment variable specifies where ccache will keep its cached compiler output. The default is $HOME/.ccache.
CCACHE_DISABLE: If you set the environment variable CCACHE_DISABLE then ccache will just call the real compiler, bypassing the cache completely.
CCACHE_EXTENSION: ccache tries to automatically determine the extension to use for
intermediate preprocessor files based on the type of file being
compiled. Unfortunately this sometimes doesn't work, for example
when using the "aCC" compiler on HP-UX. On systems like this you
can use the CCACHE_EXTENSION option to override the default. On HP-UX set this environment variable to i if you use the "aCC"
compiler.
CCACHE_EXTRAFILES: If you set the environment variable CCACHE_EXTRAFILES to a colon-separated list of paths then ccache will include the contents of those files when calculating the hash sum.
CCACHE_HARDLINK: If you set the environment variable CCACHE_HARDLINK then ccache will attempt to use hard links from the cache directory when
creating the compiler output rather than using a file copy. Using
hard links may be slightly faster in some situations, but can
confuse programs like "make" that rely on modification times.
Another thing to keep in mind is that if the resulting object file is modified in any way, this corrupts the cached object file as
well. Hard links are never made for compressed cache files. This
means that you should not set the CCACHE_COMPRESS variable if you want to use hard links.
CCACHE_HASHDIR: This tells ccache to hash the current working directory when
calculating the hash that is used to distinguish two compilations. This prevents a problem with the storage of the current working
directory in the debug info of a object file, which can lead ccache to give a cached object file that has the working directory in the debug info set incorrectly. This option is off by default as the
incorrect setting of this debug info rarely causes problems. If you strike problems with GDB not using the correct directory then
enable this option.
CCACHE_LOGFILE: If you set the CCACHE_LOGFILE environment variable then ccache will write information on what it is doing to the specified file. This
is useful for tracking down problems.
CCACHE_NLEVELS: The environment variable CCACHE_NLEVELS allows you to choose the number of levels of hash in the cache directory. The default is 2. The minimum is 1 and the maximum is 8.
CCACHE_NODIRECT: If you set the environment variable CCACHE_NODIRECT then ccache will not use the direct mode.
CCACHE_NOSTATS: If you set the environment variable CCACHE_NOSTATS then ccache will not update the statistics files on each compilation.
CCACHE_PATH: You can optionally set CCACHE_PATH to a colon-separated path where ccache will look for the real compilers. If you don't do this then ccache will look for the first executable matching the compiler
name in the normal PATH that isn't a symbolic link to ccache itself.
CCACHE_PREFIX: This option adds a prefix to the command line that ccache runs when invoking the compiler. Also see the section below on using ccache
with "distcc".
CCACHE_READONLY: The CCACHE_READONLY environment variable tells ccache to attempt to use existing cached object files, but not to try to add anything
new to the cache. If you are using this because your CCACHE_DIR is read-only, then you may find that you also need to set
CCACHE_TEMPDIR as otherwise ccache will fail to create temporary files.
CCACHE_RECACHE: This forces ccache to not use any cached results, even if it finds them. New results are still cached, but existing cache entries are ignored.
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. The
CCACHE_SLOPPINESS environment variable 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 options. Available
options are:; file_macro
Ignore __FILE__ being present in the source.; include_file_mtime
Don't check the modification time of include files in the
direct mode.; time_macros
Ignore __DATE__ and __TIME__ being present in the source code.; See the discussion under TROUBLESHOOTING for more information.
CCACHE_TEMPDIR: The CCACHE_TEMPDIR environment variable specifies where ccache will put temporary files. The default is $CCACHE_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.)
CCACHE_UMASK: This sets the umask for ccache and all child processes (such as the compiler). This is mostly useful when you wish to share your cache with other users. Note that this also affects the file permissions set on the object files created from your compilations.
CCACHE_UNIFY: If you set the environment variable CCACHE_UNIFY then ccache will use a C/C++ unifier when hashing the preprocessor output if the -g option is not used. The unifier is slower than a normal hash, so
setting this environment variable loses a little bit of speed, but it means that ccache can take advantage of not recompiling when the changes to the source code consist of reformatting only. Note that using CCACHE_UNIFY changes the hash, so cached compilations with CCACHE_UNIFY set cannot be used when CCACHE_UNIFY is not set and vice versa. The reason the unifier is off by default is that it can give incorrect line number information in compiler warning
messages. Also note that enabling the unifier implies turning off
the direct mode.

CACHE SIZE MANAGEMENT

By default ccache has a one gigabyte limit on the total size of files
in the cache and no maximum number of files. You can set different
limits using the -M/--max-size and -F/--max-files options. Use ccache -s/--show-stats to see the cache size and the currently configured limits (in addition to other various statistics).

CACHE COMPRESSION

ccache can optionally compress all files it puts into the cache using
the compression library zlib. While this involves a negligible
performance slowdown, it significantly increases the number of files
that fit in the cache. You can turn on compression by setting the
CCACHE_COMPRESS environment variable.

HOW CCACHE WORKS

The basic idea is to detect when you are compiling exactly the same
code a second time and reuse the previously produced output. The
detection is done by hashing different kinds of information that should be unique for the compilation and then using the hash sum to identify
the cached output. ccache uses MD4, a very fast cryptographic hash
algorithm, for the hashing. (MD4 is nowadays too weak to be useful in
cryptographic contexts, but it should be safe enough to be used to
identify recompilations.) On a cache hit, ccache is able to supply all of the correct compiler outputs (including all warnings, dependency
file, etc) from the cache.

ccache has two ways of doing the detection:

o the direct mode, where ccache hashes the source code and include: files directly
o the preprocessor mode, where ccache runs the preprocessor on the: source code and hashes the result
The direct mode is generally faster since running the preprocessor has some overhead.
The direct mode: In the direct mode, the hash is formed of:; o the input source file; o the command line options; o the real compiler's size and modification time (unless
CCACHE_COMPILERCHECK says something else); Based on the hash, a data structure called "manifest" is looked up in
the cache. The manifest contains:; o references to cached compilation results (object file, dependency
file, etc) that were produced by previous compilations that matched the hash; o paths to the include files that were read at the time the
compilation results were stored in the cache; o 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.; The direct mode will be disabled if any of the following holds:; o the environment variable CCACHE_NODIRECT is set; o a modification time of one of the include files is too new (needed
to avoid a race condition); o the unifier is enabled (the environment variable CCACHE_UNIFY is
set); o a compiler option not supported by the direct mode is used:

o a -Wp,X compiler option other than -Wp,-MD,path and
-Wp,-MMD,path

o -Xpreprocessor; o the string "__TIME__" is present outside comments and string
literals in the source code
The preprocessor mode: In the preprocessor mode, the hash is formed of:; o the preprocessor output from running the compiler with -E; o the command line options except options that affect include files
(-I, -include, -D, etc; the theory is that these options will change the preprocessor output if they have any effect at all); o the real compiler's size and modification time (unless
CCACHE_COMPILERCHECK says something else); o any standard error output generated by the preprocessor; Based on the hash, the cached compilation result can be looked up
directly in the cache.

COMPILING IN DIFFERENT DIRECTORIES

Some information included in the hash that identifies a unique
compilation may contain absolute paths:

o 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.
o Paths specified by compiler options (such as -I, -MF, etc) may be: absolute.
o 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. To mitigate this problem,
you can specify a "base directory" by setting the CCACHE_BASEDIR variable to an absolute path to the directory. ccache will then rewrite absolute paths that are under the base directory (i.e., paths that have the base directory as a prefix) to relative paths when constructing the hash. A typical path to use as the base directory is your home
directory or another directory that is a parent of your build
directories. (Don't use / as the base directory since that will make
ccache also rewrite paths to system header files, which doesn't gain
anything.)
The drawbacks of using CCACHE_BASEDIR are:
o If you specify an absolute path to the source code file, __FILE__: macros will be expanded to a relative path instead.
o If you specify an absolute path to the source code file and compile: with -g, the source code path stored in the object file may point to the wrong directory, which may prevent debuggers like GDB from
finding the source code. Sometimes, a work-around is to change the directory explicitly with the "cd" command in GDB.

SHARING A CACHE

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

o Use the same CCACHE_DIR environment variable setting.

o Unset the CCACHE_HARDLINK environment variable.

o Make sure everyone sets the CCACHE_UMASK environment variable to: 002. This ensures that cached files are accessible to everyone in
the group.
o Make sure that all users have write permission in the entire cache directory (and that you trust all users of the shared cache).
o 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 command "find $CCACHE_DIR -type d | xargs chmod g+s" might be useful for this.
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 the developers have CCACHE_BASEDIR set appropriately, as discussed in the 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:

o Having the cache on NFS may slow down compilation. Make sure to do: some benchmarking to see if it's worth it.
o ccache hasn't been tested very thoroughly on NFS.
A tip is to set CCACHE_TEMPDIR to a directory on the local host to avoid NFS traffic for temporary files.

USING CCACHE WITH DISTCC

"distcc" is a very useful program for distributing compilation across a range of compiler servers. It is often useful to combine distcc with
ccache so that compilations that are done are sped up by distcc, but
ccache avoids the compilation completely where possible.

The recommended way of combining distcc and ccache is by using the
CCACHE_PREFIX option. You just need to set the environment variable CCACHE_PREFIX to distcc and ccache will prefix the command line used with the compiler with the command "distcc".

It is not recommended to use the form ccache distcc compiler args as the compilation command, because ccache will in that case hash the
mtime of distcc instead of the compiler (see the CCACHE_COMPILERCHECK option), which means that compiler upgrades will not be detected
properly and that the cached results will not be shared between
compilations with and without distcc.

BUGS

o ccache doesn't handle the GNU Assembler's .incbin directive: correctly. This directive can be embedded in the source code inside an asm statement in order to include a file verbatim in the object file. If the included file is modified, ccache doesn't pick up the change since the inclusion isn't done by the preprocessor. A
workaround of this problem is to set CCACHE_EXTRAFILES to the path of the included file.

TROUBLESHOOTING

General: A general tip for getting information about what ccache is doing is to enable debug logging by setting CCACHE_LOGFILE. The log contains executed commands, important decisions that ccache makes, read and
written files, etc. Another way of keeping track of what is happening
is to check the output of ccache -s.
Performance
ccache has been written to perform well out of the box, but sometimes
you may have to do some adjustments of how you use the compiler and
ccache in order to improve performance.
Since ccache works best when I/O is fast, put the cache directory on a fast storage device if possible. Having lots of free memory so that files in the cache directory stay in the disk cache is also preferrable.
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:
o If "cache hit (preprocessed)" has been incremented instead of "cache hit (direct)", ccache has fallen back to preprocessor mode, which is generally slower. Some possible reasons are:
o The source code has been modified in such a way that the preprocessor output is not affected.
o 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.
o The compiler option -Xpreprocessor or -Wp,X (except -Wp,-MD,path and Wp,-MMD,path) is used.
o This was the first compilation with a new value of
CCACHE_BASEDIR.
o A modification time of one of the include files is too new
(created the same second as the compilation is being done).
This check is made to avoid a race condition. To fix this,
create the include file earlier in the build process, if
possible, or set CCACHE_SLOPPINESS to include_file_mtime if you are willing to take the risk. (The race condition consists of
these events: the preprocessor is run; an include file is
modified by someone; the new include file is hashed by ccache;
the real compiler is run on the preprocessor's output, which
contains data from the old header file; the wrong object file
is stored in the cache.)
o The __TIME__ preprocessor macro is (potentially) being used. ccache turns off direct mode if "__TIME__" is present in the
source code outside comments and string literals. 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 CCACHE_SLOPPINESS to time_macros.
o 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 outside
comments and string literals, 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 CCACHE_SLOPPINESS to time_macros.
o The __FILE__ preprocessor macro is (potentially) being used and the file path has changed. If "__FILE__" is present in the
source code outside comments and string literals, ccache hashes the current input file path in order to be able to produce the correct object file if the __FILE__ macro affects the output. If you know that __FILE__ isn't used in practise, or don't care if ccache produces objects where __FILE__ is expanded to the wrong path, you can set CCACHE_SLOPPINESS to file_macro.
o If "cache miss" has been 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__ or __DATE__ macros, or use of automatically generated code that contains a timestamp, build
counter or other volatile information.
o 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.
o If "preprocessor error" has been incremented, one possible reason is that precompiled headers are being used. ccache currently doesn't handle that.
o If "unsupported compiler option" has been incremented, enable debug logging and check which option was rejected.
Errors when compiling with ccache If compilation doesn't work with ccache, but it works without it, one
possible reason is that the compiler can't compile preprocessed output correctly. A workaround that may work is to set CCACHE_CPP2. This will make cache misses slower, though, so it is better to find and fix the
root cause.
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 CCACHE_PREFIX command or compiler wrapper. If this happens, you can either find out which object file is broken by reading the debug log and then delete the bad object file
from the cache, or you can simply clear the whole cache with ccache -C if you don't mind losing other cached results.
There are no reported issues about ccache producing broken object files reproducibly. That doesn't mean it can't happen, so if you find a repeatable case, please report it.

MORE INFORMATION

Credits, mailing list information, bug reporting instructions, source
code, etc, can be found on ccache's web site: http://ccache.samba.org.

AUTHOR

ccache is currently maintained by Joel Rosdahl and was originally
written by Andrew Tridgell. See http://ccache.samba.org/credits.html
for a list of contributors.

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