ccache - a fast compiler cache
ccache <compiler> [COMPILER OPTIONS]
<compiler> [COMPILER OPTIONS]
ccache is a compiler cache. It speeds up re-compilation of C/C++ code
by caching previous compiles and detecting when the same compile is
being done again.
Here is a summary of the options to ccache.
-s show statistics summary
-z zero statistics
-c run a cache cleanup
-C clear the cache completely
-F <maxfiles> set maximum files in cache
-M <maxsize> set maximum size of cache (use G, M or K)
-h this help page
-V print version number
These options only apply when you invoke ccache as "ccache". When
invoked as a compiler none of these options apply. In that case your
normal compiler options apply and you should refer to your compilers
- Print a options summary page
- Print the current statistics summary for the cache. The
statistics are stored spread across the subdirectories of the
cache. Using "ccache -s" adds up the statistics across all
subdirectories and prints the totals.
- Zero the cache statistics.
- Print the ccache version number
- Clean the cache and re-calculate the cache file count and
size totals. Normally the -c option should not be necessary as ccache
keeps the cache below the specified limits at runtime and keeps
statistics up to date on each compile. This option is mostly useful
if you manually modify the cache contents or believe that the cache
size statistics may be inaccurate.
- Clear the entire cache, removing all cached files.
- -F maxfiles
- This sets the maximum number of files allowed in
the cache. The value is stored inside the cache directory and applies
to all future compiles. Due to the way the value is stored the actual
value used is always rounded down to the nearest multiple of 16.
- -M maxsize
- This sets the maximum cache size. 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.
There are two ways to use ccache. You can either prefix your compile
commands with "ccache" or you can create a symbolic link between
ccache and the names of your compilers. 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 compiles.
To install for usage by the first method just copy ccache to somewhere
in your path.
To install for the second method do something like this:
cp ccache /usr/local/bin/
ln -s /usr/local/bin/ccache /usr/local/bin/gcc
ln -s /usr/local/bin/ccache /usr/local/bin/g++
ln -s /usr/local/bin/ccache /usr/local/bin/cc
This will work as long as /usr/local/bin comes before the path to gcc
(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 hardlink will
cause "interesting" problems.
When run as a compiler front end 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 in this parse is that any string on the command line that
exists as a file is treated as an input file name (usually a C
file). 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.
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.
- the CCACHE_DIR environment variable specifies
where ccache will keep its cached compiler output. The default is
- the CCACHE_TEMPDIR environment variable specifies
where ccache will put temporary files. The default is the same as
CCACHE_DIR. Note that the CCACHE_TEMPDIR path must be on the same
filesystem as the CCACHE_DIR path, so that renames of files between
the two directories can work.
- If you set the CCACHE_LOGFILE environment
variable then ccache will write some log information on cache hits
and misses in that file. This is useful for tracking down problems.
- 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
- 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.
- 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.
- If you set the environment variable
CCACHE_DISABLE then ccache will just call the real compiler,
bypassing the cache completely.
- 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 the
- If you set the environment variable CCACHE_CPP2
then ccache will not use the optimisation of avoiding the 2nd call to
the pre-processor by compiling the pre-processed 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
- If you set the environment variable
CCACHE_NOSTATS then ccache will not update the statistics files on
- 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.
- 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 is faster, but can confuse programs like
'make' that rely on modification times.
- 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.
- 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
- This tells ccache to hash the current working
directory when calculating the hash that is used to distinguish two
compiles. 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
- If you set the environment variable CCACHE_UNIFY
then ccache will use the C/C++ unifier when hashing the pre-processor
output if -g is not used in the compile. 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 compiles 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.
- Normally ccache tries to automatically
determine the extension to use for intermediate C pre-processor 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.
CACHE SIZE MANAGEMENT
By default ccache has a one gigabyte limit on the cache size and no
maximum number of files. You can set a different limit using the
"ccache -M" and "ccache -F" options, which set the size and number of
When these limits are reached ccache will reduce the cache to 20%
below the numbers you specified in order to avoid doing the cache
clean operation too often.
HOW IT WORKS
The basic idea is to detect when you are compiling exactly the same
code a 2nd time and use the previously compiled output. You detect
that it is the same code by forming a hash of:
- the pre-processor output from running the compiler with -E
- the command line options
- the real compilers size and modification time
- any stderr output generated by the compiler
These are hashed using md4 (a strong hash) and a cache file is formed
based on that hash result. When the same compilation is done a second
time ccache is able to supply the correct compiler output (including
all warnings etc) from the cache.
ccache has been carefully written to always produce exactly the same
compiler output that you would get without the cache. If you ever
discover a case where ccache changes the output of your compiler then
please let me know.
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 compiles that are done are sped up by distcc, but that
ccache avoids the compile completely where possible.
To use distcc with ccache I recommend 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'.
SHARING A CACHE
A group of developers can increase the cache hit rate by sharing a
cache directory. The hard links however 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. To share a cache without side effects, the
following conditions need to be met:
- Use the same CCACHE_DIR environment variable setting
- Set the CCACHE_NOLINK environment variable
- Make sure everyone sets the CCACHE_UMASK environment variable
to 002, this ensures that cached files are accessible to everyone in
- 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 command "chmod g+s `find $CCACHE_DIR -type d`" might
be useful for this.
ccache was inspired by the compilercache shell script script written
by Erik Thiele and I would like to thank him for an excellent piece of
for the Erik's scripts.
I wrote ccache because I wanted to get a bit more speed out of a
compiler cache and I wanted to remove some of the limitations of the
DIFFERENCES FROM COMPILERCACHE
The biggest differences between Erik's compilercache script and ccache
- ccache is written in C, which makes it a bit faster (calling out to
external programs is mostly what slowed down the scripts).
- ccache can automatically find the real compiler
- ccache keeps statistics on hits/misses
- ccache can do automatic cache management
- ccache can cache compiler output that includes warnings. In many
cases this gives ccache a much higher cache hit rate.
- ccache can handle a much wider ranger of compiler options
- ccache avoids a double call to cpp on a cache miss
When the cache is stored on an NFS filesystem, the filesystem must be
exported with the no_subtree_check option to make renames between
Thanks to the following people for their contributions to ccache
- Erik Thiele for the original compilercache script
- Luciano Rocha for the idea of compiling the pre-processor output
to avoid a 2nd cpp pass
- Paul Russell for many suggestions and the debian packaging
ccache was written by Andrew Tridgell
If you wish to report a problem or make a suggestion then please email
ccache is released under the GNU General Public License version 2 or
later. Please see the file COPYING for license details.