ccache(1)ccache(1)NAMEccache - a fast compiler cache
SYNOPSISccache [OPTION]
ccache <compiler> [COMPILER OPTIONS]
<compiler> [COMPILER OPTIONS]
DESCRIPTIONccache 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.
OPTIONS SUMMARY
Here is a summary of the options to ccache.
-s show statistics summary
-z zero statistics
-c run a cache cleanup
-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
OPTIONS
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 documentation.
-h Print a options summary page
-s Print the current statistics summary for the cache.
The statistics are stored spread across the subdi
rectories of the cache. Using "ccache -s" adds up
the statistics across all subdirectories and prints
the totals.
-z Zero the cache statistics.
-V Print the ccache version number
-c 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.
-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.
INSTALLATION
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 compil
ers. The first method is most convenient if you just want
to try out ccache or wish to use it for some specific pro
jects. 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/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.
ENVIRONMENT VARIABLESccache used 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_DIR
the CCACHE_DIR environment variable specifies where
ccache will keep its cached compiler output. The
default is "$HOME/.ccache".
CCACHE_LOGFILE
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.
CCACHE_PATH
You can optionally set CCACHE_PATH to a colon sepa
rated path where ccache will look for the real com
pilers. 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_DISABLE
If you set the environment variable CCACHE_DISABLE
then ccache will just call the real compiler,
bypassing the cache completely.
CCACHE_CPP2
If you set the environment variable CCACHE_CPP2
then ccache will not use the optimisation of avoid
ing 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 pri
marily 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.
CCACHE_NOSTATS
If you set the environment variable CCACHE_NOSTATS
then ccache will not update the statistics files on
each compile.
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_NOLINK
If you set the environment variable CCACHE_NOLINK
then ccache will not use hard links from the cache
directory when creating the compiler output and
will do a file copy instead. The main reason for
setting this option is to avoid the update of the
modification time on object files that are the
result of the same compilation in a different
directory.
CCACHE_UNIFY
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 warn
ing messages.
CACHE SIZE MANAGEMENT
By default ccache has no limit on the cache size. You can
set a limit using the "ccache -M" and "ccache -F" options,
which set the size and number of files limits.
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:
o the pre-processor output from running the compiler
with -E
o the command line options
o the real compilers size and modification time
o 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 with
out the cache. If you ever discover a case where ccache
changes the output of your compiler then please let me
know.
HISTORYccache was inspired by the compilercache shell script
script written by Erik Thiele and I would like to thank
him for an excellent piece of work. See
http://www.erikyyy.de/compilercache/ 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 shell-script version.
DIFFERENCES FROM COMPILERCACHE
The biggest differences between Erik's compilercache
script and ccache are:
o ccache is written in C, which makes it a bit faster
(calling out to external programs is mostly what
slowed down the scripts).
o ccache can automatically find the real compiler
o ccache keeps statistics on hits/misses
o ccache can do automatic cache management
o ccache can cache compiler output that includes
warnings. In many cases this gives ccache a much
higher cache hit rate.
o ccache can handle a much wider ranger of compiler
options
o ccache avoids a double call to cpp on a cache miss
CREDITS
Thanks to the following people for their contributions to
ccache
o Erik Thiele for the original compilercache script
o Luciano Rocha for the idea of compiling the pre-
processor output to avoid a 2nd cpp pass
o Paul Russell for many suggestions and the debian
packaging
AUTHORccache was written by Andrew Tridgell
http://samba.org/~tridge/
If you wish to report a problem or make a suggestion then
please email bugs@ccache.samba.org
ccache is released under the GNU General Public License
version 2 or later. Please see the file COPYING for
license details.
April 2002 ccache(1)
