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mailto(bugs@ccache.samba.org)
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manpage(ccache)(1)(April 2002)()()
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manpagename(ccache)(a fast compiler cache)
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ccache <compiler> [COMPILER OPTIONS]
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<compiler> [COMPILER OPTIONS]
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ccache is a compiler cache. It speeds up re-compilation of C/C++ code
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by caching previous compiles and detecting when the same compile is
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manpagesection(OPTIONS SUMMARY)
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Here is a summary of the options to ccache.
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-s show statistics summary
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-c run a cache cleanup
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-C clear the cache completely
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-F <maxfiles> set maximum files in cache
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-M <maxsize> set maximum size of cache (use G, M or K)
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-V print version number
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These options only apply when you invoke ccache as "ccache". When
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invoked as a compiler none of these options apply. In that case your
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normal compiler options apply and you should refer to your compilers
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dit(bf(-h)) Print a options summary page
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dit(bf(-s)) Print the current statistics summary for the cache. The
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statistics are stored spread across the subdirectories of the
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cache. Using "ccache -s" adds up the statistics across all
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subdirectories and prints the totals.
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dit(bf(-z)) Zero the cache statistics.
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dit(bf(-V)) Print the ccache version number
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dit(bf(-c)) Clean the cache and re-calculate the cache file count and
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size totals. Normally the -c option should not be necessary as ccache
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keeps the cache below the specified limits at runtime and keeps
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statistics up to date on each compile. This option is mostly useful
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if you manually modify the cache contents or believe that the cache
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size statistics may be inaccurate.
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dit(bf(-C)) Clear the entire cache, removing all cached files.
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dit(bf(-F maxfiles)) This sets the maximum number of files allowed in
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the cache. The value is stored inside the cache directory and applies
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to all future compiles. Due to the way the value is stored the actual
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value used is always rounded down to the nearest multiple of 16.
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dit(bf(-M maxsize)) This sets the maximum cache size. You can specify
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a value in gigabytes, megabytes or kilobytes by appending a G, M or K
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to the value. The default is gigabytes. The actual value stored is
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rounded down to the nearest multiple of 16 kilobytes.
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manpagesection(INSTALLATION)
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There are two ways to use ccache. You can either prefix your compile
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commands with "ccache" or you can create a symbolic link between
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ccache and the names of your compilers. The first method is most
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convenient if you just want to try out ccache or wish to use it for
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some specific projects. The second method is most useful for when you
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wish to use ccache for all your compiles.
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To install for usage by the first method just copy ccache to somewhere
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To install for the second method do something like this:
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cp ccache /usr/local/bin/
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ln -s /usr/local/bin/ccache /usr/local/bin/gcc
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ln -s /usr/local/bin/ccache /usr/local/bin/g++
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ln -s /usr/local/bin/ccache /usr/local/bin/cc
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This will work as long as /usr/local/bin comes before the path to gcc
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(which is usually in /usr/bin). After installing you may wish to run
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"which gcc" to make sure that the correct link is being used.
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Note! Do not use a hard link, use a symbolic link. A hardlink will
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cause "interesting" problems.
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manpagesection(EXTRA OPTIONS)
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When run as a compiler front end ccache usually just takes the same
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command line options as the compiler you are using. The only exception
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to this is the option '--ccache-skip'. That option can be used to tell
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ccache that the next option is definitely not a input filename, and
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should be passed along to the compiler as-is.
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The reason this can be important is that ccache does need to parse the
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command line and determine what is an input filename and what is a
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compiler option, as it needs the input filename to determine the name
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of the resulting object file (among other things). The heuristic
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ccache uses in this parse is that any string on the command line that
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exists as a file is treated as an input file name (usually a C
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file). By using --ccache-skip you can force an option to not be
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treated as an input file name and instead be passed along to the
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compiler as a command line option.
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manpagesection(ENVIRONMENT VARIABLES)
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ccache uses a number of environment variables to control operation. In
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most cases you won't need any of these as the defaults will be fine.
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dit(bf(CCACHE_DIR)) the CCACHE_DIR environment variable specifies
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where ccache will keep its cached compiler output. The default is
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dit(bf(CCACHE_TEMPDIR)) the CCACHE_TEMPDIR environment variable specifies
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where ccache will put temporary files. The default is the same as
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CCACHE_DIR. Note that the CCACHE_TEMPDIR path must be on the same
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filesystem as the CCACHE_DIR path, so that renames of files between
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the two directories can work.
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dit(bf(CCACHE_LOGFILE)) If you set the CCACHE_LOGFILE environment
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variable then ccache will write some log information on cache hits
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and misses in that file. This is useful for tracking down problems.
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dit(bf(CCACHE_PATH)) You can optionally set CCACHE_PATH to a colon
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separated path where ccache will look for the real compilers. If you
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don't do this then ccache will look for the first executable matching
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the compiler name in the normal PATH that isn't a symbolic link to
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dit(bf(CCACHE_CC)) You can optionally set CCACHE_CC to force the name
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of the compiler to use. If you don't do this then ccache works it out
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from the command line.
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dit(bf(CCACHE_PREFIX)) This option adds a prefix to the command line
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that ccache runs when invoking the compiler. Also see the section
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below on using ccache with distcc.
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dit(bf(CCACHE_DISABLE)) If you set the environment variable
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CCACHE_DISABLE then ccache will just call the real compiler,
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bypassing the cache completely.
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dit(bf(CCACHE_READONLY)) the CCACHE_READONLY environment variable
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tells ccache to attempt to use existing cached object files, but not
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to try to add anything new to the cache. If you are using this because
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your CCACHE_DIR is read-only, then you may find that you also need to
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set CCACHE_TEMPDIR as otherwise ccache will fail to create the
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dit(bf(CCACHE_CPP2)) If you set the environment variable CCACHE_CPP2
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then ccache will not use the optimisation of avoiding the 2nd call to
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the pre-processor by compiling the pre-processed output that was used
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for finding the hash in the case of a cache miss. This is primarily a
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debugging option, although it is possible that some unusual compilers
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will have problems with the intermediate filename extensions used in
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this optimisation, in which case this option could allow ccache to be
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dit(bf(CCACHE_NOSTATS)) If you set the environment variable
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CCACHE_NOSTATS then ccache will not update the statistics files on
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dit(bf(CCACHE_NLEVELS)) The environment variable CCACHE_NLEVELS allows
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you to choose the number of levels of hash in the cache directory. The
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default is 2. The minimum is 1 and the maximum is 8.
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dit(bf(CCACHE_HARDLINK)) If you set the environment variable
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CCACHE_HARDLINK then ccache will attempt to use hard links from the
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cache directory when creating the compiler output rather than using a
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file copy. Using hard links is faster, but can confuse programs like
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'make' that rely on modification times.
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dit(bf(CCACHE_RECACHE)) This forces ccache to not use any cached
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results, even if it finds them. New results are still cached, but
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existing cache entries are ignored.
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dit(bf(CCACHE_UMASK)) This sets the umask for ccache and all child
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processes (such as the compiler). This is mostly useful when you wish
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to share your cache with other users. Note that this also affects the
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file permissions set on the object files created from your
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dit(bf(CCACHE_HASHDIR)) This tells ccache to hash the current working
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directory when calculating the hash that is used to distinguish two
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compiles. This prevents a problem with the storage of the current
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working directory in the debug info of a object file, which can lead
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ccache to give a cached object file that has the working directory in
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the debug info set incorrectly. This option is off by default as the
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incorrect setting of this debug info rarely causes problems. If you
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strike problems with gdb not using the correct directory then enable
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dit(bf(CCACHE_UNIFY)) If you set the environment variable CCACHE_UNIFY
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then ccache will use the C/C++ unifier when hashing the pre-processor
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output if -g is not used in the compile. The unifier is slower than a
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normal hash, so setting this environment variable loses a little bit
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of speed, but it means that ccache can take advantage of not
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recompiling when the changes to the source code consist of
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reformatting only. Note that using CCACHE_UNIFY changes the hash, so
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cached compiles with CCACHE_UNIFY set cannot be used when
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CCACHE_UNIFY is not set and vice versa. The reason the unifier is off
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by default is that it can give incorrect line number information in
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compiler warning messages.
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dit(bf(CCACHE_EXTENSION)) Normally ccache tries to automatically
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determine the extension to use for intermediate C pre-processor files
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based on the type of file being compiled. Unfortunately this sometimes
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doesn't work, for example when using the aCC compiler on HP-UX. On
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systems like this you can use the CCACHE_EXTENSION option to override
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the default. On HP-UX set this environment variable to "i" if you use
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manpagesection(CACHE SIZE MANAGEMENT)
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By default ccache has a one gigabyte limit on the cache size and no
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maximum number of files. You can set a different limit using the
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"ccache -M" and "ccache -F" options, which set the size and number of
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When these limits are reached ccache will reduce the cache to 20%
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below the numbers you specified in order to avoid doing the cache
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clean operation too often.
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manpagesection(HOW IT WORKS)
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The basic idea is to detect when you are compiling exactly the same
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code a 2nd time and use the previously compiled output. You detect
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that it is the same code by forming a hash of:
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it() the pre-processor output from running the compiler with -E
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it() the command line options
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it() the real compilers size and modification time
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it() any stderr output generated by the compiler
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These are hashed using md4 (a strong hash) and a cache file is formed
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based on that hash result. When the same compilation is done a second
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time ccache is able to supply the correct compiler output (including
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all warnings etc) from the cache.
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ccache has been carefully written to always produce exactly the same
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compiler output that you would get without the cache. If you ever
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discover a case where ccache changes the output of your compiler then
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manpagesection(USING CCACHE WITH DISTCC)
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distcc is a very useful program for distributing compilation across a
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range of compiler servers. It is often useful to combine distcc with
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ccache, so that compiles that are done are sped up by distcc, but that
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ccache avoids the compile completely where possible.
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To use distcc with ccache I recommend using the CCACHE_PREFIX
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option. You just need to set the environment variable CCACHE_PREFIX to
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'distcc' and ccache will prefix the command line used with the
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compiler with the command 'distcc'.
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manpagesection(SHARING A CACHE)
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A group of developers can increase the cache hit rate by sharing a
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cache directory. The hard links however cause unwanted side effects,
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as all links to a cached file share the file's modification timestamp.
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This results in false dependencies to be triggered by timestamp-based
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build systems whenever another user links to an existing
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file. Typically, users will see that their libraries and binaries are
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relinked without reason. To share a cache without side effects, the
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following conditions need to be met:
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it() Use the same bf(CCACHE_DIR) environment variable setting
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it() Set the bf(CCACHE_NOLINK) environment variable
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it() Make sure everyone sets the CCACHE_UMASK environment variable
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to 002, this ensures that cached files are accessible to everyone in
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it() Make sure that all users have write permission in the entire
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cache directory (and that you trust all users of the shared cache).
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it() Make sure that the setgid bit is set on all directories in the
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cache. This tells the filesystem to inherit group ownership for new
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directories. The command "chmod g+s `find $CCACHE_DIR -type d`" might
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manpagesection(HISTORY)
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ccache was inspired by the compilercache shell script script written
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by Erik Thiele and I would like to thank him for an excellent piece of
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url(http://www.erikyyy.de/compilercache/)(http://www.erikyyy.de/compilercache/)
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for the Erik's scripts.
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I wrote ccache because I wanted to get a bit more speed out of a
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compiler cache and I wanted to remove some of the limitations of the
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shell-script version.
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manpagesection(DIFFERENCES FROM COMPILERCACHE)
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The biggest differences between Erik's compilercache script and ccache
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it() ccache is written in C, which makes it a bit faster (calling out to
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external programs is mostly what slowed down the scripts).
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it() ccache can automatically find the real compiler
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it() ccache keeps statistics on hits/misses
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it() ccache can do automatic cache management
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it() ccache can cache compiler output that includes warnings. In many
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cases this gives ccache a much higher cache hit rate.
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it() ccache can handle a much wider ranger of compiler options
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it() ccache avoids a double call to cpp on a cache miss
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When the cache is stored on an NFS filesystem, the filesystem must be
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exported with the bf(no_subtree_check) option to make renames between
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directories reliable.
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manpagesection(CREDITS)
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Thanks to the following people for their contributions to ccache
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it() Erik Thiele for the original compilercache script
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it() Luciano Rocha for the idea of compiling the pre-processor output
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to avoid a 2nd cpp pass
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it() Paul Russell for many suggestions and the debian packaging
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ccache was written by Andrew Tridgell
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url(http://samba.org/~tridge/)(http://samba.org/~tridge/)
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If you wish to report a problem or make a suggestion then please email
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bugs@ccache.samba.org
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ccache is released under the GNU General Public License version 2 or
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later. Please see the file COPYING for license details.
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ccache.yo
