ccache

April 2002

NAME

ccache - a fast compiler cache

SYNOPSIS

ccache [OPTION]

ccache <compiler> [COMPILER OPTIONS]

<compiler> [COMPILER OPTIONS]

DESCRIPTION

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.

OPTIONS SUMMARY

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

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 subdirectories 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.

-C
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.

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 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/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 VARIABLES

ccache 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 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_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_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 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 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_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_HASHDIR
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 this option.

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 warning messages.

CCACHE_EXTENSION
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 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:

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.

HISTORY

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 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:

CREDITS

Thanks to the following people for their contributions to ccache

AUTHOR

ccache 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.