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ccache-swig(1) ccache-swig(1)
NAME
ccache-swig - a fast compiler cache
SYNOPSIS
ccache-swig [OPTION]
ccache-swig <compiler> [COMPILER OPTIONS]
<compiler> [COMPILER OPTIONS]
DESCRIPTION
ccache-swig is a compiler cache. It speeds up re-compilation of C/C++/SWIG code by caching previous compiles and detect-
ing when the same compile is being done again. ccache-swig is ccache plus support for SWIG. ccache and ccache-swig are
used interchangeably in this document.
OPTIONS SUMMARY
Here is a summary of the options to ccache-swig.
-s show statistics summary
-z zero statistics
-c run a cache cleanup
-C clear the cache completely
-F <n> set maximum files in cache
-M <n> 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-swig". 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-swig -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 neces-
sary as ccache keeps the cache below the specified limits at runtime and keeps statistics up to date on each com-
pile. This option is mostly useful if you manually modify the cache contents or believe that the cache size sta-
tistics 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-swig" or you can create a sym-
bolic link between ccache-swig 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-swig to somewhere in your path.
To install for the second method do something like this:
cp ccache-swig /usr/local/bin/
ln -s /usr/local/bin/ccache-swig /usr/local/bin/gcc
ln -s /usr/local/bin/ccache-swig /usr/local/bin/g++
ln -s /usr/local/bin/ccache-swig /usr/local/bin/cc
ln -s /usr/local/bin/ccache-swig /usr/local/bin/swig
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.
EXTRA OPTIONS
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 file-
name 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.
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_DIR
the CCACHE_DIR environment variable specifies where ccache will keep its cached compiler output. The default is
"$HOME/.ccache".
CCACHE_TEMPDIR
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.
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_VERBOSE
If you set the CCACHE_VERBOSE environment variable then ccache will display on stdout all the compiler invocations
that it makes. This can useful for debugging unexpected 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_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_DISABLE
If you set the environment variable CCACHE_DISABLE then ccache will just call the real compiler, bypassing the
cache completely.
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 the temporary files.
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_NOCOMPRESS
If you set the environment variable CCACHE_NOCOMPRESS then there is no compression used on files that go into the
cache. However, this setting has no effect on how files are retrieved from the cache, compressed results will
still be usable.
CCACHE_NOSTATS
If you set the environment variable CCACHE_NOSTATS then ccache will not update the statistics files on each com-
pile.
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_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 is faster, but can
confuse programs like 'make' that rely on modification times. Hard links are never made for compressed cache
files.
CCACHE_RECACHE
This forces ccache to not use any cached results, even if it finds them. New results are still cached, but exist-
ing cache entries are ignored.
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_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-pro-
cessor output if -g is not used in the compile. The unifier is slower than a normal hash, so setting this environ-
ment 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 com-
piler 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_STRIPC
If you set the environment variable CCACHE_STRIPC then ccache will strip the -c option when invoking the pre-
processor. This option is primarily for the Sun Workshop C++ compiler as without this option an unwarranted warn-
ing is displayed: CC: Warning: "-E" redefines product from "object" to "source (stdout)" when -E and -c is used
together.
CCACHE_SWIG
When using SWIG as the compiler and it does not have 'swig' in the executable name, then the CCACHE_SWIG environ-
ment variable needs to be set in order for ccache to work correctly with SWIG. The use of CCACHE_CPP2 is also rec-
ommended for SWIG due to some preprocessor quirks, however, use of CCACHE_CPP2 can often be skipped -- check your
generated code with and without this option set. Known problems are using preprocessor directives within %inline
blocks and the use of '#pragma SWIG'.
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 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.
CACHE COMPRESSION
By default on most platforms ccache will compress all files it puts into the cache using the zlib compression. While this
involves a negligible performance slowdown, it significantly increases the number of files that fit in the cache. You can
turn off compression setting the CCACHE_NOCOMPRESS environment variable.
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 out-
put. 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 compila-
tion 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 com-
pletely 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:
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 accessi-
ble 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 "chmod g+s `find $CCACHE_DIR -type d`" might be useful for this.
o Set CCACHE_NOCOMPRESS for all users, if there are users with versions of ccache that do not support compression.
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. ccache-swig is a port of the
original ccache with support added for use with SWIG.
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 limi-
tations 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
AUTHOR
ccache was written by Andrew Tridgell http://samba.org/~tridge/. ccache was adapted to create ccache-swig for use with
SWIG by William Fulton.
If you wish to report a problem or make a suggestion then please email the SWIG developers on the swig-devel mailing
list, see http://www.swig.org/mail.html
ccache is released under the GNU General Public License version 2 or later. Please see the file COPYING for license
details.
ccache-swig(1)
