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 detecting 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.
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-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-swig or you can create a symbolic 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++
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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 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.
ENVIRONMENT VARIABLES
ccache uses a number of environment variables to control operation. In
most cases you wont 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.
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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 dont do
this then ccache will look for the first executable matching the
compiler name in the normal PATH that isnt 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 dont 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
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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 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_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 existing 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
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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-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 doesnt 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.
CCACHE_STRIPC
If you set the environment variable CCACHE_STRIPC then ccache
will strip the -c option when invoking the preprocessor. This
option is primarily for the Sun Workshop C++ compiler as without
this option an unwarranted warning is displayed: CC: Warning: -E
redefines product from object to source (stdout) when -E and -c
is used together.
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CCACHE_SWIG
When using SWIG as the compiler and it does not have swig in the
executable name, then the CCACHE_SWIG environment variable needs
to be set in order for ccache to work correctly with SWIG. The
use of CCACHE_CPP2 is also recommended 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 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.
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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 files 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 accessible 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 Eriks scripts.
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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 limitations of the
shell-script version.
DIFFERENCES FROM COMPILERCACHE
The biggest differences between Eriks 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.
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