ccache-man.html

ccache２．４的源代码

used.
<p><p></p><dt><strong><strong>CCACHE_NOSTATS</strong></strong><dd> If you set the environment variable
CCACHE_NOSTATS then ccache will not update the statistics files on
each compile.
<p><p></p><dt><strong><strong>CCACHE_NLEVELS</strong></strong><dd> 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. 
<p><p></p><dt><strong><strong>CCACHE_HARDLINK</strong></strong><dd> 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.
<p><p></p><dt><strong><strong>CCACHE_RECACHE</strong></strong><dd> 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.
<p><p></p><dt><strong><strong>CCACHE_UMASK</strong></strong><dd> 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.
<p><p></p><dt><strong><strong>CCACHE_HASHDIR</strong></strong><dd> 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.
<p><p></p><dt><strong><strong>CCACHE_UNIFY</strong></strong><dd> 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.
<p><p></p><dt><strong><strong>CCACHE_EXTENSION</strong></strong><dd> 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.
<p></dl>
<p><h2>CACHE SIZE MANAGEMENT</h2>
    
<p>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.
<p>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.
<p><h2>HOW IT WORKS</h2>
    
<p>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:
<p><ul>
  <li > the pre-processor output from running the compiler with -E
  <li > the command line options
  <li > the real compilers size and modification time
  <li > any stderr output generated by the compiler
</ul>
<p>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.
<p>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.
<p><h2>USING CCACHE WITH DISTCC</h2>
    
<p>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.
<p>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'. 
<p><h2>SHARING A CACHE</h2>
    
<p>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:
<p><ul>
  <li > Use the same <strong>CCACHE_DIR</strong> environment variable setting
  <li > Set the <strong>CCACHE_NOLINK</strong> environment variable
  <li > Make sure everyone sets the CCACHE_UMASK environment variable
  to 002, this ensures that cached files are accessible to everyone in
  the group.
  <li > Make sure that all users have write permission in the entire
  cache directory (and that you trust all users of the shared cache). 
  <li > 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.
</ul>
<p><h2>HISTORY</h2>
    
<p>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 
<a href="http://www.erikyyy.de/compilercache/">http://www.erikyyy.de/compilercache/</a>
for the Erik's scripts.
<p>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.
<p><h2>DIFFERENCES FROM COMPILERCACHE</h2>
    
<p>The biggest differences between Erik's compilercache script and ccache
are:
<ul>
<li > ccache is written in C, which makes it a bit faster (calling out to
  external programs is mostly what slowed down the scripts).
<li > ccache can automatically find the real compiler
<li > ccache keeps statistics on hits/misses
<li > ccache can do automatic cache management
<li > ccache can cache compiler output that includes warnings. In many
  cases this gives ccache a much higher cache hit rate.
<li > ccache can handle a much wider ranger of compiler options
<li > ccache avoids a double call to cpp on a cache miss
</ul>
<p><h2>BUGS</h2>
    
<p>When the cache is stored on an NFS filesystem, the filesystem must be
exported with the <strong>no_subtree_check</strong> option to make renames between
directories reliable.
<p><h2>CREDITS</h2>
    
<p>Thanks to the following people for their contributions to ccache
<ul>
 <li > Erik Thiele for the original compilercache script
 <li > Luciano Rocha for the idea of compiling the pre-processor output
 to avoid a 2nd cpp pass
 <li > Paul Russell for many suggestions and the debian packaging
</ul>
<p><h2>AUTHOR</h2>
    
<p>ccache was written by Andrew Tridgell
<a href="http://samba.org/~tridge/">http://samba.org/~tridge/</a>
<p>If you wish to report a problem or make a suggestion then please email
[e-mail]
<p>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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