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378 lines
14 KiB
Groff
378 lines
14 KiB
Groff
.TH "ccache" "1" "April 2002" "" ""
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.SH "NAME"
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ccache \- a fast compiler cache
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.SH "SYNOPSIS"
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.PP
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ccache [OPTION]
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.PP
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ccache <compiler> [COMPILER OPTIONS]
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.PP
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<compiler> [COMPILER OPTIONS]
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.PP
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.SH "DESCRIPTION"
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.PP
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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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being done again\&.
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.PP
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.SH "OPTIONS SUMMARY"
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.PP
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Here is a summary of the options to ccache\&.
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.PP
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.nf
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-s show statistics summary
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-z zero statistics
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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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-h this help page
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-V print version number
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.fi
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.PP
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.SH "OPTIONS"
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.PP
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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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documentation\&.
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.PP
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.IP "\fB-h\fP"
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Print a options summary page
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.IP
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.IP "\fB-s\fP"
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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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.IP
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.IP "\fB-z\fP"
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Zero the cache statistics\&.
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.IP
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.IP "\fB-V\fP"
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Print the ccache version number
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.IP
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.IP "\fB-c\fP"
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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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.IP
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.IP "\fB-C\fP"
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Clear the entire cache, removing all cached files\&.
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.IP
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.IP "\fB-F maxfiles\fP"
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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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.IP
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.IP "\fB-M maxsize\fP"
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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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.IP
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.PP
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.SH "INSTALLATION"
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.PP
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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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.PP
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To install for usage by the first method just copy ccache to somewhere
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in your path\&.
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.PP
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To install for the second method do something like this:
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.nf
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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/cc
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.fi
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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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.PP
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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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.PP
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.SH "EXTRA OPTIONS"
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.PP
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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 definately not a input filename, and
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should be passed along to the compiler as-is\&.
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.PP
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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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.PP
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.SH "ENVIRONMENT VARIABLES"
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.PP
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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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.PP
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.IP
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.IP "\fBCCACHE_DIR\fP"
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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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"$HOME/\&.ccache"\&.
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.IP
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.IP "\fBCCACHE_LOGFILE\fP"
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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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.IP
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.IP "\fBCCACHE_PATH\fP"
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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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ccache itself\&.
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.IP
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.IP "\fBCCACHE_CC\fP"
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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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.IP
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.IP "\fBCCACHE_PREFIX\fP"
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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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.IP
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.IP "\fBCCACHE_DISABLE\fP"
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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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.IP
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.IP "\fBCCACHE_CPP2\fP"
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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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used\&.
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.IP
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.IP "\fBCCACHE_NOSTATS\fP"
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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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each compile\&.
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.IP
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.IP "\fBCCACHE_NLEVELS\fP"
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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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.IP
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.IP "\fBCCACHE_HARDLINK\fP"
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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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.IP
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.IP "\fBCCACHE_RECACHE\fP"
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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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.IP
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.IP "\fBCCACHE_UMASK\fP"
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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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compilations\&.
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.IP
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.IP "\fBCCACHE_HASHDIR\fP"
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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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this option\&.
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.IP
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.IP "\fBCCACHE_UNIFY\fP"
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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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.IP
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.IP "\fBCCACHE_EXTENSION\fP"
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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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the aCC compiler\&.
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.IP
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.PP
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.SH "CACHE SIZE MANAGEMENT"
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.PP
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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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files limits\&.
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.PP
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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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.PP
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.SH "HOW IT WORKS"
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.PP
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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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.PP
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.IP o
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the pre-processor output from running the compiler with -E
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.IP o
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the command line options
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.IP o
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the real compilers size and modification time
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.IP o
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any stderr output generated by the compiler
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.PP
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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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.PP
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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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please let me know\&.
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.PP
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.SH "USING CCACHE WITH DISTCC"
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.PP
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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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.PP
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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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.PP
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.SH "SHARING A CACHE"
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.PP
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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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.PP
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.IP o
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Use the same \fBCCACHE_DIR\fP environment variable setting
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.IP o
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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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.IP o
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Make sure everyone sets the CCACHE_UMASK environment variable
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to 002, this ensures that cached files are accessible to everyone\&.
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.IP o
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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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be useful for this\&.
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.PP
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.SH "HISTORY"
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.PP
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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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work\&. See
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http://www\&.erikyyy\&.de/compilercache/
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for the Erik\&'s scripts\&.
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.PP
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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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.PP
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.SH "DIFFERENCES FROM COMPILERCACHE"
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.PP
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The biggest differences between Erik\&'s compilercache script and ccache
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are:
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.IP o
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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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.IP o
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ccache can automatically find the real compiler
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.IP o
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ccache keeps statistics on hits/misses
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.IP o
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ccache can do automatic cache management
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.IP o
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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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.IP o
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ccache can handle a much wider ranger of compiler options
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.IP o
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ccache avoids a double call to cpp on a cache miss
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.PP
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.SH "BUGS"
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.PP
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When the cache is stored on an NFS filesystem, the filesystem must be
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exported with the \fBno_subtree_check\fP option to make renames between
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directories reliable\&.
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.PP
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.SH "CREDITS"
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.PP
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Thanks to the following people for their contributions to ccache
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.IP o
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Erik Thiele for the original compilercache script
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.IP o
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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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.IP o
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Paul Russell for many suggestions and the debian packaging
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.PP
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.SH "AUTHOR"
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.PP
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ccache was written by Andrew Tridgell
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http://samba\&.org/~tridge/
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.PP
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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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.PP
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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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