ccache(1) ccache Manual ccache(1)
NAME
ccache - a fast C/C++ compiler cache
SYNOPSIS
ccache [options]
ccache compiler [compiler options]
compiler [compiler options] (via symbolic link)
DESCRIPTION
ccache is a compiler cache. It speeds up recompilation by caching the
result of previous compilations and detecting when the same compilation
is being done again. Supported languages are C, C++, Objective-C and
Objective-C++.
ccache has been carefully written to always produce exactly the same
compiler output that you would get without the cache. The only way you
should be able to tell that you are using ccache is the speed.
Currently known exceptions to this goal are listed under BUGS. If you
ever discover an undocumented case where ccache changes the output of
your compiler, please let us know.
Features
o Keeps statistics on hits/misses.
o Automatic cache size management.
o Can cache compilations that generate warnings.
o Easy installation.
o Low overhead.
o Optionally uses hard links where possible to avoid copies.
o Optionally compresses files in the cache to reduce disk space.
Limitations
o Only knows how to cache the compilation of a single
C/C++/Objective-C/Objective-C++ file. Other types of compilations
(multi-file compilation, linking, etc) will silently fall back to
running the real compiler.
o Only works with GCC and compilers that behave similar enough.
o Some compiler flags are not supported. If such a flag is detected,
ccache will silently fall back to running the real compiler.
RUN MODES
There are two ways to use ccache. You can either prefix your
compilation commands with ccache or you can let ccache masquerade as
the compiler by creating a symbolic link (named as the compiler) to
ccache. 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
compilations.
To use the first method, just make sure that ccache is in your PATH.
To use the symlinks method, do something like this:
cp ccache /usr/local/bin/
ln -s ccache /usr/local/bin/gcc
ln -s ccache /usr/local/bin/g++
ln -s ccache /usr/local/bin/cc
ln -s ccache /usr/local/bin/c++
And so forth. This will work as long as the directory with symlinks
comes before the path to the compiler (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.
Warning
The technique of letting ccache masquerade as the compiler works
well, but currently doesn't interact well with other tools that do
the same thing. See USING CCACHE WITH OTHER COMPILER WRAPPERS.
Warning
Do not use a hard link, use a symbolic link. A hard link will cause
"interesting" problems.
OPTIONS
These options only apply when you invoke ccache as "ccache". When
invoked as a compiler (via a symlink as described in the previous
section), the normal compiler options apply and you should refer to the
compiler's documentation.
-c, --cleanup
Clean up the cache by removing old cached files until the specified
file number and cache size limits are not exceeded. This also
recalculates the cache file count and size totals. Normally, it's
not needed to initiate cleanup manually as ccache keeps the cache
below the specified limits at runtime and keeps statistics up to
date on each compilation. Forcing a cleanup is mostly useful if you
manually modify the cache contents or believe that the cache size
statistics may be inaccurate.
-C, --clear
Clear the entire cache, removing all cached files.
-F, --max-files=N
Set the maximum number of files allowed in the cache. The value is
stored inside the cache directory and applies to all future
compilations. Due to the way the value is stored the actual value
used is always rounded down to the nearest multiple of 16.
-h, --help
Print an options summary page.
-M, --max-size=SIZE
Set the maximum size of the files stored in the cache. 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.
-s, --show-stats
Print the current statistics summary for the cache.
-V, --version
Print version and copyright information.
-z, --zero-stats
Zero the cache statistics (but not the configured limits).
EXTRA OPTIONS
When run as a compiler, 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 to
avoid interpreting the next option in any way and to pass it 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 when parsing the command line is that any argument that exists as
a file is treated as an input file name. 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.
Another case where --ccache-skip can be useful is if ccache interprets
an option specially but shouldn't, since the option has another meaning
for your compiler than what ccache thinks.
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_BASEDIR
If you set the environment variable CCACHE_BASEDIR to an absolute
path to a directory, ccache rewrites absolute paths into relative
paths before computing the hash that identifies the compilation,
but only for paths under the specified directory. See the
discussion under COMPILING IN DIFFERENT DIRECTORIES.
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_COMPILERCHECK
By default, ccache includes the modification time ("mtime") and
size of the compiler in the hash to ensure that results retrieved
from the cache are accurate. The CCACHE_COMPILERCHECK environment
variable can be used to select another strategy. Possible values
are:
content
Hash the content of the compiler binary. This makes ccache very
slightly slower compared to the mtime setting, but makes it
cope better with compiler upgrades during a build bootstrapping
process.
mtime
Hash the compiler's mtime and size, which is fast. This is the
default.
none
Don't hash anything. This may be good for situations where you
can safely use the cached results even though the compiler's
mtime or size has changed (e.g. if the compiler is built as
part of your build system and the compiler's source has not
changed, or if the compiler only has changes that don't affect
code generation). You should only use the none setting if you
know what you are doing.
a command string
Hash the standard output and standard error output of the
specified command. The string will be split on whitespace to
find out the command and arguments to run. No other
interpretation of the command string will be done, except that
the special word "%compiler%" will be replaced with the path to
the compiler. Several commands can be specified with semicolon
as separator. Examples:
o %compiler% -v
o %compiler% -dumpmachine; %compiler% -dumpversion
You should make sure that the specified command is as fast as
possible since it will be run once for each ccache invocation.
Identifying the compiler using a command is useful if you want
to avoid cache misses when the compiler has been rebuilt but
not changed.
Another case is when the compiler (as seen by ccache) actually
isn't the real compiler but another compiler wrapper -- in that
case, the default mtime method will hash the mtime and size of
the other compiler wrapper, which means that ccache won't be
able to detect a compiler upgrade. Using a suitable command to
identify the compiler is thus safer, but it's also slower, so
you should consider continue using the mtime method in
combination with CCACHE_PREFIX if possible. See USING CCACHE
WITH OTHER COMPILER WRAPPERS.
CCACHE_COMPRESS
If you set the environment variable CCACHE_COMPRESS then ccache
will compress object files and other compiler output it puts in the
cache. However, this setting has no effect on how files are
retrieved from the cache; compressed and uncompressed results will
still be usable regardless of this setting.
CCACHE_CPP2
If you set the environment variable CCACHE_CPP2 then ccache will
not use the optimisation of avoiding the second call to the
preprocessor by compiling the preprocessed 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 anyway.
CCACHE_DETECT_SHEBANG
The CCACHE_DETECT_SHEBANG environment variable only has meaning on
Windows. It instructs ccache to open the executable file to detect
the #!/bin/sh string, in which case ccache will search for sh.exe
in PATH and use that to launch the executable.
CCACHE_DIR
The CCACHE_DIR environment variable specifies where ccache will
keep its cached compiler output. The default is $HOME/.ccache.
CCACHE_DISABLE
If you set the environment variable CCACHE_DISABLE then ccache will
just call the real compiler, bypassing the cache completely.
CCACHE_EXTENSION
ccache tries to automatically determine the extension to use for
intermediate preprocessor 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.
CCACHE_EXTRAFILES
If you set the environment variable CCACHE_EXTRAFILES to a list of
paths then ccache will include the contents of those files when
calculating the hash sum. The list separator is semicolon in
Windows systems and colon on other systems.
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 may be slightly faster in some situations, but can
confuse programs like "make" that rely on modification times.
Another thing to keep in mind is that if the resulting object file
is modified in any way, this corrupts the cached object file as
well. Hard links are never made for compressed cache files. This
means that you should not set the CCACHE_COMPRESS variable if you
want to use hard links.
CCACHE_HASHDIR
This tells ccache to hash the current working directory when
calculating the hash that is used to distinguish two compilations.
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_LOGFILE
If you set the CCACHE_LOGFILE environment variable then ccache will
write information on what it is doing to the specified file. This
is useful for tracking down problems.
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_NODIRECT
If you set the environment variable CCACHE_NODIRECT then ccache
will not use the direct mode.
CCACHE_NOSTATS
If you set the environment variable CCACHE_NOSTATS then ccache will
not update the statistics files on each compilation.
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_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_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 temporary
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_SLOPPINESS
By default, ccache tries to give as few false cache hits as
possible. However, in certain situations it's possible that you
know things that ccache can't take for granted. The
CCACHE_SLOPPINESS environment variable makes it possible to tell
ccache to relax some checks in order to increase the hit rate. The
value should be a comma-separated string with options. Available
options are:
file_macro
Ignore __FILE__ being present in the source.
include_file_mtime
Don't check the modification time of include files in the
direct mode.
time_macros
Ignore __DATE__ and __TIME__ being present in the source code.
See the discussion under TROUBLESHOOTING for more information.
CCACHE_TEMPDIR
The CCACHE_TEMPDIR environment variable specifies where ccache will
put temporary files. The default is $CCACHE_DIR/tmp.
Note
In previous versions of ccache, CCACHE_TEMPDIR had to be on the
same filesystem as the CCACHE_DIR path, but this requirement
has been relaxed.)
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_UNIFY
If you set the environment variable CCACHE_UNIFY then ccache will
use a C/C++ unifier when hashing the preprocessor output if the -g
option is not used. 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 compilations 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. Also note that enabling the unifier implies turning off
the direct mode.
CACHE SIZE MANAGEMENT
By default ccache has a one gigabyte limit on the total size of files
in the cache and no maximum number of files. You can set different
limits using the -M/--max-size and -F/--max-files options. Use ccache
-s/--show-stats to see the cache size and the currently configured
limits (in addition to other various statistics).
CACHE COMPRESSION
ccache can optionally compress all files it puts into the cache using
the compression library zlib. While this involves a negligible
performance slowdown, it significantly increases the number of files
that fit in the cache. You can turn on compression by setting the
CCACHE_COMPRESS environment variable.
HOW CCACHE WORKS
The basic idea is to detect when you are compiling exactly the same
code a second time and reuse the previously produced output. The
detection is done by hashing different kinds of information that should
be unique for the compilation and then using the hash sum to identify
the cached output. ccache uses MD4, a very fast cryptographic hash
algorithm, for the hashing. (MD4 is nowadays too weak to be useful in
cryptographic contexts, but it should be safe enough to be used to
identify recompilations.) On a cache hit, ccache is able to supply all
of the correct compiler outputs (including all warnings, dependency
file, etc) from the cache.
ccache has two ways of doing the detection:
o the direct mode, where ccache hashes the source code and include
files directly
o the preprocessor mode, where ccache runs the preprocessor on the
source code and hashes the result
The direct mode is generally faster since running the preprocessor has
some overhead.
Common hashed information
For both modes, the following information is included in the hash:
o the extension used by the compiler for a file with preprocessor
output (normally .i for C code and .ii for C++ code)
o the compiler's size and modification time (or other
compiler-specific information specified by CCACHE_COMPILERCHECK)
o the name of the compiler
o the current directory (if CCACHE_HASHDIR is set)
o contents of files specified by CCACHE_EXTRAFILES (if any)
The direct mode
In the direct mode, the hash is formed of the common information and:
o the input source file
o the command line options
Based on the hash, a data structure called "manifest" is looked up in
the cache. The manifest contains:
o references to cached compilation results (object file, dependency
file, etc) that were produced by previous compilations that matched
the hash
o paths to the include files that were read at the time the
compilation results were stored in the cache
o hash sums of the include files at the time the compilation results
were stored in the cache
The current contents of the include files are then hashed and compared
to the information in the manifest. If there is a match, ccache knows
the result of the compilation. If there is no match, ccache falls back
to running the preprocessor. The output from the preprocessor is parsed
to find the include files that were read. The paths and hash sums of
those include files are then stored in the manifest along with
information about the produced compilation result.
The direct mode will be disabled if any of the following holds:
o the environment variable CCACHE_NODIRECT is set
o a modification time of one of the include files is too new (needed
to avoid a race condition)
o the unifier is enabled (the environment variable CCACHE_UNIFY is
set)
o a compiler option not supported by the direct mode is used:
o a -Wp,X compiler option other than -Wp,-MD,path and
-Wp,-MMD,path
o -Xpreprocessor
o the string "__TIME__" is present outside comments and string
literals in the source code
The preprocessor mode
In the preprocessor mode, the hash is formed of the common information
and:
o the preprocessor output from running the compiler with -E
o the command line options except options that affect include files
(-I, -include, -D, etc; the theory is that these options will
change the preprocessor output if they have any effect at all)
o any standard error output generated by the preprocessor
Based on the hash, the cached compilation result can be looked up
directly in the cache.
COMPILING IN DIFFERENT DIRECTORIES
Some information included in the hash that identifies a unique
compilation may contain absolute paths:
o The preprocessed source code may contain absolute paths to include
files if the compiler option -g is used or if absolute paths are
given to -I and similar compiler options.
o Paths specified by compiler options (such as -I, -MF, etc) may be
absolute.
o The source code file path may be absolute, and that path may
substituted for __FILE__ macros in the source code or included in
warnings emitted to standard error by the preprocessor.
This means that if you compile the same code in different locations,
you can't share compilation results between the different build
directories since you get cache misses because of the absolute build
directory paths that are part of the hash. To mitigate this problem,
you can specify a "base directory" by setting the CCACHE_BASEDIR
variable to an absolute path to the directory. ccache will then rewrite
absolute paths that are under the base directory (i.e., paths that have
the base directory as a prefix) to relative paths when constructing the
hash. A typical path to use as the base directory is your home
directory or another directory that is a parent of your build
directories. (Don't use / as the base directory since that will make
ccache also rewrite paths to system header files, which doesn't gain
anything.)
The drawbacks of using CCACHE_BASEDIR are:
o If you specify an absolute path to the source code file, __FILE__
macros will be expanded to a relative path instead.
o If you specify an absolute path to the source code file and compile
with -g, the source code path stored in the object file may point
to the wrong directory, which may prevent debuggers like GDB from
finding the source code. Sometimes, a work-around is to change the
directory explicitly with the "cd" command in GDB.
PRECOMPILED HEADERS
ccache has support for GCC's precompiled headers. However, you have to
do some things to make it work properly:
o You must set CCACHE_SLOPPINESS to time_macros. The reason is that
ccache can't tell whether __TIME__ or __DATE__ is used when using a
precompiled header.
o You must either:
o use the -include compiler option to include the precompiled
header (i.e., don't use #include in the source code to include
the header); or
o add the -fpch-preprocess compiler option when compiling.
If you don't do this, either the non-precompiled version of the
header file will be used (if available) or ccache will fall back to
running the real compiler and increase the statistics counter
"preprocessor error" (if the non-precompiled header file is not
available).
SHARING A CACHE
A group of developers can increase the cache hit rate by sharing a
cache directory. To share a cache without unpleasant side effects, the
following conditions should 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 "find $CCACHE_DIR -type d | xargs chmod
g+s" might be useful for this.
The reason to avoid the hard link mode is that the hard links 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.
You may also want to make sure that the developers have CCACHE_BASEDIR
set appropriately, as discussed in the previous section.
SHARING A CACHE ON NFS
It is possible to put the cache directory on an NFS filesystem (or
similar filesystems), but keep in mind that:
o Having the cache on NFS may slow down compilation. Make sure to do
some benchmarking to see if it's worth it.
o ccache hasn't been tested very thoroughly on NFS.
A tip is to set CCACHE_TEMPDIR to a directory on the local host to
avoid NFS traffic for temporary files.
USING CCACHE WITH OTHER COMPILER WRAPPERS
The recommended way of combining ccache with another compiler wrapper
(such as "distcc") is by using the CCACHE_PREFIX option. You just need
to set the environment variable CCACHE_PREFIX to the name of the
wrapper (e.g. distcc) and ccache will prefix the command line with the
specified command when running the compiler.
Unless you set CCACHE_COMPILERCHECK to a suitable command (see the
description of that configuration option), it is not recommended to use
the form ccache anotherwrapper compiler args as the compilation
command. It's also not recommended to use the masquerading technique
for the other compiler wrapper. The reason is that by default, ccache
will in both cases hash the mtime and size of the other wrapper instead
of the real compiler, which means that:
o Compiler upgrades will not be detected properly.
o The cached results will not be shared between compilations with and
without the other wrapper.
Another minor thing is that if CCACHE_PREFIX is not used, ccache will
needlessly invoke the other wrapper when running the preprocessor.
BUGS
o ccache doesn't handle the GNU Assembler's .incbin directive
correctly. This directive can be embedded in the source code inside
an asm statement in order to include a file verbatim in the object
file. If the included file is modified, ccache doesn't pick up the
change since the inclusion isn't done by the preprocessor. A
workaround of this problem is to set CCACHE_EXTRAFILES to the path
of the included file.
TROUBLESHOOTING
General
A general tip for getting information about what ccache is doing is to
enable debug logging by setting CCACHE_LOGFILE. The log contains
executed commands, important decisions that ccache makes, read and
written files, etc. Another way of keeping track of what is happening
is to check the output of ccache -s.
Performance
ccache has been written to perform well out of the box, but sometimes
you may have to do some adjustments of how you use the compiler and
ccache in order to improve performance.
Since ccache works best when I/O is fast, put the cache directory on a
fast storage device if possible. Having lots of free memory so that
files in the cache directory stay in the disk cache is also
preferrable.
A good way of monitoring how well ccache works is to run ccache -s
before and after your build and then compare the statistics counters.
Here are some common problems and what may be done to increase the hit
rate:
o If "cache hit (preprocessed)" has been incremented instead of
"cache hit (direct)", ccache has fallen back to preprocessor mode,
which is generally slower. Some possible reasons are:
o The source code has been modified in such a way that the
preprocessor output is not affected.
o Compiler arguments that are hashed in the direct mode but not
in the preprocessor mode have changed (-I, -include, -D, etc)
and they didn't affect the preprocessor output.
o The compiler option -Xpreprocessor or -Wp,X (except
-Wp,-MD,path and Wp,-MMD,path) is used.
o This was the first compilation with a new value of
CCACHE_BASEDIR.
o A modification time of one of the include files is too new
(created the same second as the compilation is being done).
This check is made to avoid a race condition. To fix this,
create the include file earlier in the build process, if
possible, or set CCACHE_SLOPPINESS to include_file_mtime if you
are willing to take the risk. (The race condition consists of
these events: the preprocessor is run; an include file is
modified by someone; the new include file is hashed by ccache;
the real compiler is run on the preprocessor's output, which
contains data from the old header file; the wrong object file
is stored in the cache.)
o The __TIME__ preprocessor macro is (potentially) being used.
ccache turns off direct mode if "__TIME__" is present in the
source code outside comments and string literals. This is done
as a safety measure since the string indicates that a __TIME__
macro may affect the output. (To be sure, ccache would have to
run the preprocessor, but the sole point of the direct mode is
to avoid that.) If you know that __TIME__ isn't used in
practise, or don't care if ccache produces objects where
__TIME__ is expanded to something in the past, you can set
CCACHE_SLOPPINESS to time_macros.
o The __DATE__ preprocessor macro is (potentially) being used and
the date has changed. This is similar to how __TIME__ is
handled. If "__DATE__" is present in the source code outside
comments and string literals, ccache hashes the current date in
order to be able to produce the correct object file if the
__DATE__ macro affects the output. If you know that __DATE__
isn't used in practise, or don't care if ccache produces
objects where __DATE__ is expanded to something in the past,
you can set CCACHE_SLOPPINESS to time_macros.
o The __FILE__ preprocessor macro is (potentially) being used and
the file path has changed. If "__FILE__" is present in the
source code outside comments and string literals, ccache hashes
the current input file path in order to be able to produce the
correct object file if the __FILE__ macro affects the output.
If you know that __FILE__ isn't used in practise, or don't care
if ccache produces objects where __FILE__ is expanded to the
wrong path, you can set CCACHE_SLOPPINESS to file_macro.
o If "cache miss" has been incremented even though the same code has
been compiled and cached before, ccache has either detected that
something has changed anyway or a cleanup has been performed
(either explicitly or implicitly when a cache limit has been
reached). Some perhaps unobvious things that may result in a cache
miss are usage of __TIME__ or __DATE__ macros, or use of
automatically generated code that contains a timestamp, build
counter or other volatile information.
o If "multiple source files" has been incremented, it's an indication
that the compiler has been invoked on several source code files at
once. ccache doesn't support that. Compile the source code files
separately if possible.
o If "unsupported compiler option" has been incremented, enable debug
logging and check which option was rejected.
o If "preprocessor error" has been incremented, one possible reason
is that precompiled headers are being used. See PRECOMPILED HEADERS
for how to remedy this.
o If "can't use precompiled header" has been incremented, see
PRECOMPILED HEADERS.
Errors when compiling with ccache
If compilation doesn't work with ccache, but it works without it, one
possible reason is that the compiler can't compile preprocessed output
correctly. A workaround that may work is to set CCACHE_CPP2. This will
make cache misses slower, though, so it is better to find and fix the
root cause.
Corrupt object files
It should be noted that ccache is susceptible to general storage
problems. If a bad object file sneaks into the cache for some reason,
it will of course stay bad. Some possible reasons for erroneous object
files are bad hardware (disk drive, disk controller, memory, etc),
buggy drivers or file systems, a bad CCACHE_PREFIX command or compiler
wrapper. If this happens, the easiest way of fixing it is this:
1. Build so that the bad object file ends up in the build tree.
2. Remove the bad object file from the build tree.
3. Rebuild with CCACHE_RECACHE set.
An alternative is to clear the whole cache with ccache -C if you don't
mind losing other cached results.
There are no reported issues about ccache producing broken object files
reproducibly. That doesn't mean it can't happen, so if you find a
repeatable case, please report it.
MORE INFORMATION
Credits, mailing list information, bug reporting instructions, source
code, etc, can be found on ccache's web site: http://ccache.samba.org.
AUTHOR
ccache was originally written by Andrew Tridgell and is currently
developed and maintained by Joel Rosdahl. See AUTHORS.txt or
AUTHORS.html and http://ccache.samba.org/credits.html for a list of
contributors.
ccache 3.1.7 01/08/2012 ccache(1)
ccache 3.1.7 - Generated Fri Jan 13 15:24:06 CST 2012