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ccache/MANUAL.txt
Joel Rosdahl 18979c420f Improve documentation of hashed information
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CCACHE(1)
=========
:man source: ccache
:man version: {revnumber}
:man manual: ccache Manual
Name
----
ccache - a fast C/C++ compiler cache
Synopsis
--------
[verse]
*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,BUGS>>. If you ever discover an undocumented case
where ccache changes the output of your compiler, please let us know.
Features
~~~~~~~~
* Keeps statistics on hits/misses.
* Automatic cache size management.
* Can cache compilations that generate warnings.
* Easy installation.
* Low overhead.
* Optionally uses hard links where possible to avoid copies.
* Optionally compresses files in the cache to reduce disk space.
Limitations
~~~~~~~~~~~
* 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.
* Only works with GCC and compilers that behave similar enough.
* 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,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,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:
+
--
* +%compiler% -v+
* +%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,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,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:
* the *direct mode*, where ccache hashes the source code and include files
directly
* 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:
* the extension used by the compiler for a file with preprocessor output
(normally *.i* for C code and *.ii* for C++ code)
* the compiler's size and modification time (or other compiler-specific
information specified by *CCACHE_COMPILERCHECK*)
* the name of the compiler
* the current directory (if *CCACHE_HASHDIR* is set)
* 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:
* the input source file
* the command line options
Based on the hash, a data structure called ``manifest'' is looked up in the
cache. The manifest contains:
* references to cached compilation results (object file, dependency file, etc)
that were produced by previous compilations that matched the hash
* paths to the include files that were read at the time the compilation results
were stored in the cache
* 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:
* the environment variable *CCACHE_NODIRECT* is set
* a modification time of one of the include files is too new (needed to avoid a
race condition)
* the unifier is enabled (the environment variable *CCACHE_UNIFY* is set)
* a compiler option not supported by the direct mode is used:
** a *-Wp,_X_* compiler option other than *-Wp,-MD,_path_* and
*-Wp,-MMD,_path_*
** *-Xpreprocessor*
* 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:
* the preprocessor output from running the compiler with *-E*
* 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)
* 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:
* 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.
* Paths specified by compiler options (such as *-I*, *-MF*, etc) may be
absolute.
* 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:
* If you specify an absolute path to the source code file, *\_\_FILE__* macros
will be expanded to a relative path instead.
* 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:
* 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.
* You must either:
+
--
** 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
** 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:
* Use the same *CCACHE_DIR* environment variable setting.
* Unset the *CCACHE_HARDLINK* environment variable.
* Make sure everyone sets the *CCACHE_UMASK* environment variable to 002. This
ensures that cached files are accessible to everyone in the group.
* Make sure that all users have write permission in the entire cache directory
(and that you trust all users of the shared cache).
* 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:
* Having the cache on NFS may slow down compilation. Make sure to do some
benchmarking to see if it's worth it.
* 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:
* Compiler upgrades will not be detected properly.
* 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
----
* 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:
* 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:
** The source code has been modified in such a way that the preprocessor output
is not affected.
** 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.
** The compiler option *-Xpreprocessor* or *-Wp,_X_* (except *-Wp,-MD,_path_*
and *Wp,-MMD,_path_*) is used.
** This was the first compilation with a new value of *CCACHE_BASEDIR*.
** 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.)
** 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*.
** 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*.
** 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*.
* 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.
* 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.
* If ``unsupported compiler option'' has been incremented, enable debug logging
and check which option was rejected.
* If ``preprocessor error'' has been incremented, one possible reason is that
precompiled headers are being used. See <<_precompiled_headers,PRECOMPILED
HEADERS>> for how to remedy this.
* If ``can't use precompiled header'' has been incremented, see
<<_precompiled_headers,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, you can either
find out which object file is broken by reading the debug log and then delete
the bad object file from the cache, or you can simply 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.
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