[platform/upstream/ccache.git] / MANUAL.txt

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 <<_caveats,CAVEATS>>. 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, there is no need 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, but keeping the
    configuration file.

*-F, --max-files*='N'::

    Set the maximum number of files allowed in the cache. Use 0 for no limit.
    The value is stored in a configuration file in the cache directory and
    applies to all future compilations.

*-h, --help*::

    Print an options summary page.

*-M, --max-size*='SIZE'::

    Set the maximum size of the files stored in the cache. 'SIZE' should be a
    number followed by an optional suffix: k, M, G, T (decimal), Ki, Mi, Gi or
    Ti (binary). The default suffix is G. Use 0 for no limit. The value is
    stored in a configuration file in the cache directory and applies to all
    future compilations.

*-o, --set-config*='KEY=VALUE'::

    Set configuration 'KEY' to 'VALUE'. See <<_configuration,CONFIGURATION>>
    for more information.

*-p, --print-config*::

    Print current configuration options and from where they originate
    (environment variable, configuration file or compile-time default).

*-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 configuration options).


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.

NOTE: *--ccache-skip* currently only tells ccache not to interpret the next
option as a special compiler option -- the option will still be included in the
direct mode hash.

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.


Configuration
-------------

ccache's default behavior can be overridden by configuration file settings,
which in turn can be overridden by environment variables with names starting
with *CCACHE_*. ccache normally reads configuration from two files: first a
system-level configuration file and secondly a cache-specific configuration
file. The priority of configuration settings is as follows (where 1 is
highest):

1. Environment variables.
2. The cache-specific configuration file *<ccachedir>/ccache.conf* (typically
   *$HOME/.ccache/ccache.conf*).
3. The system-wide configuration file *<sysconfdir>/ccache.conf* (typically
   */etc/ccache.conf* or */usr/local/etc/ccache.conf*).
4. Compile-time defaults.

As a special case, if the environment variable *CCACHE_CONFIGPATH* is set,
ccache reads configuration from the specified path instead of the default
paths.


Configuration file syntax
~~~~~~~~~~~~~~~~~~~~~~~~~

Configuration files are in a simple ``key = value'' format, one setting per
line. Lines starting with a hash sign are comments. Blank lines are ignored, as
is whitespace surrounding keys and values. Example:

-------------------------------------------------------------------------------
# Set maximum cache size to 10 GB:
max_size = 10G
-------------------------------------------------------------------------------

Boolean values
~~~~~~~~~~~~~~

Some settings are boolean values (i.e. truth values). In a configuration file,
such values must be set to the string *true* or *false*. For the corresponding
environment variables, the semantics are a bit different: a set environment
variable means ``true'' regardless of the value (even if set to the empty
string), and an unset environment variable means ``false''. Each boolean
environment variable also has a negated form starting with *CCACHE_NO*. For
example, *CCACHE_COMPRESS* can be set to force compression and
*CCACHE_NOCOMPRESS* can be set to force no compression.


Configuration settings
~~~~~~~~~~~~~~~~~~~~~~

Below is a list of available configuration settings. The corresponding
environment variable name is indicated in parentheses after each configuration
setting key.

*base_dir* (*CCACHE_BASEDIR*)::

    This setting should be an absolute path to a directory. ccache then
    rewrites absolute paths into relative paths before computing the hash that
    identifies the compilation, but only for paths under the specified
    directory. If set to the empty string (which is the default), no rewriting
    is done. 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.
+
See also the discussion under <<_compiling_in_different_directories,COMPILING
IN DIFFERENT DIRECTORIES>>.

*cache_dir* (*CCACHE_DIR*)::

    This setting specifies where ccache will keep its cached compiler outputs.
    It will only take effect if set in the system-wide configuration file or as
    an environment variable. The default is *$HOME/.ccache*.

*cache_dir_levels* (*CCACHE_NLEVELS*)::

    This setting allows you to choose the number of directory levels in the
    cache directory. The default is 2. The minimum is 1 and the maximum is 8.

*compiler* (*CCACHE_CC*)::

    This setting can be used to force the name of the compiler to use. If set
    to the empty string (which is the default), ccache works it out from the
    command line.

*compiler_check* (*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. This setting 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.
*string:value*::
    Use *value* as the string to calculate hash from. This can be the compiler
    revision number you retrieved earlier and set here via environment variable.
_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
the *prefix_command* setting if possible. See
<<_using_ccache_with_other_compiler_wrappers,USING CCACHE WITH OTHER COMPILER
WRAPPERS>>.
--
--

*compression* (*CCACHE_COMPRESS* or *CCACHE_NOCOMPRESS*, see <<_boolean_values,Boolean values>> above)::

    If true, 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. The default is false.

*compression_level* (*CCACHE_COMPRESSLEVEL*)::

    This setting determines the level at which ccache will compress object
    files. It only has effect if *compression* is enabled. The value defaults
    to 6, and must be no lower than 1 (fastest, worst compression) and no
    higher than 9 (slowest, best compression).

*cpp_extension* (*CCACHE_EXTENSION*)::

    This setting can be used to force a certain extension for the intermediate
    preprocessed file. The default is to automatically determine the extension
    to use for intermediate preprocessor files based on the type of file being
    compiled, but that sometimes doesn't work. For example, when using the
    ``aCC'' compiler on HP-UX, set the cpp extension to *i*.

*direct_mode* (*CCACHE_DIRECT* or *CCACHE_NODIRECT*, see <<_boolean_values,Boolean values>> above)::

    If true, the direct mode will be used. The default is true. See
    <<_the_direct_mode,THE DIRECT MODE>>.

*disable* (*CCACHE_DISABLE* or *CCACHE_NODISABLE*, see <<_boolean_values,Boolean values>> above)::

    When true, ccache will just call the real compiler, bypassing the cache
    completely. The default is false.

*extra_files_to_hash* (*CCACHE_EXTRAFILES*)::

    This setting is a list of paths to files that ccache will include in the
    the hash sum that identifies the build. The list separator is semicolon on
    Windows systems and colon on other systems.

*hard_link* (*CCACHE_HARDLINK* or *CCACHE_NOHARDLINK*, see <<_boolean_values,Boolean values>> above)::

    If true, 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 enable compression
    if you want to use hard links. The default is false.

*hash_dir* (*CCACHE_HASHDIR* or *CCACHE_NOHASHDIR*, see <<_boolean_values,Boolean values>> above)::

    If true (which is the default), ccache will include the current working
    directory (CWD) in the hash that is used to distinguish two compilations
    when generating debug info (compiler option *-g* with variations).
    Exception: The CWD will not be included in the hash if *base_dir* is set
    (and matches the CWD) and the compiler option *-fdebug-prefix-map* is used.
    See also the discussion under
    <<_compiling_in_different_directories,COMPILING IN DIFFERENT DIRECTORIES>>.
+
The reason for including the CWD in the hash by default is to prevent a problem
with the storage of the current working directory in the debug info of an
object file, which can lead ccache to return a cached object file that has the
working directory in the debug info set incorrectly.
+
You can disable this setting to get cache hits when compiling the same source
code in different directories if you don't mind that CWD in the debug info
might be incorrect.

*ignore_headers_in_manifest* (*CCACHE_IGNOREHEADERS*)::

    This setting is a list of paths to files (or directories with headers) that
    ccache will *not* include in the manifest list that makes up the direct
    mode. Note that this can cause stale cache hits if those headers do indeed
    change. The list separator is semicolon on Windows systems and colon on
    other systems.

*keep_comments_cpp* (*CCACHE_COMMENTS* or *CCACHE_NOCOMMENTS*, see <<_boolean_values,Boolean values>> above)::

    If true, ccache will not discard the comments before hashing preprocessor
    output. This can be used to check documentation with *-Wdocumentation*.

*limit_multiple* (*CCACHE_LIMIT_MULTIPLE*)::

    Sets the limit when cleaning up. Files are deleted (in LRU order) until the
    levels are below the limit. The default is 0.8 (= 80%). See
    <<_automatic_cleanup,AUTOMATIC CLEANUP>> for more information.

*log_file* (*CCACHE_LOGFILE*)::

    If set to a file path, ccache will write information on what it is doing to
    the specified file. This is useful for tracking down problems.

*max_files* (*CCACHE_MAXFILES*)::

    This option specifies the maximum number of files to keep in the cache. Use
    0 for no limit (which is the default). See also
    <<_cache_size_management,CACHE SIZE MANAGEMENT>>.

*max_size* (*CCACHE_MAXSIZE*)::

    This option specifies the maximum size of the cache. Use 0 for no limit.
    The default value is 5G. Available suffixes: k, M, G, T (decimal) and Ki,
    Mi, Gi, Ti (binary). The default suffix is "G". See also
    <<_cache_size_management,CACHE SIZE MANAGEMENT>>.

*path* (*CCACHE_PATH*)::

    If set, ccache will search directories in this list when looking for the
    real compiler. The list separator is semicolon on Windows systems and colon
    on other systems. If not set, ccache will look for the first executable
    matching the compiler name in the normal *PATH* that isn't a symbolic link
    to ccache itself.

*prefix_command* (*CCACHE_PREFIX*)::

    This option adds a list of prefixes (separated by space) to the command
    line that ccache uses when invoking the compiler. See also
    <<_using_ccache_with_other_compiler_wrappers,USING CCACHE WITH OTHER
    COMPILER WRAPPERS>>.

*prefix_command_cpp* (*CCACHE_PREFIX_CPP*)::

    This option adds a list of prefixes (separated by space) to the command
    line that ccache uses when invoking the preprocessor.

*read_only* (*CCACHE_READONLY* or *CCACHE_NOREADONLY*, see <<_boolean_values,Boolean values>> above)::

    If true, ccache will attempt to use existing cached object files, but it
    will not to try to add anything new to the cache. If you are using this
    because your ccache directory is read-only, then you need to set
    *temporary_dir* as otherwise ccache will fail to create temporary files.

*read_only_direct* (*CCACHE_READONLY_DIRECT* or *CCACHE_NOREADONLY_DIRECT*, see <<_boolean_values,Boolean values>> above)::

    Just like *read_only* except that ccache will only try to retrieve results
    from the cache using the direct mode, not the preprocessor mode. See
    documentation for *read_only* regarding using a read-only ccache directory.

*recache* (*CCACHE_RECACHE* or *CCACHE_NORECACHE*, see <<_boolean_values,Boolean values>> above)::

    If true, ccache will not use any previously stored result. New results will
    still be cached, possibly overwriting any pre-existing results.

*run_second_cpp* (*CCACHE_CPP2* or *CCACHE_NOCPP2*, see <<_boolean_values,Boolean values>> above)::

    If true, ccache will first run the preprocessor to preprocess the source
    code (see <<_the_preprocessor_mode,THE PREPROCESSOR MODE>>) and then on a
    cache miss run the compiler on the source code to get hold of the object
    file. This is the default.
+
If false, ccache will first run preprocessor to preprocess the source code and
then on a cache miss run the compiler on the _preprocessed source code_ instead
of the original source code. This makes cache misses slightly faster since the
source code only has to be preprocessed once. The downside is that some
compilers won't produce the same result (for instance diagnostics warnings)
when compiling preprocessed source code.

*sloppiness* (*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. This setting 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.
*file_stat_matches*::
    ccache normally examines a file's contents to determine whether it matches
    the cached version. With this option set, ccache will consider a file as
    matching its cached version if the mtimes and ctimes match.
*include_file_ctime*::
    By default, ccache also will not cache a file if it includes a header whose
    ctime is too new. This option disables that check.
*include_file_mtime*::
    By default, ccache will not cache a file if it includes a header whose
    mtime is too new. This option disables that check.
*no_system_headers*::
    By default, ccache will also include all system headers in the manifest.
    With this option set, ccache will only include system headers in the hash
    but not add the system header files to the list of include files.
*pch_defines*::
    Be sloppy about **#define**s when precompiling a header file. See
    <<_precompiled_headers,PRECOMPILED HEADERS>> for more information.
*time_macros*::
    Ignore *\_\_DATE\__* and *\_\_TIME__* being present in the source code.
--
+
See the discussion under <<_troubleshooting,TROUBLESHOOTING>> for more
information.

*stats* (*CCACHE_STATS* or *CCACHE_NOSTATS*, see <<_boolean_values,Boolean values>> above)::

    If true, ccache will update the statistics counters on each compilation.
    The default is true.

*temporary_dir* (*CCACHE_TEMPDIR*)::

    This setting specifies where ccache will put temporary files. The default
    is *<cache_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.)

*umask* (*CCACHE_UMASK*)::

    This setting specifies 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.

*unify* (*CCACHE_UNIFY* or *CCACHE_NOUNIFY*, see <<_boolean_values,Boolean values>> above)::

    If true, 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 enabling the
    unifier changes the hash, so cached compilations produced when the unifier
    is enabled cannot be reused when the unifier is disabled, and vice versa.
    Enabling the unifier may result in incorrect line number information in
    compiler warning messages and expansions of the *\_\_LINE__* macro. Also
    note that enabling the unifier implies turning off the direct mode.


Cache size management
---------------------

By default, ccache has a 5 GB limit on the total size of files in the cache and
no limit on the 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).

Cleanup can be triggered in two different ways: automatic and manual.


Automatic cleanup
~~~~~~~~~~~~~~~~~

ccache maintains counters for various statistics about the cache, including the
size and number of all cached files. In order to improve performance and reduce
issues with concurrent ccache invocations, there is one statistics file for
each of the sixteen subdirectories in the cache.

After a new compilation result has been written to the cache, ccache will
update the size and file number statistics for the subdirectory (one of
sixteen) to which the result was written. Then, if the size counter for said
subdirectory is greater than *max_size / 16* or the file number counter is
greater than *max_files / 16*, automatic cleanup is triggered.

When automatic cleanup is triggered for a subdirectory in the cache, ccache
will:

1. Count all files in the subdirectory and compute their aggregated size.
2. Remove files in LRU (least recently used) order until the size is at most
   *limit_multiple * max_size / 16* and the number of files is at most
   *limit_multiple * max_files / 16*, where *limit_multiple*, *max_size* and
   *max_files* are configuration settings.
3. Set the size and file number counters to match the files that were kept.

The reason for removing more files than just those needed to not exceed the max
limits is that a cleanup is a fairly slow operation, so it would not be a good
idea to trigger it often, like after each cache miss.


Manual cleanup
~~~~~~~~~~~~~~

You can run *ccache -c/--cleanup* to force cleanup of the whole cache, i.e. all
of the sixteen subdirectories. This will recalculate the statistics counters
and make sure that the *max_size* and *max_files* settings are not exceeded.
Note that *limit_multiple* is not taken into account for manual cleanup.


Cache compression
-----------------

ccache can optionally compress all files it puts into the cache using the
compression library zlib. While this may involve a tiny performance slowdown,
it increases the number of files that fit in the cache. You can turn on
compression with the *compression* configuration setting and you can also tweak
the compression level with *compression_level*.


Cache statistics
----------------

*ccache -s/--show-stats* can show the following statistics:

[options="header",cols="30%,70%"]
|==============================================================================
|Name | Description
| autoconf compile/link |
Uncachable compilation or linking by an autoconf test.

| bad compiler arguments |
Malformed compiler argument, e.g. missing a value for an option that requires
an argument or failure to read a file specified by an option argument.

| cache file missing |
A file was unexpectedly missing from the cache. This only happens in rare
situations, e.g. if one ccache instance is about to get a file from the cache
while another instance removed the file as part of cache cleanup.

| cache hit (direct) |
A result was successfully found using <<_the_direct_mode,the direct mode>>.

| cache hit (preprocessed) |
A result was successfully found using <<_the_preprocessor_mode,the preprocessor
mode>>.

| cache miss |
No result was found.

| cache size |
Current size of the cache.

| called for link |
The compiler was called for linking, not compiling.

| called for preprocessing |
The compiler was called for preprocessing, not compiling.

| can't use precompiled header |
Preconditions for using <<_precompiled_headers,precompiled headers>> were not
fulfilled.

| ccache internal error |
Unexpected failure, e.g. due to problems reading/writing the cache.

| cleanups performed |
Number of cleanups performed, either implicitly due to the cache size limit
being reached or due to explicit *ccache -c/--cleanup* calls.

| compile failed |
The compilation failed. No result stored in the cache.

| compiler check failed |
A compiler check program specified by *compiler_check* (*CCACHE_COMPILERCHECK*)
failed.

| compiler produced empty output |
The compiler's output file (typically an object file) was empty after
compilation.

| compiler produced no output |
The compiler's output file (typically an object file) was missing after
compilation.

| compiler produced stdout |
The compiler wrote data to standard output. This is something that compilers
normally never do, so ccache is not designed to store such output in the cache.

| couldn't find the compiler |
The compiler to execute could not be found.

| error hashing extra file |
Failure reading a file specified by *extra_files_to_hash*
(*CCACHE_EXTRAFILES*).

| files in cache |
Current number of files in the cache.

| multiple source files |
The compiler was called to compile multiple source files in one go. This is not
supported by ccache.

| no input file |
No input file was specified to the compiler.

| output to a non-regular file |
The output path specified with *-o* is not a file (e.g. a directory or a device
node).

| output to stdout |
The compiler was instructed to write its output to standard output using *-o
-*. This is not supported by ccache.

| preprocessor error |
Preprocessing the source code using the compiler's *-E* option failed.

| unsupported code directive |
Code like the assembler ``.incbin'' directive was found. This is not supported
by ccache.

| unsupported compiler option |
A compiler option not supported by ccache was found.

| unsupported source language |
A source language e.g. specified with *-x* was unsupported by ccache.

|==============================================================================


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 the *compiler_check* setting)
* the name of the compiler
* the current directory (if the *hash_dir* setting is enabled)
* contents of files specified by the *extra_files_to_hash* setting (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.

There is a catch with the direct mode: header files that were used by the
compiler are recorded, but header files that were *not* used, but would have
been used if they existed, are not. So, when ccache checks if a result can be
taken from the cache, it currently can't check if the existence of a new header
file should invalidate the result. In practice, the direct mode is safe to use
in the absolute majority of cases.

The direct mode will be disabled if any of the following holds:

* the configuration setting *direct_mode* is false
* a modification time of one of the include files is too new (needed to avoid a
  race condition)
* the unifier is enabled (the configuration setting *unify* is true)
* a compiler option not supported by the direct mode is used:
** a *-Wp,_X_* compiler option other than *-Wp,-MD,_path_*,
   *-Wp,-MMD,_path_* and *-Wp,-D_define_*
** *-Xpreprocessor*
* the string ``\_\_TIME__'' is present 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 can
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) on the command
  line 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.

Here's what can be done to enable cache hits between different build
directories:

* If you build with *-g* (or similar) to add debug information to the object
  file, you must either:
+
--
** use the *-fdebug-prefix-map=_old_=_new_* option for relocating debug info to
   a common prefix (e.g. *-fdebug-prefix-map=$PWD=.*); or
** set *hash_dir = false*.
--
* If you use absolute paths anywhere on the command line (e.g. the source code
  file path or an argument to compiler options like *-I* and *-MF*), you must
  to set *base_dir* to an absolute path to a ``base directory''. ccache will
  then rewrite absolute paths under that directory to relative before computing
  the hash.


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 *sloppiness* to *pch_defines,time_macros*. The reason is that
  ccache can't tell whether *\_\_TIME\__* or *\_\_DATE__* is used when using a
  precompiled header. Further, it can't detect changes in **#define**s in the
  source code because of how preprocessing works in combination with
  precompiled headers.
* 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
** (for the Clang compiler) use the *-include-pch* compiler option to include
   the PCH file generated from the precompiled 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 cache directory.
* Make sure that the configuration setting *hard_link* is false (which is the
  default).
* Make sure that all users are in the same group.
* Set the configuration setting *umask* 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
  following command might be useful for this:
+
--
----
find $CCACHE_DIR -type d | xargs chmod g+s
----
--

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 a base directory is set appropriately, as
discussed in a 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 *temporary_dir* 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 letting ccache execute the compiler wrapper. This is
accomplished by defining the configuration setting *prefix_command*, for
example by setting the environment variable *CCACHE_PREFIX* to the name of the
wrapper (e.g. *distcc*). ccache will then prefix the command line with the
specified command when running the compiler. To specify several prefix
commands, set *prefix_command* to a colon-separated list of commands.

Unless you set *compiler_check* 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 *prefix_command* is used, ccache will not invoke
the other wrapper when running the preprocessor, which increases performance.
You can use the *prefix_command_cpp* configuration setting if you also want to
invoke the other wrapper when doing preprocessing (normally by adding *-E*).


Caveats
-------

* The direct mode fails to pick up new header files in some rare scenarios. See
  <<_the_direct_mode,THE DIRECT MODE>> above.


Troubleshooting
---------------

General
~~~~~~~

A general tip for getting information about what ccache is doing is to enable
debug logging by setting *log_file*. 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 preferable.

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_*,
   *-Wp,-MMD,_path_*, and *-Wp,-D_define_*) is used.
** This was the first compilation with a new value of the base directory
   setting.
** 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 *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. 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 *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, 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 *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,
   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 *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>>.


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
*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: <https://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
<https://ccache.samba.org/credits.html> for a list of contributors.