void-packages/srcpkgs/ccache/files/ccache.1

'\" t
.\"     Title: ccache
.\"    Author: [see the "AUTHOR(S)" section]
.\" Generator: Asciidoctor 2.0.15
.\"      Date: 2022-01-05
.\"    Manual: \ \&
.\"    Source: Ccache 4.5.1
.\"  Language: English
.\"
.TH "CCACHE" "1" "2022-01-05" "Ccache 4.5.1" "\ \&"
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
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.de URL
\fI\\$2\fP <\\$1>\\$3
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.\}
.SH "NAME"
ccache \- a fast C/C++ compiler cache
.SH "SYNOPSIS"
.sp
.nf
\fBccache\fP [\fIoptions\fP]
\fBccache\fP \fIcompiler\fP [\fIcompiler options\fP]
\fIcompiler\fP [\fIcompiler options\fP]                   (via symbolic link)
.fi
.br
.SH "DESCRIPTION"
.sp
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.
.sp
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 \fICaveats\fP. If you discover an undocumented case where
ccache changes the output of your compiler, please let us know.
.SH "RUN MODES"
.sp
There are two ways to use ccache. You can either prefix your compilation
commands with \fBccache\fP 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.
.sp
To use the first method, just make sure that \fBccache\fP is in your \fBPATH\fP.
.sp
To use the symlinks method, do something like this:
.sp
.if n .RS 4
.nf
.fam C
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++
.fam
.fi
.if n .RE
.sp
And so forth. This will work as long as the directory with symlinks comes
before the path to the compiler (which is usually in \fB/usr/bin\fP). After
installing you may wish to run \(lqwhich gcc\(rq to make sure that the correct link
is being used.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Warning
.ps -1
.br
.sp
The technique of letting ccache masquerade as the compiler works well,
but currently doesn\(cqt interact well with other tools that do the same thing. See
\fI[_using_ccache_with_other_compiler_wrappers]\fP.
.sp .5v
.RE
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Warning
.ps -1
.br
.sp
Use a symbolic links for masquerading, not hard links.
.sp .5v
.RE
.SH "COMMAND LINE OPTIONS"
.sp
These command line options only apply when you invoke ccache as \(lqccache\(rq.
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\(cqs documentation.
.SS "Common options"
.sp
\fB\-c\fP, \fB\-\-cleanup\fP
.RS 4
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.
.RE
.sp
\fB\-C\fP, \fB\-\-clear\fP
.RS 4
Clear the entire cache, removing all cached files, but keeping the
configuration file.
.RE
.sp
\fB\-\-config\-path\fP \fIPATH\fP
.RS 4
Let the command line options operate on configuration file \fIPATH\fP instead of
the default. Using this option has the same effect as setting the
environment variable \fBCCACHE_CONFIGPATH\fP temporarily.
.RE
.sp
\fB\-d\fP, \fB\-\-dir\fP \fIPATH\fP
.RS 4
Let the command line options operate on cache directory \fIPATH\fP instead of
the default. For example, to show statistics for a cache directory at
\fB/shared/ccache\fP you can run \fBccache \-d /shared/ccache \-s\fP. Using this option
has the same effect as setting the environment variable \fBCCACHE_DIR\fP
temporarily.
.RE
.sp
\fB\-\-evict\-namespace\fP \fINAMESPACE\fP
.RS 4
Remove files created in the given \fBnamespace\fP from the
cache.
.RE
.sp
\fB\-\-evict\-older\-than\fP \fIAGE\fP
.RS 4
Remove files older than \fIAGE\fP from the cache. \fIAGE\fP should be an unsigned
integer with a \fBd\fP (days) or \fBs\fP (seconds) suffix. If combined with
\fB\-\-evict\-namespace\fP, only remove old files within that namespace.
.RE
.sp
\fB\-h\fP, \fB\-\-help\fP
.RS 4
Print a summary of command line options.
.RE
.sp
\fB\-F\fP \fINUM\fP, \fB\-\-max\-files\fP \fINUM\fP
.RS 4
Set the maximum number of files allowed in the cache to \fINUM\fP. Use 0 for no
limit. The value is stored in a configuration file in the cache directory
and applies to all future compilations.
.RE
.sp
\fB\-M\fP \fISIZE\fP, \fB\-\-max\-size\fP \fISIZE\fP
.RS 4
Set the maximum size of the files stored in the cache. \fISIZE\fP 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.
.RE
.sp
\fB\-X\fP \fILEVEL\fP, \fB\-\-recompress\fP \fILEVEL\fP
.RS 4
Recompress the cache to level \fILEVEL\fP using the Zstandard algorithm. The
level can be an integer, with the same semantics as the
\fBcompression_level\fP configuration option), or
the special value \fBuncompressed\fP for no compression. See
\fI[_cache_compression]\fP for more information. This can potentionally take a
long time since all files in the cache need to be visited. Only files that
are currently compressed with a different level than \fILEVEL\fP will be
recompressed.
.RE
.sp
\fB\-o\fP \fIKEY=VALUE\fP, \fB\-\-set\-config\fP \fIKEY\fP=\fIVALUE\fP
.RS 4
Set configuration option \fIKEY\fP to \fIVALUE\fP. See \fI[_configuration]\fP for more
information.
.RE
.sp
\fB\-x\fP, \fB\-\-show\-compression\fP
.RS 4
Print cache compression statistics. See \fI[_cache_compression]\fP for more
information. This can potentionally take a long time since all files in the
cache need to be visited.
.RE
.sp
\fB\-p\fP, \fB\-\-show\-config\fP
.RS 4
Print current configuration options and from where they originate
(environment variable, configuration file or compile\-time default) in
human\-readable format.
.RE
.sp
\fB\-\-show\-log\-stats\fP
.RS 4
Print statistics counters from the stats log in human\-readable format. See
\fBstats_log\fP. Use \fB\-v\fP/\fB\-\-verbose\fP once or twice for
more details.
.RE
.sp
\fB\-s\fP, \fB\-\-show\-stats\fP
.RS 4
Print a summary of configuration and statistics counters in human\-readable
format. Use \fB\-v\fP/\fB\-\-verbose\fP once or twice for more details.
.RE
.sp
\fB\-v\fP, \fB\-\-verbose\fP
.RS 4
Increase verbosity. The option can be given multiple times.
.RE
.sp
\fB\-V\fP, \fB\-\-version\fP
.RS 4
Print version and copyright information.
.RE
.sp
\fB\-z\fP, \fB\-\-zero\-stats\fP
.RS 4
Zero the cache statistics (but not the configuration options).
.RE
.SS "Options for secondary storage"
.sp
\fB\-\-trim\-dir\fP \fIPATH\fP
.RS 4
Remove old files from directory \fIPATH\fP until it is at most the size specified
by \fB\-\-trim\-max\-size\fP.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Warning
.ps -1
.br
.sp
Don\(cqt use this option to trim the primary cache. To trim the primary
cache directory to a certain size, use \fBCCACHE_MAXSIZE=\fISIZE\fP ccache \-c\fP.
.sp .5v
.RE
.RE
.sp
\fB\-\-trim\-max\-size\fP \fISIZE\fP
.RS 4
Specify the maximum size for \fB\-\-trim\-dir\fP. \fISIZE\fP 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.
.RE
.sp
\fB\-\-trim\-method\fP \fIMETHOD\fP
.RS 4
Specify the method to trim a directory with \fB\-\-trim\-dir\fP. Possible values
are:
.sp
\fBatime\fP
.RS 4
LRU (least recently used) using the file access timestamp. This is the
default.
.RE
.sp
\fBmtime\fP
.RS 4
LRU (least recently used) using the file modification timestamp.
.RE
.RE
.SS "Options for scripting or debugging"
.sp
\fB\-\-checksum\-file\fP \fIPATH\fP
.RS 4
Print the checksum (128 bit XXH3) of the file at \fIPATH\fP (\fB\-\fP for standard
input).
.RE
.sp
\fB\-\-dump\-manifest\fP \fIPATH\fP
.RS 4
Dump manifest file at \fIPATH\fP (\fB\-\fP for standard input) in text format to
standard output. This is only useful when debugging ccache and its behavior.
.RE
.sp
\fB\-\-dump\-result\fP \fIPATH\fP
.RS 4
Dump result file at \fIPATH\fP (\fB\-\fP for standard input) in text format to
standard output. This is only useful when debugging ccache and its behavior.
.RE
.sp
\fB\-\-extract\-result\fP \fIPATH\fP
.RS 4
Extract data stored in the result file at \fIPATH\fP (\fB\-\fP for standard input).
The data will be written to \fBccache\-result.*\fP files in to the current
working directory. This is only useful when debugging ccache and its
behavior.
.RE
.sp
\fB\-k\fP \fIKEY\fP, \fB\-\-get\-config\fP \fIKEY\fP
.RS 4
Print the value of configuration option \fIKEY\fP. See \fI[_configuration]\fP for
more information.
.RE
.sp
\fB\-\-hash\-file\fP \fIPATH\fP
.RS 4
Print the hash (160 bit BLAKE3) of the file at \fIPATH\fP (\fB\-\fP for standard
input). This is only useful when debugging ccache and its behavior.
.RE
.sp
\fB\-\-print\-stats\fP
.RS 4
Print statistics counter IDs and corresponding values in machine\-parsable
(tab\-separated) format.
.RE
.SS "Extra options"
.sp
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
\fB\-\-ccache\-skip\fP. 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.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Note
.ps -1
.br
.sp
\fB\-\-ccache\-skip\fP currently only tells ccache not to interpret the next
option as a special compiler option \(em the option will still be included in the
direct mode hash.
.sp .5v
.RE
.sp
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 \fB\-\-ccache\-skip\fP 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.
.sp
Another case where \fB\-\-ccache\-skip\fP can be useful is if ccache interprets an
option specially but shouldn\(cqt, since the option has another meaning for your
compiler than what ccache thinks.
.SH "CONFIGURATION"
.sp
Ccache\(cqs default behavior can be overridden by options in configuration files,
which in turn can be overridden by environment variables with names starting
with \fBCCACHE_\fP. Ccache normally reads configuration from two files: first a
system\-level configuration file and secondly a cache\-specific configuration
file. The priorities of configuration options are as follows (where 1 is
highest):
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 1." 4.2
.\}
Environment variables.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 2." 4.2
.\}
The primary (cache\-specific) configuration file (see below).
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 3." 4.2
.\}
The secondary (system\-wide read\-only) configuration file
\fB<sysconfdir>/ccache.conf\fP (typically \fB/etc/ccache.conf\fP or
\fB/usr/local/etc/ccache.conf\fP).
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 4.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 4." 4.2
.\}
Compile\-time defaults.
.RE
.sp
As a special case, if the the environment variable \fBCCACHE_CONFIGPATH\fP is set
it specifies the primary configuration file and the secondary (system\-wide)
configuration file won\(cqt be read.
.SS "Location of the primary configuration file"
.sp
The location of the primary (cache\-specific) configuration is determined like
this:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 1." 4.2
.\}
If \fBCCACHE_CONFIGPATH\fP is set, use that path.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 2." 4.2
.\}
Otherwise, if the environment variable \fBCCACHE_DIR\fP is set then use
\fB$CCACHE_DIR/ccache.conf\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 3." 4.2
.\}
Otherwise, if \fBcache_dir\fP is set in the secondary
(system\-wide) configuration file then use \fB<cache_dir>/ccache.conf\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 4.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 4." 4.2
.\}
Otherwise, if there is a legacy \fB$HOME/.ccache\fP directory then use
\fB$HOME/.ccache/ccache.conf\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 5.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 5." 4.2
.\}
Otherwise, if \fBXDG_CONFIG_HOME\fP is set then use
\fB$XDG_CONFIG_HOME/ccache/ccache.conf\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 6.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 6." 4.2
.\}
Otherwise, use \fB%APPDATA%/ccache/ccache.conf\fP (Windows),
\fB$HOME/Library/Preferences/ccache/ccache.conf\fP (macOS) or
\fB$HOME/.config/ccache/ccache.conf\fP (other systems).
.RE
.SS "Configuration file syntax"
.sp
Configuration files are in a simple \(lqkey = value\(rq format, one option per
line. Lines starting with a hash sign are comments. Blank lines are ignored, as
is whitespace surrounding keys and values. Example:
.sp
.if n .RS 4
.nf
.fam C
# Set maximum cache size to 10 GB:
max_size = 10G
.fam
.fi
.if n .RE
.SS "Boolean values"
.sp
Some configuration options are boolean values (i.e. truth values). In a
configuration file, such values must be set to the string \fBtrue\fP or \fBfalse\fP.
For the corresponding environment variables, the semantics are a bit different:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
A set environment variable means \(lqtrue\(rq (even if set to the empty string).
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The following case\-insensitive negative values are considered an error
(instead of surprising the user): \fB0\fP, \fBfalse\fP, \fBdisable\fP and \fBno\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
An unset environment variable means \(lqfalse\(rq.
.RE
.sp
Each boolean environment variable also has a negated form starting with
\fBCCACHE_NO\fP. For example, \fBCCACHE_COMPRESS\fP can be set to force compression and
\fBCCACHE_NOCOMPRESS\fP can be set to force no compression.
.SS "Configuration options"
.sp
Below is a list of available configuration options. The corresponding
environment variable name is indicated in parentheses after each configuration
option key.
.sp
\fBabsolute_paths_in_stderr\fP (\fBCCACHE_ABSSTDERR\fP)
.RS 4
This option specifies whether ccache should rewrite relative paths in the
compiler\(cqs standard error output to absolute paths. This can be useful if
you use \fBbase_dir\fP with a build system (e.g. CMake with
the "Unix Makefiles" generator) that executes the compiler in a different
working directory, which makes relative paths in compiler errors or
warnings incorrect. The default is false.
.RE
.sp
\fBbase_dir\fP (\fBCCACHE_BASEDIR\fP)
.RS 4
This option should be an absolute path to a directory. If set, ccache will
rewrite absolute paths into paths relative to the current working directory,
but only absolute paths that begin with \fBbase_dir\fP. Cache results can then
be shared for compilations in different directories even if the project uses
absolute paths in the compiler command line. See also the discussion under
\fI[_compiling_in_different_directories]\fP. If set to the empty string (which
is the default), no rewriting is done.
.sp
A typical path to use as \fBbase_dir\fP is your home directory or another directory
that is a parent of your project directories. Don\(cqt use \fB/\fP as the base
directory since that will make ccache also rewrite paths to system header
files, which typically is contraproductive.
.sp
For example, say that Alice\(cqs current working directory is
\fB/home/alice/project1/build\fP and that she compiles like this:
.sp
.if n .RS 4
.nf
.fam C
ccache gcc \-I/usr/include/example \-I/home/alice/project2/include \-c /home/alice/project1/src/example.c
.fam
.fi
.if n .RE
.sp
Here is what ccache will actually execute for different \fBbase_dir\fP values:
.sp
.if n .RS 4
.nf
.fam C
# Current working directory: /home/alice/project1/build

# With base_dir = /:
gcc \-I../../../../usr/include/example \-I../../project2/include \-c ../src/example.c

# With base_dir = /home or /home/alice:
gcc \-I/usr/include/example \-I../../project2/include \-c ../src/example.c

# With base_dir = /home/alice/project1 or /home/alice/project1/src:
gcc \-I/usr/include/example \-I/home/alice/project2/include \-c ../src/example.c
.fam
.fi
.if n .RE
.sp
If Bob has put \fBproject1\fP and \fBproject2\fP in \fB/home/bob/stuff\fP and both users
have set \fBbase_dir\fP to \fB/home\fP or \fB/home/$USER\fP, then Bob will get a cache hit
(if they share ccache directory) since the actual command line will be
identical to that of Alice:
.sp
.if n .RS 4
.nf
.fam C
# Current working directory: /home/bob/stuff/project1/build

# With base_dir = /home or /home/bob:
gcc \-I/usr/include/example \-I../../project2/include \-c ../src/example.c
.fam
.fi
.if n .RE
.sp
Without \fBbase_dir\fP there will be a cache miss since the absolute paths will
differ. With \fBbase_dir\fP set to \fB/\fP there will be a cache miss since the
relative path to \fB/usr/include/example\fP will be different. With \fBbase_dir\fP set
to \fB/home/bob/stuff/project1\fP there will a cache miss since the path to
project2 will be a different absolute path.
.RE
.sp
\fBcache_dir\fP (\fBCCACHE_DIR\fP)
.RS 4
This option specifies where ccache will keep its cached compiler outputs.
The default is \fB$XDG_CACHE_HOME/ccache\fP if \fBXDG_CACHE_HOME\fP is set,
otherwise \fB$HOME/.cache/ccache\fP. Exception: If the legacy directory
\fB$HOME/.ccache\fP exists then that directory is the default.
.sp
See also \fI[_location_of_the_primary_configuration_file]\fP.
.sp
If you want to use another \fBCCACHE_DIR\fP value temporarily for one ccache
invocation you can use the \fB\-d\fP/\fB\-\-dir\fP command line option instead.
.RE
.sp
\fBcompiler\fP (\fBCCACHE_COMPILER\fP or (deprecated) \fBCCACHE_CC\fP)
.RS 4
This option 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.
.RE
.sp
\fBcompiler_check\fP (\fBCCACHE_COMPILERCHECK\fP)
.RS 4
By default, ccache includes the modification time (\(lqmtime\(rq) and size of
the compiler in the hash to ensure that results retrieved from the cache
are accurate. This option can be used to select another strategy. Possible
values are:
.sp
\fBcontent\fP
.RS 4
Hash the content of the compiler binary. This makes ccache very slightly
slower compared to \fBmtime\fP, but makes it cope better with compiler upgrades
during a build bootstrapping process.
.RE
.sp
\fBmtime\fP
.RS 4
Hash the compiler\(cqs mtime and size, which is fast. This is the default.
.RE
.sp
\fBnone\fP
.RS 4
Don\(cqt hash anything. This may be good for situations where you can safely
use the cached results even though the compiler\(cqs mtime or size has changed
(e.g. if the compiler is built as part of your build system and the
compiler\(cqs source has not changed, or if the compiler only has changes that
don\(cqt affect code generation). You should only use \fBnone\fP if you know what
you are doing.
.RE
.sp
\fBstring:value\fP
.RS 4
Hash \fBvalue\fP. This can for instance be a compiler revision number or
another string that the build system generates to identify the compiler.
.RE
.sp
\fIa command string\fP
.RS 4
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 \fB%compiler%\fP will be replaced with the
path to the compiler. Several commands can be specified with semicolon as
separator. Examples:
.sp
.if n .RS 4
.nf
.fam C
%compiler% \-v
.fam
.fi
.if n .RE
.sp
.if n .RS 4
.nf
.fam C
%compiler% \-dumpmachine; %compiler% \-dumpversion
.fam
.fi
.if n .RE
.sp
You should make sure that the specified command is as fast as possible since it
will be run once for each ccache invocation.
.sp
Identifying the compiler using a command is useful if you want to avoid cache
misses when the compiler has been rebuilt but not changed.
.sp
Another case is when the compiler (as seen by ccache) actually isn\(cqt the real
compiler but another compiler wrapper \(em in that case, the default \fBmtime\fP
method will hash the mtime and size of the other compiler wrapper, which means
that ccache won\(cqt be able to detect a compiler upgrade. Using a suitable command
to identify the compiler is thus safer, but it\(cqs also slower, so you should
consider continue using the \fBmtime\fP method in combination with the
\fBprefix_command\fP option if possible. See
\fI[_using_ccache_with_other_compiler_wrappers]\fP.
.RE
.RE
.sp
\fBcompiler_type\fP (\fBCCACHE_COMPILERTYPE\fP)
.RS 4
Ccache normally guesses the compiler type based on the compiler name. The
\fBcompiler_type\fP option lets you force a compiler type. This can be useful
if the compiler has a non\-standard name but is actually one of the known
compiler types. Possible values are:
.sp
\fBauto\fP
.RS 4
Guess one of the types below based on the compiler name (following
symlinks). This is the default.
.RE
.sp
\fBclang\fP
.RS 4
Clang\-based compiler.
.RE
.sp
\fBgcc\fP
.RS 4
GCC\-based compiler.
.RE
.sp
\fBnvcc\fP
.RS 4
NVCC (CUDA) compiler.
.RE
.sp
\fBother\fP
.RS 4
Any compiler other than the known types.
.RE
.sp
\fBpump\fP
.RS 4
distcc\(cqs \(lqpump\(rq script.
.RE
.RE
.sp
\fBcompression\fP (\fBCCACHE_COMPRESS\fP or \fBCCACHE_NOCOMPRESS\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will compress data it puts in the cache. However, this
option has no effect on how files are retrieved from the cache; compressed
and uncompressed results will still be usable regardless of this option.
The default is true.
.sp
Compression is done using the Zstandard algorithm. The algorithm is fast enough
that there should be little reason to turn off compression to gain performance.
One exception is if the cache is located on a compressed file system, in which
case the compression performed by ccache of course is redundant.
.sp
Compression will be disabled if file cloning (the
\fBfile_clone\fP option) or hard linking (the
\fBhard_link\fP option) is enabled.
.RE
.sp
\fBcompression_level\fP (\fBCCACHE_COMPRESSLEVEL\fP)
.RS 4
This option determines the level at which ccache will compress object files
using the real\-time compression algorithm Zstandard. It only has effect if
\fBcompression\fP is enabled (which it is by default).
Zstandard is extremely fast for decompression and very fast for compression
for lower compression levels. The default is 0.
.sp
Semantics of \fBcompression_level\fP:
.sp
\fB> 0\fP
.RS 4
A positive value corresponds to normal Zstandard compression levels. Lower
levels (e.g. \fB1\fP) mean faster compression but worse compression ratio.
Higher levels (e.g. \fB19\fP) mean slower compression but better compression
ratio. The maximum possible value depends on the libzstd version, but at
least up to 19 is available for all versions. Decompression speed is
essentially the same for all levels. As a rule of thumb, use level 5 or
lower since higher levels may slow down compilations noticeably. Higher
levels are however useful when recompressing the cache with command line
option \fB\-X\fP/\fB\-\-recompress\fP.
.RE
.sp
\fB< 0\fP
.RS 4
A negative value corresponds to Zstandard\(cqs \(lqultra\-fast\(rq compression
levels, which are even faster than level 1 but with less good compression
ratios. For instance, level \fB\-3\fP corresponds to \fB\-\-fast=3\fP for the \fBzstd\fP
command line tool. In practice, there is little use for levels lower than
\fB\-5\fP or so.
.RE
.sp
\fB0\fP (default)
.RS 4
The value \fB0\fP means that ccache will choose a suitable level, currently
\fB1\fP.
.RE
.sp
See the \c
.URL "http://zstd.net" "Zstandard documentation" ""
for more information.
.RE
.sp
\fBcpp_extension\fP (\fBCCACHE_EXTENSION\fP)
.RS 4
This option 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\(cqt work. For example, when using the
\(lqaCC\(rq compiler on HP\-UX, set the cpp extension to \fBi\fP.
.RE
.sp
\fBdebug\fP (\fBCCACHE_DEBUG\fP or \fBCCACHE_NODEBUG\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, enable the debug mode. The debug mode creates per\-object debug
files that are helpful when debugging unexpected cache misses. Note however
that ccache performance will be reduced slightly. See \fI[_cache_debugging]\fP
for more information. The default is false.
.RE
.sp
\fBdebug_dir\fP (\fBCCACHE_DEBUGDIR\fP)
.RS 4
Specifies where to write per\-object debug files if the debug
mode is enabled. If set to the empty string, the files will be written
next to the object file. If set to a directory, the debug files will be
written with full absolute paths in that directory, creating it if needed.
The default is the empty string.
.sp
For example, if \fBdebug_dir\fP is set to \fB/example\fP, the current working directory
is \fB/home/user\fP and the object file is \fBbuild/output.o\fP then the debug log will
be written to \fB/example/home/user/build/output.o.ccache\-log\fP. See also
\fI[_cache_debugging]\fP.
.RE
.sp
\fBdepend_mode\fP (\fBCCACHE_DEPEND\fP or \fBCCACHE_NODEPEND\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, the depend mode will be used. The default is false. See
\fI[_the_depend_mode]\fP.
.RE
.sp
\fBdirect_mode\fP (\fBCCACHE_DIRECT\fP or \fBCCACHE_NODIRECT\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, the direct mode will be used. The default is true. See
\fI[_the_direct_mode]\fP.
.RE
.sp
\fBdisable\fP (\fBCCACHE_DISABLE\fP or \fBCCACHE_NODISABLE\fP, see \fI[_boolean_values]\fP above)
.RS 4
When true, ccache will just call the real compiler, bypassing the cache
completely. The default is false.
.RE
.sp
\fBextra_files_to_hash\fP (\fBCCACHE_EXTRAFILES\fP)
.RS 4
This option 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.
.RE
.sp
\fBfile_clone\fP (\fBCCACHE_FILECLONE\fP or \fBCCACHE_NOFILECLONE\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will attempt to use file cloning (also known as \(lqcopy on
write\(rq, \(lqCoW\(rq or \(lqreflinks\(rq) to store and fetch cached compiler
results. \fBfile_clone\fP has priority over \fBhard_link\fP.
The default is false.
.sp
Files stored by cloning cannot be compressed, so the cache size will likely be
significantly larger if this option is enabled. However, performance may be
improved depending on the use case.
.sp
Unlike the \fBhard_link\fP option, \fBfile_clone\fP is completely
safe to use, but not all file systems support the feature. For such file
systems, ccache will fall back to use plain copying (or hard links if
\fBhard_link\fP is enabled).
.RE
.sp
\fBhard_link\fP (\fBCCACHE_HARDLINK\fP or \fBCCACHE_NOHARDLINK\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will attempt to use hard links to store and fetch cached
object files. The default is false.
.sp
Files stored via hard links cannot be compressed, so the cache size will likely
be significantly larger if this option is enabled. However, performance may be
improved depending on the use case.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Warning
.ps -1
.br
.sp
Do not enable this option unless you are aware of these caveats:
.sp .5v
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If the resulting file is modified, the file in the cache will also be
modified since they share content, which corrupts the cache entry. As of
version 4.0, ccache makes stored and fetched object files read\-only as a
safety measure guard. Furthermore, a simple integrity check is made for
cached object files by verifying that their sizes are correct. This means
that mistakes like \fBstrip file.o\fP or \fBecho >file.o\fP will be detected even if
the object file is made writeable, but a modification that doesn\(cqt change the
file size will not.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Programs that don\(cqt expect that files from two different identical
compilations are hard links to each other can fail.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Programs that rely on modification times (like \fBmake\fP) can be confused if
several users (or one user with several build trees) use the same cache
directory. The reason for this is that the object files share i\-nodes and
therefore modification times. If \fBfile.o\fP is in build tree \fBA\fP (hard\-linked
from the cache) and \fBfile.o\fP then is produced by ccache in build tree \fBB\fP by
hard\-linking from the cache, the modification timestamp will be updated for
\fBfile.o\fP in build tree \fBA\fP as well. This can retrigger relinking in build tree
\fBA\fP even though nothing really has changed.
.RE
.RE
.sp
\fBhash_dir\fP (\fBCCACHE_HASHDIR\fP or \fBCCACHE_NOHASHDIR\fP, see \fI[_boolean_values]\fP above)
.RS 4
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 \fB\-g\fP with variations).
Exception: The CWD will not be included in the hash if
\fBbase_dir\fP is set (and matches the CWD) and the
compiler option \fB\-fdebug\-prefix\-map\fP is used. See also the discussion under
\fI[_compiling_in_different_directories]\fP.
.sp
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.
.sp
You can disable this option to get cache hits when compiling the same source
code in different directories if you don\(cqt mind that CWD in the debug info
might be incorrect.
.RE
.sp
\fBignore_headers_in_manifest\fP (\fBCCACHE_IGNOREHEADERS\fP)
.RS 4
This option is a list of paths to files (or directories with headers) that
ccache will \fBnot\fP 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.
.RE
.sp
\fBignore_options\fP (\fBCCACHE_IGNOREOPTIONS\fP)
.RS 4
This option is a space\-delimited list of compiler options that ccache will
exclude from the hash. Excluding a compiler option from the hash can be
useful when you know it doesn\(cqt affect the result (but ccache doesn\(cqt know
that), or when it does and you don\(cqt care. If a compiler option in the list
is suffixed with an asterisk (\fB*\fP) it will be matched as a prefix. For
example, \fB\-fmessage\-length=*\fP will match both \fB\-fmessage\-length=20\fP and
\fB\-fmessage\-length=70\fP.
.RE
.sp
\fBinode_cache\fP (\fBCCACHE_INODECACHE\fP or \fBCCACHE_NOINODECACHE\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, enables caching of source file hashes based on device, inode and
timestamps. This will reduce the time spent on hashing included files as
the result can be resused between compilations.
.sp
The feature is still experimental and thus off by default. It is currently not
available on Windows.
.sp
The feature requires \fBtemporary_dir\fP to be located on a local filesystem.
.RE
.sp
\fBkeep_comments_cpp\fP (\fBCCACHE_COMMENTS\fP or \fBCCACHE_NOCOMMENTS\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will not discard the comments before hashing preprocessor
output. This can be used to check documentation with \fB\-Wdocumentation\fP.
.RE
.sp
\fBlimit_multiple\fP (\fBCCACHE_LIMIT_MULTIPLE\fP)
.RS 4
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
\fI[_automatic_cleanup]\fP for more information.
.RE
.sp
\fBlog_file\fP (\fBCCACHE_LOGFILE\fP)
.RS 4
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.
.sp
If set to \fBsyslog\fP, ccache will log using \fBsyslog()\fP instead of to a file. If
you use rsyslogd, you can add something like this to \fB/etc/rsyslog.conf\fP or a
file in \fB/etc/rsyslog.d\fP:
.sp
.if n .RS 4
.nf
.fam C
# log ccache to file
:programname, isequal, "ccache"         /var/log/ccache
# remove from syslog
& ~
.fam
.fi
.if n .RE
.RE
.sp
\fBmax_files\fP (\fBCCACHE_MAXFILES\fP)
.RS 4
This option specifies the maximum number of files to keep in the cache. Use
0 for no limit (which is the default). See also \fI[_cache_size_management]\fP.
.RE
.sp
\fBmax_size\fP (\fBCCACHE_MAXSIZE\fP)
.RS 4
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
\fI[_cache_size_management]\fP.
.RE
.sp
\fBnamespace\fP (\fBCCACHE_NAMESPACE\fP)
.RS 4
    If set, the namespace string will be added to the hashed data for each
compilation. This will make the associated cache entries logically separate
from cache entries with other namespaces, but they will still share the same
storage space. Cache entries can also be selectively removed from the
primary cache with the command line option \fB\-\-evict\-namespace\fP, potentially
in combination with \fB\-\-evict\-older\-than\fP.
\&.
For instance, if you use the same primary cache for several disparate projects,
you can use a unique namespace string for each one. This allows you to remove
cache entries that belong to a certain project if stop working with that
project.
.RE
.sp
\fBpath\fP (\fBCCACHE_PATH\fP)
.RS 4
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 \fBPATH\fP that isn\(cqt a symbolic link
to ccache itself.
.RE
.sp
\fBpch_external_checksum\fP (\fBCCACHE_PCH_EXTSUM\fP or \fBCCACHE_NOPCH_EXTSUM\fP, see \fI[_boolean_values]\fP above)
.RS 4
When this option is set, and ccache finds a precompiled header file,
ccache will look for a file with the extension \(lq.sum\(rq added
(e.g. \(lqpre.h.gch.sum\(rq), and if found, it will hash this file instead
of the precompiled header itself to work around the performance
penalty of hashing very large files.
.RE
.sp
\fBprefix_command\fP (\fBCCACHE_PREFIX\fP)
.RS 4
This option adds a list of prefixes (separated by space) to the command line
that ccache uses when invoking the compiler. See also
\fI[_using_ccache_with_other_compiler_wrappers]\fP.
.RE
.sp
\fBprefix_command_cpp\fP (\fBCCACHE_PREFIX_CPP\fP)
.RS 4
This option adds a list of prefixes (separated by space) to the command
line that ccache uses when invoking the preprocessor.
.RE
.sp
\fBread_only\fP (\fBCCACHE_READONLY\fP or \fBCCACHE_NOREADONLY\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will attempt to use existing cached results, but it will not
add new results to any cache backend. Statistics counters will still be
updated, though, unless the \fBstats\fP option is set to
\fBfalse\fP.
.sp
If you are using this because your ccache directory is read\-only, you need to
set \fBtemporary_dir\fP since ccache will fail to create
temporary files otherwise. You may also want to set \fBstats\fP to
\fBfalse\fP make ccache not even try to update stats files.
.RE
.sp
\fBread_only_direct\fP (\fBCCACHE_READONLY_DIRECT\fP or \fBCCACHE_NOREADONLY_DIRECT\fP, see \fI[_boolean_values]\fP above)
.RS 4
Just like \fBread_only\fP except that ccache will only try
to retrieve results from the cache using the direct mode, not the
preprocessor mode. See documentation for \fBread_only\fP
regarding using a read\-only ccache directory.
.RE
.sp
\fBrecache\fP (\fBCCACHE_RECACHE\fP or \fBCCACHE_NORECACHE\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will not use any previously stored result. New results will
still be cached, possibly overwriting any pre\-existing results.
.RE
.sp
\fBreshare\fP (\fBCCACHE_RESHARE\fP or \fBCCACHE_NORESHARE\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will write results to secondary storage even for primary
storage cache hits. The default is false.
.RE
.sp
\fBrun_second_cpp\fP (\fBCCACHE_CPP2\fP or \fBCCACHE_NOCPP2\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will first run the preprocessor to preprocess the source
code (see \fI[_the_preprocessor_mode]\fP) and then on a cache miss run the
compiler on the source code to get hold of the object file. This is the
default.
.sp
If false, ccache will first run preprocessor to preprocess the source code and
then on a cache miss run the compiler on the \fIpreprocessed source code\fP 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\(cqt produce the same result (for instance diagnostics warnings)
when compiling preprocessed source code.
.sp
A solution to the above mentioned downside is to set \fBrun_second_cpp\fP to false
and pass \fB\-fdirectives\-only\fP (for GCC) or \fB\-frewrite\-includes\fP (for Clang) to
the compiler. This will cause the compiler to leave the macros and other
preprocessor information, and only process the \fB#include\fP directives. When run
in this way, the preprocessor arguments will be passed to the compiler since it
still has to do \fIsome\fP preprocessing (like macros).
.RE
.sp
\fBsecondary_storage\fP (\fBCCACHE_SECONDARY_STORAGE\fP)
.RS 4
This option specifies one or several storage backends (separated by space)
to query after the primary cache storage. See
\fI[_secondary_storage_backends]\fP for documentation of syntax and available
backends.
.sp
Examples:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBfile:/shared/nfs/directory\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBfile:///shared/nfs/one|read\-only file:///shared/nfs/two\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBhttp://example.com/cache\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBredis://example.com\fP
.RE
.RE
.sp
\fBsloppiness\fP (\fBCCACHE_SLOPPINESS\fP)
.RS 4
By default, ccache tries to give as few false cache hits as possible.
However, in certain situations it\(cqs possible that you know things that
ccache can\(cqt take for granted. This option 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 one or several of the following
values:
.sp
\fBclang_index_store\fP
.RS 4
Ignore the Clang compiler option \fB\-index\-store\-path\fP and its argument when
computing the manifest hash. This is useful if you use Xcode, which uses an
index store path derived from the local project path. Note that the index
store won\(cqt be updated correctly on cache hits if you enable this
sloppiness.
.RE
.sp
\fBfile_stat_matches\fP
.RS 4
Ccache normally examines a file\(cqs contents to determine whether it matches
the cached version. With this sloppiness set, ccache will consider a file
as matching its cached version if the mtimes and ctimes match.
.RE
.sp
\fBfile_stat_matches_ctime\fP
.RS 4
Ignore ctimes when \fBfile_stat_matches\fP is enabled. This can be useful when
backdating files\(aq mtimes in a controlled way.
.RE
.sp
\fBinclude_file_ctime\fP
.RS 4
By default, ccache will not cache a file if it includes a header whose ctime
is too new. This sloppiness disables that check. See also
\fI[_handling_of_newly_created_header_files]\fP.
.RE
.sp
\fBinclude_file_mtime\fP
.RS 4
By default, ccache will not cache a file if it includes a header whose mtime
is too new. This sloppiness disables that check. See also
\fI[_handling_of_newly_created_header_files]\fP.
.RE
.sp
\fBivfsoverlay\fP
.RS 4
Ignore the Clang compiler option \fB\-ivfsoverlay\fP and its argument. This is
useful if you use Xcode, which uses a virtual file system (VFS) for things
like combining Objective\-C and Swift code.
.RE
.sp
\fBlocale\fP
.RS 4
Ccache includes the environment variables \fBLANG\fP, \fBLC_ALL\fP, \fBLC_CTYPE\fP and
\fBLC_MESSAGES\fP in the hash by default since they may affect localization of
compiler warning messages. Set this sloppiness to tell ccache not to do
that.
.RE
.sp
\fBpch_defines\fP
.RS 4
Be sloppy about \fB#define\fP directives when precompiling a header file. See
\fI[_precompiled_headers]\fP for more information.
.RE
.sp
\fBmodules\fP
.RS 4
By default, ccache will not cache compilations if \fB\-fmodules\fP is used since
it cannot hash the state of compiler\(cqs internal representation of relevant
modules. This sloppiness allows caching in such a case. See
\fI[_c_modules]\fP for more information.
.RE
.sp
\fBsystem_headers\fP
.RS 4
By default, ccache will also include all system headers in the manifest.
With this sloppiness set, ccache will only include system headers in the
hash but not add the system header files to the list of include files.
.RE
.sp
\fBtime_macros\fP
.RS 4
Ignore \fB__DATE__\fP, \fB__TIME__\fP and \fB__TIMESTAMP__\fP being present in the
source code.
.RE
.sp
See the discussion under \fI[_troubleshooting]\fP for more information.
.RE
.sp
\fBstats\fP (\fBCCACHE_STATS\fP or \fBCCACHE_NOSTATS\fP, see \fI[_boolean_values]\fP above)
.RS 4
If true, ccache will update the statistics counters on each compilation.
The default is true.
.RE
.sp
\fBstats_log\fP (\fBCCACHE_STATSLOG\fP)
.RS 4
If set to a file path, ccache will write statistics counter updates to the
specified file. This is useful for getting statistics for individual builds.
To show a summary of the current stats log, use \fBccache \-\-show\-log\-stats\fP.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Note
.ps -1
.br
.sp
Lines in the stats log starting with a hash sign (\fB#\fP) are comments.
.sp .5v
.RE
.RE
.sp
\fBtemporary_dir\fP (\fBCCACHE_TEMPDIR\fP)
.RS 4
This option specifies where ccache will put temporary files. The default is
\fB/run/user/<UID>/ccache\-tmp\fP if \fB/run/user/<UID>\fP exists, otherwise
\fB<cache_dir>/tmp\fP.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Note
.ps -1
.br
.sp
In previous versions of ccache, \fBCCACHE_TEMPDIR\fP had to be on the same
filesystem as the \fBCCACHE_DIR\fP path, but this requirement has been relaxed.
.sp .5v
.RE
.RE
.sp
\fBumask\fP (\fBCCACHE_UMASK\fP)
.RS 4
This option (an octal integer) specifies the umask for files and directories
in the cache directory. This is mostly useful when you wish to share your
cache with other users.
.RE
.SH "SECONDARY STORAGE BACKENDS"
.sp
The \fBsecondary_storage\fP option lets you configure
ccache to use one or several other storage backends in addition to the primary
cache storage located in \fBcache_dir\fP. Note that cache
statistics counters will still be kept in the primary cache directory \(em secondary storage backends only store cache results and manifests.
.sp
A secondary storage backend is specified with a URL, optionally followed by a
pipe (\fB|\fP) and a pipe\-separated list of attributes. An attribute is
\fIkey\fP=\fIvalue\fP or just \fIkey\fP as a short form of \fIkey\fP=\fBtrue\fP. Attribute values
must be \c
.URL "https://en.wikipedia.org/wiki/Percent\-encoding" "percent\-encoded" ""
if they
contain percent, pipe or space characters.
.SS "Attributes for all backends"
.sp
These optional attributes are available for all secondary storage backends:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBread\-only\fP: If \fBtrue\fP, only read from this backend, don\(cqt write. The default
is \fBfalse\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBshards\fP: A comma\-separated list of names for sharding (partitioning) the
cache entries using
.URL "https://en.wikipedia.org/wiki/Rendezvous_hashing" "Rendezvous hashing" ","
typically to spread the cache over a server cluster. When set, the storage URL
must contain an asterisk (\fB*\fP), which will be replaced by one of the shard
names to form a real URL. A shard name can optionally have an appended weight
within parentheses to indicate how much of the key space should be associated
with that shard. A shard with weight \fBw\fP will contain \fBw\fP/\fBS\fP of the cache,
where \fBS\fP is the sum of all shard weights. A weight could for instance be set
to represent the available memory for a memory cache on a specific server. The
default weight is \fB1\fP.
.sp
Examples:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBredis://cache\-*.example.com|shards=a(3),b(1),c(1.5)\fP will put 55% (3/5.5)
of the cache on \fBredis://cache\-a.example.com\fP, 18% (1/5.5) on
\fBredis://cache\-b.example.com\fP and 27% (1.5/5.5) on
\fBredis://cache\-c.example.com\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBhttp://example.com/*|shards=alpha,beta\fP will put 50% of the cache on
\fBhttp://example.com/alpha\fP and 50% on \fBhttp://example.com/beta\fP.
.RE
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBshare\-hits\fP: If \fBtrue\fP, write hits for this backend to primary storage. The
default is \fBtrue\fP.
.RE
.SS "Storage interaction"
.sp
The table below describes the interaction between primary and secondary storage
on cache hits and misses:
.TS
allbox tab(:);
lt lt lt.
T{
.sp
\fBPrimary storage\fP
T}:T{
.sp
\fBSecondary storage\fP
T}:T{
.sp
\fBWhat happens\fP
T}
T{
.sp
miss
T}:T{
.sp
miss
T}:T{
.sp
Compile, write to primary, write to secondary[1]
T}
T{
.sp
miss
T}:T{
.sp
hit
T}:T{
.sp
Read from secondary, write to primary[2]
T}
T{
.sp
hit
T}:T{
.sp
\-
T}:T{
.sp
Read from primary, don\(cqt write to secondary[3]
T}
.TE
.sp
.sp
[1] Unless secondary storage has attribute \fBread\-only=true\fP.
.br
[2] Unless secondary storage has attribute \fBshare\-hits=false\fP.
.br
[3] Unless primary storage is set to share its cache hits with the
\fBreshare\fP option.
.SS "File storage backend"
.sp
URL format: \fBfile:DIRECTORY\fP or \fBfile://DIRECTORY\fP
.sp
This backend stores data as separate files in a directory structure below
\fBDIRECTORY\fP (an absolute path), similar (but not identical) to the primary cache
storage. A typical use case for this backend would be sharing a cache on an NFS
directory.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Important
.ps -1
.br
.sp
ccache will not perform any cleanup of the storage \(em that has to be
done by other means, for instance by running \fBccache \-\-trim\-dir\fP periodically.
.sp .5v
.RE
.sp
Examples:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBfile:/shared/nfs/directory\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBfile:///shared/nfs/directory|umask=002|update\-mtime=true\fP
.RE
.sp
Optional attributes:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBlayout\fP: How to store file under the cache directory. Available values:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBflat\fP: Store all files directly under the cache directory.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBsubdirs\fP: Store files in 256 subdirectories of the cache directory.
.RE
.sp
The default is \fBsubdirs\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBumask\fP: This attribute (an octal integer) overrides the umask to use for
files and directories in the cache directory.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBupdate\-mtime\fP: If \fBtrue\fP, update the modification time (mtime) of cache
entries that are read. The default is \fBfalse\fP.
.RE
.SS "HTTP storage backend"
.sp
URL format: \fBhttp://HOST[:PORT][/PATH]\fP
.sp
This backend stores data in an HTTP\-compatible server. The required HTTP methods
are \fBGET\fP, \fBPUT\fP and \fBDELETE\fP.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Important
.ps -1
.br
.sp
ccache will not perform any cleanup of the storage \(em that has to be
done by other means, for instance by running \fBccache \-\-trim\-dir\fP periodically.
.sp .5v
.RE
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Note
.ps -1
.br
.sp
HTTPS is not supported.
.sp .5v
.RE
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Tip
.ps -1
.br
.sp
See \c
.URL "https://ccache.dev/howto/http\-storage.html" "How to set up HTTP storage"
for hints on how to set up an HTTP server for use with ccache.
.sp .5v
.RE
.sp
Examples:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBhttp://localhost\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBhttp://someusername:p4ssw0rd@example.com/cache/\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBhttp://localhost:8080|layout=bazel|connect\-timeout=50\fP
.RE
.sp
Optional attributes:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBconnect\-timeout\fP: Timeout (in ms) for network connection. The default is 100.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBkeep\-alive\fP: If \fBtrue\fP, keep the HTTP connection to the storage server open
to avoid reconnects. The default is \fBfalse\fP.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Note
.ps -1
.br
.sp
Connection keep\-alive is disabled by default because with the current
HTTP implementation uploads to the remote end might fail in case the server
closes the connection due to a keep\-alive timeout. If the general case with
short compilation times should be accelerated or the server is configured with
a long\-enough timeout, then connection keep\-alive could be enabled.
.sp .5v
.RE
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBlayout\fP: How to map key names to the path part of the URL. Available values:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBbazel\fP: Store values in a format compatible with the Bazel HTTP caching
protocol. More specifically, the entries will be stored as 64 hex digits
under the \fB/ac/\fP part of the cache.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Note
.ps -1
.br
.sp
You may have to disable verification of action cache values in the server
for this to work since ccache entries are not valid action result metadata
values.
.sp .5v
.RE
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBflat\fP: Append the key directly to the path part of the URL (with a leading
slash if needed).
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBsubdirs\fP: Append the first two characters of the key to the URL (with a
leading slash if needed), followed by a slash and the rest of the key. This
divides the entries into 256 buckets.
.RE
.sp
The default is \fBsubdirs\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBoperation\-timeout\fP: Timeout (in ms) for HTTP requests. The default is 10000.
.RE
.SS "Redis storage backend"
.sp
URL format: \fBredis://[[USERNAME:]PASSWORD@]HOST[:PORT][/DBNUMBER]\fP
.sp
This backend stores data in a \c
.URL "https://redis.io" "Redis" ""
(or Redis\-compatible)
server. There are implementations for both memory\-based and disk\-based storage.
\fBPORT\fP defaults to \fB6379\fP and \fBDBNUMBER\fP defaults to \fB0\fP.
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Note
.ps -1
.br
.sp
ccache will not perform any cleanup of the Redis storage, but you can
.URL "https://redis.io/topics/lru\-cache" "configure LRU eviction" "."
.sp .5v
.RE
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Tip
.ps -1
.br
.sp
See \c
.URL "https://ccache.dev/howto/redis\-storage.html" "How to set up Redis
storage"  for hints on setting up a Redis server for use with ccache.
.sp .5v
.RE
.if n .sp
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
.B Tip
.ps -1
.br
.sp
You can set up a cluster of Redis servers using the \fBshards\fP attribute
described in \fI[_secondary_storage_backends]\fP.
.sp .5v
.RE
.sp
Examples:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBredis://localhost\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBredis://p4ssw0rd@cache.example.com:6379/0|connect\-timeout=50\fP
.RE
.sp
Optional attributes:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBconnect\-timeout\fP: Timeout (in ms) for network connection. The default is 100.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBoperation\-timeout\fP: Timeout (in ms) for Redis commands. The default is 10000.
.RE
.SH "CACHE SIZE MANAGEMENT"
.sp
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 command
line options \fB\-M\fP/\fB\-\-max\-size\fP and \fB\-F\fP/\fB\-\-max\-files\fP. Use the
\fB\-s\fP/\fB\-\-show\-stats\fP option to see the cache size and the currently configured
limits (in addition to other various statistics).
.sp
Cleanup can be triggered in two different ways: automatic and manual.
.SS "Automatic cleanup"
.sp
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.
.sp
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 \fBmax_size / 16\fP or the file number counter is
greater than \fBmax_files / 16\fP, automatic cleanup is triggered.
.sp
When automatic cleanup is triggered for a subdirectory in the cache, ccache
will:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 1." 4.2
.\}
Count all files in the subdirectory and compute their aggregated size.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 2." 4.2
.\}
Remove files in LRU (least recently used) order until the size is at most
\fBlimit_multiple * max_size / 16\fP and the number of files is at most
\fBlimit_multiple * max_files / 16\fP, where
\fBlimit_multiple\fP, \fBmax_size\fP
and \fBmax_files\fP are configuration options.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 3." 4.2
.\}
Set the size and file number counters to match the files that were kept.
.RE
.sp
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.
.SS "Manual cleanup"
.sp
You can run \fBccache \-c/\-\-cleanup\fP 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 configuration options \fBmax_size\fP and
\fBmax_files\fP are not exceeded. Note that
\fBlimit_multiple\fP is not taken into account for manual
cleanup.
.SH "CACHE COMPRESSION"
.sp
Ccache will by default compress all data it puts into the cache using the
compression algorithm \c
.URL "http://zstd.net" "Zstandard" ""
(zstd) using compression level
1. The algorithm is fast enough that there should be little reason to turn off
compression to gain performance. One exception is if the cache is located on a
compressed file system, in which case the compression performed by ccache of
course is redundant. See the documentation for the configuration options
\fBcompression\fP and
\fBcompression_level\fP for more information.
.sp
You can use the command line option \fB\-x\fP/\fB\-\-show\-compression\fP to print
information related to compression. Example:
.sp
.if n .RS 4
.nf
.fam C
Total data:           14.8 GB (16.0 GB disk blocks)
Compressed data:      11.3 GB (30.6% of original size)
  Original size:      36.9 GB
  Compression ratio: 3.267 x  (69.4% space savings)
Incompressible data:   3.5 GB
.fam
.fi
.if n .RE
.sp
Notes:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The \(lqdisk blocks\(rq size is the cache size when taking disk block size into
account. This value should match the \(lqCache size\(rq value from \(lqccache
\-\-show\-stats\(rq. The other size numbers refer to actual content sizes.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\(lqCompressed data\(rq refers to result and manifest files stored in the cache.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\(lqIncompressible data\(rq refers to files that are always stored uncompressed
(triggered by enabling \fBfile_clone\fP or
\fBhard_link\fP) or unknown files (for instance files
created by older ccache versions).
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The compression ratio is affected by
\fBcompression_level\fP.
.RE
.sp
The cache data can also be recompressed to another compression level (or made
uncompressed) with the command line option \fB\-X\fP/\fB\-\-recompress\fP. If you choose to
disable compression by default or to use a low compression level, you can
(re)compress newly cached data with a higher compression level after the build
or at another time when there are more CPU cycles available, for instance every
night. Full recompression potentially takes a lot of time, but only files that
are currently compressed with a different level than the target level will be
recompressed.
.SH "CACHE STATISTICS"
.sp
\fBccache \-\-show\-stats\fP shows a summary of statistics, including cache size,
cleanups (number of performed cleanups, either implicitly due to a cache size
limit being reached or due to explicit \fBccache \-c\fP calls), overall hit rate, hit
rate for direct/preprocessed modes
and hit rate for primary and secondary storage.
.sp
The summary also includes counters called \(lqErrors\(rq and \(lqUncacheable\(rq, which
are sums of more detailed counters. To see those detailed counters, use the
\fB\-v\fP/\fB\-\-verbose\fP flag. The verbose mode can show the following counters:
.TS
allbox tab(:);
lt lt.
T{
.sp
\fBCounter\fP
T}:T{
.sp
\fBDescription\fP
T}
T{
.sp
Autoconf compile/link
T}:T{
.sp
Uncacheable compilation or linking by an Autoconf test.
T}
T{
.sp
Bad compiler arguments
T}:T{
.sp
Malformed compiler argument, e.g. missing a value for a compiler option that
requires an argument or failure to read a file specified by a compiler option
argument.
T}
T{
.sp
Called for linking
T}:T{
.sp
The compiler was called for linking, not compiling. Ccache only supports
compilation of a single file, i.e. calling the compiler with the \fB\-c\fP option to
produce a single object file from a single source file.
T}
T{
.sp
Called for preprocessing
T}:T{
.sp
The compiler was called for preprocessing, not compiling.
T}
T{
.sp
Could not use modules
T}:T{
.sp
Preconditions for using [_c_modules] were not fulfilled.
T}
T{
.sp
Could not use precompiled header
T}:T{
.sp
Preconditions for using precompiled headers were not
fulfilled.
T}
T{
.sp
Could not write to output file
T}:T{
.sp
The output path specified with \fB\-o\fP is not a file (e.g. a directory or a device
node).
T}
T{
.sp
Compilation failed
T}:T{
.sp
The compilation failed. No result stored in the cache.
T}
T{
.sp
Compiler check failed
T}:T{
.sp
A compiler check program specified by
\fBcompiler_check\fP (\fBCCACHE_COMPILERCHECK\fP) failed.
T}
T{
.sp
Compiler produced empty output
T}:T{
.sp
The compiler\(cqs output file (typically an object file) was empty after
compilation.
T}
T{
.sp
Compiler produced no output
T}:T{
.sp
The compiler\(cqs output file (typically an object file) was missing after
compilation.
T}
T{
.sp
Compiler produced stdout
T}:T{
.sp
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.
T}
T{
.sp
Could not find the compiler
T}:T{
.sp
The compiler to execute could not be found.
T}
T{
.sp
Error hashing extra file
T}:T{
.sp
Failure reading a file specified by
\fBextra_files_to_hash\fP (\fBCCACHE_EXTRAFILES\fP).
T}
T{
.sp
Forced recache
T}:T{
.sp
\fBCCACHE_RECACHE\fP was used to overwrite an existing result.
T}
T{
.sp
Internal error
T}:T{
.sp
Unexpected failure, e.g. due to problems reading/writing the cache.
T}
T{
.sp
Missing cache file
T}:T{
.sp
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.
T}
T{
.sp
Multiple source files
T}:T{
.sp
The compiler was called to compile multiple source files in one go. This is not
supported by ccache.
T}
T{
.sp
No input file
T}:T{
.sp
No input file was specified to the compiler.
T}
T{
.sp
Output to stdout
T}:T{
.sp
The compiler was instructed to write its output to standard output using \fB\-o \-\fP.
This is not supported by ccache.
T}
T{
.sp
Preprocessing failed
T}:T{
.sp
Preprocessing the source code using the compiler\(cqs \fB\-E\fP option failed.
T}
T{
.sp
Unsupported code directive
T}:T{
.sp
Code like the assembler \fB.incbin\fP directive was found. This is not supported
by ccache.
T}
T{
.sp
Unsupported compiler option
T}:T{
.sp
A compiler option not supported by ccache was found.
T}
T{
.sp
Unsupported source language
T}:T{
.sp
A source language e.g. specified with \fB\-x\fP was unsupported by ccache.
T}
.TE
.sp
.SH "HOW CCACHE WORKS"
.sp
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 BLAKE3, a very fast cryptographic hash algorithm, for the hashing. On a
cache hit, ccache is able to supply all of the correct compiler outputs
(including all warnings, dependency file, etc) from the cache. Data stored in
the cache is checksummed with XXH3, an extremely fast non\-cryptographic
algorithm, to detect corruption.
.sp
Ccache has two ways of gathering information used to look up results in the
cache:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the \fBpreprocessor mode\fP, where ccache runs the preprocessor on the source
code and hashes the result
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the \fBdirect mode\fP, where ccache hashes the source code and include files
directly
.RE
.sp
The direct mode is generally faster since running the preprocessor has some
overhead.
.sp
If no previous result is detected (i.e., there is a cache miss) using the direct
mode, ccache will fall back to the preprocessor mode unless the \fBdepend mode\fP is
enabled. In the depend mode, ccache never runs the preprocessor, not even on
cache misses. Read more in \fI[_the_depend_mode]\fP below.
.SS "Common hashed information"
.sp
The following information is always included in the hash:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the extension used by the compiler for a file with preprocessor output
(normally \fB.i\fP for C code and \fB.ii\fP for C++ code)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the compiler\(cqs size and modification time (or other compiler\-specific
information specified by \fBcompiler_check\fP)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the name of the compiler
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the current directory (if \fBhash_dir\fP is enabled)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
contents of files specified by
\fBextra_files_to_hash\fP (if any)
.RE
.SS "The preprocessor mode"
.sp
In the preprocessor mode, the hash is formed of the common information and:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the preprocessor output from running the compiler with \fB\-E\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the command line options except those that affect include files (\fB\-I\fP,
\fB\-include\fP, \fB\-D\fP, etc; the theory is that these command line options will
change the preprocessor output if they have any effect at all)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
any standard error output generated by the preprocessor
.RE
.sp
Based on the hash, the cached compilation result can be looked up directly in
the cache.
.SS "The direct mode"
.sp
In the direct mode, the hash is formed of the common information and:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the input source file
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the compiler options
.RE
.sp
Based on the hash, a data structure called \(lqmanifest\(rq is looked up in the
cache. The manifest contains:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
references to cached compilation results (object file, dependency file, etc)
that were produced by previous compilations that matched the hash
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
paths to the include files that were read at the time the compilation results
were stored in the cache
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
hash sums of the include files at the time the compilation results were
stored in the cache
.RE
.sp
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.
.sp
There is a catch with the direct mode: header files that were used by the
compiler are recorded, but header files that were \fBnot\fP 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\(cqt 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.
.sp
The direct mode will be disabled if any of the following holds:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBdirect_mode\fP is false
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
a modification time of one of the include files is too new (needed to avoid a
race condition)
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
a compiler option not supported by the direct mode is used:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
a \fB\-Wp,*\fP compiler option other than \fB\-Wp,\-MD,<path>\fP, \fB\-Wp,\-MMD,<path>\fP
and \fB\-Wp,\-D<define>\fP
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fB\-Xpreprocessor\fP
.RE
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
the string \fB__TIME__\fP is present in the source code
.RE
.SS "The depend mode"
.sp
If the depend mode is enabled, ccache will not use the preprocessor at all. The
hash used to identify results in the cache will be based on the direct mode
hash described above plus information about include files read from the
dependency file generated by the compiler with \fB\-MD\fP or \fB\-MMD\fP.
.sp
Advantages:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The ccache overhead of a cache miss will be much smaller.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Not running the preprocessor at all can be good if compilation is performed
remotely, for instance when using distcc or similar; ccache then won\(cqt make
potentially costly preprocessor calls on the local machine.
.RE
.sp
Disadvantages:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The cache hit rate will likely be lower since any change to compiler options
or source code will make the hash different. Compare this with the default
setup where ccache will fall back to the preprocessor mode, which is tolerant
to some types of changes of compiler options and source code changes.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If \-MD is used, the manifest entries will include system header files as
well, thus slowing down cache hits slightly, just as using \-MD slows down
make.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If \-MMD is used, the manifest entries will not include system header files,
which means ccache will ignore changes in them.
.RE
.sp
The depend mode will be disabled if any of the following holds:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBdepend_mode\fP is false.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
\fBrun_second_cpp\fP is false.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The compiler is not generating dependencies using \fB\-MD\fP or \fB\-MMD\fP.
.RE
.SH "HANDLING OF NEWLY CREATED HEADER FILES"
.sp
If modification time (mtime) or status change time (ctime) of one of the include
files is the same second as the time compilation is being done, ccache disables
the direct mode (or, in the case of a precompiled
header, disables caching completely). This done as a safety measure to avoid a
race condition (see below).
.sp
To be able to use a newly created header files in direct mode (or use a newly
precompiled header), either:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
create the include file earlier in the build process, or
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
set \fBsloppiness\fP to
\fBinclude_file_ctime,include_file_mtime\fP if you are willing to take the risk,
for instance if you know that your build system is robust enough not to
trigger the race condition.
.RE
.sp
For reference, the race condition mentioned above consists of these events:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 1." 4.2
.\}
The preprocessor is run.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 2." 4.2
.\}
An include file is modified by someone.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 3." 4.2
.\}
The new include file is hashed by ccache.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 4.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 4." 4.2
.\}
The real compiler is run on the preprocessor\(cqs output, which contains data
from the old header file.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 5.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 5." 4.2
.\}
The wrong object file is stored in the cache.
.RE
.SH "CACHE DEBUGGING"
.sp
To find out what information ccache actually is hashing, you can enable the
debug mode via the configuration option \fBdebug\fP or by setting
\fBCCACHE_DEBUG\fP in the environment. This can be useful if you are investigating
why you don\(cqt get cache hits. Note that performance will be reduced slightly.
.sp
When the debug mode is enabled, ccache will create up to five additional files
next to the object file:
.TS
allbox tab(:);
lt lt.
T{
.sp
\fBFilename\fP
T}:T{
.sp
\fBDescription\fP
T}
T{
.sp
\fB<objectfile>.ccache\-input\-c\fP
T}:T{
.sp
Binary input hashed by both the direct mode and the preprocessor mode.
T}
T{
.sp
\fB<objectfile>.ccache\-input\-d\fP
T}:T{
.sp
Binary input only hashed by the direct mode.
T}
T{
.sp
\fB<objectfile>.ccache\-input\-p\fP
T}:T{
.sp
Binary input only hashed by the preprocessor mode.
T}
T{
.sp
\fB<objectfile>.ccache\-input\-text\fP
T}:T{
.sp
Human\-readable combined diffable text version of the three files above.
T}
T{
.sp
\fB<objectfile>.ccache\-log\fP
T}:T{
.sp
Log for this object file.
T}
.TE
.sp
.sp
If \fBconfig_dir\fP (environment variable \fBCCACHE_DEBUGDIR\fP) is
set, the files above will be written to that directory with full absolute paths
instead of next to the object file.
.sp
In the direct mode, ccache uses the 160 bit BLAKE3 hash of the
\(lqccache\-input\-c\(rq + \(lqccache\-input\-d\(rq data (where \fB+\fP means concatenation),
while the \(lqccache\-input\-c\(rq + \(lqccache\-input\-p\(rq data is used in the
preprocessor mode.
.sp
The \(lqccache\-input\-text\(rq file is a combined text version of the three binary
input files. It has three sections (\(lqCOMMON\(rq, \(lqDIRECT MODE\(rq and
\(lqPREPROCESSOR MODE\(rq), which is turn contain annotations that say what kind of
data comes next.
.sp
To debug why you don\(cqt get an expected cache hit for an object file, you can do
something like this:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 1." 4.2
.\}
Build with debug mode enabled.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 2." 4.2
.\}
Save the \fB<objectfile>.ccache\-*\fP files.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 3." 4.2
.\}
Build again with debug mode enabled.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 4.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 4." 4.2
.\}
Compare \fB<objectfile>.ccache\-input\-text\fP for the two builds. This together
with the \fB<objectfile>.ccache\-log\fP files should give you some clues about
what is happening.
.RE
.SH "COMPILING IN DIFFERENT DIRECTORIES"
.sp
Some information included in the hash that identifies a unique compilation can
contain absolute paths:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The preprocessed source code may contain absolute paths to include files if
the compiler option \fB\-g\fP is used or if absolute paths are given to \fB\-I\fP and
similar compiler options.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Paths specified by compiler options (such as \fB\-I\fP, \fB\-MF\fP, etc) on the command
line may be absolute.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The source code file path may be absolute, and that path may substituted for
\fB__FILE__\fP macros in the source code or included in warnings emitted to
standard error by the preprocessor.
.RE
.sp
This means that if you compile the same code in different locations, you can\(cqt
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.
.sp
Here\(cqs what can be done to enable cache hits between different build
directories:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If you build with \fB\-g\fP (or similar) to add debug information to the object
file, you must either:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
use the compiler option \fB\-fdebug\-prefix\-map=<old>=<new>\fP for relocating
debug info to a common prefix (e.g. \fB\-fdebug\-prefix\-map=$PWD=.\fP); or
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
set \fBhash_dir = false\fP.
.RE
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If you use absolute paths anywhere on the command line (e.g. the source code
file path or an argument to compiler options like \fB\-I\fP and \fB\-MF\fP), you must
set \fBbase_dir\fP to an absolute path to a \(lqbase
directory\(rq. Ccache will then rewrite absolute paths under that directory to
relative before computing the hash.
.RE
.SH "PRECOMPILED HEADERS"
.sp
Ccache has support for GCC\(cqs precompiled headers. However, you have to do some
things to make it work properly:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
You must set \fBsloppiness\fP to \fBpch_defines,time_macros\fP.
The reason is that ccache can\(cqt tell whether \fB__TIME__\fP, \fB__DATE__\fP or
\fB__TIMESTAMP__\fP is used when using a precompiled header. Further, it can\(cqt
detect changes in \fB#define\fPs in the source code because of how preprocessing
works in combination with precompiled headers.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
You may also want to include \fBinclude_file_mtime,include_file_ctime\fP in
\fBsloppiness\fP. See
\fI[_handling_of_newly_created_header_files]\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
You must either:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
use the compiler option \fB\-include\fP to include the precompiled header (i.e.,
don\(cqt use \fB#include\fP in the source code to include the header; the filename
itself must be sufficient to find the header, i.e. \fB\-I\fP paths are not
searched); or
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
(for the Clang compiler) use the compiler option \fB\-include\-pch\fP to include
the PCH file generated from the precompiled header; or
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
(for the GCC compiler) add the compiler option \fB\-fpch\-preprocess\fP when
compiling.
.RE
.sp
If you don\(cqt 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 \(lqPreprocessing failed\(rq (if the non\-precompiled
header file is not available).
.RE
.SH "C++ MODULES"
.sp
Ccache has support for Clang\(cqs \fB\-fmodules\fP option. In practice ccache only
additionally hashes \fBmodule.modulemap\fP files; it does not know how Clang
handles its cached binary form of modules so those are ignored. This should not
matter in practice: as long as everything else (including \fBmodule.modulemap\fP
files) is the same the cached result should work. Still, you must set
\fBsloppiness\fP to \fBmodules\fP to allow caching.
.sp
You must use both \fBdirect mode\fP and
\fBdepend mode\fP. When using
the preprocessor mode Clang does not provide enough
information to allow hashing of \fBmodule.modulemap\fP files.
.SH "SHARING A CACHE"
.sp
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:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Use the same cache directory.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Make sure that the configuration option \fBhard_link\fP is
false (which is the default).
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Make sure that all users are in the same group.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Set the configuration option \fBumask\fP to \fB002\fP. This ensures
that cached files are accessible to everyone in the group.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Make sure that all users have write permission in the entire cache directory
(and that you trust all users of the shared cache).
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
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:
.sp
.if n .RS 4
.nf
.fam C
find $CCACHE_DIR \-type d | xargs chmod g+s
.fam
.fi
.if n .RE
.RE
.sp
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\(cqs 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.
.sp
You may also want to make sure that a base directory is set appropriately, as
discussed in a previous section.
.SH "SHARING A CACHE ON NFS"
.sp
It is possible to put the cache directory on an NFS filesystem (or similar
filesystems), but keep in mind that:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Having the cache on NFS may slow down compilation. Make sure to do some
benchmarking to see if it\(cqs worth it.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Ccache hasn\(cqt been tested very thoroughly on NFS.
.RE
.sp
A tip is to set \fBtemporary_dir\fP to a directory on the
local host to avoid NFS traffic for temporary files.
.sp
It is recommended to use the same operating system version when using a shared
cache. If operating system versions are different then system include files
will likely be different and there will be few or no cache hits between the
systems. One way of improving cache hit rate in that case is to set
\fBsloppiness\fP to \fBsystem_headers\fP to ignore system
headers.
.sp
An alternative to putting the main cache directory on NFS is to set up a
secondary storage file cache.
.SH "USING CCACHE WITH OTHER COMPILER WRAPPERS"
.sp
The recommended way of combining ccache with another compiler wrapper (such as
\(lqdistcc\(rq) is by letting ccache execute the compiler wrapper. This is
accomplished by defining \fBprefix_command\fP, for
example by setting the environment variable \fBCCACHE_PREFIX\fP to the name of the
wrapper (e.g. \fBdistcc\fP). Ccache will then prefix the command line with the
specified command when running the compiler. To specify several prefix
commands, set \fBprefix_command\fP to a colon\-separated
list of commands.
.sp
Unless you set \fBcompiler_check\fP to a suitable command
(see the description of that configuration option), it is not recommended to use
the form \fBccache anotherwrapper compiler args\fP as the compilation command. It\(cqs
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:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Compiler upgrades will not be detected properly.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The cached results will not be shared between compilations with and without
the other wrapper.
.RE
.sp
Another minor thing is that if \fBprefix_command\fP is
used, ccache will not invoke the other wrapper when running the preprocessor,
which increases performance. You can use
\fBprefix_command_cpp\fP if you also want to invoke
the other wrapper when doing preprocessing (normally by adding \fB\-E\fP).
.SH "CAVEATS"
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The direct mode fails to pick up new header files in some rare scenarios. See
\fI[_the_direct_mode]\fP above.
.RE
.SH "TROUBLESHOOTING"
.SS "General"
.sp
A general tip for getting information about what ccache is doing is to enable
debug logging by setting the configuration option \fBdebug\fP (or
the environment variable \fBCCACHE_DEBUG\fP); see \fI[_cache_debugging]\fP
for more information. Another way of keeping track of what is
happening is to check the output of \fBccache \-s\fP.
.SS "Performance"
.sp
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.
.sp
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.
.sp
A good way of monitoring how well ccache works is to run \fBccache \-s\fP 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:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If the counter for preprocessed cache hits has been incremented instead of the
one for direct cache hits, ccache has fallen back to preprocessor mode, which
is generally slower. Some possible reasons are:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The source code has been modified in such a way that the preprocessor output
is not affected.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
Compiler arguments that are hashed in the direct mode but not in the
preprocessor mode have changed (\fB\-I\fP, \fB\-include\fP, \fB\-D\fP, etc) and they didn\(cqt
affect the preprocessor output.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The compiler option \fB\-Xpreprocessor\fP or \fB\-Wp,*\fP (except \fB\-Wp,\-MD,<path>\fP,
\fB\-Wp,\-MMD,<path>\fP, and \fB\-Wp,\-D<define>\fP) is used.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
This was the first compilation with a new value of the
base directory.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
A modification or status change time of one of the include files is too new
(created the same second as the compilation is being done). See
\fI[_handling_of_newly_created_header_files]\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The \fB__TIME__\fP preprocessor macro is (potentially) being used. Ccache turns
off direct mode if \fB__TIME__\fP is present in the source code. This is done
as a safety measure since the string indicates that a \fB__TIME__\fP macro
\fImay\fP 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 \fB__TIME__\fP isn\(cqt used in practise, or don\(cqt care if ccache
produces objects where \fB__TIME__\fP is expanded to something in the past, you
can set \fBsloppiness\fP to \fBtime_macros\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The \fB__DATE__\fP preprocessor macro is (potentially) being used and the date
has changed. This is similar to how \fB__TIME__\fP is handled. If \fB__DATE__\fP
is present in the source code, ccache hashes the current date in order to be
able to produce the correct object file if the \fB__DATE__\fP macro affects the
output. If you know that \fB__DATE__\fP isn\(cqt used in practise, or don\(cqt care
if ccache produces objects where \fB__DATE__\fP is expanded to something in the
past, you can set \fBsloppiness\fP to \fBtime_macros\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The \fB__TIMESTAMP__\fP preprocessor macro is (potentially) being used and the
source file\(cqs modification time has changed. This is similar to how
\fB__TIME__\fP is handled. If \fB__TIMESTAMP__\fP is present in the source code,
ccache hashes the string representation of the source file\(cqs modification
time in order to be able to produce the correct object file if the
\fB__TIMESTAMP__\fP macro affects the output. If you know that
\fB__TIMESTAMP__\fP isn\(cqt used in practise, or don\(cqt care if ccache produces
objects where \fB__TIMESTAMP__\fP is expanded to something in the past, you can
set \fBsloppiness\fP to \fBtime_macros\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
The input file path has changed. Ccache includes the input file path in the
direct mode hash to be able to take relative include files into account and
to produce a correct object file if the source code includes a \fB__FILE__\fP
macro.
.RE
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If a cache hit counter was not 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 \fB__TIME__\fP, \fB__DATE__\fP or
\fB__TIMESTAMP__\fP macros, or use of automatically generated code that contains
a timestamp, build counter or other volatile information.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If \(lqMultiple source files\(rq has been incremented, it\(cqs an indication that the
compiler has been invoked on several source code files at once. Ccache doesn\(cqt
support that. Compile the source code files separately if possible.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If \(lqUnsupported compiler option\(rq has been incremented, enable debug logging
and check which compiler option was rejected.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If \(lqPreprocessing failed\(rq has been incremented, one possible reason is that
precompiled headers are being used. See \fI[_precompiled_headers]\fP for how to
remedy this.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If \(lqCould not use precompiled header\(rq has been incremented, see
\fI[_precompiled_headers]\fP.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.  sp -1
.  IP \(bu 2.3
.\}
If \(lqCould not use modules\(rq has been incremented, see \fI[_c_modules]\fP.
.RE
.SS "Corrupt object files"
.sp
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
\fBprefix_command\fP or compiler wrapper. If this
happens, the easiest way of fixing it is this:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 1." 4.2
.\}
Build so that the bad object file ends up in the build tree.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 2." 4.2
.\}
Remove the bad object file from the build tree.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.  sp -1
.  IP " 3." 4.2
.\}
Rebuild with \fBCCACHE_RECACHE\fP set.
.RE
.sp
An alternative is to clear the whole cache with \fBccache \-C\fP if you don\(cqt mind
losing other cached results.
.sp
There are no reported issues about ccache producing broken object files
reproducibly. That doesn\(cqt mean it can\(cqt happen, so if you find a repeatable
case, please report it.
.SH "MORE INFORMATION"
.sp
Credits, mailing list information, bug reporting instructions, source code,
etc, can be found on ccache\(cqs web site: \c
.URL "https://ccache.dev" "" "."
.SH "AUTHOR"
.sp
Ccache was originally written by Andrew Tridgell and is currently developed and
maintained by Joel Rosdahl. See AUTHORS.txt or AUTHORS.html and
.URL "https://ccache.dev/credits.html" "" ""
for a list of contributors.