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<?xml version="1.0"?>
<guide self="general-concepts/use-flags/">
<chapter>
<title>USE Flags</title>
<body>
<p>
<c>USE</c> flags are to control optional dependencies and settings which
the user may reasonably want to select. For example, <c>app-editors/vim</c>
can optionally build with support for the <c>ruby</c> interpreter, and it
needs <c>dev-lang/ruby</c> installed to do this <d/> we use the ruby
<c>USE</c> flag to provide this option. On the other hand,
<c>app-text/glark</c> requires <c>ruby</c> no matter what, so no <c>USE</c>
flag is used there.
</p>
<p>
No combination of <c>USE</c> flags should cause a package to fail to build
because users can set any combination of flags.
</p>
<p>
Packages should not configure and link based upon what is available at
compile time <d/> any autodetection must be overridden. This is commonly
referred to as the dependency being "automagic" - This is bad because the
dependency is not detected by the package manager tools and can easily
break, among other issues.
</p>
<p>
The usage of a <c>USE</c> flag should not control runtime dependencies when
the package does not link to it. Doing so will create extra
configuration for the package and re-compilation for no underlying file
change on disk. This should be avoided and instead can be conveyed to the
user via post install messages if needed.
</p>
<note>
The status of USE flags is saved in the VDB, and their value in
<c>pkg_prerm</c> and <c>pkg_postrm</c> is taken from there. This means that
setting or unsetting a USE flag between merge and unmerge has no effect.
</note>
</body>
<section>
<title><c>noblah</c> USE Flags</title>
<body>
<p>
Avoid <c>noblah</c> style <c>USE</c> flags. These break <c>use.mask</c> and
cause all sorts of complications for arch developers. Here's why:
</p>
<p>
Consider a hypothetical package named 'vplayer', which plays videos. This
package has optional support, via <c>USE</c> flags, for various sound and
video output methods, various video codecs and so on.
</p>
<p>
One of vplayer's optional features is support for the 'fakemedia' codec,
which is unfortunately only available as a dodgy x86 binary. We <e>could</e>
handle this by doing something like:
</p>
<codesample lang="ebuild">
RDEPEND="x86? ( fakemedia? ( >=media-libs/fakemedia-1.1 ) )"
</codesample>
<p>
Except this is pretty nasty <d/> what happens when an AMD64 binary is made
as well? Also, users on other archs will see fakemedia listed in
<c>emerge -pv</c> output, even though it is not actually available.
</p>
<p>
Similarly, say vplayer supports output via the ALSA codec as one option.
However, ALSA isn't (or wasn't when this example was written) available on
SPARC or Alpha. So we could do:
</p>
<codesample lang="ebuild">
DEPEND="!sparc? ( !alpha? ( alsa? ( media-libs/alsa-lib ) ) )"
</codesample>
<p>
Again, it's messy, and ALSA still shows up in the <c>emerge -p</c> output.
Also, once ALSA starts working on SPARC, every ebuild that does this would
have to be manually edited.
</p>
<p>
The solution is <c>use.mask</c>, which is documented in
<uri link="::profiles/use.mask/"/>. Each profile can have a <c>use.mask</c>
file which can be used to forcibly disable certain USE flags on a given
arch (or subarch, or subprofile). So, if the <c>fakemedia</c> USE flag was
use.masked on every non-x86 profile, the following would be totally legal
and wouldn't break anything:
</p>
<codesample lang="ebuild">
RDEPEND="fakemedia? ( >=media-libs/fakemedia-1-1 )"
</codesample>
<p>
Users of non-x86 would see the following when doing
<c>emerge -pv vplayer</c>:
</p>
<pre>
[ebuild R ] media-video/vplayer-1.2 alsa -blah (-fakemedia) xyz
</pre>
<p>
To get a flag added to <c>use.mask</c>, ask the relevant arch team.
</p>
</body>
</section>
<section>
<title>Local and Global USE Flags</title>
<body>
<p>
USE flags are categorised as either local or global. A global USE flag must
satisfy several criteria:
</p>
<ul>
<li>It is used by many different packages, at least 5 seems to be agreed
upon.</li>
<li>It has a general non-specific purpose.</li>
</ul>
<p>
The second point is important. If the effect of the USE flag upon
<c>pkg-one</c> is substantially different from the effect it has upon
<c>pkg-two</c>, then the flag is not a suitable candidate for being made a
global flag. In particular, note that if <c>client</c> and <c>server</c>
USE flags are ever introduced, they can not be global USE flags for this
reason.
</p>
<p>
Before introducing a new global USE flag, it must be discussed on the
gentoo-dev mailing list.
</p>
</body>
</section>
<section>
<title>USE Flag Descriptions</title>
<body>
<p>
All USE flags must be described in either <c>use.desc</c> in the
<c>profiles/</c> directory or <c>metadata.xml</c> in the package's
directory. See <c>man portage</c> or the comments in these files for an
explanation of the format. Remember to keep these files sorted. The file
<c>use.local.desc</c> is automatically generated from entries in the
package's <c>metadata.xml</c> and may be used by tools that parse the tree.
Since <c>use.local.desc</c> is automatically generated it must never be
manually editted in the tree.
See <uri link="https://www.gentoo.org/glep/glep-0056.html">GLEP 56</uri>
for more info.
</p>
<p>
The exceptions to this are <c>USE_EXPAND</c> flags, which must be
documented in the <c>profiles/desc/</c> directory. One file per
<c>USE_EXPAND</c> variable is required, which must contain descriptions of
the possible values this variable can take. See the comments in these files
for the format, and remember to keep them sorted.
</p>
</body>
</section>
<section>
<title>Conflicting USE Flags</title>
<body>
<p>
Occasionally, ebuilds will have conflicting USE flags for functionality.
Checking for them and returning an error is <e>not</e> a viable solution.
Instead, you must pick one of the USE flags in conflict to favour and should
alert the user that a particular flag is being used instead.
</p>
<p>
One example comes from the <c>mail-mta/msmtp</c> ebuilds. The package can
use either SSL with GnuTLS, SSL with OpenSSL, or no SSL at all. Because
GnuTLS is more featureful than OpenSSL, it is favoured:
</p>
<codesample lang="ebuild">
src_compile() {
local myconf
if use ssl && use gnutls ; then
myconf="${myconf} --enable-ssl --with-ssl=gnutls"
elif use ssl && ! use gnutls ; then
myconf="${myconf} --enable-ssl --with-ssl=openssl"
else
myconf="${myconf} --disable-ssl"
fi
econf \
# Other stuff
${myconf}
emake || die "make failed"
}
</codesample>
<p>
In some exceptional cases, above policy would break reverse USE
dependencies. To avoid this, the ebuild can specify allowed USE flag
combinations with <c>REQUIRED_USE</c> (available in EAPI 4). See section
<uri link="::ebuild-writing/eapi/#eapi=4" /> for a description
of its syntax.
</p>
<p>
For example, if a package <c>dev-libs/foo</c> can be built with either
<c>USE="a"</c> or <c>USE="b"</c> but not with both, then preferring one of
the flags would break packages that depend on either <c>dev-libs/foo[a]</c>
or <c>dev-libs/foo[b]</c>. Therefore, the ebuild should specify
<c>REQUIRED_USE="a? ( !b )"</c> in this case.
</p>
<note>
In order to avoid forcing users to micro-manage flags too much,
<c>REQUIRED_USE</c> should be used sparingly. Follow the normal policy
whenever it is possible to do a build that will presumably suit the user's
needs.
</note>
</body>
</section>
<section>
<title>USE_EXPAND and ARCH USE Flags</title>
<body>
<p>
The <c>VIDEO_CARDS</c>, <c>INPUT_DEVICES</c> and <c>LINGUAS</c> variables
are automatically expanded into USE flags. These are known as
<c>USE_EXPAND</c> variables. If the user has <c>LINGUAS="en fr"</c> in
<c>make.conf</c>, for example, then <c>USE="linguas_en linguas_fr"</c> will
automatically be set by Portage.
</p>
<p>
The <c>USE_EXPAND</c> list is set in <c>profiles/base/make.defaults</c> as of
Portage 2.0.51.20. This must not be modified without discussion on the
gentoo-dev list, and it must not be modified in any subprofile.
</p>
<p>
The current architecture (e.g. <c>x86</c>, <c>sparc</c>, <c>ppc-macos</c>)
will automatically be set as a USE flag as well. See
<c>profiles/arch.list</c> for a full list of valid architecture keywords,
and <uri link="https://www.gentoo.org/glep/glep-0022.html">GLEP 22</uri>
for an explanation of the format.
</p>
<warning>
It is a common misconception that the architecture variable is somehow
related to <c>ACCEPT_KEYWORDS</c>. It isn't. Accepting <c>x86</c> keywords
on <c>sparc</c>, for example, won't set <c>USE="x86"</c>. Similarly, there
are no <c>~arch</c> USE flags, so don't try <c>if use ~x86</c>.
</warning>
</body>
</section>
</chapter>
</guide>
|
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