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<?xml version="1.0"?>
<guide self="ebuild-writing/functions/src_compile/build-environment/">
<chapter>
<title>Configuring Build Environment</title>
<body>
<p>
Sometimes it is necessary to manipulate certain aspects of the user's
build environment before running <c>./configure</c>. The <c>flag-o-matic</c>
eclass is the best choice for manipulating <c>CFLAGS</c>, <c>LDFLAGS</c>
and suchlike.
</p>
<note>
Except where otherwise specified, any function which operates on
<c>CFLAGS</c> also operates on <c>CXXFLAGS</c>.
</note>
<p>
Ebuilds must not simply ignore user CFLAGS, CXXFLAGS or LDFLAGS <d/> see
<uri link="::general-concepts/user-environment#Not Filtering Variables"/>.
</p>
</body>
<section>
<title>Guidelines for Flag Filtering</title>
<body>
<p>
If a package breaks with any reasonable <c>CFLAGS</c>, it is best to filter the
problematic flag if a bug report is received. Reasonable <c>CFLAGS</c> are
<c>-march=</c>, <c>-mcpu=</c>, <c>-mtune=</c> (depending upon
arch), <c>-O2</c>, <c>-Os</c> and <c>-fomit-frame-pointer</c>. Note that
<c>-Os</c> should usually be replaced with <c>-O2</c> rather than being
stripped entirely. The <c>-fstack-protector</c> flag should probably be
in this group too, although our hardened team claim that this flag
never ever breaks anything...
</p>
<p>
The <c>-pipe</c> flag doesn't affect the generated code, so it's not really
relevant here, but it's a sensible flag to have set globally.
</p>
<p>
If a package breaks with other (insane) <c>CFLAGS</c>, it is perfectly
OK to close the bug with a <b>WONTFIX</b> suggesting that the user
picks more sensible global <c>CFLAGS</c>. Similarly, if you suspect
that a bug is caused by insane <c>CFLAGS</c>, an <b>INVALID</b> resolution is
suitable.
</p>
<p>
All of the following assumes that the ebuild has an <c>inherit flag-o-matic</c>
line in the correct place.
</p>
</body>
</section>
<section>
<title>Simple Flag Stripping</title>
<body>
<p>
The <c>filter-flags</c> function can be used to remove a particular flag from
<c>{C,CPP,CXX,CCAS,F,FC,LD}FLAGS</c>. Multiple arguments can be supplied; each is the name of a flag to
remove.
</p>
<codesample lang="ebuild">
# -fomit-frame-pointer leads to nasty broken code on sparc thanks to a
# rather icky asm function
use sparc && filter-flags -fomit-frame-pointer
</codesample>
<p>
There is a <c>filter-ldflags</c> function available which does the equivalent
for <c>LDFLAGS</c>.
</p>
<p>
If a package is known to be very <c>CFLAGS</c> sensitive, the
<c>strip-flags</c> function will remove most flags, leaving only a minimal
conservative set of flags.
</p>
<codesample lang="ebuild">
# Our app hates screwy flags
strip-flags
</codesample>
</body>
</section>
<section>
<title>Flag Replacement</title>
<body>
<p>
To replace a flag with a different one, use <c>replace-flags</c>. This
is most commonly used to replace <c>-Os</c> with <c>-O2</c>
(or <c>-O3</c> with <c>-O2</c> if you are feeling kind).
</p>
<codesample lang="ebuild">
# Seems to have issues with -Os, switch to -O2
replace-flags -Os -O2
</codesample>
<p>
There is also a special function named <c>replace-cpu-flags</c> for
replacing CPU (<c>-mtune</c>, <c>-mcpu</c>, <c>-march</c>) designation
flags. The last argument is the flag to use; previous arguments are
the flags to be replaced.
</p>
<codesample lang="ebuild">
# Can't use ultrasparc or ultrasparc3 code, drop to v9
replace-cpu-flags ultrasparc ultrasparc3 v9
</codesample>
</body>
</section>
<section>
<title>Adding Additional Flags</title>
<body>
<p>
Sometimes it is necessary to add in additional <c>CFLAGS</c>
or <c>LDFLAGS</c>. The <c>append-flags</c> and <c>append-ldflags</c>
functions can be used here.
</p>
<codesample lang="ebuild">
# If we're using selinux, we need to add a -D
use selinux && append-flags "-DWITH_SELINUX"
# Secure linking needed, since we're setuid root
append-ldflags -Wl,-z,now
</codesample>
<p>
See <uri link="::eclass-reference/flag-o-matic.eclass/"/> for a full reference.
</p>
</body>
</section>
</chapter>
</guide>
|
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