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doc: Describe CC Wrapper in more detail

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The main motivation for this is to have something to google for LD=$CC.

Eventually, this should probably be moved to another section, but we
can deal with that later.
This commit is contained in:
John Ericson 2017-08-18 17:48:30 -04:00
parent b6a746daa1
commit 0578dda8e1
1 changed files with 31 additions and 8 deletions
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35
doc/stdenv.xml
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@ -1,3 +1,4 @@
<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-stdenv">
@ -1342,12 +1343,34 @@ someVar=$(stripHash $name)
<variablelist>
<varlistentry>
<term>GCC wrapper</term>
<listitem><para>Adds the <filename>include</filename> subdirectory
of each build input to the <envar>NIX_CFLAGS_COMPILE</envar>
environment variable, and the <filename>lib</filename> and
<filename>lib64</filename> subdirectories to
<envar>NIX_LDFLAGS</envar>.</para></listitem>
<term>CC Wrapper</term>
<listitem>
<para>
CC Wrapper wraps a C toolchain for a bunch of miscellaneous purposes.
Specifically, a C compiler (GCC or Clang), Binutils (or the CCTools + binutils mashup when targetting Darwin), and a C standard library (glibc or Darwin's libSystem) are all fed in, and dependency finding, hardening (see below), and purity checks for each are handled by CC Wrapper.
Packages typically depend on only CC Wrapper, instead of those 3 inputs directly.
</para>
<para>
Dependency finding is undoubtedly the main task of CC wrapper.
It is currently accomplished by collecting directories of host-platform dependencies (i.e. <varname>buildInputs</varname> and <varname>nativeBuildInputs</varname>) in environment variables.
CC wrapper's setup hook causes any <filename>include</filename> subdirectory of such a dependency to be added to <envar>NIX_CFLAGS_COMPILE</envar>, and any <filename>lib</filename> and <filename>lib64</filename> subdirectories to <envar>NIX_LDFLAGS</envar>.
The setup hook itself contains some lengthy comments describing the exact convoluted mechanism by which this is accomplished.
</para>
<para>
A final task of the setup hook is defining a number of standard environment variables to tell build systems which executables full-fill which purpose.
They are defined to just be the base name of the tools, under the assumption that CC Wrapper's binaries will be on the path.
Firstly, this helps poorly-written packages, e.g. ones that look for just <command>gcc</command> when <envar>CC</envar> isn't defined yet <command>clang</command> is to be used.
Secondly, this helps packages not get confused when cross-compiling, in which case multiple CC wrappers may be simultaneous in use (targeting different platforms).
<envar>BUILD_</envar>- and <envar>TARGET_</envar>-prefixed versions of the normal environment variable are defined for the additional CC Wrappers, properly disambiguating them.
</para>
<para>
A problem with this final task is that CC Wrapper is honest and defines <envar>LD</envar> as <command>ld</command>.
Most packages, however, firstly use the C compiler for linking, secondly use <envar>LD</envar> anyways, defining it as the C compiler, and thirdly, only so define <envar>LD</envar> when it is undefined as a fallback.
This triple-threat means CC Wrapper will break those packages, as LD is already defined as the actually linker which the package won't override yet doesn't want to use.
The workaround is to define, just for the problematic package, <envar>LD</envar> as the C compiler.
A good way to do this would be <command>preConfigure = "LD=$CC"</command>.
</para>
</listitem>
</varlistentry>
<varlistentry>