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What is gcc-config?
-------------------
Gentoo allows switching gcc as system runs via gcc-config:
$ gcc-config x86_64-pc-linux-gnu-8.1.0
$ gcc-config x86_64-pc-linux-gnu-7.2.0
Ideally changes should be visible instantly and atomically
without shell restart. Practically see TODOs and BUGs on PATH/ROOTPATH.
Files, variables, things.
-------------------------
- Wrappers (copies of /usr/$(libexec)/gcc-config/wrapper)
/usr/bin/gcc
/usr/bin/g++
/usr/bin/${CTARGET}-gcc
...
(all files from /usr/${CTARGET}/gcc-bin/$GCC_VERSION/*)
Wrapper reads env config and re-execs binary from `GCC_PATH`.
See `gcc-config` script for wrapping details.
- private gcc configs (provided by `toolchain.eclass`, gcc ebuilds)
/etc/env.d/gcc/.NATIVE -> x86_64-pc-linux-gnu-8.1.0 (link to target config)
/etc/env.d/gcc/x86_64-pc-linux-gnu-8.1.0
Contains variables that describe toolchain layout:
LDPATH="/usr/lib/gcc/x86_64-pc-linux-gnu/8.1.0"
MANPATH="/usr/share/gcc-data/x86_64-pc-linux-gnu/8.1.0/man"
INFOPATH="/usr/share/gcc-data/x86_64-pc-linux-gnu/8.1.0/info"
STDCXX_INCDIR="g++-v8"
CTARGET="x86_64-pc-linux-gnu"
GCC_SPECS=""
MULTIOSDIRS="../lib64"
GCC_PATH="/usr/x86_64-pc-linux-gnu/gcc-bin/8.1.0"
Used by gcc-config to generate wrappers and 05gcc- env.d files.
Used by wrapper to extract GCC_PATH and re-exec().
- gcc env.d compiler entries (provided by gcc-config)
/etc/env.d/04gcc-${CTARGET} (native)
Populates paths for native-compilers
PATH="/usr/x86_64-pc-linux-gnu/gcc-bin/8.2.0"
ROOTPATH="/usr/x86_64-pc-linux-gnu/gcc-bin/8.2.0"
GCC_SPECS=""
MANPATH="/usr/share/gcc-data/x86_64-pc-linux-gnu/8.2.0/man"
INFOPATH="/usr/share/gcc-data/x86_64-pc-linux-gnu/8.2.0/info"
Used by env-update to populate PATH and more (TODO: remove PATH population).
/etc/env.d/05gcc-${CTARGET} (cross)
Populates paths for cross-compilers
PATH="/usr/x86_64-pc-linux-gnu/powerpc64le-unknown-linux-gnu/gcc-bin/7.3.0"
ROOTPATH="/usr/x86_64-pc-linux-gnu/powerpc64le-unknown-linux-gnu/gcc-bin/7.3.0"
Used by wrapper to extract PATH and re-exec().
Used by env-update to populate PATH (TODO: remove PATH population).
How does gcc-config work?
-------------------------
/usr/bin/gcc (or /usr/bin/<tool>) is a wrapper (wrapper.c).
gcc-config allows runtime gcc switch via level of indirection.
Tool name is ${CTARGET}-<tool> or <tool>. TODO: doesn't it break
on ${CTARGET}-<tool>-<version>? Doesn't ${CTARGET} get misdetected?
Today's resolution order is the following:
1. Searches PATH for <tool> and reexecute if found. Some
target paths are ignored:
- itself (/usr/bin/<tool>) is ignored to avoid recursion
- */gcc-bin/* (/usr/${CHOST}/${CTARGET}/gcc-bin/7.3.0) is ignored
as those should precede in PATH to avoid wrapping entirely or be
selected via gcc-config.
2. If [1.] failed search for /etc/env.d/: /etc/env.d/gcc/.NATIVE (native compiler),
/etc/env.d/05gcc-${CTARGET} (cross-compiler).
There GCC_PATH= (native) or PATH= (cross) directory entry is searched.
Usually it's GCC_PATH="${EPREFIX}/usr/${CHOST}/gcc-bin/<gcc-version>" (native)
or GCC_PATH="${EPREFIX}/usr/${CHOST}/${CTARGET}/gcc-bin/<gcc-version>" (cross)
3. If [2.] failed run 'ROOT= gcc-config --get-bin-path' and use it's output.
4. Prepend path fetched from above to PATH environment variable and re-exec().
5. Done
`gcc-config` script maintains binaries in `/usr/bin/` to make the above model work.
To make compiler switchable in active shell `/usr/bin/` path must precede
'/usr/${CHOST}/gcc-bin/<gcc-version>'. Otherwise wrapper is skipped.
TODOs
-----
- Write proper `gcc-config` manpage off this readme to be more discoverable.
- Make /usr/bin/<tool> a symlink of /usr/$(libexec)/gcc-config/wrapper,
not a file copy. This will make the fact that Gentoo uses wrappers more obvious.
It's essential to know for people using ccache cache or similar.
- Move /etc/env.d/05gcc-${CTARGET} and /etc/env.d/04gcc-${CTARGET} after
/usr/bin PATH injection to restore gcc-config wrapping.
See https://bugs.gentoo.org/255695 for some details.
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