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WIP: cc-wrapper: -nostdlib doessn't mean not C++ #85513

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WIP: cc-wrapper: -nostdlib doessn't mean not C++ #85513

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Ericson2314
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Motivation for this change

This was introduced in 87607af, which
made the wrapper aware of -x <lang> flags. I am not sure why this new
case was added, especially with the -l that looks like a mistake.

FWIW, https://github.com/briansmith/nostdlib is an example of writing C++
with -nosdlib.

Things done
  • Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
  • Built on platform(s)
    • NixOS
    • macOS
    • other Linux distributions
  • Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • Tested execution of all binary files (usually in ./result/bin/)
  • Determined the impact on package closure size (by running nix path-info -S before and after)
  • Ensured that relevant documentation is up to date
  • Fits CONTRIBUTING.md.

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@Ericson2314
cc-wrapper: -nostdlib doessn't mean not C++

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ae175a8 
This was introduced in 87607af, which
made the wrapper aware of `-x <lang>` flags. I am not sure why this new
case was added, especially with the `-l` that looks like a mistake.

FWIW, https://github.com/briansmith/nostdlib is an example of writing C++
with -nosdlib.
@veprbl
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veprbl commented Apr 18, 2020

I don't see the contradiction. "isCpp=-1" disables linking to the standard c++ libraries, you can write C++ code with -nosdlib, but you would not be able to use the libraries anymore. Perhaps, should rename the variable instead.

@Ericson2314 Ericson2314 changed the title cc-wrapper: -nostdlib doessn't mean not C++ WIP: cc-wrapper: -nostdlib doessn't mean not C++ Apr 18, 2020
@Ericson2314
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Do those flags with GCC affect search paths? Right now we control some -l and -L together which also seems wrong. I will mark this WIP while we figure out what should happen.

@Ericson2314
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@Mic92 Might you have some insight here, since you just did the -nostdinc fix?

@Mic92
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Mic92 commented Aug 3, 2020

Maybe it could set a noCppLink but still allow to treat the file as c++.

@Ericson2314
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Well does -nostdlib actually affect lookup paths, or just what libraries are linked by the default? https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html doen't mention search paths so I think it's just the latter.

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Mic92 commented Aug 4, 2020 •
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Well does -nostdlib actually affect lookup paths, or just what libraries are linked by the default? gcc.gnu.org/onlinedocs/gcc/Link-Options.html doen't mention search paths so I think it's just the latter.

It skip linking startup files, search paths are kept intact. The following builds:

$ printf '#include <optional>\n extern \"C\" { void _start(){} }' | g++  -nostdlib -o foo -xc++ -

@Ericson2314
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Great, thanks for checking! I think that means this PR is correct, do you?

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Mic92 commented Aug 5, 2020 •
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Great, thanks for checking! I think that means this PR is correct, do you?

I don't think it is a complete fix for it. Since the wrapper adds NIX_CXXSTDLIB_LINK to NIX_CFLAGS_LINK it would still link those libraries since they have been passed manually from the compilers point of view. Therefore I would say we need a dedicated variable set that will not add those linker flags while still having c++ header in the search path.

@Ericson2314
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Oh right! Because we manually do e.g. -lc++abi to force some odd combinations of GCC and LLVM stuff. Thanks! Now I know why this was written this way in the first place---as a crude fix around that.

@ryantm ryantm marked this pull request as draft October 23, 2020 03:01
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stale bot commented Jun 4, 2021

I marked this as stale due to inactivity. → More info

@stale stale bot added the 2.status: stale https://github.com/NixOS/nixpkgs/blob/master/.github/STALE-BOT.md label Jun 4, 2021
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veprbl commented Jun 17, 2021

Fixed by #121527

@veprbl veprbl closed this Jun 17, 2021
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