[root@asterinas:~]# cat /proc/version
Linux version 5.13.0 (root@kiryca) (rustc 1.94.0-nightly) #1 Wed Dec 17 02:57:56 UTC 2025

Hi all,
I'm trying to write a derivation for an embedded C++ microcontroller application.
For this, I must use a copy of libstdc++ with something like --enable-cxx-flags='-fno-exceptions -fno-rtti' (docs here).
Is there a way to configure Nixpkgs to use these settings for libstdc++ for the host platform? Something to add to the following, for example?

import <nixpkgs> {
  crossSystem = {
    config = "arm-none-eabihf";
    libc = "newlib-nano";
    gcc = {
      cpu = "cortex-m7";
      fpu = "fpv5-d16";
      float-abi = "hard";
      thumb = true;
    };
    rust.rustcTarget = "thumbv7em-none-eabihf";
    isStatic = true;
  }
}

I'm looking at something like this patch to Nixpkgs at the moment, but hopefully there's an existing mechanism I'm yet to discover

--- a/pkgs/development/compilers/gcc/common/platform-flags.nix
+++ b/pkgs/development/compilers/gcc/common/platform-flags.nix
@@ -17,6 +17,7 @@ lib.concatLists [
   (lib.optional (p ? fpu) "--with-fpu=${p.fpu}")
   (lib.optional (p ? float) "--with-float=${p.float}")
   (lib.optional (p ? mode) "--with-mode=${p.mode}")
+  (lib.optional (p ? enableCxxFlags) "--enable-cxx-flags=${builtins.concatStringsSep ''\ '' p.enableCxxFlags}")
   (lib.optionals targetPlatform.isPower64
     # musl explicitly rejects 128-bit long double on
     # powerpc64; see musl/arch/powerpc64/bits/float.h

on commit 67f0c66 I run the command nom build .#base.vmClosureInfo --out-link .gcroots/vm --max-jobs 1 as the readme says and the build fails with the following output:

┏━ 2 Errors:
┃ error: Cannot build '/nix/store/2l49vw52k9gd2y2iw2psd87jnnadxj9x-closure-info.drv'.
┃        Reason: 1 dependency failed.
┃        Output paths:
┃          /nix/store/8v213znd8vm4qff7830g2l17y10vbhaa-closure-info
┃ error: Cannot build '/nix/store/5sq1f0ji91j1czjx0km33g72pjg6g2cx-lowdown-2.0.2.drv'.
┃        Reason: builder failed with exit code 1.
┃        Output paths:
┃          /nix/store/1ysm97d9py1a37k0mwbp4ph83p3adszx-lowdown-2.0.2-lib
┃          /nix/store/801d76j8vkn15cvwnhrdrqig532m57hh-lowdown-2.0.2
┃          /nix/store/dvjbv8rjy9wrw9nryjlf777m1ljhyzsp-lowdown-2.0.2-man
┃          /nix/store/ka6ippkbsr6r1w3rnxbaj1ba2dfsjrjq-lowdown-2.0.2-dev
┃        Last 14 log lines:
┃        > Running phase: unpackPhase
┃        > unpacking source archive /nix/store/0wzq3ghg2csxz804r9jdf1vnjbga6agr-lowdown-2.0.2.tar.gz
┃        > source root is lowdown-2.0.2
┃        > setting SOURCE_DATE_EPOCH to timestamp 1739739200 of file "lowdown-2.0.2/regress/template/var2.xml"
┃        > Running phase: patchPhase
┃        > applying patch /nix/store/8s6y8pq08z8nsvyhy5fs097k0fhdnlms-fix-cygwin-build.patch
┃        > patching file Makefile
┃        > Hunk #1 FAILED at 161.
┃        > Hunk #2 FAILED at 190.
┃        > Hunk #3 FAILED at 202.
┃        > Hunk #4 succeeded at 206 (offset -34 lines).
┃        > Hunk #5 FAILED at 273.
┃        > Hunk #6 FAILED at 406.
┃        > 5 out of 6 hunks FAILED -- saving rejects to file Makefile.rej
┃        For full logs, run:
┃          nix log /nix/store/5sq1f0ji91j1czjx0km33g72pjg6g2cx-lowdown-2.0.2.drv
┣━ Dependency Graph:
┃          ┌─ ⏸ sdl2-compat-2.32.60 waiting for 1 ⏵
┃       ┌─ ⏸ qemu-host-cpu-only-10.1.2
┃       │  ┌─ ⏸ closure-info waiting for 1 ⏵
┃       │  ├─ ⏸ closure-info waiting for 1 ⏵
┃       │  │              ┌─ ⏸ fcrontab waiting for 1 ⏵
┃       │  │           ┌─ ⏸ fcron
┃       │  │        ┌─ ⏸ rc.d
┃       │  │        │                 ┌─ ⏸ nix-cmd-2.34.0pre waiting for 1 ⏵
┃       │  │        │              ┌─ ⏸ nix-2.34.0pre20251214_9799fcb
┃       │  │        │           ┌─ ⏸ lazy-options.json
┃       │  │        │        ┌─ ⏸ options.json
┃       │  │        │     ┌─ ⏸ nixos-configuration-reference-manpage
┃       │  │        │  ┌─ ⏸ system-path
┃       │  │        ├─ ⏸ fcron.conf
┃       │  │     ┌─ ⏸ etc
┃       │  │  ┌─ ⏸ nixos-system-nixbsd-base-x86_64-unknown-freebsd
┃       │  ├─ ⏸ espDerivation
┃       ├─ ⏸ nixbsd-disk-image
┃    ┌─ ⏸ run-nixos-vm
┃ ┌─ ⏸ nixos-vm
┃ ├─ ⏵ openblas-0.3.30
┃ ├─ ⏵ lowdown-2.0.2 (patchPhase)
┃ ⏸ closure-info
┣━━━ Builds
┗━ ∑ ⏵ 2 │ ✔ 0 │ ⏸ 1125 │ ⚠ Exited with 2 errors reported by nix at 22:10:47 after 4s

some patch failed while building lowdown. Do any of yall know why this happened? Is this something silly or should I maybe file an error report?

it's not on lix, rather cppnix. The flake inputs section is as follows:

it's not on lix, rather cppnix. The flake inputs section is as follows:

inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/staging";
    # I can't figure out how to rebase our lix fork easily. Let's use cppnix for now.
    # lix = {
    #   url = "git+https://git.lix.systems/artemist/lix.git?ref=freebsd-build";
    #   inputs.nixpkgs.follows = "nixpkgs";
    #   # We don't need another nixpkgs clone, it won't evaluate anyway
    #   inputs.nixpkgs-regression.follows = "nixpkgs";
    # };
    cppnix = {
      url = "github:rhelmot/nix/freebsd";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    mini-tmpfiles = {
      url = "github:nixos-bsd/mini-tmpfiles";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    flake-compat.url = "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz";
  };

  nixConfig = {
    extra-substituters = [ "https://attic.mildlyfunctional.gay/nixbsd" ];
    extra-trusted-public-keys =
      [ "nixbsd:gwcQlsUONBLrrGCOdEboIAeFq9eLaDqfhfXmHZs1mgc=" ];
  };