From 6ea224f538ca7e40a43564d3d7089293478abaab Mon Sep 17 00:00:00 2001
From: Erwin Jansen <jansene@google.com>
Date: Sat, 25 Oct 2025 09:47:54 -0700
Subject: [PATCH 1/2] test(arm-linux): Add failing C atomics compilation test

Verify compilation of a C file that uses C11 atomics, which currently
fails due to the toolchain incorrectly including C++ headers when
compiling a C file.

This test documents the existing issue where C files pick up C++
include paths, resulting in build failures. It also adds the new
target to the `arm_linux_test` build test exposing the issue.
---
 examples/bzlmod/BUILD               |  5 ++++-
 examples/bzlmod/arm-linux/BUILD     | 11 +++++++----
 examples/bzlmod/arm-linux/atomics.c | 21 +++++++++++++++++++++
 3 files changed, 32 insertions(+), 5 deletions(-)
 create mode 100644 examples/bzlmod/arm-linux/atomics.c

diff --git a/examples/bzlmod/BUILD b/examples/bzlmod/BUILD
index 9e1996d..08d13f7 100644
--- a/examples/bzlmod/BUILD
+++ b/examples/bzlmod/BUILD
@@ -6,7 +6,10 @@ load("@bazel_skylib//rules:build_test.bzl", "build_test")
 
 build_test(
     name = "arm_linux_test",
-    targets = ["//arm-linux:hex"],
+    targets = [
+        "//arm-linux:hex",
+        "//arm-linux:atomics",
+    ],
 )
 
 build_test(
diff --git a/examples/bzlmod/arm-linux/BUILD b/examples/bzlmod/arm-linux/BUILD
index 128131e..2e054ae 100644
--- a/examples/bzlmod/arm-linux/BUILD
+++ b/examples/bzlmod/arm-linux/BUILD
@@ -25,10 +25,13 @@ cc_library(
         "-mthumb",
     ],
     includes = ["includes"],
-    target_compatible_with = [
-        "@platforms//cpu:arm",
-        "@platforms//os:linux",
-    ],
+   )
+
+cc_library(
+    name = "atomics",
+    srcs = ["atomics.c"],
+    copts = ["-std=c11"],
+    visibility = ["//visibility:public"],
 )
 
 cc_binary(
diff --git a/examples/bzlmod/arm-linux/atomics.c b/examples/bzlmod/arm-linux/atomics.c
new file mode 100644
index 0000000..30ff021
--- /dev/null
+++ b/examples/bzlmod/arm-linux/atomics.c
@@ -0,0 +1,21 @@
+#include <stdio.h>
+
+// This should pick up the C-variant, not C++ one,
+#include <stdatomic.h>
+
+/**
+ * Minimal C11 stdatomic.h example without threads.
+ *
+ * This example is designed to test the compiler's support for the
+ * C11 _Atomic type specifier and the functions provided by <stdatomic.h>.
+ */
+int main(void) {
+    atomic_int counter = ATOMIC_VAR_INIT(0);
+    atomic_fetch_add(&counter, 1);
+    int final_value = atomic_load(&counter);
+
+    printf("Atomic counter initialized to 0, incremented once.\n");
+    printf("Final value (read atomically): %d\n", final_value);
+
+    return 0;
+}
\ No newline at end of file

From a9cf01056ac94cbf509de1be36351a4ad3fead24 Mon Sep 17 00:00:00 2001
From: Erwin Jansen <jansene@google.com>
Date: Sat, 25 Oct 2025 11:28:48 -0700
Subject: [PATCH 2/2] fix(toolchain): Remove explicit gcc_tool from toolchain
 definition

Remove the hardcoded `gcc_tool = "g++"` argument from the
`arm_none_eabi_toolchain` definition.

This explicit setting was incorrectly forcing the C compiler to use
the C++ frontend (g++) for C files, which led to compilation failures
when attempting to use C-specific features like C11 atomics.

By removing this argument, Bazel is allowed to correctly infer the
appropriate C compiler (`gcc`) for C sources and the C++ compiler
(`g++`) for C++ sources, resolving the build issue documented in the
previous test commit.

Fixes: 83
---
 examples/bzlmod/custom/toolchain/BUILD | 1 -
 1 file changed, 1 deletion(-)

diff --git a/examples/bzlmod/custom/toolchain/BUILD b/examples/bzlmod/custom/toolchain/BUILD
index ad3960c..882ca00 100644
--- a/examples/bzlmod/custom/toolchain/BUILD
+++ b/examples/bzlmod/custom/toolchain/BUILD
@@ -32,7 +32,6 @@ arm_none_eabi_toolchain(
         "-mfloat-abi=hard",
         "-mfpu=fpv4-sp-d16",
     ],
-    gcc_tool = "g++",
     linkopts = [
         "-mcpu=cortex-m4",
         "-mthumb",