diff --git a/src/libexpr-c/nix_api_expr.cc b/src/libexpr-c/nix_api_expr.cc
index 02e901de9..a028202ae 100644
--- a/src/libexpr-c/nix_api_expr.cc
+++ b/src/libexpr-c/nix_api_expr.cc
@@ -40,6 +40,8 @@ static T * unsafe_new_with_self(F && init)
     return new (p) T(init(static_cast<T *>(p)));
 }
 
+extern "C" {
+
 nix_err nix_libexpr_init(nix_c_context * context)
 {
     if (context)
@@ -287,3 +289,5 @@ void nix_gc_register_finalizer(void * obj, void * cd, void (*finalizer)(void * o
     GC_REGISTER_FINALIZER(obj, finalizer, cd, 0, 0);
 #endif
 }
+
+} // extern "C"
diff --git a/src/libexpr-c/nix_api_expr_internal.h b/src/libexpr-c/nix_api_expr_internal.h
index a26595cec..3aa1d9932 100644
--- a/src/libexpr-c/nix_api_expr_internal.h
+++ b/src/libexpr-c/nix_api_expr_internal.h
@@ -8,6 +8,8 @@
 #include "nix_api_value.h"
 #include "nix/expr/search-path.hh"
 
+extern "C" {
+
 struct nix_eval_state_builder
 {
     nix::ref<nix::Store> store;
@@ -61,4 +63,6 @@ struct nix_realised_string
     std::vector<StorePath> storePaths;
 };
 
+} // extern "C"
+
 #endif // NIX_API_EXPR_INTERNAL_H
diff --git a/src/libexpr-c/nix_api_external.cc b/src/libexpr-c/nix_api_external.cc
index ecb67cfb4..ff2950448 100644
--- a/src/libexpr-c/nix_api_external.cc
+++ b/src/libexpr-c/nix_api_external.cc
@@ -14,6 +14,8 @@
 
 #include <nlohmann/json.hpp>
 
+extern "C" {
+
 void nix_set_string_return(nix_string_return * str, const char * c)
 {
     str->str = c;
@@ -40,6 +42,8 @@ nix_err nix_external_add_string_context(nix_c_context * context, nix_string_cont
     NIXC_CATCH_ERRS
 }
 
+} // extern "C"
+
 class NixCExternalValue : public nix::ExternalValueBase
 {
     NixCExternalValueDesc & desc;
@@ -170,6 +174,8 @@ public:
     virtual ~NixCExternalValue() override {};
 };
 
+extern "C" {
+
 ExternalValue * nix_create_external_value(nix_c_context * context, NixCExternalValueDesc * desc, void * v)
 {
     if (context)
@@ -198,3 +204,5 @@ void * nix_get_external_value_content(nix_c_context * context, ExternalValue * b
     }
     NIXC_CATCH_ERRS_NULL
 }
+
+} // extern "C"
diff --git a/src/libexpr-c/nix_api_value.cc b/src/libexpr-c/nix_api_value.cc
index fb90e2872..0f6595e49 100644
--- a/src/libexpr-c/nix_api_value.cc
+++ b/src/libexpr-c/nix_api_value.cc
@@ -111,6 +111,8 @@ static void nix_c_primop_wrapper(
     v = vTmp;
 }
 
+extern "C" {
+
 PrimOp * nix_alloc_primop(
     nix_c_context * context,
     PrimOpFun fun,
@@ -651,3 +653,5 @@ const StorePath * nix_realised_string_get_store_path(nix_realised_string * s, si
 {
     return &s->storePaths[i];
 }
+
+} // extern "C"
diff --git a/src/libexpr-tests/meson.build b/src/libexpr-tests/meson.build
index a876e9705..c5dafe0de 100644
--- a/src/libexpr-tests/meson.build
+++ b/src/libexpr-tests/meson.build
@@ -55,6 +55,7 @@ sources = files(
   'nix_api_expr.cc',
   'nix_api_external.cc',
   'nix_api_value.cc',
+  'nix_api_value_internal.cc',
   'primops.cc',
   'search-path.cc',
   'trivial.cc',
diff --git a/src/libexpr-tests/nix_api_expr.cc b/src/libexpr-tests/nix_api_expr.cc
index 529c2f584..5e0868b6e 100644
--- a/src/libexpr-tests/nix_api_expr.cc
+++ b/src/libexpr-tests/nix_api_expr.cc
@@ -1,7 +1,5 @@
 #include "nix_api_store.h"
-#include "nix_api_store_internal.h"
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix_api_expr.h"
 #include "nix_api_value.h"
 
@@ -151,8 +149,8 @@ TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_value)
     assert_ctx_ok();
     auto r = nix_string_realise(ctx, state, value, false);
     ASSERT_EQ(nullptr, r);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("cannot coerce")));
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("cannot coerce"));
 }
 
 TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_build)
@@ -168,8 +166,8 @@ TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_build)
     assert_ctx_ok();
     auto r = nix_string_realise(ctx, state, value, false);
     ASSERT_EQ(nullptr, r);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("failed with exit code 1")));
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("failed with exit code 1"));
 }
 
 TEST_F(nix_api_expr_test, nix_expr_realise_context)
@@ -381,12 +379,11 @@ TEST_F(nix_api_expr_test, nix_expr_primop_bad_no_return)
     nix_value * result = nix_alloc_value(ctx, state);
     assert_ctx_ok();
     nix_value_call(ctx, state, primopValue, three, result);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
     ASSERT_THAT(
-        ctx->last_err,
-        testing::Optional(
-            testing::HasSubstr("Implementation error in custom function: return value was not initialized")));
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("badNoReturn")));
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("Implementation error in custom function: return value was not initialized"));
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("badNoReturn"));
 }
 
 static void primop_bad_return_thunk(
@@ -419,12 +416,11 @@ TEST_F(nix_api_expr_test, nix_expr_primop_bad_return_thunk)
     assert_ctx_ok();
     NIX_VALUE_CALL(ctx, state, result, primopValue, toString, four);
 
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
     ASSERT_THAT(
-        ctx->last_err,
-        testing::Optional(
-            testing::HasSubstr("Implementation error in custom function: return value must not be a thunk")));
-    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("badReturnThunk")));
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("Implementation error in custom function: return value must not be a thunk"));
+    ASSERT_THAT(nix_err_msg(nullptr, ctx, nullptr), testing::HasSubstr("badReturnThunk"));
 }
 
 TEST_F(nix_api_expr_test, nix_value_call_multi_no_args)
diff --git a/src/libexpr-tests/nix_api_external.cc b/src/libexpr-tests/nix_api_external.cc
index 93da3ca39..ec19f1212 100644
--- a/src/libexpr-tests/nix_api_external.cc
+++ b/src/libexpr-tests/nix_api_external.cc
@@ -1,9 +1,6 @@
 #include "nix_api_store.h"
-#include "nix_api_store_internal.h"
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix_api_expr.h"
-#include "nix_api_expr_internal.h"
 #include "nix_api_value.h"
 #include "nix_api_external.h"
 
@@ -39,7 +36,7 @@ private:
         std::string type_string = "nix-external<MyExternalValueDesc( ";
         type_string += std::to_string(obj->_x);
         type_string += " )>";
-        res->str = &*type_string.begin();
+        nix_set_string_return(res, &*type_string.begin());
     }
 };
 
diff --git a/src/libexpr-tests/nix_api_value.cc b/src/libexpr-tests/nix_api_value.cc
index 5d85ed68d..af95224de 100644
--- a/src/libexpr-tests/nix_api_value.cc
+++ b/src/libexpr-tests/nix_api_value.cc
@@ -1,10 +1,7 @@
 #include "nix_api_store.h"
-#include "nix_api_store_internal.h"
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix_api_expr.h"
 #include "nix_api_value.h"
-#include "nix_api_expr_internal.h"
 
 #include "nix/expr/tests/nix_api_expr.hh"
 #include "nix/util/tests/string_callback.hh"
@@ -16,14 +13,6 @@
 
 namespace nixC {
 
-TEST_F(nix_api_expr_test, as_nix_value_ptr)
-{
-    // nix_alloc_value casts nix::Value to nix_value
-    // It should be obvious from the decl that that works, but if it doesn't,
-    // the whole implementation would be utterly broken.
-    ASSERT_EQ(sizeof(nix::Value), sizeof(nix_value));
-}
-
 TEST_F(nix_api_expr_test, nix_value_get_int_invalid)
 {
     ASSERT_EQ(0, nix_get_int(ctx, nullptr));
@@ -320,8 +309,10 @@ TEST_F(nix_api_expr_test, nix_value_init_apply_error)
 
     // Evaluate it
     nix_value_force(ctx, state, v);
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
-    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("attempt to call something which is not a function but"));
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("attempt to call something which is not a function but"));
 
     // Clean up
     nix_gc_decref(ctx, some_string);
@@ -380,7 +371,9 @@ TEST_F(nix_api_expr_test, nix_value_init_apply_lazy_arg)
     // nix_get_attr_byname isn't lazy (it could have been) so it will throw the exception
     nix_value * foo = nix_get_attr_byname(ctx, r, state, "foo");
     ASSERT_EQ(nullptr, foo);
-    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("error message for test case nix_value_init_apply_lazy_arg"));
+    ASSERT_THAT(
+        nix_err_msg(nullptr, ctx, nullptr),
+        testing::HasSubstr("error message for test case nix_value_init_apply_lazy_arg"));
 
     // Clean up
     nix_gc_decref(ctx, f);
diff --git a/src/libexpr-tests/nix_api_value_internal.cc b/src/libexpr-tests/nix_api_value_internal.cc
new file mode 100644
index 000000000..34db6ac81
--- /dev/null
+++ b/src/libexpr-tests/nix_api_value_internal.cc
@@ -0,0 +1,25 @@
+#include "nix_api_store.h"
+#include "nix_api_util.h"
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+#include "nix_api_expr_internal.h"
+
+#include "nix/expr/tests/nix_api_expr.hh"
+#include "nix/util/tests/string_callback.hh"
+
+#include <gmock/gmock.h>
+#include <cstddef>
+#include <cstdlib>
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+TEST_F(nix_api_expr_test, as_nix_value_ptr)
+{
+    // nix_alloc_value casts nix::Value to nix_value
+    // It should be obvious from the decl that that works, but if it doesn't,
+    // the whole implementation would be utterly broken.
+    ASSERT_EQ(sizeof(nix::Value), sizeof(nix_value));
+}
+
+} // namespace nixC
diff --git a/src/libfetchers-c/nix_api_fetchers.cc b/src/libfetchers-c/nix_api_fetchers.cc
index 4e8037a5e..7fefedb0c 100644
--- a/src/libfetchers-c/nix_api_fetchers.cc
+++ b/src/libfetchers-c/nix_api_fetchers.cc
@@ -2,6 +2,8 @@
 #include "nix_api_fetchers_internal.hh"
 #include "nix_api_util_internal.h"
 
+extern "C" {
+
 nix_fetchers_settings * nix_fetchers_settings_new(nix_c_context * context)
 {
     try {
@@ -17,3 +19,5 @@ void nix_fetchers_settings_free(nix_fetchers_settings * settings)
 {
     delete settings;
 }
+
+} // extern "C"
diff --git a/src/libflake-c/nix_api_flake.cc b/src/libflake-c/nix_api_flake.cc
index ad8f0bf4e..2de0e667e 100644
--- a/src/libflake-c/nix_api_flake.cc
+++ b/src/libflake-c/nix_api_flake.cc
@@ -10,6 +10,8 @@
 
 #include "nix/flake/flake.hh"
 
+extern "C" {
+
 nix_flake_settings * nix_flake_settings_new(nix_c_context * context)
 {
     nix_clear_err(context);
@@ -203,3 +205,5 @@ nix_value * nix_locked_flake_get_output_attrs(
     }
     NIXC_CATCH_ERRS_NULL
 }
+
+} // extern "C"
diff --git a/src/libmain-c/nix_api_main.cc b/src/libmain-c/nix_api_main.cc
index eacb80455..2d4f588a8 100644
--- a/src/libmain-c/nix_api_main.cc
+++ b/src/libmain-c/nix_api_main.cc
@@ -5,6 +5,8 @@
 
 #include "nix/main/plugin.hh"
 
+extern "C" {
+
 nix_err nix_init_plugins(nix_c_context * context)
 {
     if (context)
@@ -14,3 +16,5 @@ nix_err nix_init_plugins(nix_c_context * context)
     }
     NIXC_CATCH_ERRS
 }
+
+} // extern "C"
diff --git a/src/libstore-c/nix_api_store.cc b/src/libstore-c/nix_api_store.cc
index 4f91f5332..7ce63f5c2 100644
--- a/src/libstore-c/nix_api_store.cc
+++ b/src/libstore-c/nix_api_store.cc
@@ -10,6 +10,8 @@
 
 #include "nix/store/globals.hh"
 
+extern "C" {
+
 nix_err nix_libstore_init(nix_c_context * context)
 {
     if (context)
@@ -91,7 +93,7 @@ nix_store_get_version(nix_c_context * context, Store * store, nix_get_string_cal
     NIXC_CATCH_ERRS
 }
 
-bool nix_store_is_valid_path(nix_c_context * context, Store * store, StorePath * path)
+bool nix_store_is_valid_path(nix_c_context * context, Store * store, const StorePath * path)
 {
     if (context)
         context->last_err_code = NIX_OK;
@@ -129,7 +131,7 @@ nix_err nix_store_realise(
     Store * store,
     StorePath * path,
     void * userdata,
-    void (*callback)(void * userdata, const char *, const char *))
+    void (*callback)(void * userdata, const char *, const StorePath *))
 {
     if (context)
         context->last_err_code = NIX_OK;
@@ -144,8 +146,8 @@ nix_err nix_store_realise(
         if (callback) {
             for (const auto & result : results) {
                 for (const auto & [outputName, realisation] : result.builtOutputs) {
-                    auto op = store->ptr->printStorePath(realisation.outPath);
-                    callback(userdata, outputName.c_str(), op.c_str());
+                    StorePath p{realisation.outPath};
+                    callback(userdata, outputName.c_str(), &p);
                 }
             }
         }
@@ -180,3 +182,5 @@ nix_err nix_store_copy_closure(nix_c_context * context, Store * srcStore, Store
     }
     NIXC_CATCH_ERRS
 }
+
+} // extern "C"
diff --git a/src/libstore-c/nix_api_store.h b/src/libstore-c/nix_api_store.h
index ad3d7b22a..51bd1bc89 100644
--- a/src/libstore-c/nix_api_store.h
+++ b/src/libstore-c/nix_api_store.h
@@ -148,7 +148,7 @@ void nix_store_path_free(StorePath * p);
  * @param[in] path Path to check
  * @return true or false, error info in context
  */
-bool nix_store_is_valid_path(nix_c_context * context, Store * store, StorePath * path);
+bool nix_store_is_valid_path(nix_c_context * context, Store * store, const StorePath * path);
 
 /**
  * @brief Get the physical location of a store path
@@ -190,7 +190,7 @@ nix_err nix_store_realise(
     Store * store,
     StorePath * path,
     void * userdata,
-    void (*callback)(void * userdata, const char * outname, const char * out));
+    void (*callback)(void * userdata, const char * outname, const StorePath * out));
 
 /**
  * @brief get the version of a nix store.
diff --git a/src/libstore-c/nix_api_store_internal.h b/src/libstore-c/nix_api_store_internal.h
index b0194bfd3..cbe04b2c7 100644
--- a/src/libstore-c/nix_api_store_internal.h
+++ b/src/libstore-c/nix_api_store_internal.h
@@ -2,6 +2,8 @@
 #define NIX_API_STORE_INTERNAL_H
 #include "nix/store/store-api.hh"
 
+extern "C" {
+
 struct Store
 {
     nix::ref<nix::Store> ptr;
@@ -12,4 +14,6 @@ struct StorePath
     nix::StorePath path;
 };
 
+} // extern "C"
+
 #endif
diff --git a/src/libstore-tests/derivation-parser-bench.cc b/src/libstore-tests/derivation-parser-bench.cc
index 61c9807a6..1709eed1c 100644
--- a/src/libstore-tests/derivation-parser-bench.cc
+++ b/src/libstore-tests/derivation-parser-bench.cc
@@ -51,18 +51,10 @@ static void BM_UnparseRealDerivationFile(benchmark::State & state, const std::st
 
 // Register benchmarks for actual test derivation files if they exist
 BENCHMARK_CAPTURE(
-    BM_ParseRealDerivationFile,
-    hello,
-    getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value_or(NIX_UNIT_TEST_DATA) + "/derivation/hello.drv");
+    BM_ParseRealDerivationFile, hello, getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value() + "/derivation/hello.drv");
 BENCHMARK_CAPTURE(
-    BM_ParseRealDerivationFile,
-    firefox,
-    getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value_or(NIX_UNIT_TEST_DATA) + "/derivation/firefox.drv");
+    BM_ParseRealDerivationFile, firefox, getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value() + "/derivation/firefox.drv");
 BENCHMARK_CAPTURE(
-    BM_UnparseRealDerivationFile,
-    hello,
-    getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value_or(NIX_UNIT_TEST_DATA) + "/derivation/hello.drv");
+    BM_UnparseRealDerivationFile, hello, getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value() + "/derivation/hello.drv");
 BENCHMARK_CAPTURE(
-    BM_UnparseRealDerivationFile,
-    firefox,
-    getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value_or(NIX_UNIT_TEST_DATA) + "/derivation/firefox.drv");
+    BM_UnparseRealDerivationFile, firefox, getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value() + "/derivation/firefox.drv");
diff --git a/src/libstore-tests/meson.build b/src/libstore-tests/meson.build
index fced20269..4c2840ab7 100644
--- a/src/libstore-tests/meson.build
+++ b/src/libstore-tests/meson.build
@@ -130,10 +130,13 @@ if get_option('benchmarks')
     link_args : linker_export_flags,
     install : true,
     cpp_pch : do_pch ? [ 'pch/precompiled-headers.hh' ] : [],
-    cpp_args : [
-      '-DNIX_UNIT_TEST_DATA="' + meson.current_source_dir() + '/data"',
-    ],
   )
 
-  benchmark('nix-store-benchmarks', benchmark_exe)
+  benchmark(
+    'nix-store-benchmarks',
+    benchmark_exe,
+    env : {
+      '_NIX_TEST_UNIT_DATA' : meson.current_source_dir() / 'data',
+    },
+  )
 endif
diff --git a/src/libstore-tests/nix_api_store.cc b/src/libstore-tests/nix_api_store.cc
index c7146f977..c14fb6d9f 100644
--- a/src/libstore-tests/nix_api_store.cc
+++ b/src/libstore-tests/nix_api_store.cc
@@ -1,7 +1,5 @@
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix_api_store.h"
-#include "nix_api_store_internal.h"
 
 #include "nix/store/tests/nix_api_store.hh"
 #include "nix/util/tests/string_callback.hh"
@@ -65,7 +63,7 @@ TEST_F(nix_api_store_test, nix_store_get_storedir)
 TEST_F(nix_api_store_test, InvalidPathFails)
 {
     nix_store_parse_path(ctx, store, "invalid-path");
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
 }
 
 TEST_F(nix_api_store_test, ReturnsValidStorePath)
@@ -80,7 +78,7 @@ TEST_F(nix_api_store_test, ReturnsValidStorePath)
 TEST_F(nix_api_store_test, SetsLastErrCodeToNixOk)
 {
     StorePath * path = nix_store_parse_path(ctx, store, (nixStoreDir + PATH_SUFFIX).c_str());
-    ASSERT_EQ(ctx->last_err_code, NIX_OK);
+    ASSERT_EQ(nix_err_code(ctx), NIX_OK);
     nix_store_path_free(path);
 }
 
@@ -103,7 +101,7 @@ TEST_F(nix_api_util_context, nix_store_open_dummy)
 {
     nix_libstore_init(ctx);
     Store * store = nix_store_open(ctx, "dummy://", nullptr);
-    ASSERT_EQ(NIX_OK, ctx->last_err_code);
+    ASSERT_EQ(NIX_OK, nix_err_code(ctx));
     ASSERT_STREQ("dummy://", store->ptr->config.getReference().render(/*withParams=*/true).c_str());
 
     std::string str;
@@ -117,7 +115,7 @@ TEST_F(nix_api_util_context, nix_store_open_invalid)
 {
     nix_libstore_init(ctx);
     Store * store = nix_store_open(ctx, "invalid://", nullptr);
-    ASSERT_EQ(NIX_ERR_NIX_ERROR, ctx->last_err_code);
+    ASSERT_EQ(NIX_ERR_NIX_ERROR, nix_err_code(ctx));
     ASSERT_EQ(nullptr, store);
     nix_store_free(store);
 }
diff --git a/src/libutil-c/nix_api_util.cc b/src/libutil-c/nix_api_util.cc
index 2254f18fa..a43e7103b 100644
--- a/src/libutil-c/nix_api_util.cc
+++ b/src/libutil-c/nix_api_util.cc
@@ -9,6 +9,8 @@
 
 #include "nix_api_util_config.h"
 
+extern "C" {
+
 nix_c_context * nix_c_context_create()
 {
     return new nix_c_context();
@@ -156,3 +158,5 @@ nix_err call_nix_get_string_callback(const std::string str, nix_get_string_callb
     callback(str.c_str(), str.size(), user_data);
     return NIX_OK;
 }
+
+} // extern "C"
diff --git a/src/libutil-c/nix_api_util_internal.h b/src/libutil-c/nix_api_util_internal.h
index 664cd6e23..92bb9c1d2 100644
--- a/src/libutil-c/nix_api_util_internal.h
+++ b/src/libutil-c/nix_api_util_internal.h
@@ -7,6 +7,8 @@
 #include "nix/util/error.hh"
 #include "nix_api_util.h"
 
+extern "C" {
+
 struct nix_c_context
 {
     nix_err last_err_code = NIX_OK;
@@ -47,4 +49,6 @@ nix_err call_nix_get_string_callback(const std::string str, nix_get_string_callb
     }
 #define NIXC_CATCH_ERRS_NULL NIXC_CATCH_ERRS_RES(nullptr)
 
+} // extern "C"
+
 #endif // NIX_API_UTIL_INTERNAL_H
diff --git a/src/libutil-test-support/include/nix/util/tests/nix_api_util.hh b/src/libutil-test-support/include/nix/util/tests/nix_api_util.hh
index 57f7f1ecf..cc1d244f5 100644
--- a/src/libutil-test-support/include/nix/util/tests/nix_api_util.hh
+++ b/src/libutil-test-support/include/nix/util/tests/nix_api_util.hh
@@ -54,4 +54,12 @@ protected:
 #define assert_ctx_err() assert_ctx_err(__FILE__, __LINE__)
 };
 
+static inline auto createOwnedNixContext()
+{
+    return std::unique_ptr<nix_c_context, decltype([](nix_c_context * ctx) {
+                               if (ctx)
+                                   nix_c_context_free(ctx);
+                           })>(nix_c_context_create(), {});
+}
+
 } // namespace nixC
diff --git a/src/libutil-tests/meson.build b/src/libutil-tests/meson.build
index 0e2a2e468..ff71d2215 100644
--- a/src/libutil-tests/meson.build
+++ b/src/libutil-tests/meson.build
@@ -63,6 +63,7 @@ sources = files(
   'lru-cache.cc',
   'monitorfdhup.cc',
   'nix_api_util.cc',
+  'nix_api_util_internal.cc',
   'pool.cc',
   'position.cc',
   'processes.cc',
diff --git a/src/libutil-tests/nix_api_util.cc b/src/libutil-tests/nix_api_util.cc
index 9693ab3a5..48f85c403 100644
--- a/src/libutil-tests/nix_api_util.cc
+++ b/src/libutil-tests/nix_api_util.cc
@@ -1,7 +1,6 @@
 #include "nix/util/config-global.hh"
 #include "nix/util/args.hh"
 #include "nix_api_util.h"
-#include "nix_api_util_internal.h"
 #include "nix/util/tests/nix_api_util.hh"
 #include "nix/util/tests/string_callback.hh"
 
@@ -13,41 +12,6 @@
 
 namespace nixC {
 
-TEST_F(nix_api_util_context, nix_context_error)
-{
-    std::string err_msg_ref;
-    try {
-        throw nix::Error("testing error");
-    } catch (nix::Error & e) {
-        err_msg_ref = e.what();
-        nix_context_error(ctx);
-    }
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
-    ASSERT_EQ(ctx->name, "nix::Error");
-    ASSERT_EQ(*ctx->last_err, err_msg_ref);
-    ASSERT_EQ(ctx->info->msg.str(), "testing error");
-
-    try {
-        throw std::runtime_error("testing exception");
-    } catch (std::exception & e) {
-        err_msg_ref = e.what();
-        nix_context_error(ctx);
-    }
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_UNKNOWN);
-    ASSERT_EQ(*ctx->last_err, err_msg_ref);
-
-    nix_clear_err(ctx);
-    ASSERT_EQ(ctx->last_err_code, NIX_OK);
-}
-
-TEST_F(nix_api_util_context, nix_set_err_msg)
-{
-    ASSERT_EQ(ctx->last_err_code, NIX_OK);
-    nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "unknown test error");
-    ASSERT_EQ(ctx->last_err_code, NIX_ERR_UNKNOWN);
-    ASSERT_EQ(*ctx->last_err, "unknown test error");
-}
-
 TEST(nix_api_util, nix_version_get)
 {
     ASSERT_EQ(std::string(nix_version_get()), PACKAGE_VERSION);
@@ -61,17 +25,9 @@ struct MySettings : nix::Config
 MySettings mySettings;
 static nix::GlobalConfig::Register rs(&mySettings);
 
-static auto createOwnedNixContext()
-{
-    return std::unique_ptr<nix_c_context, decltype([](nix_c_context * ctx) {
-                               if (ctx)
-                                   nix_c_context_free(ctx);
-                           })>(nix_c_context_create(), {});
-}
-
 TEST_F(nix_api_util_context, nix_setting_get)
 {
-    ASSERT_EQ(ctx->last_err_code, NIX_OK);
+    ASSERT_EQ(nix_err_code(ctx), NIX_OK);
     std::string setting_value;
     nix_err result = nix_setting_get(ctx, "invalid-key", OBSERVE_STRING(setting_value));
     ASSERT_EQ(result, NIX_ERR_KEY);
@@ -114,40 +70,6 @@ TEST_F(nix_api_util_context, nix_err_msg)
     ASSERT_EQ(sz, err_msg.size());
 }
 
-TEST_F(nix_api_util_context, nix_err_info_msg)
-{
-    std::string err_info;
-
-    // no error
-    EXPECT_THROW(nix_err_info_msg(NULL, ctx, OBSERVE_STRING(err_info)), nix::Error);
-
-    try {
-        throw nix::Error("testing error");
-    } catch (...) {
-        nix_context_error(ctx);
-    }
-    auto new_ctx = createOwnedNixContext();
-    nix_err_info_msg(new_ctx.get(), ctx, OBSERVE_STRING(err_info));
-    ASSERT_STREQ("testing error", err_info.c_str());
-}
-
-TEST_F(nix_api_util_context, nix_err_name)
-{
-    std::string err_name;
-
-    // no error
-    EXPECT_THROW(nix_err_name(NULL, ctx, OBSERVE_STRING(err_name)), nix::Error);
-
-    try {
-        throw nix::Error("testing error");
-    } catch (...) {
-        nix_context_error(ctx);
-    }
-    auto new_ctx = createOwnedNixContext();
-    nix_err_name(new_ctx.get(), ctx, OBSERVE_STRING(err_name));
-    ASSERT_EQ(std::string(err_name), "nix::Error");
-}
-
 TEST_F(nix_api_util_context, nix_err_code)
 {
     ASSERT_EQ(nix_err_code(ctx), NIX_OK);
diff --git a/src/libutil-tests/nix_api_util_internal.cc b/src/libutil-tests/nix_api_util_internal.cc
new file mode 100644
index 000000000..6fb0a623f
--- /dev/null
+++ b/src/libutil-tests/nix_api_util_internal.cc
@@ -0,0 +1,85 @@
+#include "nix/util/config-global.hh"
+#include "nix/util/args.hh"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix/util/tests/nix_api_util.hh"
+#include "nix/util/tests/string_callback.hh"
+
+#include <gtest/gtest.h>
+
+#include <memory>
+
+#include "util-tests-config.hh"
+
+namespace nixC {
+
+TEST_F(nix_api_util_context, nix_context_error)
+{
+    std::string err_msg_ref;
+    try {
+        throw nix::Error("testing error");
+    } catch (nix::Error & e) {
+        err_msg_ref = e.what();
+        nix_context_error(ctx);
+    }
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_NIX_ERROR);
+    ASSERT_EQ(ctx->name, "nix::Error");
+    ASSERT_EQ(*ctx->last_err, err_msg_ref);
+    ASSERT_EQ(ctx->info->msg.str(), "testing error");
+
+    try {
+        throw std::runtime_error("testing exception");
+    } catch (std::exception & e) {
+        err_msg_ref = e.what();
+        nix_context_error(ctx);
+    }
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_UNKNOWN);
+    ASSERT_EQ(*ctx->last_err, err_msg_ref);
+
+    nix_clear_err(ctx);
+    ASSERT_EQ(nix_err_code(ctx), NIX_OK);
+}
+
+TEST_F(nix_api_util_context, nix_set_err_msg)
+{
+    ASSERT_EQ(nix_err_code(ctx), NIX_OK);
+    nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "unknown test error");
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_UNKNOWN);
+    ASSERT_EQ(*ctx->last_err, "unknown test error");
+}
+
+TEST_F(nix_api_util_context, nix_err_info_msg)
+{
+    std::string err_info;
+
+    // no error
+    EXPECT_THROW(nix_err_info_msg(NULL, ctx, OBSERVE_STRING(err_info)), nix::Error);
+
+    try {
+        throw nix::Error("testing error");
+    } catch (...) {
+        nix_context_error(ctx);
+    }
+    auto new_ctx = createOwnedNixContext();
+    nix_err_info_msg(new_ctx.get(), ctx, OBSERVE_STRING(err_info));
+    ASSERT_STREQ("testing error", err_info.c_str());
+}
+
+TEST_F(nix_api_util_context, nix_err_name)
+{
+    std::string err_name;
+
+    // no error
+    EXPECT_THROW(nix_err_name(NULL, ctx, OBSERVE_STRING(err_name)), nix::Error);
+
+    try {
+        throw nix::Error("testing error");
+    } catch (...) {
+        nix_context_error(ctx);
+    }
+    auto new_ctx = createOwnedNixContext();
+    nix_err_name(new_ctx.get(), ctx, OBSERVE_STRING(err_name));
+    ASSERT_EQ(std::string(err_name), "nix::Error");
+}
+
+} // namespace nixC