From 7e4608a3f8112451cd5597577a0ac73744c8980e Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Tue, 2 Sep 2025 01:54:27 -0400
Subject: [PATCH] More `extern "C"` for FFI

This allows us to catch the header and file getting out of sync, because
we are not doing overloading by mistake.
---
 src/libexpr-c/nix_api_expr.cc           | 4 ++++
 src/libexpr-c/nix_api_expr_internal.h   | 4 ++++
 src/libexpr-c/nix_api_external.cc       | 8 ++++++++
 src/libexpr-c/nix_api_value.cc          | 4 ++++
 src/libfetchers-c/nix_api_fetchers.cc   | 4 ++++
 src/libflake-c/nix_api_flake.cc         | 4 ++++
 src/libmain-c/nix_api_main.cc           | 4 ++++
 src/libstore-c/nix_api_store.cc         | 4 ++++
 src/libstore-c/nix_api_store_internal.h | 4 ++++
 src/libutil-c/nix_api_util.cc           | 4 ++++
 src/libutil-c/nix_api_util_internal.h   | 4 ++++
 11 files changed, 48 insertions(+)

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/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..1026c2227 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)
@@ -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_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/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