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

@@ -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"

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

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"

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"