Add void_obj option

include/refcount/refcount.h

@@ -51,6 +51,8 @@ typedef void (*refcount_destroy_callback_t)(void *obj, void *user_data);
  */
 typedef struct RefcountContext {
     size_t entry_offset; //!< Offset to the #RefcountEntry member.
+    void *void_val; //!< Value returned by #refcount_context_unref when a
+                    //!< reference count falls to 0.
     refcount_held_refs_callback_t
         held_refs_callback; //!< Callback to list an object's held references.
     refcount_destroy_callback_t
@@ -75,7 +77,7 @@ typedef struct RefcountContext {
 extern RefcountContext *refcount_default_context;
 
 RefcountContext *
-refcount_make_context(size_t entry_offset,
+refcount_make_context(size_t entry_offset, void *void_val,
                       refcount_held_refs_callback_t held_refs_callback,
                       refcount_destroy_callback_t destroy_callback,
                       void *user_data, const RefcountAllocator *alloc);

src/refcount.c

@@ -59,7 +59,7 @@ RefcountContext *refcount_default_context = NULL;
  * @return The new context, or NULL in the case of an error
  */
 RefcountContext *
-refcount_make_context(size_t entry_offset,
+refcount_make_context(size_t entry_offset, void *void_val,
                       refcount_held_refs_callback_t held_refs_callback,
                       refcount_destroy_callback_t destroy_callback,
                       void *user_data, const RefcountAllocator *alloc) {
@@ -82,6 +82,7 @@ refcount_make_context(size_t entry_offset,
     }
 #endif
     ctx->entry_offset = entry_offset;
+    ctx->void_val = void_val;
     ctx->held_refs_callback = held_refs_callback;
     ctx->destroy_callback = destroy_callback;
     ctx->user_data = user_data;
@@ -355,7 +356,7 @@ end:
  */
 void *refcount_context_ref(const RefcountContext *ctx, void *obj) {
     if (!obj) {
-        return NULL;
+        return ctx->void_val;
     }
     if (!lock_entry_mtx(ENTRY)) {
         return obj;
@@ -419,7 +420,7 @@ static void remove_gc_root(RefcountContext *ctx, void *obj) {
 static void *unref_to_queue(RefcountContext *ctx, void *obj,
                             RefcountList **queue) {
     if (!obj) {
-        return NULL;
+        return ctx->void_val;
     } else if (ENTRY->is_static) {
         return obj;
     }
@@ -437,7 +438,7 @@ static void *unref_to_queue(RefcountContext *ctx, void *obj,
             *queue = refcount_list_push_full(*queue, obj, &ctx->alloc);
         }
         unlock_entry_mtx(ENTRY);
-        return NULL;
+        return ctx->void_val;
     } else {
         --ENTRY->impl.counted.ref_count;
         track_gc_root(ctx, obj);
@@ -523,7 +524,7 @@ static void process_unref_queue(RefcountContext *ctx, RefcountList *queue,
  */
 void *refcount_context_unref(RefcountContext *ctx, void *obj) {
     if (!obj) {
-        return NULL;
+        return ctx->void_val;
     }
     RefcountList *queue = NULL;
     void *retval = unref_to_queue(ctx, obj, &queue);

test/test_refcount.c

@@ -50,7 +50,7 @@ int main(int argc, const char **argv) {
     struct ContextAndFlag ctx_and_flag = {.should_be_doing_gc = false};
 
     RefcountContext *c = refcount_make_context(
-        offsetof(A, refcount), held_refs_callback, destroy_callback,
+        offsetof(A, refcount), NULL, held_refs_callback, destroy_callback,
         &ctx_and_flag, &COUNTING_ALLOCATOR);
 
     ctx_and_flag.ctx = c;