Initial garbage colletor implementation

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.10)
+cmake_minimum_required(VERSION 3.11)
 
 set(CMAKE_C_STANDARD 11)
 
@@ -7,13 +7,25 @@ project(
   VERSION 1.0
   LANGUAGES C)
 
+include(FetchContent)
+FetchContent_Declare(
+  ht
+  GIT_REPOSITORY https://git.zander.im/Zander671/ht.git
+  GIT_TAG 9a1b271cfdc8ab203f9d6aa6a83cc4523de422be)
+
+FetchContent_MakeAvailable(ht)
+
 if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
   include(CTest)
 endif()
 
+add_compile_options(-fsanitize=address,leak,undefined)
+add_link_options(-fsanitize=address,leak,undefined)
+
 add_library(refcount src/allocator.c src/list.c src/refcount.c)
+target_link_libraries(refcount PUBLIC ht)
 target_include_directories(refcount PUBLIC include/)
-target_compile_options(refcount PRIVATE -Wall)
+target_compile_options(refcount PRIVATE -Wall -Wpedantic)
 
 if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
   add_subdirectory(test/)

include/refcount/allocator.h

@@ -4,6 +4,8 @@
 #ifndef INCLUDED_REFCOUNT_ALLOCATOR_H
 #define INCLUDED_REFCOUNT_ALLOCATOR_H
 
+#include <ht.h>
+#include <stdbool.h>
 #include <stddef.h>
 
 #ifdef __cplusplus
@@ -14,12 +16,27 @@ extern "C" {
  * A set of the three main C standard allocators. Each member function should
  * behave the same as its standard library counterpart. That is, for example,
  * free should allow NULL as an input.
+ *
+ * To allow e.g. keeping track of allocations in different contexts, each
+ * function also takes a second user_data pointer argument.
  */
 typedef struct RefcountAllocator {
-    void *(*malloc)(size_t); //!< malloc implementation for the allocator.
+    union {
-    void *(*realloc)(void *,
+        void *(*malloc)(size_t size); //!< malloc implementation which takes no
-                     size_t); //!< realloc implementation for the allocator.
+                                      //!< data.
-    void (*free)(void *); //!< free implementation for the allocator.
+        void *(*malloc_with_data)(
+            size_t size,
+            void *user_data); //!< malloc implementation that takes a data
+                              //!< argument.
+    };
+    union {
+        void (*free)(void *ptr); //!< free implementation which takes no data.
+        void (*free_with_data)(void *ptr,
+                               void *user_data); //!< free implementation that
+                                                 //!< takes a data argument.
+    };
+    bool pass_data;
+    void *user_data;
 } RefcountAllocator;
 
 extern const RefcountAllocator *refcount_global_allocator;
@@ -30,22 +47,12 @@ extern const RefcountAllocator *refcount_global_allocator;
  * @param size The number of bytes to allocate
  * @return The newly allocate pointer, or NULL if allocation failed
  */
-static inline void *refcount_allocator_malloc(const RefcountAllocator *alloc,
+static inline void *refcount_malloc(const RefcountAllocator *alloc,
                                     size_t size) {
-    return alloc->malloc(size);
+    if (alloc->pass_data) {
+        return alloc->malloc_with_data(size, alloc->user_data);
     }
-
+    return alloc->malloc(size);
-/**
- * Reallocate memory using a #RefcountAllocator.
- * @param alloc The allocator to use
- * @param old_ptr The pointer to reallocate
- * @param size The number of bytes to allocate
- * @return The resized pointer, a newly allocated pointer, or NULL if
- * allocation failed
- */
-static inline void *refcount_allocator_realloc(const RefcountAllocator *alloc,
-                                               void *old_ptr, size_t size) {
-    return alloc->realloc(old_ptr, size);
 }
 
 /**
@@ -53,44 +60,16 @@ static inline void *refcount_allocator_realloc(const RefcountAllocator *alloc,
  * @param alloc The allocator to use
  * @param ptr The pointer to free
  */
-static inline void refcount_allocator_free(const RefcountAllocator *alloc,
+static inline void refcount_free(const RefcountAllocator *alloc, void *ptr) {
-                                           void *ptr) {
+    if (alloc->pass_data) {
+        alloc->free_with_data(ptr, alloc->user_data);
+    } else {
         alloc->free(ptr);
     }
-
-/**
- * Allocate memory using the global #RefcountAllocator.
- * @param size The number of bytes to allocate
- * @return The newly allocate pointer, or NULL if allocation failed
- *
- * @see refcount_allocator_malloc
- */
-static inline void *refcount_malloc(size_t size) {
-    return refcount_allocator_malloc(refcount_global_allocator, size);
 }
 
-/**
+void refcount_allocator_to_ht_allocator(const RefcountAllocator *src,
- * Reallocate memory using the global #RefcountAllocator.
+                                        HTAllocator *dest);
- * @param old_ptr The pointer to reallocate
- * @param size The number of bytes to allocate
- * @return The resized pointer, a newly allocated pointer, or NULL if
- * allocation failed
- *
- * @see refcount_allocator_realloc
- */
-static inline void *refcount_realloc(void *old_ptr, size_t size) {
-    return refcount_allocator_realloc(refcount_global_allocator, old_ptr, size);
-}
-
-/**
- * Free memory using the global #RefcountAllocator.
- * @param ptr The pointer to free
- *
- * @see refcount_allocator_free
- */
-static inline void refcount_free(void *ptr) {
-    refcount_allocator_free(refcount_global_allocator, ptr);
-}
 
 #ifdef __cplusplus
 }

include/refcount/list.h

@@ -5,6 +5,7 @@
 #define INCLUDED_REFCOUNT_LIST_H
 
 #include <refcount/allocator.h>
+#include <stdarg.h>
 #include <stddef.h> // for NULL
 
 #ifdef __cplusplus
@@ -24,42 +25,130 @@ typedef struct RefcountList {
 
 /**
  * Callback to used to free list elements.
+ * @param ptr The pointer to free
  * @see refcount_list_free
  */
-typedef void (*refcount_list_destroy_callback_t)(void *);
+typedef void (*refcount_list_destroy_callback_t)(void *ptr);
 /**
- * Callback to used when looping over the list with extra data.
+ * Callback to used to free list elements.
- * @see refcount_list_free_with_data
+ * @param ptr The pointer to free
+ * @param user_data Arbitrary user specified data
+ * @see refcount_list_free
  */
-typedef void (*refcount_list_foreach_callback_t)(void *, void *);
+typedef void (*refcount_list_destroy_with_data_callback_t)(void *ptr,
+                                                           void *user_data);
 /**
  * Callback used when copying elements from one list to another.
+ * @param ptr The element to copy
+ * @param user_data Arbitrary user specified data
+ * @return The new copy, or NULL if an error occurred
  * @see refcount_list_copy
  */
-typedef void *(*refcount_list_copy_callback_t)(void *);
+typedef void *(*refcount_list_copy_callback_t)(const void *ptr,
+                                               void *user_data);
 
-RefcountList *refcount_list_build(int count, ...);
+RefcountList *refcount_list_build_full_va(int count,
+                                          const RefcountAllocator *alloc,
+                                          va_list args);
 
-RefcountList *refcount_list_copy(RefcountList *list,
+/**
-                                 refcount_list_copy_callback_t callback);
+ * Like #refcount_list_build, but lets you specify the allocator to use.
+ * @param count The number of elements given
+ * @param alloc The allocator to use
+ * @param ... The elements from which to build the list
+ * @return The built list, or NULL if a memory allocation error occurred
+ */
+static inline RefcountList *
+refcount_list_build_full(int count, const RefcountAllocator *alloc, ...) {
+    va_list args;
+    va_start(args, alloc);
+    RefcountList *retval = refcount_list_build_full_va(count, alloc, args);
+    va_end(args);
+    return retval;
+}
+
+/**
+ * Build a #RefcountList from a number of elements.
+ *
+ * > that this will return NULL if count is 0 or if a memory allocation error
+ * > occurred. However, as no allocation is preformed is count is 0, these
+ * > errors should be mutually exclusive.
+ *
+ * @param count The number of elements given
+ * @param ... The elements from which to build the list
+ * @return The built list, or NULL if a memory allocation error occurred
+ */
+static inline RefcountList *refcount_list_build(int count, ...) {
+    va_list args;
+    va_start(args, count);
+    RefcountList *retval =
+        refcount_list_build_full_va(count, refcount_global_allocator, args);
+    va_end(args);
+    return retval;
+}
+
+RefcountList *refcount_list_copy_full(const RefcountList *list,
+                                      refcount_list_copy_callback_t callback,
+                                      void *user_data,
+                                      const RefcountAllocator *alloc);
+
+/**
+ * Return a new #RefcountList that is a copy of list. This does no allocation if
+ * the input list is NULL.
+ * @param list The list to copy
+ * @param callback The function to copy each element of the list, or NULL
+ * @param user_data Extra data to be passed to the copy callback
+ * @return The copy, or NULL if an allocation error occurred
+ */
+static inline RefcountList *
+refcount_list_copy(const RefcountList *list,
+                   refcount_list_copy_callback_t callback, void *user_data) {
+    return refcount_list_copy_full(list, callback, user_data,
+                                   refcount_global_allocator);
+}
 
 RefcountList *refcount_list_reverse(RefcountList *list);
 
-void refcount_list_free(RefcountList *list,
+void refcount_list_free_full(RefcountList *list,
-                        refcount_list_destroy_callback_t callback);
+                             refcount_list_destroy_callback_t callback,
+                             const RefcountAllocator *alloc);
 
-void refcount_list_free_with_data(RefcountList *list,
+/**
-                                  refcount_list_foreach_callback_t callback,
+ * Free a #RefcountList.
-                                  void *data);
+ * @param list The list to free
+ * @param callback The function to call to free each member, or NULL
+ */
+static inline void
+refcount_list_free(RefcountList *list,
+                   refcount_list_destroy_callback_t callback) {
+    refcount_list_free_full(list, callback, refcount_global_allocator);
+}
 
-size_t refcount_list_length(RefcountList *list);
+void refcount_list_free_with_data_full(
+    RefcountList *list, refcount_list_destroy_with_data_callback_t callback,
+    void *data, const RefcountAllocator *alloc);
+
+/**
+ * Free a #RefcountList.
+ * @param list The list to free
+ * @param callback The function to call to free each member, or NULL
+ * @param data Extra data to pass to the second argument of the callback
+ */
+static inline void refcount_list_free_with_data(
+    RefcountList *list, refcount_list_destroy_with_data_callback_t callback,
+    void *data) {
+    refcount_list_free_with_data_full(list, callback, data,
+                                      refcount_global_allocator);
+}
+
+size_t refcount_list_length(const RefcountList *list);
 
 /**
  * Return the first element in a #RefcountList.
  * @param list The list
  * @return The first object in the list
  */
-static inline void *refcount_list_peek(RefcountList *list) {
+static inline void *refcount_list_peek(const RefcountList *list) {
     if (!list) {
         return NULL;
     }
@@ -68,21 +157,171 @@ static inline void *refcount_list_peek(RefcountList *list) {
 
 RefcountList *refcount_list_drop(RefcountList *list, size_t n);
 
-void *refcount_list_nth(RefcountList *list, size_t n);
+void *refcount_list_nth(const RefcountList *list, size_t n);
 
-RefcountList *refcount_list_pop(RefcountList *list,
+RefcountList *
-                                refcount_list_destroy_callback_t callback);
+refcount_list_remove_full(RefcountList *list, RefcountList *link,
+                          refcount_list_destroy_callback_t callback,
+                          const RefcountAllocator *alloc);
 
-RefcountList *refcount_list_remove(RefcountList *list, RefcountList *link,
+/**
-                                   refcount_list_destroy_callback_t callback);
+ * Remove a link from a #RefcountList. Note that you must save the return
+ * value as it is the new value of the list.
+ *
+ * If the given list or link is NULL, this does nothing. It is undefined
+ * behavior if link is not in list.
+ *
+ * @param list The list
+ * @param link The link to remove, or NULL
+ * @param callback The function to call to free the member, or NULL
+ * @return The new value of the list
+ */
+static inline RefcountList *
+refcount_list_remove(RefcountList *list, RefcountList *link,
+                     refcount_list_destroy_callback_t callback) {
+    return refcount_list_remove_full(list, link, callback,
+                                     refcount_global_allocator);
+}
 
-RefcountList *refcount_list_push(RefcountList *list, void *element);
+/**
+ * Same as #refcount_list_remove_with_data, but lets you specify the allocator
+ * to use.
+ * @param list The list
+ * @param callback The function to call to free the member, or NULL
+ * @param link The link to remove, or NULL
+ * @param user_data Extra data to pass to the callback
+ * @param alloc The allocator to use
+ * @return The new value of the list
+ */
+static inline RefcountList *refcount_list_remove_with_data_full(
+    RefcountList *list, RefcountList *link,
+    refcount_list_destroy_with_data_callback_t callback, void *user_data,
+    const RefcountAllocator *alloc) {
+    if (list && link && callback) {
+        callback(link->data, user_data);
+    }
+    return refcount_list_remove_full(list, link, NULL, alloc);
+}
+
+/**
+ * Same as #refcount_list_remove except that the callback takes an additional
+ * argument. This also does nothing if either of list or link is NULL.
+ * @param list The list
+ * @param callback The function to call to free the member, or NULL
+ * @param link The link to remove, or NULL
+ * @param user_data Extra data to pass to the callback
+ * @return The new value of the list
+ */
+static inline RefcountList *refcount_list_remove_with_data(
+    RefcountList *list, RefcountList *link,
+    refcount_list_destroy_with_data_callback_t callback, void *user_data) {
+    return refcount_list_remove_with_data_full(list, link, callback, user_data,
+                                               refcount_global_allocator);
+}
+
+/**
+ * Same as #refcount_list_pop, but lets you specify the allocator to use.
+ *
+ * @param list The list
+ * @param callback The function to call to free the removed, or NULL
+ * @param alloc The allocator to use
+ * @return The new value of the list
+ *
+ * @see refcount_list_remove
+ */
+static inline RefcountList *
+refcount_list_pop_full(RefcountList *list,
+                       refcount_list_destroy_callback_t callback,
+                       const RefcountAllocator *alloc) {
+    return refcount_list_remove_full(list, list, callback, alloc);
+}
+
+/**
+ * Remove the first element from a #RefcountList. Note that you must save the
+ * return value as it is the new value of the list. If the list is a sub-list,
+ * the state of the larger list is undefined after this call.
+ *
+ * @param list The list
+ * @param callback The function to call to free the removed, or NULL
+ * @return The new value of the list
+ *
+ * @see refcount_list_remove
+ */
+static inline RefcountList *
+refcount_list_pop(RefcountList *list,
+                  refcount_list_destroy_callback_t callback) {
+    return refcount_list_pop_full(list, callback, refcount_global_allocator);
+}
+
+/**
+ * Same as #refcount_list_pop_with_data, but lets specify the allocator to use.
+ * @param list The list
+ * @param callback The function to call to free the member, or NULL
+ * @param user_data Extra data to pass to the callback
+ * @param alloc The allocator to use
+ * @return The new value of the list
+ */
+static inline RefcountList *refcount_list_pop_with_data_full(
+    RefcountList *list, refcount_list_destroy_with_data_callback_t callback,
+    void *user_data, const RefcountAllocator *alloc) {
+    if (list && callback) {
+        callback(list->data, user_data);
+    }
+    return refcount_list_pop_full(list, NULL, alloc);
+}
+
+/**
+ * Same as #refcount_list_pop except that the callback takes an additional
+ * argument.
+ * @param list The list
+ * @param callback The function to call to free the member, or NULL
+ * @param user_data Extra data to pass to the callback
+ * @return The new value of the list
+ */
+static inline RefcountList *
+refcount_list_pop_with_data(RefcountList *list,
+                            refcount_list_destroy_with_data_callback_t callback,
+                            void *user_data) {
+    return refcount_list_pop_with_data_full(list, callback, user_data,
+                                            refcount_global_allocator);
+}
+
+RefcountList *refcount_list_push_full(RefcountList *list, void *element,
+                                      const RefcountAllocator *alloc);
+
+/**
+ * Add an element to the start of a #RefcountList. Note that you must save the
+ * return value as it is the new value of the list.
+ * @param list The list
+ * @param element The element to add
+ * @return The new value of the list, or NULL if allocation of the new node
+ * failed
+ */
+static inline RefcountList *refcount_list_push(RefcountList *list,
+                                               void *element) {
+    return refcount_list_push_full(list, element, refcount_global_allocator);
+}
 
 RefcountList *refcount_list_tail(RefcountList *list);
 
 RefcountList *refcount_list_join(RefcountList *list1, RefcountList *list2);
 
-RefcountList *refcount_list_push_back(RefcountList *list, void *element);
+RefcountList *refcount_list_push_back_full(RefcountList *list, void *element,
+                                           const RefcountAllocator *alloc);
+
+/**
+ * Add an element to the end of a #RefcountList. Note that you must save the
+ * return value as it is the new value of the list.
+ * @param list The list
+ * @param element The element to add
+ * @return The new value of the list, or NULL if allocation of the new node
+ * failed
+ */
+static inline RefcountList *refcount_list_push_back(RefcountList *list,
+                                                    void *element) {
+    return refcount_list_push_back_full(list, element,
+                                        refcount_global_allocator);
+}
 
 #ifdef __cplusplus
 }

include/refcount/refcount.h

@@ -4,6 +4,7 @@
 #ifndef INCLUDED_REFCOUNT_REFCOUNT_H
 #define INCLUDED_REFCOUNT_REFCOUNT_H
 
+#include <ht.h>
 #include <refcount/list.h>
 #include <stdbool.h>
 #include <stdint.h>
@@ -14,15 +15,29 @@ extern "C" {
 
 /**
  * Callback for listing all references held by the passed object. The function
- * should take an objects and return a #RefcountList of references held by that
+ * should take an object and return a #RefcountList of references held by that
- * object.
+ * object. The second parameter is user supplied data.
+ *
+ * > The list in the refs parameter will probably not be empty, so it is
+ * > important that you update it, not replace it. If an errors occurs, you
+ * > mustn't free anything that you did not add to the list yourself.
+ *
+ * @param obj The object
+ * @param refs A pointer to a #RefcountList that should be updated with the held
+ * references.
+ * @param user_data User supplied data pointer
+ * @return True if the operation was successful, false otherwise
  */
-typedef RefcountList *(*refcount_held_refs_callback_t)(void *);
+typedef bool (*refcount_held_refs_callback_t)(void *obj, RefcountList **refs,
+                                              void *user_data);
 /**
  * Callback for freeing an object after its reference count drops to 0. It is
- * safe the free the object from this callback.
+ * safe the free the object from this callback. The second parameter is user
+ * supplied data.
+ * @param obj The object to free
+ * @param user_data User supplied data pointer
  */
-typedef void (*refcount_destroy_callback_t)(void *);
+typedef void (*refcount_destroy_callback_t)(void *obj, void *user_data);
 
 /**
  * Context for reference counting a specific type of object. This should be
@@ -33,9 +48,11 @@ typedef struct RefcountContext {
     refcount_held_refs_callback_t
         held_refs_callback; //!< Callback to list an object's held references.
     refcount_destroy_callback_t
-        destroy_callback; //!< Callback to free an object
+        destroy_callback; //!< Callback to free an object after its reference
-                          //!< after its reference count
+                          //!< count drops to 0.
-                          //!< dropped to 0.
+    void *user_data; //<! User data to pass to other callbacks.
+    RefcountAllocator alloc; //<! Allocator to use for this context.
+    HTAllocator ht_alloc; //<! ht version of the context's allocator.
 
     RefcountList *static_objects; //!< List of static objects registered with
                                   //!< this context.
@@ -47,7 +64,6 @@ typedef struct RefcountContext {
  */
 typedef enum {
     REFCOUNT_OPER_DEINIT_STATIC,
-    REFCOUNT_OPER_UNREF,
     REFCOUNT_OPER_FLOAT,
 } RefcountOperation;
 
@@ -55,9 +71,12 @@ typedef enum {
  * Type for a function to be called when some kind of error occurs. It should
  * take a #RefcountOperation, a #RefcountContext pointer, and the object on
  * which the error occurred.
+ * @param oper The operation that failed
+ * @param ctx The context the operation was performed on
+ * @param obj The object that caused the failure
  */
-typedef void (*refcount_debug_callback_t)(RefcountOperation, RefcountContext *,
+typedef void (*refcount_debug_callback_t)(RefcountOperation oper,
-                                          void *);
+                                          RefcountContext *ctx, void *obj);
 
 extern refcount_debug_callback_t refcount_debug_breakpoint;
 
@@ -66,7 +85,8 @@ extern RefcountContext *refcount_default_context;
 RefcountContext *
 refcount_make_context(size_t entry_offset,
                       refcount_held_refs_callback_t held_refs_callback,
-                      refcount_destroy_callback_t destroy_callback);
+                      refcount_destroy_callback_t destroy_callback,
+                      void *user_data, const RefcountAllocator *alloc);
 
 void refcount_context_destroy(RefcountContext *ctx);
 
@@ -100,7 +120,7 @@ typedef struct RefcountEntry {
  * @return A pointer to the objects #RefcountEntry
  */
 #define REFCOUNT_OBJECT_ENTRY(ctx, obj) \
-    ((RefcountEntry *) (((void *) (obj)) + (ctx)->entry_offset))
+    ((RefcountEntry *) (((char *) (obj)) + (ctx)->entry_offset))
 
 /**
  * Test whether an object is static or not. The object must have been
@@ -128,9 +148,9 @@ static inline size_t refcount_context_num_refs(RefcountContext *ctx,
 
 void refcount_context_init_obj(RefcountContext *ctx, void *obj);
 
-void refcount_context_init_static(RefcountContext *ctx, void *obj);
+bool refcount_context_init_static(RefcountContext *ctx, void *obj);
 
-void refcount_context_deinit_static(RefcountContext *ctx, void *obj);
+bool refcount_context_deinit_static(RefcountContext *ctx, void *obj);
 
 /**
  * Unregister a static object in a context. This is suitable to be passed as a
@@ -158,6 +178,8 @@ static inline void refcount_context_unref_as_callback(void *obj, void *ctx) {
 }
 void *refcount_context_float(RefcountContext *ctx, void *obj);
 
+bool refcount_context_garbage_collect(RefcountContext *ctx);
+
 /**
  * Same as #refcount_context_is_static, but only operates on the global
  * context.
@@ -242,6 +264,15 @@ static inline void *refcount_float(void *obj) {
     return refcount_context_float(refcount_default_context, obj);
 }
 
+/**
+ * Same as #refcount_context_garbage_collect, but only operates on the globa
+ * context.
+ * @return False if an error occured, true otherwise
+ */
+static inline bool refcount_garbage_collect(void) {
+    return refcount_context_garbage_collect(refcount_default_context);
+}
+
 #ifdef __cplusplus
 }
 #endif

src/allocator.c

@@ -9,14 +9,46 @@
 /**
  * Initial value of #refcount_global_allocator.
  */
-static const RefcountAllocator default_allocator = {
+static const RefcountAllocator DEFAULT_ALLOCATOR = {
     .malloc = malloc,
-    .realloc = realloc,
     .free = free,
+    .pass_data = false,
+    .user_data = NULL,
 };
 
 /**
  * The global #RefcountAllocator used by other parts of RefCount. The default
- * value just calls the C standard functions malloc, realloc, and free.
+ * value just calls the C standard functions malloc and free.
  */
-const RefcountAllocator *refcount_global_allocator = &default_allocator;
+const RefcountAllocator *refcount_global_allocator = &DEFAULT_ALLOCATOR;
+
+/**
+ * HTAllocator malloc function that delegates to #refcount_malloc.
+ * @param size The number of bytes to allocate
+ * @parma alloc The #RefcountAllocator to use
+ * @return The result of #refcount_malloc
+ */
+static void *refcount_ht_malloc(size_t size, void *alloc) {
+    return refcount_malloc(alloc, size);
+}
+
+/**
+ * HTAllocator free function that delegates to #refcount_free.
+ * @param ptr The pointer to free
+ * @parma alloc The #RefcountAllocator to use
+ */
+static void refcount_ht_free(void *ptr, void *alloc) {
+    refcount_free(alloc, ptr);
+}
+
+/**
+ * Create a new HTAllocator that delegates to src.
+ * @param src The #RefcountAllocator to delegate to
+ * @param dest A pointer to a #HTAllocator to initialize
+ */
+void refcount_allocator_to_ht_allocator(const RefcountAllocator *src,
+                                        HTAllocator *dest) {
+    dest->malloc = refcount_ht_malloc;
+    dest->free = refcount_ht_free;
+    dest->user_data = (void *) src;
+}

src/list.c

@@ -4,31 +4,26 @@
  */
 #include "refcount/list.h"
 
-#include <stdarg.h>
 #include <stdbool.h>
 #include <stdlib.h>
 
 /**
- * Build a #RefcountList from a number of elements.
+ * Like #refcount_list_build, but lets you specify the allocator to use and
- *
+ * takes a va_list instead of variable arguments.
- * > that this will return NULL if count is 0 or if a memory allocation error
- * > occurred. However, as no allocation is preformed is count is 0, these
- * > errors should be mutually exclusive.
- *
  * @param count The number of elements given
- * @param ... The elements from which to build the list
+ * @param alloc The allocator to use
+ * @param args The va_list to use to get the elements
  * @return The built list, or NULL if a memory allocation error occurred
  */
-RefcountList *refcount_list_build(int count, ...) {
+RefcountList *refcount_list_build_full_va(int count,
-    va_list args;
+                                          const RefcountAllocator *alloc,
-    va_start(args, count);
+                                          va_list args) {
     RefcountList *start = NULL;
     RefcountList *end;
     while (count--) {
-        RefcountList *new_list = refcount_malloc(sizeof(RefcountList));
+        RefcountList *new_list = refcount_malloc(alloc, sizeof(RefcountList));
         if (!new_list) {
             refcount_list_free(start, NULL);
-            va_end(args);
             return NULL;
         }
         new_list->data = va_arg(args, void *);
@@ -43,28 +38,30 @@ RefcountList *refcount_list_build(int count, ...) {
             end = new_list;
         }
     }
-    va_end(args);
     return start;
 }
 
 /**
- * Return a new #RefcountList that is a copy of list. This does no allocation if
+ * Same as #refcount_list_copy, but lets you specify the allocator.
- * the input list is NULL.
  * @param list The list to copy
  * @param callback The function to copy each element of the list, or NULL
+ * @param user_data Extra data to be passed to the copy callback
+ * @param alloc The allocator to use
  * @return The copy, or NULL if an allocation error occurred
  */
-RefcountList *refcount_list_copy(RefcountList *list,
+RefcountList *refcount_list_copy_full(const RefcountList *list,
-                                 refcount_list_copy_callback_t callback) {
+                                      refcount_list_copy_callback_t callback,
+                                      void *user_data,
+                                      const RefcountAllocator *alloc) {
     RefcountList *start = NULL;
     RefcountList *end;
     while (list) {
-        RefcountList *new_end = refcount_malloc(sizeof(RefcountList));
+        RefcountList *new_end = refcount_malloc(alloc, sizeof(RefcountList));
         if (!new_end) {
-            refcount_list_free(start, NULL);
+            refcount_list_free_full(start, NULL, alloc);
             return NULL;
         }
-        new_end->data = callback ? callback(list->data) : list->data;
+        new_end->data = callback ? callback(list->data, user_data) : list->data;
         new_end->next = NULL;
         if (!start) {
             start = new_end;
@@ -101,16 +98,18 @@ RefcountList *refcount_list_reverse(RefcountList *list) {
 /**
  * Free a #RefcountList.
  * @param list The list to free
+ * @param alloc The allocator to use
  * @param callback The function to call to free each member, or NULL
  */
-void refcount_list_free(RefcountList *list,
+void refcount_list_free_full(RefcountList *list,
-                        refcount_list_destroy_callback_t callback) {
+                             refcount_list_destroy_callback_t callback,
+                             const RefcountAllocator *alloc) {
     while (list) {
         RefcountList *next = list->next;
         if (callback) {
             callback(list->data);
         }
-        refcount_free(list);
+        refcount_free(alloc, list);
         list = next;
     }
 }
@@ -120,16 +119,17 @@ void refcount_list_free(RefcountList *list,
  * @param list The list to free
  * @param callback The function to call to free each member, or NULL
  * @param data Extra data to pass to the second argument of the callback
+ * @param alloc The allocator to use
  */
-void refcount_list_free_with_data(RefcountList *list,
+void refcount_list_free_with_data_full(
-                                  refcount_list_foreach_callback_t callback,
+    RefcountList *list, refcount_list_destroy_with_data_callback_t callback,
-                                  void *data) {
+    void *data, const RefcountAllocator *alloc) {
     while (list) {
         RefcountList *next = list->next;
         if (callback) {
             callback(list->data, data);
         }
-        refcount_free(list);
+        refcount_free(alloc, list);
         list = next;
     }
 }
@@ -139,7 +139,7 @@ void refcount_list_free_with_data(RefcountList *list,
  * @param list The list
  * @return The length of the list
  */
-size_t refcount_list_length(RefcountList *list) {
+size_t refcount_list_length(const RefcountList *list) {
     size_t len = 0;
     while (list) {
         list = list->next;
@@ -171,41 +171,23 @@ RefcountList *refcount_list_drop(RefcountList *list, size_t n) {
  * @param n The index of the element
  * @return The Nth element of the list
  */
-void *refcount_list_nth(RefcountList *list, size_t n) {
+void *refcount_list_nth(const RefcountList *list, size_t n) {
-    RefcountList *link = refcount_list_drop(list, n);
+    RefcountList *link = refcount_list_drop((RefcountList *) list, n);
     return link ? link->data : NULL;
 }
 
 /**
- * Remove the first element from a #RefcountList. Note that you must save the
+ * The same as #refcount_list_remove, but lets you specify the allocator to use.
- * return value as it is the new value of the list. If the list is a sub-list,
- * the state of the larger list is undefined after this call.
- *
- * @param list The list
- * @param callback The function to call to free the removed, or NULL
- * @return The new value of the list
- *
- * @see refcount_list_remove
- */
-RefcountList *refcount_list_pop(RefcountList *list,
-                                refcount_list_destroy_callback_t callback) {
-    return refcount_list_remove(list, list, callback);
-}
-
-/**
- * Remove a link from a #RefcountList. Note that you must save the return
- * value as it is the new value of the list.
- *
- * If the given list or link is NULL, this does nothing. It is undefined
- * behavior if link is not in list.
- *
  * @param list The list
  * @param link The link to remove, or NULL
  * @param callback The function to call to free the member, or NULL
+ * @param alloc The allocator to use
  * @return The new value of the list
  */
-RefcountList *refcount_list_remove(RefcountList *list, RefcountList *link,
+RefcountList *
-                                   refcount_list_destroy_callback_t callback) {
+refcount_list_remove_full(RefcountList *list, RefcountList *link,
+                          refcount_list_destroy_callback_t callback,
+                          const RefcountAllocator *alloc) {
     if (!list || !link) {
         return list;
     }
@@ -219,20 +201,22 @@ RefcountList *refcount_list_remove(RefcountList *list, RefcountList *link,
     if (callback) {
         callback(link->data);
     }
-    refcount_free(link);
+    refcount_free(alloc, link);
     return list == link ? rest : list;
 }
 
 /**
- * Add an element to the start of a #RefcountList. Note that you must save the
+ * The same as #refcount_list_push, but lets you specify the allocator to
- * return value as it is the new value of the list.
+ * use.
  * @param list The list
  * @param element The element to add
+ * @param alloc The allocator to use
  * @return The new value of the list, or NULL if allocation of the new node
  * failed
  */
-RefcountList *refcount_list_push(RefcountList *list, void *element) {
+RefcountList *refcount_list_push_full(RefcountList *list, void *element,
-    RefcountList *new_head = refcount_malloc(sizeof(RefcountList));
+                                      const RefcountAllocator *alloc) {
+    RefcountList *new_head = refcount_malloc(alloc, sizeof(RefcountList));
     if (!new_head) {
         return NULL;
     }
@@ -283,15 +267,17 @@ RefcountList *refcount_list_join(RefcountList *list1, RefcountList *list2) {
 }
 
 /**
- * Add an element to the end of a #RefcountList. Note that you must save the
+ * The same as #refcount_list_push_back, but lets you specify the allocator to
- * return value as it is the new value of the list.
+ * use.
  * @param list The list
  * @param element The element to add
+ * @param alloc The allocator to use
  * @return The new value of the list, or NULL if allocation of the new node
  * failed
  */
-RefcountList *refcount_list_push_back(RefcountList *list, void *element) {
+RefcountList *refcount_list_push_back_full(RefcountList *list, void *element,
-    RefcountList *new_end = refcount_malloc(sizeof(RefcountList));
+                                           const RefcountAllocator *alloc) {
+    RefcountList *new_end = refcount_malloc(alloc, sizeof(RefcountList));
     if (!new_end) {
         return NULL;
     }

src/refcount.c

@@ -45,14 +45,21 @@ RefcountContext *refcount_default_context = NULL;
 RefcountContext *
 refcount_make_context(size_t entry_offset,
                       refcount_held_refs_callback_t held_refs_callback,
-                      refcount_destroy_callback_t destroy_callback) {
+                      refcount_destroy_callback_t destroy_callback,
-    RefcountContext *ctx = refcount_malloc(sizeof(RefcountContext));
+                      void *user_data, const RefcountAllocator *alloc) {
+    if (!alloc) {
+        alloc = refcount_global_allocator;
+    }
+    RefcountContext *ctx = refcount_malloc(alloc, sizeof(RefcountContext));
     if (!ctx) {
         return NULL;
     }
     ctx->entry_offset = entry_offset;
     ctx->held_refs_callback = held_refs_callback;
     ctx->destroy_callback = destroy_callback;
+    ctx->user_data = user_data;
+    ctx->alloc = *alloc;
+    refcount_allocator_to_ht_allocator(&ctx->alloc, &ctx->ht_alloc);
 
     ctx->static_objects = NULL;
     ctx->gc_roots = NULL;
@@ -66,12 +73,13 @@ refcount_make_context(size_t entry_offset,
  * @param ctx The #RefcountContext
  */
 void refcount_context_destroy(RefcountContext *ctx) {
-    refcount_list_free_with_data(
+    refcount_list_free_with_data_full(
-        ctx->static_objects, refcount_context_deinit_static_as_callback, ctx);
+        ctx->static_objects, refcount_context_deinit_static_as_callback, ctx,
+        &ctx->alloc);
 
-    // TODO process gc_roots
+    refcount_context_garbage_collect(ctx);
 
-    refcount_free(ctx);
+    refcount_free(&ctx->alloc, ctx);
 }
 
 /**
@@ -85,37 +93,65 @@ void refcount_context_destroy(RefcountContext *ctx) {
  * @param obj The object to initialize
  */
 void refcount_context_init_obj(RefcountContext *ctx, void *obj) {
+    if (obj) {
         ENTRY->is_static = false;
         ENTRY->gc_root = NULL;
         ENTRY->ref_count = 0;
     }
+}
 
 /**
  * Register a static object in a context.
  * @param ctx The #RefcountContext
  * @param obj The object to register
+ * @return True on success, false otherwise
  */
-void refcount_context_init_static(RefcountContext *ctx, void *obj) {
+bool refcount_context_init_static(RefcountContext *ctx, void *obj) {
     ENTRY->is_static = true;
-    ctx->static_objects = refcount_list_push(ctx->static_objects, obj);
+    ctx->static_objects =
+        refcount_list_push_full(ctx->static_objects, obj, &ctx->alloc);
+    if (!ctx->static_objects) {
+        return false;
+    }
     ENTRY->static_entry = ctx->static_objects;
+    return true;
+}
+
+/**
+ * Return the references held by an object.
+ * @param ctx The #RefcountContext
+ * @param obj The object
+ * @param refs Where to store the refs
+ * @return True on success
+ */
+static inline bool obj_held_refs(RefcountContext *ctx, void *obj,
+                                 RefcountList **refs) {
+    if (ctx->held_refs_callback) {
+        return ctx->held_refs_callback(obj, refs, ctx->user_data);
+    }
+    return true;
 }
 
 /**
  * Unregister a static object in a context.
  * @param ctx The #RefcountContext
  * @param obj The object to unregister
+ * @return True on success, false otherwise
  */
-void refcount_context_deinit_static(RefcountContext *ctx, void *obj) {
+bool refcount_context_deinit_static(RefcountContext *ctx, void *obj) {
     if (refcount_context_is_static(ctx, obj)) {
-        ctx->static_objects = refcount_list_remove(ctx->static_objects,
+        RefcountList *held_refs = NULL;
-                                                   ENTRY->static_entry, NULL);
+        if (!obj_held_refs(ctx, obj, &held_refs)) {
-        RefcountList *held_refs =
+            return false;
-            ctx->held_refs_callback ? ctx->held_refs_callback(obj) : NULL;
+        }
-        refcount_list_free_with_data(held_refs,
+        ctx->static_objects = refcount_list_remove_full(
-                                     refcount_context_unref_as_callback, ctx);
+            ctx->static_objects, ENTRY->static_entry, NULL, &ctx->alloc);
+        refcount_list_free_with_data_full(
+            held_refs, refcount_context_unref_as_callback, ctx, &ctx->alloc);
+        return true;
     } else {
         BREAKPOINT(DEINIT_STATIC, ctx, obj)
+        return false;
     }
 }
 
@@ -126,7 +162,9 @@ void refcount_context_deinit_static(RefcountContext *ctx, void *obj) {
  * @return The input object
  */
 void *refcount_context_ref(RefcountContext *ctx, void *obj) {
-    if (!ENTRY->is_static) {
+    if (!obj) {
+        return NULL;
+    } else if (!ENTRY->is_static) {
         ++ENTRY->ref_count;
     }
     return obj;
@@ -137,12 +175,18 @@ void *refcount_context_ref(RefcountContext *ctx, void *obj) {
  * already tracked object.
  * @param ctx the #RefcountContext
  * @param obj The object to track
+ * @return True on success
  */
-static void track_gc_root(RefcountContext *ctx, void *obj) {
+static bool track_gc_root(RefcountContext *ctx, void *obj) {
     if (!ENTRY->gc_root) {
-        ctx->gc_roots = refcount_list_push(ctx->gc_roots, obj);
+        ctx->gc_roots =
+            refcount_list_push_full(ctx->gc_roots, obj, &ctx->alloc);
+        if (!ctx->gc_roots) {
+            return false;
+        }
         ENTRY->gc_root = ctx->gc_roots;
     }
+    return true;
 }
 
 /**
@@ -152,7 +196,8 @@ static void track_gc_root(RefcountContext *ctx, void *obj) {
  * @param obj The object to untrack
  */
 static void remove_gc_root(RefcountContext *ctx, void *obj) {
-    ctx->gc_roots = refcount_list_remove(ctx->gc_roots, ENTRY->gc_root, NULL);
+    ctx->gc_roots = refcount_list_remove_full(ctx->gc_roots, ENTRY->gc_root,
+                                              NULL, &ctx->alloc);
     ENTRY->gc_root = NULL;
 }
 
@@ -162,14 +207,19 @@ static void remove_gc_root(RefcountContext *ctx, void *obj) {
  * @param ctx The #RefcountContext
  * @param obj The object to unref
  * @param queue A double pointer to a #RefcountList acting as a queue
+ * @param toplevel The toplevel object that triggered this unref. It is ignored
+ * so that it is not freed twice in the specific case that an object with a
+ * floating ref and a reference cycle is freed
  * @return NULL if the reference count fell to 0, the given object otherwise
  */
 static void *unref_to_queue(RefcountContext *ctx, void *obj,
-                            RefcountList **queue) {
+                            RefcountList **queue, void *toplevel) {
     if (ENTRY->is_static) {
         return obj;
+    } else if (obj == toplevel) {
+        return NULL;
     } else if (ENTRY->ref_count <= 1) {
-        *queue = refcount_list_push(*queue, obj);
+        *queue = refcount_list_push_full(*queue, obj, &ctx->alloc);
         return NULL;
     }
     --ENTRY->ref_count;
@@ -183,6 +233,7 @@ static void *unref_to_queue(RefcountContext *ctx, void *obj,
  */
 struct ContextAndQueue {
     RefcountContext *ctx; //!< The context.
+    void *toplevel; //!< Toplevel object that triggered the unref.
     RefcountList **queue; //!< The queue.
 };
 
@@ -193,27 +244,49 @@ struct ContextAndQueue {
  */
 static void unref_to_queue_as_callback(void *obj, void *ctx_and_queue_raw) {
     struct ContextAndQueue *ctx_and_queue = ctx_and_queue_raw;
-    unref_to_queue(ctx_and_queue->ctx, obj, ctx_and_queue->queue);
+    unref_to_queue(ctx_and_queue->ctx, obj, ctx_and_queue->queue,
+                   ctx_and_queue->toplevel);
+}
+
+/**
+ * Destroy an object by calling it's destructor.
+ * @param ctx The #RefcountContext
+ * @param obj The object to destroy
+ */
+static inline void destroy_object(RefcountContext *ctx, void *obj) {
+    remove_gc_root(ctx, obj);
+    if (ctx->destroy_callback) {
+        ctx->destroy_callback(obj, ctx->user_data);
+    }
 }
 
 /**
  * Continually release held references and object held in a queue.
  * @param ctx The #RefcountContext
  * @param queue The queue
+ * @param toplevel Toplevel object that triggered the unref
  */
-static void process_unref_queue(RefcountContext *ctx, RefcountList *queue) {
+static void process_unref_queue(RefcountContext *ctx, RefcountList *queue,
-    struct ContextAndQueue ctx_and_queue = {.ctx = ctx, .queue = &queue};
+                                void *toplevel) {
+    struct ContextAndQueue ctx_and_queue = {
+        .ctx = ctx, .queue = &queue, .toplevel = toplevel};
     while (queue) {
         void *cur = refcount_list_peek(queue);
-        queue = refcount_list_pop(queue, NULL);
+        RefcountList *held_refs = NULL;
-        RefcountList *held_refs =
+        queue = refcount_list_pop_full(queue, NULL, &ctx->alloc);
-            ctx->held_refs_callback ? ctx->held_refs_callback(cur) : NULL;
+        if (!cur) {
-        refcount_list_free_with_data(held_refs, unref_to_queue_as_callback,
+            continue;
-                                     &ctx_and_queue);
-        remove_gc_root(ctx, cur);
-        if (ctx->destroy_callback) {
-            ctx->destroy_callback(cur);
         }
+        if (obj_held_refs(ctx, cur, &held_refs)) {
+            // I don't really know how else to handle this as I can't think of a
+            // good way to undo all the unrefs that have already been processed,
+            // so we can't really make this atomic without going over all
+            // objects twice.
+            refcount_list_free_with_data_full(held_refs,
+                                              unref_to_queue_as_callback,
+                                              &ctx_and_queue, &ctx->alloc);
+        }
+        destroy_object(ctx, cur);
     }
 }
 
@@ -226,10 +299,13 @@ static void process_unref_queue(RefcountContext *ctx, RefcountList *queue) {
  * object
  */
 void *refcount_context_unref(RefcountContext *ctx, void *obj) {
+    if (!obj) {
+        return NULL;
+    }
     RefcountList *queue = NULL;
-    obj = unref_to_queue(ctx, obj, &queue);
+    void *retval = unref_to_queue(ctx, obj, &queue, NULL);
-    process_unref_queue(ctx, queue);
+    process_unref_queue(ctx, queue, obj);
-    return obj;
+    return retval;
 }
 
 /**
@@ -242,10 +318,13 @@ void *refcount_context_unref(RefcountContext *ctx, void *obj) {
  * @return The input object
  */
 void *refcount_context_float(RefcountContext *ctx, void *obj) {
-    if (ENTRY->is_static) {
+    if (!obj) {
+        return NULL;
+    } else if (ENTRY->is_static) {
         return obj;
     } else if (!ENTRY->ref_count) {
         BREAKPOINT(FLOAT, ctx, obj);
+        return obj;
     }
     if (--ENTRY->ref_count) {
         track_gc_root(ctx, obj);
@@ -254,3 +333,108 @@ void *refcount_context_float(RefcountContext *ctx, void *obj) {
     }
     return obj;
 }
+
+/**
+ * Set of hash table functions used in #gc_check_root.
+ */
+static const HTTableFunctions ROOT_COUNTS_FNS = {
+    .hash = ht_intptr_hash_callback,
+    .equal = ht_intptr_equal_callback,
+    .destroy_key = NULL,
+    .destroy_value = NULL,
+    .user_data = NULL,
+};
+
+struct ContextAndRootPtr {
+    RefcountContext *ctx;
+    RefcountList **root_ptr;
+    bool did_update;
+};
+
+static bool free_roots_foreach(void *obj, void *ignored, void *user_data) {
+    struct ContextAndRootPtr *data = user_data;
+    if (*data->root_ptr == REFCOUNT_OBJECT_ENTRY(data->ctx, obj)->gc_root) {
+        *data->root_ptr = (*data->root_ptr)->next;
+        data->did_update = true;
+    }
+    destroy_object(data->ctx, obj);
+    return false;
+}
+
+/**
+ * Check the root pointed to by the double pointer root_ptr. After the call,
+ * root_ptr is set to the next root to be checked.
+ * @param ctx The context
+ * @param root_ptr Double pointer to one of the GC roots
+ * @return True on success
+ */
+static bool check_gc_root(RefcountContext *ctx, RefcountList **root_ptr) {
+    HTTable *counts = ht_new(&ROOT_COUNTS_FNS, &ctx->ht_alloc, NULL);
+    if (!counts) {
+        *root_ptr = (*root_ptr)->next;
+        return false;
+    }
+    RefcountList *root = *root_ptr;
+    RefcountList *queue = NULL;
+    if (!obj_held_refs(ctx, root->data, &queue)) {
+        ht_free(counts);
+        *root_ptr = (*root_ptr)->next;
+        return false;
+    }
+    size_t seen_objects = 0;
+    size_t clear_objects = 0;
+    // ignore allocation errors until I decide how to deal with them (never)
+    while (queue) {
+        void *obj = queue->data;
+        if (!obj || refcount_context_is_static(ctx, obj)) {
+            goto next;
+        }
+        uintptr_t count;
+        if (ht_has(counts, obj)) {
+            count = HT_UUNSTUFF(ht_get(counts, obj));
+        } else {
+            count = REFCOUNT_OBJECT_ENTRY(ctx, obj)->ref_count;
+            ++seen_objects;
+        }
+        if (count > 0) {
+            ht_insert(counts, obj, HT_STUFF(--count));
+            if (count == 0) {
+                ++clear_objects;
+            }
+            obj_held_refs(ctx, root->data, &queue);
+        }
+    next:
+        queue = refcount_list_pop_full(queue, NULL, &ctx->alloc);
+    }
+    if (seen_objects == clear_objects) {
+        struct ContextAndRootPtr data = {
+            .ctx = ctx, .root_ptr = root_ptr, .did_update = false};
+        ht_foreach(counts, free_roots_foreach, &data);
+        if (!data.did_update) {
+            *root_ptr = (*root_ptr)->next;
+        }
+    } else {
+        *root_ptr = (*root_ptr)->next;
+    }
+    ht_free(counts);
+    return true;
+}
+
+/**
+ * Run the garbage collector on a context.
+ * @param cts The #RefcountContext
+ * @return False if an error occurred, true otherwise
+ */
+bool refcount_context_garbage_collect(RefcountContext *ctx) {
+    if (!ctx->held_refs_callback) {
+        // no loops possible
+        return true;
+    }
+    RefcountList *root = ctx->gc_roots;
+    while (root) {
+        if (!check_gc_root(ctx, &root)) {
+            return false;
+        }
+    }
+    return true;
+}

test/CMakeLists.txt

@@ -1,5 +1,5 @@
 link_libraries(refcount)
-add_compile_options(-Wall)
+add_compile_options(-Wall -Wpedantic)
 
 add_executable(test_list test_list.c)
 add_test(NAME list COMMAND test_list)

test/alloc.h

@@ -68,7 +68,6 @@ static UNUSED char *counting_strdup(const char *str) {
 
 static UNUSED const RefcountAllocator COUNTING_ALLOCATOR = {
     .malloc = counting_malloc,
-    .realloc = counting_realloc,
     .free = counting_free,
 };
 

test/test_list.c

@@ -4,7 +4,11 @@
 #include <refcount/list.h>
 #include <string.h>
 
-int main() {
+void *counting_strdup_callback(const void *str, void *ignored) {
+    return counting_strdup(str);
+}
+
+int main(int argc, const char **argv) {
     refcount_global_allocator = &COUNTING_ALLOCATOR;
 
     assert(refcount_list_length(NULL) == 0);
@@ -77,7 +81,7 @@ int main() {
     assert(strcmp(refcount_list_nth(l, 3), "str4") == 0);
     assert(strcmp(refcount_list_nth(l, 4), "str5") == 0);
 
-    RefcountList *l2 = refcount_list_copy(l, (void *) counting_strdup);
+    RefcountList *l2 = refcount_list_copy(l, counting_strdup_callback, NULL);
 
     refcount_list_free(l, counting_free);
 

test/test_refcount.c

@@ -7,49 +7,78 @@ typedef struct A {
     int num;
     RefcountEntry refcount;
     char *str;
+    struct A *next;
 } A;
 
 A *make_a(RefcountContext *c, int n, const char *s) {
     A *a = counting_malloc(sizeof(A));
     a->num = n;
     a->str = counting_strdup(s);
+    a->next = NULL;
     refcount_context_init_obj(c, a);
     return a;
 }
 
-void destroy_callback(void *a_raw) {
+bool held_refs_callback(void *a_raw, RefcountList **out, void *ignored) {
+    A *a = a_raw;
+    if (a->next) {
+        *out = refcount_list_push_full(*out, a->next, &COUNTING_ALLOCATOR);
+    }
+    return true;
+}
+
+void destroy_callback(void *a_raw, void *ignored) {
     A *a = a_raw;
     counting_free(a->str);
     counting_free(a);
 }
 
-int main() {
+int main(int argc, const char **argv) {
-    refcount_global_allocator = &COUNTING_ALLOCATOR;
-
     RefcountContext *c =
-        refcount_make_context(offsetof(A, refcount), NULL, destroy_callback);
+        refcount_make_context(offsetof(A, refcount), held_refs_callback,
+                              destroy_callback, NULL, &COUNTING_ALLOCATOR);
 
     A *a = make_a(c, 10, "Hello world\n");
     assert(!refcount_context_is_static(c, a));
     assert(refcount_context_num_refs(c, a) == 0);
 
-    refcount_context_ref(c, a);
+    a = refcount_context_ref(c, a);
     assert(refcount_context_num_refs(c, a) == 1);
 
-    refcount_context_ref(c, a);
+    a = refcount_context_ref(c, a);
     assert(refcount_context_num_refs(c, a) == 2);
 
-    refcount_context_unref(c, a);
+    a = refcount_context_unref(c, a);
     assert(refcount_context_num_refs(c, a) == 1);
 
-    refcount_context_float(c, a);
+    a = refcount_context_float(c, a);
+    assert(a);
     assert(refcount_context_num_refs(c, a) == 0);
 
     refcount_context_ref(c, a);
     assert(refcount_context_num_refs(c, a) == 1);
 
+    a = refcount_context_unref(c, a);
+    assert(!a);
+
+    a = make_a(c, 10, "Hello World\n");
+    A *b = make_a(c, 42, "The answer!");
+    a->next = refcount_context_ref(c, b);
+    assert(refcount_context_num_refs(c, a->next) == 1);
+    assert(refcount_context_num_refs(c, a) == 0);
+
     refcount_context_unref(c, a);
 
+    a = make_a(c, 'a', "a");
+    a->next = refcount_context_ref(c, a);
+    assert(refcount_context_num_refs(c, a) == 1);
+
+    refcount_context_ref(c, a);
+    refcount_context_unref(c, a);
+    assert(refcount_context_num_refs(c, a) == 1);
+
+    refcount_context_garbage_collect(c);
+
     refcount_context_destroy(c);
 
     check_allocator_status();