Implement a more explicit object system

Make every dynamically allocated, opaque data structure inherit from
a common sqfs_object_t structure with common entry points (e.g. destroy).

This removes tons of public API functions and replaces them with a
simple sqfs_destroy instead. If semantics of the (until now implicit)
object system need to be extended, it can be much more conveniantely
done this way.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

48 files changed, 340 insertions, 336 deletions

diff --git a/difftool/sqfsdiff.c b/difftool/sqfsdiff.c
index 58ddb4b..c5d2bf2 100644
--- a/difftool/sqfsdiff.c
+++ b/difftool/sqfsdiff.c
@@ -103,28 +103,28 @@ static int open_sfqs(sqfs_state_t *state, const char *path)
 
 	return 0;
 fail_data:
-	sqfs_data_reader_destroy(state->data);
+	sqfs_destroy(state->data);
 fail_tree:
 	sqfs_dir_tree_destroy(state->root);
 fail_dr:
-	sqfs_dir_reader_destroy(state->dr);
+	sqfs_destroy(state->dr);
 fail_id:
-	sqfs_id_table_destroy(state->idtbl);
+	sqfs_destroy(state->idtbl);
 fail_cmp:
-	state->cmp->destroy(state->cmp);
+	sqfs_destroy(state->cmp);
 fail_file:
-	state->file->destroy(state->file);
+	sqfs_destroy(state->file);
 	return -1;
 }
 
 static void close_sfqs(sqfs_state_t *state)
 {
-	sqfs_data_reader_destroy(state->data);
+	sqfs_destroy(state->data);
 	sqfs_dir_tree_destroy(state->root);
-	sqfs_dir_reader_destroy(state->dr);
-	sqfs_id_table_destroy(state->idtbl);
-	state->cmp->destroy(state->cmp);
-	state->file->destroy(state->file);
+	sqfs_destroy(state->dr);
+	sqfs_destroy(state->idtbl);
+	sqfs_destroy(state->cmp);
+	sqfs_destroy(state->file);
 }
 
 int main(int argc, char **argv)
diff --git a/extras/browse.c b/extras/browse.c
index 49ddb7e..0bca183 100644
--- a/extras/browse.c
+++ b/extras/browse.c
@@ -599,16 +599,16 @@ int main(int argc, char **argv)
 	status = EXIT_SUCCESS;
 	free(buffer);
 out:
-	sqfs_data_reader_destroy(data);
+	sqfs_destroy(data);
 out_dir:
 	if (working_dir != NULL)
 		free(working_dir);
-	sqfs_dir_reader_destroy(dr);
+	sqfs_destroy(dr);
 out_id:
-	sqfs_id_table_destroy(idtbl);
+	sqfs_destroy(idtbl);
 out_cmp:
-	cmp->destroy(cmp);
+	sqfs_destroy(cmp);
 out_fd:
-	file->destroy(file);
+	sqfs_destroy(file);
 	return status;
 }
diff --git a/extras/list_files.c b/extras/list_files.c
index de390f0..5e181f2 100644
--- a/extras/list_files.c
+++ b/extras/list_files.c
@@ -134,12 +134,12 @@ int main(int argc, char **argv)
 out:
 	if (root != NULL)
 		sqfs_dir_tree_destroy(root);
-	sqfs_dir_reader_destroy(dr);
+	sqfs_destroy(dr);
 out_id:
-	sqfs_id_table_destroy(idtbl);
+	sqfs_destroy(idtbl);
 out_cmp:
-	cmp->destroy(cmp);
+	sqfs_destroy(cmp);
 out_fd:
-	file->destroy(file);
+	sqfs_destroy(file);
 	return status;
 }
diff --git a/extras/mk42sqfs.c b/extras/mk42sqfs.c
index 0dbcc4e..53cf2f0 100644
--- a/extras/mk42sqfs.c
+++ b/extras/mk42sqfs.c
@@ -189,16 +189,16 @@ int main(void)
 
 	/* cleanup */
 	status = EXIT_SUCCESS;
-	sqfs_id_table_destroy(idtbl);
+	sqfs_destroy(idtbl);
 out_dirwr:
-	sqfs_dir_writer_destroy(dirwr);
+	sqfs_destroy(dirwr);
 out_dm:
-	sqfs_meta_writer_destroy(dir_m);
+	sqfs_destroy(dir_m);
 out_im:
-	sqfs_meta_writer_destroy(inode_m);
+	sqfs_destroy(inode_m);
 out_cmp:
-	cmp->destroy(cmp);
+	sqfs_destroy(cmp);
 out_file:
-	file->destroy(file);
+	sqfs_destroy(file);
 	return status;
 }
diff --git a/extras/mknastyfs.c b/extras/mknastyfs.c
index 2a66bc9..b11f8f9 100644
--- a/extras/mknastyfs.c
+++ b/extras/mknastyfs.c
@@ -168,16 +168,16 @@ int main(void)
 
 	/* cleanup */
 	status = EXIT_SUCCESS;
-	sqfs_id_table_destroy(idtbl);
+	sqfs_destroy(idtbl);
 out_dirwr:
-	sqfs_dir_writer_destroy(dirwr);
+	sqfs_destroy(dirwr);
 out_dm:
-	sqfs_meta_writer_destroy(dir_m);
+	sqfs_destroy(dir_m);
 out_im:
-	sqfs_meta_writer_destroy(inode_m);
+	sqfs_destroy(inode_m);
 out_cmp:
-	cmp->destroy(cmp);
+	sqfs_destroy(cmp);
 out_file:
-	file->destroy(file);
+	sqfs_destroy(file);
 	return status;
 }
diff --git a/include/sqfs/block_processor.h b/include/sqfs/block_processor.h
index 41cb3d3..cef4459 100644
--- a/include/sqfs/block_processor.h
+++ b/include/sqfs/block_processor.h
@@ -33,6 +33,8 @@
  *
  * @brief Abstracts generating of file data and fragment blocks.
  *
+ * @implements sqfs_object_t
+ *
  * This data structure provides a simple begin/append/end interface
  * to generate file data blocks (see @ref sqfs_block_processor_begin_file,
  * @ref sqfs_block_processor_append and @ref sqfs_block_processor_end
@@ -130,15 +132,6 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 						    sqfs_frag_table_t *tbl);
 
 /**
- * @brief Destroy a data writer and free all memory used by it.
- *
- * @memberof sqfs_block_processor_t
- *
- * @param proc A pointer to a data writer object.
- */
-SQFS_API void sqfs_block_processor_destroy(sqfs_block_processor_t *proc);
-
-/**
  * @brief Start writing a file.
  *
  * @memberof sqfs_block_processor_t
diff --git a/include/sqfs/block_writer.h b/include/sqfs/block_writer.h
index e921141..bd1ddfc 100644
--- a/include/sqfs/block_writer.h
+++ b/include/sqfs/block_writer.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_block_writer_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts writing and deduplicating of data and fragment blocks.
  */
 
@@ -190,15 +192,6 @@ SQFS_API int sqfs_block_writer_set_hooks(sqfs_block_writer_t *wr,
 					 const sqfs_block_hooks_t *hooks);
 
 /**
- * @brief Destroy a block writer object.
- *
- * @memberof sqfs_block_writer_t
- *
- * @param wr A pointer to a block writer object.
- */
-SQFS_API void sqfs_block_writer_destroy(sqfs_block_writer_t *wr);
-
-/**
  * @brief Submit a data block to a blokc writer.
  *
  * @memberof sqfs_block_writer_t
diff --git a/include/sqfs/compressor.h b/include/sqfs/compressor.h
index 5e0be5d..c0fafda 100644
--- a/include/sqfs/compressor.h
+++ b/include/sqfs/compressor.h
@@ -32,16 +32,13 @@
 /**
  * @interface sqfs_compressor_t
  *
+ * @extends sqfs_object_t
+ *
  * @brief Encapsultes a compressor with a simple interface to compress or
  *        extract chunks of data.
  */
 struct sqfs_compressor_t {
-	/**
-	 * @brief Destroy a compressor and free all memory used by it.
-	 *
-	 * @param cmp A pointer to a compressor object.
-	 */
-	void (*destroy)(sqfs_compressor_t *cmp);
+	sqfs_object_t base;
 
 	/**
 	 * @brief Write compressor options to disk if non-default settings
diff --git a/include/sqfs/data_reader.h b/include/sqfs/data_reader.h
index 0c4918c..e058017 100644
--- a/include/sqfs/data_reader.h
+++ b/include/sqfs/data_reader.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_data_reader_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts access to data blocks stored in a SquashFS image.
  *
  * A SquashFS image can contain a series of file data blocks between the
@@ -69,15 +71,6 @@ SQFS_API sqfs_data_reader_t *sqfs_data_reader_create(sqfs_file_t *file,
 						     sqfs_compressor_t *cmp);
 
 /**
- * @brief Destroy a data reader instance and free all memory used by it.
- *
- * @memberof sqfs_data_reader_t
- *
- * @param data A pointer to a data reader object.
- */
-SQFS_API void sqfs_data_reader_destroy(sqfs_data_reader_t *data);
-
-/**
  * @brief Read and decode the fragment table from disk.
  *
  * @memberof sqfs_data_reader_t
diff --git a/include/sqfs/dir_reader.h b/include/sqfs/dir_reader.h
index 82121ed..7b55eb0 100644
--- a/include/sqfs/dir_reader.h
+++ b/include/sqfs/dir_reader.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_dir_reader_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts reading of directory entries
  *
  * SquashFS stores directory listings and inode structures separated from
@@ -167,13 +169,6 @@ SQFS_API sqfs_dir_reader_t *sqfs_dir_reader_create(const sqfs_super_t *super,
 						   sqfs_file_t *file);
 
 /**
- * @brief Cleanup a directory reader and free all its memory.
- *
- * @memberof sqfs_dir_reader_t
- */
-SQFS_API void sqfs_dir_reader_destroy(sqfs_dir_reader_t *rd);
-
-/**
  * @brief Navigate a directory reader to the location of a directory
  *        represented by an inode.
  *
diff --git a/include/sqfs/dir_writer.h b/include/sqfs/dir_writer.h
index 05d054b..e4a26c7 100644
--- a/include/sqfs/dir_writer.h
+++ b/include/sqfs/dir_writer.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_dir_writer_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts generating of directory entries
  *
  * SquashFS stores directory entries and inodes separated from each other. The
@@ -102,15 +104,6 @@ SQFS_API sqfs_dir_writer_t *sqfs_dir_writer_create(sqfs_meta_writer_t *dm,
 						   sqfs_u32 flags);
 
 /**
- * @brief Destroy a directory writer and free all its memory.
- *
- * @memberof sqfs_dir_writer_t
- *
- * @param writer A pointer to a directory writer object.
- */
-SQFS_API void sqfs_dir_writer_destroy(sqfs_dir_writer_t *writer);
-
-/**
  * @brief Begin writing a directory, i.e. reset and initialize all internal
  *        state neccessary.
  *
diff --git a/include/sqfs/frag_table.h b/include/sqfs/frag_table.h
index 45ba645..9a66692 100644
--- a/include/sqfs/frag_table.h
+++ b/include/sqfs/frag_table.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_frag_table_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts reading, writing and management of the fragment table.
  */
 
@@ -50,13 +52,6 @@ extern "C" {
 SQFS_API sqfs_frag_table_t *sqfs_frag_table_create(sqfs_u32 flags);
 
 /**
- * @brief Destroy a fragment table and release all associated memory.
- *
- * @memberof sqfs_frag_table_t
- */
-SQFS_API void sqfs_frag_table_destroy(sqfs_frag_table_t *tbl);
-
-/**
  * @brief Load a fragment table from a SquashFS image.
  *
  * @memberof sqfs_frag_table_t
diff --git a/include/sqfs/id_table.h b/include/sqfs/id_table.h
index 67990db..706687f 100644
--- a/include/sqfs/id_table.h
+++ b/include/sqfs/id_table.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_id_table_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief A simple data structure that encapsulates ID to index mapping for
  *        user and group IDs.
  *
@@ -57,15 +59,6 @@ extern "C" {
 SQFS_API sqfs_id_table_t *sqfs_id_table_create(sqfs_u32 flags);
 
 /**
- * @brief Destroy an ID table object and free all memory used by it.
- *
- * @memberof sqfs_id_table_t
- *
- * @param tbl A pointer to an ID table object.
- */
-SQFS_API void sqfs_id_table_destroy(sqfs_id_table_t *tbl);
-
-/**
  * @brief Resolve a 32 bit ID to a unique 16 bit index.
  *
  * @memberof sqfs_id_table_t
diff --git a/include/sqfs/io.h b/include/sqfs/io.h
index 1108a61..578c4fb 100644
--- a/include/sqfs/io.h
+++ b/include/sqfs/io.h
@@ -60,15 +60,12 @@ typedef enum {
 /**
  * @interface sqfs_file_t
  *
+ * @extends sqfs_object_t
+ *
  * @brief Abstracts file I/O to make it easy to embedd SquashFS.
  */
 struct sqfs_file_t {
-	/**
-	 * @brief Close the file and destroy the interface implementation.
-	 *
-	 * @param file A pointer to the file object.
-	 */
-	void (*destroy)(sqfs_file_t *file);
+	sqfs_object_t base;
 
 	/**
 	 * @brief Read a chunk of data from an absolute position.
diff --git a/include/sqfs/meta_reader.h b/include/sqfs/meta_reader.h
index e2203de..e8b40d4 100644
--- a/include/sqfs/meta_reader.h
+++ b/include/sqfs/meta_reader.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_meta_reader_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts reading of meta data blocks.
  *
  * SquashFS stores meta data by dividing it into fixed size (8k) chunks
@@ -70,15 +72,6 @@ SQFS_API sqfs_meta_reader_t *sqfs_meta_reader_create(sqfs_file_t *file,
 						     sqfs_u64 limit);
 
 /**
- * @brief Destroy a meta data reader and free all memory used by it.
- *
- * @memberof sqfs_meta_reader_t
- *
- * @param m A pointer to a meta data reader.
- */
-SQFS_API void sqfs_meta_reader_destroy(sqfs_meta_reader_t *m);
-
-/**
  * @brief Seek to a specific meta data block and offset.
  *
  * @memberof sqfs_meta_reader_t
diff --git a/include/sqfs/meta_writer.h b/include/sqfs/meta_writer.h
index cf8e16c..be151cc 100644
--- a/include/sqfs/meta_writer.h
+++ b/include/sqfs/meta_writer.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_meta_writer_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts generating of meta data blocks, either in memory or
  *        directly on disk.
  *
@@ -88,15 +90,6 @@ SQFS_API sqfs_meta_writer_t *sqfs_meta_writer_create(sqfs_file_t *file,
 						     sqfs_u32 flags);
 
 /**
- * @brief Destroy a meta data writer and free all memory used by it.
- *
- * @memberof sqfs_meta_writer_t
- *
- * @param m A pointer to a meta data writer.
- */
-SQFS_API void sqfs_meta_writer_destroy(sqfs_meta_writer_t *m);
-
-/**
  * @brief Finish the current block, even if it isn't full yet.
  *
  * @memberof sqfs_meta_writer_t
diff --git a/include/sqfs/predef.h b/include/sqfs/predef.h
index d8cc293..6ca3503 100644
--- a/include/sqfs/predef.h
+++ b/include/sqfs/predef.h
@@ -57,6 +57,12 @@
 	#endif
 #endif
 
+#ifdef _MSC_VER
+	#define SQFS_INLINE __forceinline
+#else
+	#define SQFS_INLINE __inline__ __attribute__((always_inline))
+#endif
+
 typedef uint8_t sqfs_u8;
 typedef uint16_t sqfs_u16;
 typedef uint32_t sqfs_u32;
@@ -108,4 +114,25 @@ typedef struct sqfs_xattr_value_t sqfs_xattr_value_t;
 typedef struct sqfs_xattr_id_t sqfs_xattr_id_t;
 typedef struct sqfs_xattr_id_table_t sqfs_xattr_id_table_t;
 
+/**
+ * @interface sqfs_object_t
+ *
+ * @brief Base interface for all libsquashfs in-memory data structures.
+ */
+typedef struct sqfs_object_t {
+	void (*destroy)(struct sqfs_object_t *instance);
+} sqfs_object_t;
+
+/**
+ * @brief Destroy an object and free all its memory
+ *
+ * @memberof sqfs_object_t
+ *
+ * @param obj A pointer to an object
+ */
+static SQFS_INLINE void sqfs_destroy(void *obj)
+{
+	((sqfs_object_t *)obj)->destroy(obj);
+}
+
 #endif /* SQFS_PREDEF_H */
diff --git a/include/sqfs/xattr_reader.h b/include/sqfs/xattr_reader.h
index 6544a21..7912de4 100644
--- a/include/sqfs/xattr_reader.h
+++ b/include/sqfs/xattr_reader.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_xattr_reader_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts read access to extended attributes in a SquashFS filesystem
  *
  * SquashFS stores extended attributes using multiple levels of indirection.
@@ -95,15 +97,6 @@ SQFS_API sqfs_xattr_reader_t *sqfs_xattr_reader_create(sqfs_file_t *file,
 						       sqfs_compressor_t *cmp);
 
 /**
- * @brief Destroy an xattr reader and free all memory used by it
- *
- * @memberof sqfs_xattr_reader_t
- *
- * @param xr A pointer to an xattr reader instance
- */
-SQFS_API void sqfs_xattr_reader_destroy(sqfs_xattr_reader_t *xr);
-
-/**
  * @brief Load the locations of the xattr meta data blocks into memory
  *
  * @memberof sqfs_xattr_reader_t
diff --git a/include/sqfs/xattr_writer.h b/include/sqfs/xattr_writer.h
index 5b18659..b25215e 100644
--- a/include/sqfs/xattr_writer.h
+++ b/include/sqfs/xattr_writer.h
@@ -31,6 +31,8 @@
 /**
  * @struct sqfs_xattr_writer_t
  *
+ * @implements sqfs_object_t
+ *
  * @brief Abstracts writing of extended attributes to a SquashFS filesystem.
  *
  * This data structure provides a simple, abstract interface to recording
@@ -64,15 +66,6 @@ extern "C" {
 SQFS_API sqfs_xattr_writer_t *sqfs_xattr_writer_create(void);
 
 /**
- * @brief Destroy an xattr writer instance and free all memory it used.
- *
- * @memberof sqfs_xattr_writer_t
- *
- * @param xwr A pointer to an xattr writer instance.
- */
-SQFS_API void sqfs_xattr_writer_destroy(sqfs_xattr_writer_t *xwr);
-
-/**
  * @brief Begin recording a block of key-value pairs.
  *
  * @memberof sqfs_xattr_writer_t
diff --git a/lib/common/comp_lzo.c b/lib/common/comp_lzo.c
index f371fb6..a0c838a 100644
--- a/lib/common/comp_lzo.c
+++ b/lib/common/comp_lzo.c
@@ -195,7 +195,7 @@ static sqfs_compressor_t *lzo_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)lzo;
 }
 
-static void lzo_destroy(sqfs_compressor_t *base)
+static void lzo_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -247,11 +247,11 @@ sqfs_compressor_t *lzo_compressor_create(const sqfs_compressor_config_t *cfg)
 	lzo->buf_size = scratch_size;
 	lzo->work_size = lzo_algs[cfg->opt.lzo.algorithm].bufsize;
 
-	base->destroy = lzo_destroy;
 	base->do_block = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) ?
 		lzo_uncomp_block : lzo_comp_block;
 	base->write_options = lzo_write_options;
 	base->read_options = lzo_read_options;
 	base->create_copy = lzo_create_copy;
+	((sqfs_object_t *)base)->destroy = lzo_destroy;
 	return base;
 }
diff --git a/lib/common/io_stdin.c b/lib/common/io_stdin.c
index ca6bf49..469e300 100644
--- a/lib/common/io_stdin.c
+++ b/lib/common/io_stdin.c
@@ -21,7 +21,7 @@ typedef struct {
 } sqfs_file_stdinout_t;
 
 
-static void stdinout_destroy(sqfs_file_t *base)
+static void stdinout_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -159,7 +159,7 @@ sqfs_file_t *sqfs_get_stdin_file(FILE *fp, const sparse_map_t *map,
 	file->map = map;
 	file->fp = fp;
 
-	base->destroy = stdinout_destroy;
+	((sqfs_object_t *)base)->destroy = stdinout_destroy;
 	base->write_at = stdin_write_at;
 	base->get_size = stdinout_get_size;
 	base->truncate = stdinout_truncate;
diff --git a/lib/common/writer.c b/lib/common/writer.c
index 84a40d2..4762b82 100644
--- a/lib/common/writer.c
+++ b/lib/common/writer.c
@@ -90,7 +90,7 @@ int sqfs_writer_init(sqfs_writer_t *sqfs, const sqfs_writer_cfg_t *wrcfg)
 #ifdef WITH_LZO
 	if (cfg.id == SQFS_COMP_LZO) {
 		if (sqfs->cmp != NULL)
-			sqfs->cmp->destroy(sqfs->cmp);
+			sqfs_destroy(sqfs->cmp);
 
 		sqfs->cmp = lzo_compressor_create(&cfg);
 	}
@@ -187,26 +187,26 @@ int sqfs_writer_init(sqfs_writer_t *sqfs, const sqfs_writer_cfg_t *wrcfg)
 
 	return 0;
 fail_dm:
-	sqfs_meta_writer_destroy(sqfs->dm);
+	sqfs_destroy(sqfs->dm);
 fail_im:
-	sqfs_meta_writer_destroy(sqfs->im);
+	sqfs_destroy(sqfs->im);
 fail_xwr:
 	if (sqfs->xwr != NULL)
-		sqfs_xattr_writer_destroy(sqfs->xwr);
+		sqfs_destroy(sqfs->xwr);
 fail_id:
-	sqfs_id_table_destroy(sqfs->idtbl);
+	sqfs_destroy(sqfs->idtbl);
 fail_data:
-	sqfs_block_processor_destroy(sqfs->data);
+	sqfs_destroy(sqfs->data);
 fail_fragtbl:
-	sqfs_frag_table_destroy(sqfs->fragtbl);
+	sqfs_destroy(sqfs->fragtbl);
 fail_blkwr:
-	sqfs_block_writer_destroy(sqfs->blkwr);
+	sqfs_destroy(sqfs->blkwr);
 fail_cmp:
-	sqfs->cmp->destroy(sqfs->cmp);
+	sqfs_destroy(sqfs->cmp);
 fail_fs:
 	fstree_cleanup(&sqfs->fs);
 fail_file:
-	sqfs->outfile->destroy(sqfs->outfile);
+	sqfs_destroy(sqfs->outfile);
 	return -1;
 }
 
@@ -299,16 +299,16 @@ int sqfs_writer_finish(sqfs_writer_t *sqfs, const sqfs_writer_cfg_t *cfg)
 void sqfs_writer_cleanup(sqfs_writer_t *sqfs)
 {
 	if (sqfs->xwr != NULL)
-		sqfs_xattr_writer_destroy(sqfs->xwr);
-
-	sqfs_dir_writer_destroy(sqfs->dirwr);
-	sqfs_meta_writer_destroy(sqfs->dm);
-	sqfs_meta_writer_destroy(sqfs->im);
-	sqfs_id_table_destroy(sqfs->idtbl);
-	sqfs_block_processor_destroy(sqfs->data);
-	sqfs_block_writer_destroy(sqfs->blkwr);
-	sqfs_frag_table_destroy(sqfs->fragtbl);
-	sqfs->cmp->destroy(sqfs->cmp);
+		sqfs_destroy(sqfs->xwr);
+
+	sqfs_destroy(sqfs->dirwr);
+	sqfs_destroy(sqfs->dm);
+	sqfs_destroy(sqfs->im);
+	sqfs_destroy(sqfs->idtbl);
+	sqfs_destroy(sqfs->data);
+	sqfs_destroy(sqfs->blkwr);
+	sqfs_destroy(sqfs->fragtbl);
+	sqfs_destroy(sqfs->cmp);
 	fstree_cleanup(&sqfs->fs);
-	sqfs->outfile->destroy(sqfs->outfile);
+	sqfs_destroy(sqfs->outfile);
 }
diff --git a/lib/sqfs/block_processor/internal.h b/lib/sqfs/block_processor/internal.h
index b3d76ba..71f8fab 100644
--- a/lib/sqfs/block_processor/internal.h
+++ b/lib/sqfs/block_processor/internal.h
@@ -50,6 +50,8 @@ typedef struct sqfs_block_t {
 } sqfs_block_t;
 
 struct sqfs_block_processor_t {
+	sqfs_object_t obj;
+
 	/* synchronization primitives */
 #ifdef WITH_PTHREAD
 	pthread_mutex_t mtx;
diff --git a/lib/sqfs/block_processor/serial.c b/lib/sqfs/block_processor/serial.c
index e18b67e..469388c 100644
--- a/lib/sqfs/block_processor/serial.c
+++ b/lib/sqfs/block_processor/serial.c
@@ -7,6 +7,11 @@
 #define SQFS_BUILDING_DLL
 #include "internal.h"
 
+static void block_processor_destroy(sqfs_object_t *obj)
+{
+	block_processor_cleanup((sqfs_block_processor_t *)obj);
+}
+
 sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 						    sqfs_compressor_t *cmp,
 						    unsigned int num_workers,
@@ -27,14 +32,10 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 		return NULL;
 	}
 
+	((sqfs_object_t *)proc)->destroy = block_processor_destroy;
 	return proc;
 }
 
-void sqfs_block_processor_destroy(sqfs_block_processor_t *proc)
-{
-	block_processor_cleanup(proc);
-}
-
 int test_and_set_status(sqfs_block_processor_t *proc, int status)
 {
 	if (proc->status == 0)
diff --git a/lib/sqfs/block_processor/winpthread.c b/lib/sqfs/block_processor/winpthread.c
index 85416ab..a91f001 100644
--- a/lib/sqfs/block_processor/winpthread.c
+++ b/lib/sqfs/block_processor/winpthread.c
@@ -94,6 +94,35 @@ static THREAD_TYPE worker_proc(THREAD_ARG arg)
 }
 
 #if defined(_WIN32) || defined(__WINDOWS__)
+static void block_processor_destroy(sqfs_object_t *obj)
+{
+	sqfs_block_processor_t *proc = (sqfs_block_processor_t *)obj;
+	unsigned int i;
+
+	EnterCriticalSection(&proc->mtx);
+	proc->status = -1;
+	WakeAllConditionVariable(&proc->queue_cond);
+	LeaveCriticalSection(&proc->mtx);
+
+	for (i = 0; i < proc->num_workers; ++i) {
+		if (proc->workers[i] == NULL)
+			continue;
+
+		if (proc->workers[i]->thread != NULL) {
+			WaitForSingleObject(proc->workers[i]->thread, INFINITE);
+			CloseHandle(proc->workers[i]->thread);
+		}
+
+		if (proc->workers[i]->cmp != NULL)
+			sqfs_destroy(proc->workers[i]->cmp);
+
+		free(proc->workers[i]);
+	}
+
+	DeleteCriticalSection(&proc->mtx);
+	block_processor_cleanup(proc);
+}
+
 sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 						    sqfs_compressor_t *cmp,
 						    unsigned int num_workers,
@@ -112,6 +141,8 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 	if (proc == NULL)
 		return NULL;
 
+	((sqfs_object_t *)obj)->destroy = block_processor_destroy;
+
 	InitializeCriticalSection(&proc->mtx);
 	InitializeConditionVariable(&proc->queue_cond);
 	InitializeConditionVariable(&proc->done_cond);
@@ -142,38 +173,34 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 
 	return proc;
 fail:
-	sqfs_block_processor_destroy(proc);
+	block_processor_destroy(proc);
 	return NULL;
 }
-
-void sqfs_block_processor_destroy(sqfs_block_processor_t *proc)
+#else
+static void block_processor_destroy(sqfs_object_t *obj)
 {
+	sqfs_block_processor_t *proc = (sqfs_block_processor_t *)obj;
 	unsigned int i;
 
-	EnterCriticalSection(&proc->mtx);
+	pthread_mutex_lock(&proc->mtx);
 	proc->status = -1;
-	WakeAllConditionVariable(&proc->queue_cond);
-	LeaveCriticalSection(&proc->mtx);
+	pthread_cond_broadcast(&proc->queue_cond);
+	pthread_mutex_unlock(&proc->mtx);
 
 	for (i = 0; i < proc->num_workers; ++i) {
-		if (proc->workers[i] == NULL)
-			continue;
-
-		if (proc->workers[i]->thread != NULL) {
-			WaitForSingleObject(proc->workers[i]->thread, INFINITE);
-			CloseHandle(proc->workers[i]->thread);
-		}
-
-		if (proc->workers[i]->cmp != NULL)
-			proc->workers[i]->cmp->destroy(proc->workers[i]->cmp);
+		pthread_join(proc->workers[i]->thread, NULL);
 
+		sqfs_destroy(proc->workers[i]->cmp);
 		free(proc->workers[i]);
 	}
 
-	DeleteCriticalSection(&proc->mtx);
+	pthread_cond_destroy(&proc->done_cond);
+	pthread_cond_destroy(&proc->queue_cond);
+	pthread_mutex_destroy(&proc->mtx);
+
 	block_processor_cleanup(proc);
 }
-#else
+
 sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 						    sqfs_compressor_t *cmp,
 						    unsigned int num_workers,
@@ -194,6 +221,7 @@ sqfs_block_processor_t *sqfs_block_processor_create(size_t max_block_size,
 	if (proc == NULL)
 		return NULL;
 
+	((sqfs_object_t *)proc)->destroy = block_processor_destroy;
 	proc->mtx = (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
 	proc->queue_cond = (pthread_cond_t)PTHREAD_COND_INITIALIZER;
 	proc->done_cond = (pthread_cond_t)PTHREAD_COND_INITIALIZER;
@@ -246,10 +274,8 @@ fail_thread:
 fail_init:
 	for (i = 0; i < num_workers; ++i) {
 		if (proc->workers[i] != NULL) {
-			if (proc->workers[i]->cmp != NULL) {
-				proc->workers[i]->cmp->
-					destroy(proc->workers[i]->cmp);
-			}
+			if (proc->workers[i]->cmp != NULL)
+				sqfs_destroy(proc->workers[i]->cmp);
 
 			free(proc->workers[i]);
 		}
@@ -260,29 +286,6 @@ fail_init:
 	block_processor_cleanup(proc);
 	return NULL;
 }
-
-void sqfs_block_processor_destroy(sqfs_block_processor_t *proc)
-{
-	unsigned int i;
-
-	pthread_mutex_lock(&proc->mtx);
-	proc->status = -1;
-	pthread_cond_broadcast(&proc->queue_cond);
-	pthread_mutex_unlock(&proc->mtx);
-
-	for (i = 0; i < proc->num_workers; ++i) {
-		pthread_join(proc->workers[i]->thread, NULL);
-
-		proc->workers[i]->cmp->destroy(proc->workers[i]->cmp);
-		free(proc->workers[i]);
-	}
-
-	pthread_cond_destroy(&proc->done_cond);
-	pthread_cond_destroy(&proc->queue_cond);
-	pthread_mutex_destroy(&proc->mtx);
-
-	block_processor_cleanup(proc);
-}
 #endif
 
 int append_to_work_queue(sqfs_block_processor_t *proc, sqfs_block_t *block,
diff --git a/lib/sqfs/block_writer.c b/lib/sqfs/block_writer.c
index f77fcf8..63f8c61 100644
--- a/lib/sqfs/block_writer.c
+++ b/lib/sqfs/block_writer.c
@@ -26,6 +26,7 @@ typedef struct {
 } blk_info_t;
 
 struct sqfs_block_writer_t {
+	sqfs_object_t *base;
 	sqfs_file_t *file;
 
 	size_t num_blocks;
@@ -115,6 +116,12 @@ static int align_file(sqfs_block_writer_t *wr)
 	return store_block_location(wr, size, 0, 0);
 }
 
+static void block_writer_destroy(sqfs_object_t *wr)
+{
+	free(((sqfs_block_writer_t *)wr)->blocks);
+	free(wr);
+}
+
 sqfs_block_writer_t *sqfs_block_writer_create(sqfs_file_t *file,
 					      size_t devblksz, sqfs_u32 flags)
 {
@@ -127,6 +134,7 @@ sqfs_block_writer_t *sqfs_block_writer_create(sqfs_file_t *file,
 	if (wr == NULL)
 		return NULL;
 
+	((sqfs_object_t *)wr)->destroy = block_writer_destroy;
 	wr->file = file;
 	wr->devblksz = devblksz;
 	wr->max_blocks = INIT_BLOCK_COUNT;
@@ -153,12 +161,6 @@ int sqfs_block_writer_set_hooks(sqfs_block_writer_t *wr, void *user_ptr,
 	return 0;
 }
 
-void sqfs_block_writer_destroy(sqfs_block_writer_t *wr)
-{
-	free(wr->blocks);
-	free(wr);
-}
-
 int sqfs_block_writer_write(sqfs_block_writer_t *wr, sqfs_u32 size,
 			    sqfs_u32 checksum, sqfs_u32 flags,
 			    const sqfs_u8 *data, sqfs_u64 *location)
diff --git a/lib/sqfs/comp/gzip.c b/lib/sqfs/comp/gzip.c
index 7183e5a..37491bb 100644
--- a/lib/sqfs/comp/gzip.c
+++ b/lib/sqfs/comp/gzip.c
@@ -31,7 +31,7 @@ typedef struct {
 	gzip_options_t opt;
 } gzip_compressor_t;
 
-static void gzip_destroy(sqfs_compressor_t *base)
+static void gzip_destroy(sqfs_object_t *base)
 {
 	gzip_compressor_t *gzip = (gzip_compressor_t *)base;
 
@@ -264,10 +264,10 @@ sqfs_compressor_t *gzip_compressor_create(const sqfs_compressor_config_t *cfg)
 	gzip->compress = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) == 0;
 	gzip->block_size = cfg->block_size;
 	base->do_block = gzip_do_block;
-	base->destroy = gzip_destroy;
 	base->write_options = gzip_write_options;
 	base->read_options = gzip_read_options;
 	base->create_copy = gzip_create_copy;
+	((sqfs_object_t *)base)->destroy = gzip_destroy;
 
 	if (gzip->compress) {
 		ret = deflateInit2(&gzip->strm, cfg->opt.gzip.level,
diff --git a/lib/sqfs/comp/lz4.c b/lib/sqfs/comp/lz4.c
index dd9b6ac..7656d67 100644
--- a/lib/sqfs/comp/lz4.c
+++ b/lib/sqfs/comp/lz4.c
@@ -110,7 +110,7 @@ static sqfs_compressor_t *lz4_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)lz4;
 }
 
-static void lz4_destroy(sqfs_compressor_t *base)
+static void lz4_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -132,11 +132,11 @@ sqfs_compressor_t *lz4_compressor_create(const sqfs_compressor_config_t *cfg)
 
 	lz4->high_compression = (cfg->flags & SQFS_COMP_FLAG_LZ4_HC) != 0;
 
-	base->destroy = lz4_destroy;
 	base->do_block = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) ?
 		lz4_uncomp_block : lz4_comp_block;
 	base->write_options = lz4_write_options;
 	base->read_options = lz4_read_options;
 	base->create_copy = lz4_create_copy;
+	((sqfs_object_t *)base)->destroy = lz4_destroy;
 	return base;
 }
diff --git a/lib/sqfs/comp/lzma.c b/lib/sqfs/comp/lzma.c
index f7ab428..900cc14 100644
--- a/lib/sqfs/comp/lzma.c
+++ b/lib/sqfs/comp/lzma.c
@@ -153,7 +153,7 @@ static sqfs_compressor_t *lzma_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)copy;
 }
 
-static void lzma_destroy(sqfs_compressor_t *base)
+static void lzma_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -176,11 +176,11 @@ sqfs_compressor_t *lzma_compressor_create(const sqfs_compressor_config_t *cfg)
 	if (lzma->block_size < SQFS_META_BLOCK_SIZE)
 		lzma->block_size = SQFS_META_BLOCK_SIZE;
 
-	base->destroy = lzma_destroy;
 	base->do_block = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) ?
 		lzma_uncomp_block : lzma_comp_block;
 	base->write_options = lzma_write_options;
 	base->read_options = lzma_read_options;
 	base->create_copy = lzma_create_copy;
+	((sqfs_object_t *)base)->destroy = lzma_destroy;
 	return base;
 }
diff --git a/lib/sqfs/comp/xz.c b/lib/sqfs/comp/xz.c
index 91d48da..d612729 100644
--- a/lib/sqfs/comp/xz.c
+++ b/lib/sqfs/comp/xz.c
@@ -207,7 +207,7 @@ static sqfs_compressor_t *xz_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)xz;
 }
 
-static void xz_destroy(sqfs_compressor_t *base)
+static void xz_destroy(sqfs_object_t *base)
 {
 	free(base);
 }
@@ -233,11 +233,11 @@ sqfs_compressor_t *xz_compressor_create(const sqfs_compressor_config_t *cfg)
 	xz->flags = cfg->flags;
 	xz->dict_size = cfg->opt.xz.dict_size;
 	xz->block_size = cfg->block_size;
-	base->destroy = xz_destroy;
 	base->do_block = (cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS) ?
 		xz_uncomp_block : xz_comp_block;
 	base->write_options = xz_write_options;
 	base->read_options = xz_read_options;
 	base->create_copy = xz_create_copy;
+	((sqfs_object_t *)base)->destroy = xz_destroy;
 	return base;
 }
diff --git a/lib/sqfs/comp/zstd.c b/lib/sqfs/comp/zstd.c
index a850033..4bd9456 100644
--- a/lib/sqfs/comp/zstd.c
+++ b/lib/sqfs/comp/zstd.c
@@ -110,7 +110,7 @@ static sqfs_compressor_t *zstd_create_copy(sqfs_compressor_t *cmp)
 	return (sqfs_compressor_t *)zstd;
 }
 
-static void zstd_destroy(sqfs_compressor_t *base)
+static void zstd_destroy(sqfs_object_t *base)
 {
 	zstd_compressor_t *zstd = (zstd_compressor_t *)base;
 
@@ -142,11 +142,11 @@ sqfs_compressor_t *zstd_compressor_create(const sqfs_compressor_config_t *cfg)
 		return NULL;
 	}
 
-	base->destroy = zstd_destroy;
 	base->do_block = cfg->flags & SQFS_COMP_FLAG_UNCOMPRESS ?
 		zstd_uncomp_block : zstd_comp_block;
 	base->write_options = zstd_write_options;
 	base->read_options = zstd_read_options;
 	base->create_copy = zstd_create_copy;
+	((sqfs_object_t *)base)->destroy = zstd_destroy;
 	return base;
 }
diff --git a/lib/sqfs/data_reader.c b/lib/sqfs/data_reader.c
index c8886c5..8c9bb2e 100644
--- a/lib/sqfs/data_reader.c
+++ b/lib/sqfs/data_reader.c
@@ -21,6 +21,8 @@
 #include <string.h>
 
 struct sqfs_data_reader_t {
+	sqfs_object_t obj;
+
 	sqfs_frag_table_t *frag_tbl;
 	sqfs_compressor_t *cmp;
 	sqfs_file_t *file;
@@ -125,6 +127,16 @@ static int precache_fragment_block(sqfs_data_reader_t *data, size_t idx)
 			 &data->frag_blk_size, &data->frag_block);
 }
 
+static void data_reader_destroy(sqfs_object_t *obj)
+{
+	sqfs_data_reader_t *data = (sqfs_data_reader_t *)obj;
+
+	sqfs_destroy(data->frag_tbl);
+	free(data->data_block);
+	free(data->frag_block);
+	free(data);
+}
+
 sqfs_data_reader_t *sqfs_data_reader_create(sqfs_file_t *file,
 					    size_t block_size,
 					    sqfs_compressor_t *cmp)
@@ -140,6 +152,7 @@ sqfs_data_reader_t *sqfs_data_reader_create(sqfs_file_t *file,
 		return NULL;
 	}
 
+	((sqfs_object_t *)data)->destroy = data_reader_destroy;
 	data->file = file;
 	data->block_size = block_size;
 	data->cmp = cmp;
@@ -164,14 +177,6 @@ int sqfs_data_reader_load_fragment_table(sqfs_data_reader_t *data,
 	return 0;
 }
 
-void sqfs_data_reader_destroy(sqfs_data_reader_t *data)
-{
-	sqfs_frag_table_destroy(data->frag_tbl);
-	free(data->data_block);
-	free(data->frag_block);
-	free(data);
-}
-
 int sqfs_data_reader_get_block(sqfs_data_reader_t *data,
 			       const sqfs_inode_generic_t *inode,
 			       size_t index, size_t *size, sqfs_u8 **out)
diff --git a/lib/sqfs/dir_reader.c b/lib/sqfs/dir_reader.c
index 8eae5bd..39e3e57 100644
--- a/lib/sqfs/dir_reader.c
+++ b/lib/sqfs/dir_reader.c
@@ -20,6 +20,8 @@
 #include <stdlib.h>
 
 struct sqfs_dir_reader_t {
+	sqfs_object_t base;
+
 	sqfs_meta_reader_t *meta_dir;
 	sqfs_meta_reader_t *meta_inode;
 	const sqfs_super_t *super;
@@ -34,6 +36,15 @@ struct sqfs_dir_reader_t {
 	sqfs_u16 inode_offset;
 };
 
+static void dir_reader_destroy(sqfs_object_t *obj)
+{
+	sqfs_dir_reader_t *rd = (sqfs_dir_reader_t *)obj;
+
+	sqfs_destroy(rd->meta_inode);
+	sqfs_destroy(rd->meta_dir);
+	free(rd);
+}
+
 sqfs_dir_reader_t *sqfs_dir_reader_create(const sqfs_super_t *super,
 					  sqfs_compressor_t *cmp,
 					  sqfs_file_t *file)
@@ -66,22 +77,16 @@ sqfs_dir_reader_t *sqfs_dir_reader_create(const sqfs_super_t *super,
 	rd->meta_dir = sqfs_meta_reader_create(file, cmp, start, limit);
 
 	if (rd->meta_dir == NULL) {
-		sqfs_meta_reader_destroy(rd->meta_inode);
+		sqfs_destroy(rd->meta_inode);
 		free(rd);
 		return NULL;
 	}
 
+	((sqfs_object_t *)rd)->destroy = dir_reader_destroy;
 	rd->super = super;
 	return rd;
 }
 
-void sqfs_dir_reader_destroy(sqfs_dir_reader_t *rd)
-{
-	sqfs_meta_reader_destroy(rd->meta_inode);
-	sqfs_meta_reader_destroy(rd->meta_dir);
-	free(rd);
-}
-
 int sqfs_dir_reader_open_dir(sqfs_dir_reader_t *rd,
 			     const sqfs_inode_generic_t *inode)
 {
diff --git a/lib/sqfs/dir_writer.c b/lib/sqfs/dir_writer.c
index 53e5800..9dd7ad2 100644
--- a/lib/sqfs/dir_writer.c
+++ b/lib/sqfs/dir_writer.c
@@ -37,6 +37,8 @@ typedef struct index_ent_t {
 } index_ent_t;
 
 struct sqfs_dir_writer_t {
+	sqfs_object_t base;
+
 	dir_entry_t *list;
 	dir_entry_t *list_end;
 
@@ -136,6 +138,15 @@ static int add_export_table_entry(sqfs_dir_writer_t *writer,
 	return 0;
 }
 
+static void dir_writer_destroy(sqfs_object_t *obj)
+{
+	sqfs_dir_writer_t *writer = (sqfs_dir_writer_t *)obj;
+
+	writer_reset(writer);
+	free(writer->export_tbl);
+	free(writer);
+}
+
 sqfs_dir_writer_t *sqfs_dir_writer_create(sqfs_meta_writer_t *dm,
 					  sqfs_u32 flags)
 {
@@ -162,17 +173,11 @@ sqfs_dir_writer_t *sqfs_dir_writer_create(sqfs_meta_writer_t *dm,
 		       sizeof(writer->export_tbl[0]) * writer->export_tbl_max);
 	}
 
+	((sqfs_object_t *)writer)->destroy = dir_writer_destroy;
 	writer->dm = dm;
 	return writer;
 }
 
-void sqfs_dir_writer_destroy(sqfs_dir_writer_t *writer)
-{
-	writer_reset(writer);
-	free(writer->export_tbl);
-	free(writer);
-}
-
 int sqfs_dir_writer_begin(sqfs_dir_writer_t *writer, sqfs_u32 flags)
 {
 	sqfs_u32 offset;
diff --git a/lib/sqfs/frag_table.c b/lib/sqfs/frag_table.c
index 200f3d5..58851c5 100644
--- a/lib/sqfs/frag_table.c
+++ b/lib/sqfs/frag_table.c
@@ -27,6 +27,8 @@ typedef struct {
 
 
 struct sqfs_frag_table_t {
+	sqfs_object_t base;
+
 	size_t capacity;
 	size_t used;
 	sqfs_fragment_t *table;
@@ -37,6 +39,15 @@ struct sqfs_frag_table_t {
 	chunk_info_t *chunk_list;
 };
 
+static void frag_table_destroy(sqfs_object_t *obj)
+{
+	sqfs_frag_table_t *tbl = (sqfs_frag_table_t *)obj;
+
+	free(tbl->chunk_list);
+	free(tbl->table);
+	free(tbl);
+}
+
 sqfs_frag_table_t *sqfs_frag_table_create(sqfs_u32 flags)
 {
 	sqfs_frag_table_t *tbl;
@@ -48,16 +59,10 @@ sqfs_frag_table_t *sqfs_frag_table_create(sqfs_u32 flags)
 	if (tbl == NULL)
 		return NULL;
 
+	((sqfs_object_t *)tbl)->destroy = frag_table_destroy;
 	return tbl;
 }
 
-void sqfs_frag_table_destroy(sqfs_frag_table_t *tbl)
-{
-	free(tbl->chunk_list);
-	free(tbl->table);
-	free(tbl);
-}
-
 int sqfs_frag_table_read(sqfs_frag_table_t *tbl, sqfs_file_t *file,
 			 const sqfs_super_t *super, sqfs_compressor_t *cmp)
 {
diff --git a/lib/sqfs/id_table.c b/lib/sqfs/id_table.c
index 8c30964..86f2809 100644
--- a/lib/sqfs/id_table.c
+++ b/lib/sqfs/id_table.c
@@ -17,23 +17,35 @@
 #include <string.h>
 
 struct sqfs_id_table_t {
+	sqfs_object_t base;
+
 	sqfs_u32 *ids;
 	size_t num_ids;
 	size_t max_ids;
 };
 
+static void id_table_destroy(sqfs_object_t *obj)
+{
+	sqfs_id_table_t *tbl = (sqfs_id_table_t *)obj;
+
+	free(tbl->ids);
+	free(tbl);
+}
+
 sqfs_id_table_t *sqfs_id_table_create(sqfs_u32 flags)
 {
+	sqfs_id_table_t *tbl;
+
 	if (flags != 0)
 		return NULL;
 
-	return calloc(1, sizeof(sqfs_id_table_t));
-}
+	tbl = calloc(1, sizeof(sqfs_id_table_t));
 
-void sqfs_id_table_destroy(sqfs_id_table_t *tbl)
-{
-	free(tbl->ids);
-	free(tbl);
+	if (tbl != NULL) {
+		((sqfs_object_t *)tbl)->destroy = id_table_destroy;
+	}
+
+	return tbl;
 }
 
 int sqfs_id_table_id_to_index(sqfs_id_table_t *tbl, sqfs_u32 id, sqfs_u16 *out)
diff --git a/lib/sqfs/meta_reader.c b/lib/sqfs/meta_reader.c
index 19e856a..8fd7f21 100644
--- a/lib/sqfs/meta_reader.c
+++ b/lib/sqfs/meta_reader.c
@@ -18,6 +18,8 @@
 #include <string.h>
 
 struct sqfs_meta_reader_t {
+	sqfs_object_t base;
+
 	sqfs_u64 start;
 	sqfs_u64 limit;
 	size_t data_used;
@@ -44,6 +46,11 @@ struct sqfs_meta_reader_t {
 	sqfs_u8 scratch[SQFS_META_BLOCK_SIZE];
 };
 
+static void meta_reader_destroy(sqfs_object_t *m)
+{
+	free(m);
+}
+
 sqfs_meta_reader_t *sqfs_meta_reader_create(sqfs_file_t *file,
 					    sqfs_compressor_t *cmp,
 					    sqfs_u64 start, sqfs_u64 limit)
@@ -53,6 +60,7 @@ sqfs_meta_reader_t *sqfs_meta_reader_create(sqfs_file_t *file,
 	if (m == NULL)
 		return NULL;
 
+	((sqfs_object_t *)m)->destroy = meta_reader_destroy;
 	m->start = start;
 	m->limit = limit;
 	m->file = file;
@@ -60,11 +68,6 @@ sqfs_meta_reader_t *sqfs_meta_reader_create(sqfs_file_t *file,
 	return m;
 }
 
-void sqfs_meta_reader_destroy(sqfs_meta_reader_t *m)
-{
-	free(m);
-}
-
 int sqfs_meta_reader_seek(sqfs_meta_reader_t *m, sqfs_u64 block_start,
 			  size_t offset)
 {
diff --git a/lib/sqfs/meta_writer.c b/lib/sqfs/meta_writer.c
index acb2235..46a67cd 100644
--- a/lib/sqfs/meta_writer.c
+++ b/lib/sqfs/meta_writer.c
@@ -25,6 +25,8 @@ typedef struct meta_block_t {
 } meta_block_t;
 
 struct sqfs_meta_writer_t {
+	sqfs_object_t base;
+
 	/* A byte offset into the uncompressed data of the current block */
 	size_t offset;
 
@@ -53,6 +55,20 @@ static int write_block(sqfs_file_t *file, meta_block_t *outblk)
 	return file->write_at(file, off, outblk->data, count + 2);
 }
 
+static void meta_writer_destroy(sqfs_object_t *obj)
+{
+	sqfs_meta_writer_t *m = (sqfs_meta_writer_t *)obj;
+	meta_block_t *blk;
+
+	while (m->list != NULL) {
+		blk = m->list;
+		m->list = blk->next;
+		free(blk);
+	}
+
+	free(m);
+}
+
 sqfs_meta_writer_t *sqfs_meta_writer_create(sqfs_file_t *file,
 					    sqfs_compressor_t *cmp,
 					    sqfs_u32 flags)
@@ -66,25 +82,13 @@ sqfs_meta_writer_t *sqfs_meta_writer_create(sqfs_file_t *file,
 	if (m == NULL)
 		return NULL;
 
+	((sqfs_object_t *)m)->destroy = meta_writer_destroy;
 	m->cmp = cmp;
 	m->file = file;
 	m->flags = flags;
 	return m;
 }
 
-void sqfs_meta_writer_destroy(sqfs_meta_writer_t *m)
-{
-	meta_block_t *blk;
-
-	while (m->list != NULL) {
-		blk = m->list;
-		m->list = blk->next;
-		free(blk);
-	}
-
-	free(m);
-}
-
 int sqfs_meta_writer_flush(sqfs_meta_writer_t *m)
 {
 	meta_block_t *outblk;
diff --git a/lib/sqfs/read_table.c b/lib/sqfs/read_table.c
index 096ac1a..1c9c8ad 100644
--- a/lib/sqfs/read_table.c
+++ b/lib/sqfs/read_table.c
@@ -76,12 +76,12 @@ int sqfs_read_table(sqfs_file_t *file, sqfs_compressor_t *cmp,
 		table_size -= diff;
 	}
 
-	sqfs_meta_reader_destroy(m);
+	sqfs_destroy(m);
 	free(locations);
 	*out = data;
 	return 0;
 fail:
-	sqfs_meta_reader_destroy(m);
+	sqfs_destroy(m);
 fail_idx:
 	free(locations);
 fail_data:
diff --git a/lib/sqfs/unix/io_file.c b/lib/sqfs/unix/io_file.c
index f93af4d..1c181ab 100644
--- a/lib/sqfs/unix/io_file.c
+++ b/lib/sqfs/unix/io_file.c
@@ -25,7 +25,7 @@ typedef struct {
 } sqfs_file_stdio_t;
 
 
-static void stdio_destroy(sqfs_file_t *base)
+static void stdio_destroy(sqfs_object_t *base)
 {
 	sqfs_file_stdio_t *file = (sqfs_file_stdio_t *)base;
 
@@ -154,10 +154,10 @@ sqfs_file_t *sqfs_open_file(const char *filename, sqfs_u32 flags)
 
 	file->size = sb.st_size;
 
-	base->destroy = stdio_destroy;
 	base->read_at = stdio_read_at;
 	base->write_at = stdio_write_at;
 	base->get_size = stdio_get_size;
 	base->truncate = stdio_truncate;
+	((sqfs_object_t *)base)->destroy = stdio_destroy;
 	return base;
 }
diff --git a/lib/sqfs/win32/io_file.c b/lib/sqfs/win32/io_file.c
index 951a0fe..c1849a8 100644
--- a/lib/sqfs/win32/io_file.c
+++ b/lib/sqfs/win32/io_file.c
@@ -24,7 +24,7 @@ typedef struct {
 } sqfs_file_stdio_t;
 
 
-static void stdio_destroy(sqfs_file_t *base)
+static void stdio_destroy(sqfs_object_t *base)
 {
 	sqfs_file_stdio_t *file = (sqfs_file_stdio_t *)base;
 
@@ -161,10 +161,10 @@ sqfs_file_t *sqfs_open_file(const char *filename, sqfs_u32 flags)
 	}
 
 	file->size = size.QuadPart;
-	base->destroy = stdio_destroy;
 	base->read_at = stdio_read_at;
 	base->write_at = stdio_write_at;
 	base->get_size = stdio_get_size;
 	base->truncate = stdio_truncate;
+	((sqfs_object_t *)base)->destroy = stdio_destroy;
 	return base;
 }
diff --git a/lib/sqfs/write_table.c b/lib/sqfs/write_table.c
index 1931b83..b198941 100644
--- a/lib/sqfs/write_table.c
+++ b/lib/sqfs/write_table.c
@@ -74,7 +74,7 @@ int sqfs_write_table(sqfs_file_t *file, sqfs_compressor_t *cmp,
 	/* cleanup */
 	ret = 0;
 out:
-	sqfs_meta_writer_destroy(m);
+	sqfs_destroy(m);
 out_idx:
 	free(locations);
 	return ret;
diff --git a/lib/sqfs/xattr_reader.c b/lib/sqfs/xattr_reader.c
index d5b003b..97cf821 100644
--- a/lib/sqfs/xattr_reader.c
+++ b/lib/sqfs/xattr_reader.c
@@ -21,6 +21,8 @@
 #include <errno.h>
 
 struct sqfs_xattr_reader_t {
+	sqfs_object_t base;
+
 	sqfs_u64 xattr_start;
 
 	size_t num_id_blocks;
@@ -34,6 +36,20 @@ struct sqfs_xattr_reader_t {
 	sqfs_file_t *file;
 };
 
+static void xattr_reader_destroy(sqfs_object_t *obj)
+{
+	sqfs_xattr_reader_t *xr = (sqfs_xattr_reader_t *)obj;
+
+	if (xr->kvrd != NULL)
+		sqfs_destroy(xr->kvrd);
+
+	if (xr->idrd != NULL)
+		sqfs_destroy(xr->idrd);
+
+	free(xr->id_block_starts);
+	free(xr);
+}
+
 int sqfs_xattr_reader_load_locations(sqfs_xattr_reader_t *xr)
 {
 	sqfs_xattr_id_table_t idtbl;
@@ -242,18 +258,6 @@ int sqfs_xattr_reader_get_desc(sqfs_xattr_reader_t *xr, sqfs_u32 idx,
 	return 0;
 }
 
-void sqfs_xattr_reader_destroy(sqfs_xattr_reader_t *xr)
-{
-	if (xr->kvrd != NULL)
-		sqfs_meta_reader_destroy(xr->kvrd);
-
-	if (xr->idrd != NULL)
-		sqfs_meta_reader_destroy(xr->idrd);
-
-	free(xr->id_block_starts);
-	free(xr);
-}
-
 sqfs_xattr_reader_t *sqfs_xattr_reader_create(sqfs_file_t *file,
 					      sqfs_super_t *super,
 					      sqfs_compressor_t *cmp)
@@ -263,6 +267,7 @@ sqfs_xattr_reader_t *sqfs_xattr_reader_create(sqfs_file_t *file,
 	if (xr == NULL)
 		return NULL;
 
+	((sqfs_object_t *)xr)->destroy = xattr_reader_destroy;
 	xr->file = file;
 	xr->super = super;
 
@@ -286,6 +291,6 @@ sqfs_xattr_reader_t *sqfs_xattr_reader_create(sqfs_file_t *file,
 
 	return xr;
 fail:
-	sqfs_xattr_reader_destroy(xr);
+	sqfs_destroy(xr);
 	return NULL;
 }
diff --git a/lib/sqfs/xattr_writer.c b/lib/sqfs/xattr_writer.c
index 047e411..827d0c5 100644
--- a/lib/sqfs/xattr_writer.c
+++ b/lib/sqfs/xattr_writer.c
@@ -99,6 +99,8 @@ typedef struct kv_block_desc_t {
 } kv_block_desc_t;
 
 struct sqfs_xattr_writer_t {
+	sqfs_object_t base;
+
 	str_table_t keys;
 	str_table_t values;
 
@@ -113,6 +115,23 @@ struct sqfs_xattr_writer_t {
 };
 
 
+static void xattr_writer_destroy(sqfs_object_t *obj)
+{
+	sqfs_xattr_writer_t *xwr = (sqfs_xattr_writer_t *)obj;
+	kv_block_desc_t *blk;
+
+	while (xwr->kv_blocks != NULL) {
+		blk = xwr->kv_blocks;
+		xwr->kv_blocks = xwr->kv_blocks->next;
+		free(blk);
+	}
+
+	free(xwr->kv_pairs);
+	str_table_cleanup(&xwr->values);
+	str_table_cleanup(&xwr->keys);
+	free(xwr);
+}
+
 sqfs_xattr_writer_t *sqfs_xattr_writer_create(void)
 {
 	sqfs_xattr_writer_t *xwr = calloc(1, sizeof(*xwr));
@@ -129,6 +148,7 @@ sqfs_xattr_writer_t *sqfs_xattr_writer_create(void)
 	if (xwr->kv_pairs == NULL)
 		goto fail_pairs;
 
+	((sqfs_object_t *)xwr)->destroy = xattr_writer_destroy;
 	return xwr;
 fail_pairs:
 	str_table_cleanup(&xwr->values);
@@ -139,22 +159,6 @@ fail_keys:
 	return NULL;
 }
 
-void sqfs_xattr_writer_destroy(sqfs_xattr_writer_t *xwr)
-{
-	kv_block_desc_t *blk;
-
-	while (xwr->kv_blocks != NULL) {
-		blk = xwr->kv_blocks;
-		xwr->kv_blocks = xwr->kv_blocks->next;
-		free(blk);
-	}
-
-	free(xwr->kv_pairs);
-	str_table_cleanup(&xwr->values);
-	str_table_cleanup(&xwr->keys);
-	free(xwr);
-}
-
 int sqfs_xattr_writer_begin(sqfs_xattr_writer_t *xwr)
 {
 	xwr->kv_start = xwr->num_pairs;
@@ -603,6 +607,6 @@ int sqfs_xattr_writer_flush(sqfs_xattr_writer_t *xwr, sqfs_file_t *file,
 				   locations, count);
 out:
 	free(locations);
-	sqfs_meta_writer_destroy(mw);
+	sqfs_destroy(mw);
 	return err;
 }
diff --git a/mkfs/mkfs.c b/mkfs/mkfs.c
index 5e350c8..357dad7 100644
--- a/mkfs/mkfs.c
+++ b/mkfs/mkfs.c
@@ -96,7 +96,7 @@ static int pack_files(sqfs_block_processor_t *data, fstree_t *fs,
 		    SZ_ADD_OV(sizeof(*inode), size, &size)) {
 			fputs("creating file inode: too many blocks\n",
 			      stderr);
-			file->destroy(file);
+			sqfs_destroy(file);
 			free(node_path);
 			return -1;
 		}
@@ -104,7 +104,7 @@ static int pack_files(sqfs_block_processor_t *data, fstree_t *fs,
 		inode = calloc(1, size);
 		if (inode == NULL) {
 			perror("creating file inode");
-			file->destroy(file);
+			sqfs_destroy(file);
 			free(node_path);
 			return -1;
 		}
@@ -121,7 +121,7 @@ static int pack_files(sqfs_block_processor_t *data, fstree_t *fs,
 			flags |= SQFS_BLK_DONT_FRAGMENT;
 
 		ret = write_data_from_file(path, data, inode, file, flags);
-		file->destroy(file);
+		sqfs_destroy(file);
 		free(node_path);
 
 		if (ret)
diff --git a/tar/sqfs2tar.c b/tar/sqfs2tar.c
index 2fb30cb..5a786dd 100644
--- a/tar/sqfs2tar.c
+++ b/tar/sqfs2tar.c
@@ -688,17 +688,17 @@ out_tree:
 		sqfs_dir_tree_destroy(root);
 out_xr:
 	if (xr != NULL)
-		sqfs_xattr_reader_destroy(xr);
+		sqfs_destroy(xr);
 out_dr:
-	sqfs_dir_reader_destroy(dr);
+	sqfs_destroy(dr);
 out_data:
-	sqfs_data_reader_destroy(data);
+	sqfs_destroy(data);
 out_id:
-	sqfs_id_table_destroy(idtbl);
+	sqfs_destroy(idtbl);
 out_cmp:
-	cmp->destroy(cmp);
+	sqfs_destroy(cmp);
 out_fd:
-	file->destroy(file);
+	sqfs_destroy(file);
 out_dirs:
 	for (i = 0; i < num_subdirs; ++i)
 		free(subdirs[i]);
diff --git a/tar/tar2sqfs.c b/tar/tar2sqfs.c
index 856a1f4..51987fe 100644
--- a/tar/tar2sqfs.c
+++ b/tar/tar2sqfs.c
@@ -303,7 +303,7 @@ static int write_file(tar_header_decoded_t *hdr, file_info_t *fi,
 		flags |= SQFS_BLK_DONT_FRAGMENT;
 
 	ret = write_data_from_file(hdr->name, sqfs.data, inode, file, 0);
-	file->destroy(file);
+	sqfs_destroy(file);
 
 	if (ret)
 		return -1;
diff --git a/unpack/rdsquashfs.c b/unpack/rdsquashfs.c
index 14a88ae..99e51fb 100644
--- a/unpack/rdsquashfs.c
+++ b/unpack/rdsquashfs.c
@@ -179,18 +179,18 @@ int main(int argc, char **argv)
 out:
 	sqfs_dir_tree_destroy(n);
 out_data:
-	sqfs_data_reader_destroy(data);
+	sqfs_destroy(data);
 out_dr:
-	sqfs_dir_reader_destroy(dirrd);
+	sqfs_destroy(dirrd);
 out_id:
-	sqfs_id_table_destroy(idtbl);
+	sqfs_destroy(idtbl);
 out_xr:
 	if (xattr != NULL)
-		sqfs_xattr_reader_destroy(xattr);
+		sqfs_destroy(xattr);
 out_cmp:
-	cmp->destroy(cmp);
+	sqfs_destroy(cmp);
 out_file:
-	file->destroy(file);
+	sqfs_destroy(file);
 out_cmd:
 	free(opt.cmdpath);
 	return status;