thodg/libgit2

diff --git a/src/config_file.c b/src/config_file.c
index 7f8dd65..a36c0aa 100644
--- a/src/config_file.c
+++ b/src/config_file.c
@@ -227,9 +227,9 @@ static void backend_free(git_config_backend *_backend)
 
 static int config_iterator_new(
 	git_config_iterator **iter,
-	struct git_config_backend* backend)
+	struct git_config_backend *backend)
 {
-	diskfile_header *bh = (diskfile_header *) backend;
+	diskfile_header *bh = GIT_CONTAINER_OF(backend, diskfile_header, parent);
 	git_config_entries *entries;
 	int error;
 
diff --git a/src/iterator.c b/src/iterator.c
index 0db9292..d00b8aa 100644
--- a/src/iterator.c
+++ b/src/iterator.c
@@ -1521,7 +1521,7 @@ static void filesystem_iterator_set_current(
 static int filesystem_iterator_current(
 	const git_index_entry **out, git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 
 	if (!iterator__has_been_accessed(i))
 		return iter->base.cb->advance(out, i);
@@ -1568,7 +1568,7 @@ done:
 static int filesystem_iterator_advance(
 	const git_index_entry **out, git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 	bool is_dir;
 	int error = 0;
 
@@ -1627,7 +1627,7 @@ static int filesystem_iterator_advance(
 static int filesystem_iterator_advance_into(
 	const git_index_entry **out, git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 	filesystem_iterator_frame *frame;
 	filesystem_iterator_entry *prev_entry;
 	int error;
@@ -1664,7 +1664,7 @@ static int filesystem_iterator_advance_into(
 
 int git_iterator_current_workdir_path(git_buf **out, git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 	const git_index_entry *entry;
 
 	if (i->type != GIT_ITERATOR_TYPE_FS &&
@@ -1724,15 +1724,19 @@ GIT_INLINE(bool) filesystem_iterator_current_is_ignored(
 
 bool git_iterator_current_is_ignored(git_iterator *i)
 {
+	filesystem_iterator *iter = NULL;
+
 	if (i->type != GIT_ITERATOR_TYPE_WORKDIR)
 		return false;
 
-	return filesystem_iterator_current_is_ignored((filesystem_iterator *)i);
+	iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
+
+	return filesystem_iterator_current_is_ignored(iter);
 }
 
 bool git_iterator_current_tree_is_ignored(git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 	filesystem_iterator_frame *frame;
 
 	if (i->type != GIT_ITERATOR_TYPE_WORKDIR)
@@ -1747,7 +1751,7 @@ static int filesystem_iterator_advance_over(
 	git_iterator_status_t *status,
 	git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 	filesystem_iterator_frame *current_frame;
 	filesystem_iterator_entry *current_entry;
 	const git_index_entry *entry = NULL;
@@ -1866,7 +1870,7 @@ static int filesystem_iterator_init(filesystem_iterator *iter)
 
 static int filesystem_iterator_reset(git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 
 	filesystem_iterator_clear(iter);
 	return filesystem_iterator_init(iter);
@@ -1874,7 +1878,7 @@ static int filesystem_iterator_reset(git_iterator *i)
 
 static void filesystem_iterator_free(git_iterator *i)
 {
-	filesystem_iterator *iter = (filesystem_iterator *)i;
+	filesystem_iterator *iter = GIT_CONTAINER_OF(i, filesystem_iterator, base);
 	git__free(iter->root);
 	git_buf_dispose(&iter->current_path);
 	git_tree_free(iter->tree);
@@ -2083,7 +2087,7 @@ static int index_iterator_skip_pseudotree(index_iterator *iter)
 static int index_iterator_advance(
 	const git_index_entry **out, git_iterator *i)
 {
-	index_iterator *iter = (index_iterator *)i;
+	index_iterator *iter = GIT_CONTAINER_OF(i, index_iterator, base);
 	const git_index_entry *entry = NULL;
 	bool is_submodule;
 	int error = 0;
@@ -2156,7 +2160,7 @@ static int index_iterator_advance(
 static int index_iterator_advance_into(
 	const git_index_entry **out, git_iterator *i)
 {
-	index_iterator *iter = (index_iterator *)i;
+	index_iterator *iter = GIT_CONTAINER_OF(i, index_iterator, base);
 
 	if (! S_ISDIR(iter->tree_entry.mode)) {
 		if (out)
@@ -2174,7 +2178,7 @@ static int index_iterator_advance_over(
 	git_iterator_status_t *status,
 	git_iterator *i)
 {
-	index_iterator *iter = (index_iterator *)i;
+	index_iterator *iter = GIT_CONTAINER_OF(i, index_iterator, base);
 	const git_index_entry *entry;
 	int error;
 
@@ -2203,7 +2207,7 @@ static int index_iterator_init(index_iterator *iter)
 
 static int index_iterator_reset(git_iterator *i)
 {
-	index_iterator *iter = (index_iterator *)i;
+	index_iterator *iter = GIT_CONTAINER_OF(i, index_iterator, base);
 
 	index_iterator_clear(iter);
 	return index_iterator_init(iter);
@@ -2211,7 +2215,7 @@ static int index_iterator_reset(git_iterator *i)
 
 static void index_iterator_free(git_iterator *i)
 {
-	index_iterator *iter = (index_iterator *)i;
+	index_iterator *iter = GIT_CONTAINER_OF(i, index_iterator, base);
 
 	git_index_snapshot_release(&iter->entries, iter->base.index);
 	git_buf_dispose(&iter->tree_buf);
diff --git a/src/refdb_fs.c b/src/refdb_fs.c
index 30aa1a8..c8533e6 100644
--- a/src/refdb_fs.c
+++ b/src/refdb_fs.c
@@ -327,7 +327,7 @@ static int refdb_fs_backend__exists(
 	git_refdb_backend *_backend,
 	const char *ref_name)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	git_buf ref_path = GIT_BUF_INIT;
 	int error;
 
@@ -469,7 +469,7 @@ static int refdb_fs_backend__lookup(
 	git_refdb_backend *_backend,
 	const char *ref_name)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	int error;
 
 	assert(backend);
@@ -502,7 +502,7 @@ typedef struct {
 
 static void refdb_fs_backend__iterator_free(git_reference_iterator *_iter)
 {
-	refdb_fs_iter *iter = (refdb_fs_iter *) _iter;
+	refdb_fs_iter *iter = GIT_CONTAINER_OF(_iter, refdb_fs_iter, parent);
 
 	git_vector_free(&iter->loose);
 	git_pool_clear(&iter->pool);
@@ -591,8 +591,8 @@ static int refdb_fs_backend__iterator_next(
 	git_reference **out, git_reference_iterator *_iter)
 {
 	int error = GIT_ITEROVER;
-	refdb_fs_iter *iter = (refdb_fs_iter *)_iter;
-	refdb_fs_backend *backend = (refdb_fs_backend *)iter->parent.db->backend;
+	refdb_fs_iter *iter = GIT_CONTAINER_OF(_iter, refdb_fs_iter, parent);
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(iter->parent.db->backend, refdb_fs_backend, parent);
 	struct packref *ref;
 
 	while (iter->loose_pos < iter->loose.length) {
@@ -632,8 +632,8 @@ static int refdb_fs_backend__iterator_next_name(
 	const char **out, git_reference_iterator *_iter)
 {
 	int error = GIT_ITEROVER;
-	refdb_fs_iter *iter = (refdb_fs_iter *)_iter;
-	refdb_fs_backend *backend = (refdb_fs_backend *)iter->parent.db->backend;
+	refdb_fs_iter *iter = GIT_CONTAINER_OF(_iter, refdb_fs_iter, parent);
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(iter->parent.db->backend, refdb_fs_backend, parent);
 	struct packref *ref;
 
 	while (iter->loose_pos < iter->loose.length) {
@@ -674,7 +674,7 @@ static int refdb_fs_backend__iterator_next_name(
 static int refdb_fs_backend__iterator(
 	git_reference_iterator **out, git_refdb_backend *_backend, const char *glob)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	refdb_fs_iter *iter = NULL;
 	int error;
 
@@ -838,7 +838,7 @@ static int refdb_fs_backend__lock(void **out, git_refdb_backend *_backend, const
 {
 	int error;
 	git_filebuf *lock;
-	refdb_fs_backend *backend = (refdb_fs_backend *) _backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 
 	lock = git__calloc(1, sizeof(git_filebuf));
 	GIT_ERROR_CHECK_ALLOC(lock);
@@ -1248,7 +1248,7 @@ static int refdb_fs_backend__write(
 	const git_oid *old_id,
 	const char *old_target)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	git_filebuf file = GIT_FILEBUF_INIT;
 	int error = 0;
 
@@ -1274,7 +1274,7 @@ static int refdb_fs_backend__write_tail(
 	const git_oid *old_id,
 	const char *old_target)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	int error = 0, cmp = 0, should_write;
 	const char *new_target = NULL;
 	const git_oid *new_id = NULL;
@@ -1364,7 +1364,7 @@ static int refdb_fs_backend__delete(
 	const char *ref_name,
 	const git_oid *old_id, const char *old_target)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	git_filebuf file = GIT_FILEBUF_INIT;
 	int error = 0;
 
@@ -1387,7 +1387,7 @@ static int refdb_fs_backend__delete_tail(
 	const char *ref_name,
 	const git_oid *old_id, const char *old_target)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	git_buf loose_path = GIT_BUF_INIT;
 	size_t pack_pos;
 	int error = 0, cmp = 0;
@@ -1455,7 +1455,7 @@ static int refdb_fs_backend__rename(
 	const git_signature *who,
 	const char *message)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	git_reference *old, *new;
 	git_filebuf file = GIT_FILEBUF_INIT;
 	int error;
@@ -1511,7 +1511,7 @@ static int refdb_fs_backend__rename(
 static int refdb_fs_backend__compress(git_refdb_backend *_backend)
 {
 	int error;
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 
 	assert(backend);
 
@@ -1525,7 +1525,7 @@ static int refdb_fs_backend__compress(git_refdb_backend *_backend)
 
 static void refdb_fs_backend__free(git_refdb_backend *_backend)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 
 	assert(backend);
 
@@ -1703,7 +1703,7 @@ static int refdb_reflog_fs__ensure_log(git_refdb_backend *_backend, const char *
 
 	assert(_backend && name);
 
-	backend = (refdb_fs_backend *) _backend;
+	backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	repo = backend->repo;
 
 	if ((error = retrieve_reflog_path(&path, repo, name)) < 0)
@@ -1736,7 +1736,7 @@ static int refdb_reflog_fs__has_log(git_refdb_backend *_backend, const char *nam
 
 	assert(_backend && name);
 
-	backend = (refdb_fs_backend *) _backend;
+	backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 
 	return has_reflog(backend->repo, name);
 }
@@ -1752,7 +1752,7 @@ static int refdb_reflog_fs__read(git_reflog **out, git_refdb_backend *_backend, 
 
 	assert(out && _backend && name);
 
-	backend = (refdb_fs_backend *) _backend;
+	backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	repo = backend->repo;
 
 	if (reflog_alloc(&log, name) < 0)
@@ -1862,7 +1862,7 @@ static int refdb_reflog_fs__write(git_refdb_backend *_backend, git_reflog *reflo
 
 	assert(_backend && reflog);
 
-	backend = (refdb_fs_backend *) _backend;
+	backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 
 	if ((error = lock_reflog(&fbuf, backend, reflog->ref_name)) < 0)
 		return -1;
@@ -1984,7 +1984,7 @@ static int refdb_reflog_fs__rename(git_refdb_backend *_backend, const char *old_
 
 	assert(_backend && old_name && new_name);
 
-	backend = (refdb_fs_backend *) _backend;
+	backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	repo = backend->repo;
 
 	if ((error = git_reference__normalize_name(
@@ -2055,7 +2055,7 @@ cleanup:
 
 static int refdb_reflog_fs__delete(git_refdb_backend *_backend, const char *name)
 {
-	refdb_fs_backend *backend = (refdb_fs_backend *) _backend;
+	refdb_fs_backend *backend = GIT_CONTAINER_OF(_backend, refdb_fs_backend, parent);
 	git_buf path = GIT_BUF_INIT;
 	int error;
 
diff --git a/src/transports/http.c b/src/transports/http.c
index 80ba5ba..9d77e62 100644
--- a/src/transports/http.c
+++ b/src/transports/http.c
@@ -1144,7 +1144,7 @@ static int http_stream_write_chunked(
 	const char *buffer,
 	size_t len)
 {
-	http_stream *s = (http_stream *)stream;
+	http_stream *s = GIT_CONTAINER_OF(stream, http_stream, parent);
 	http_subtransport *t = OWNING_SUBTRANSPORT(s);
 
 	assert(t->connected);
@@ -1218,7 +1218,7 @@ static int http_stream_write_single(
 	const char *buffer,
 	size_t len)
 {
-	http_stream *s = (http_stream *)stream;
+	http_stream *s = GIT_CONTAINER_OF(stream, http_stream, parent);
 	http_subtransport *t = OWNING_SUBTRANSPORT(s);
 	git_buf request = GIT_BUF_INIT;
 
@@ -1252,7 +1252,7 @@ on_error:
 
 static void http_stream_free(git_smart_subtransport_stream *stream)
 {
-	http_stream *s = (http_stream *)stream;
+	http_stream *s = GIT_CONTAINER_OF(stream, http_stream, parent);
 
 	if (s->chunk_buffer)
 		git__free(s->chunk_buffer);
@@ -1365,7 +1365,7 @@ static int http_action(
 	const char *url,
 	git_smart_service_t action)
 {
-	http_subtransport *t = (http_subtransport *)subtransport;
+	http_subtransport *t = GIT_CONTAINER_OF(subtransport, http_subtransport, parent);
 	int ret;
 
 	assert(stream);
@@ -1419,7 +1419,7 @@ static void free_auth_contexts(git_vector *contexts)
 
 static int http_close(git_smart_subtransport *subtransport)
 {
-	http_subtransport *t = (http_subtransport *) subtransport;
+	http_subtransport *t = GIT_CONTAINER_OF(subtransport, http_subtransport, parent);
 
 	clear_parser_state(t);
 
@@ -1459,7 +1459,7 @@ static int http_close(git_smart_subtransport *subtransport)
 
 static void http_free(git_smart_subtransport *subtransport)
 {
-	http_subtransport *t = (http_subtransport *) subtransport;
+	http_subtransport *t = GIT_CONTAINER_OF(subtransport, http_subtransport, parent);
 
 	http_close(subtransport);
 
diff --git a/src/transports/smart.c b/src/transports/smart.c
index fd7113c..7f66ae0 100644
--- a/src/transports/smart.c
+++ b/src/transports/smart.c
@@ -63,7 +63,7 @@ static int git_smart__set_callbacks(
 	git_transport_certificate_check_cb certificate_check_cb,
 	void *message_cb_payload)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 
 	t->progress_cb = progress_cb;
 	t->error_cb = error_cb;
@@ -128,7 +128,7 @@ static int git_smart__set_custom_headers(
 	git_transport *transport,
 	const git_strarray *custom_headers)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 	size_t i;
 
 	if (t->custom_headers.count)
@@ -212,7 +212,7 @@ static int git_smart__connect(
 	int direction,
 	int flags)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 	git_smart_subtransport_stream *stream;
 	int error;
 	git_pkt *pkt;
@@ -315,7 +315,7 @@ cleanup:
 
 static int git_smart__ls(const git_remote_head ***out, size_t *size, git_transport *transport)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 
 	if (!t->have_refs) {
 		git_error_set(GIT_ERROR_NET, "the transport has not yet loaded the refs");
@@ -330,7 +330,7 @@ static int git_smart__ls(const git_remote_head ***out, size_t *size, git_transpo
 
 int git_smart__negotiation_step(git_transport *transport, void *data, size_t len)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 	git_smart_subtransport_stream *stream;
 	int error;
 
@@ -387,21 +387,21 @@ int git_smart__get_push_stream(transport_smart *t, git_smart_subtransport_stream
 
 static void git_smart__cancel(git_transport *transport)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 
 	git_atomic_set(&t->cancelled, 1);
 }
 
 static int git_smart__is_connected(git_transport *transport)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 
 	return t->connected;
 }
 
 static int git_smart__read_flags(git_transport *transport, int *flags)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 
 	*flags = t->flags;
 
@@ -410,7 +410,7 @@ static int git_smart__read_flags(git_transport *transport, int *flags)
 
 static int git_smart__close(git_transport *transport)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 	git_vector *common = &t->common;
 	unsigned int i;
 	git_pkt *p;
@@ -447,7 +447,7 @@ static int git_smart__close(git_transport *transport)
 
 static void git_smart__free(git_transport *transport)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 	git_vector *refs = &t->refs;
 	unsigned int i;
 	git_pkt *p;
@@ -479,7 +479,7 @@ static int ref_name_cmp(const void *a, const void *b)
 
 int git_transport_smart_certificate_check(git_transport *transport, git_cert *cert, int valid, const char *hostname)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 
 	assert(transport && cert && hostname);
 
@@ -491,7 +491,7 @@ int git_transport_smart_certificate_check(git_transport *transport, git_cert *ce
 
 int git_transport_smart_credentials(git_cred **out, git_transport *transport, const char *user, int methods)
 {
-	transport_smart *t = (transport_smart *)transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 
 	assert(out && transport);
 
@@ -503,7 +503,7 @@ int git_transport_smart_credentials(git_cred **out, git_transport *transport, co
 
 int git_transport_smart_proxy_options(git_proxy_options *out, git_transport *transport)
 {
-	transport_smart *t = (transport_smart *) transport;
+	transport_smart *t = GIT_CONTAINER_OF(transport, transport_smart, parent);
 	return git_proxy_options_dup(out, &t->proxy);
 }
 
diff --git a/src/transports/ssh.c b/src/transports/ssh.c
index 5a6058f..9b5d8a5 100644
--- a/src/transports/ssh.c
+++ b/src/transports/ssh.c
@@ -132,7 +132,7 @@ static int ssh_stream_read(
 	size_t *bytes_read)
 {
 	int rc;
-	ssh_stream *s = (ssh_stream *)stream;
+	ssh_stream *s = GIT_CONTAINER_OF(stream, ssh_stream, parent);
 
 	*bytes_read = 0;
 
@@ -170,7 +170,7 @@ static int ssh_stream_write(
 	const char *buffer,
 	size_t len)
 {
-	ssh_stream *s = (ssh_stream *)stream;
+	ssh_stream *s = GIT_CONTAINER_OF(stream, ssh_stream, parent);
 	size_t off = 0;
 	ssize_t ret = 0;
 
@@ -196,7 +196,7 @@ static int ssh_stream_write(
 
 static void ssh_stream_free(git_smart_subtransport_stream *stream)
 {
-	ssh_stream *s = (ssh_stream *)stream;
+	ssh_stream *s = GIT_CONTAINER_OF(stream, ssh_stream, parent);
 	ssh_subtransport *t;
 
 	if (!stream)
@@ -479,7 +479,7 @@ static int _git_ssh_session_create(
 {
 	int rc = 0;
 	LIBSSH2_SESSION* s;
-	git_socket_stream *socket = (git_socket_stream *) io;
+	git_socket_stream *socket = GIT_CONTAINER_OF(io, git_socket_stream, parent);
 
 	assert(session);
 
@@ -730,7 +730,7 @@ static int _ssh_action(
 	const char *url,
 	git_smart_service_t action)
 {
-	ssh_subtransport *t = (ssh_subtransport *) subtransport;
+	ssh_subtransport *t = GIT_CONTAINER_OF(subtransport, ssh_subtransport, parent);
 
 	switch (action) {
 		case GIT_SERVICE_UPLOADPACK_LS:
@@ -752,7 +752,7 @@ static int _ssh_action(
 
 static int _ssh_close(git_smart_subtransport *subtransport)
 {
-	ssh_subtransport *t = (ssh_subtransport *) subtransport;
+	ssh_subtransport *t = GIT_CONTAINER_OF(subtransport, ssh_subtransport, parent);
 
 	assert(!t->current_stream);
 
@@ -763,7 +763,7 @@ static int _ssh_close(git_smart_subtransport *subtransport)
 
 static void _ssh_free(git_smart_subtransport *subtransport)
 {
-	ssh_subtransport *t = (ssh_subtransport *) subtransport;
+	ssh_subtransport *t = GIT_CONTAINER_OF(subtransport, ssh_subtransport, parent);
 
 	assert(!t->current_stream);
 
diff --git a/src/util.h b/src/util.h
index d4fe6ee..b9ab652 100644
--- a/src/util.h
+++ b/src/util.h
@@ -30,6 +30,17 @@
 # define max(a,b) ((a) > (b) ? (a) : (b))
 #endif
 
+#if defined(__GNUC__)
+# define GIT_CONTAINER_OF(ptr, type, member) \
+	__builtin_choose_expr( \
+	    __builtin_offsetof(type, member) == 0 && \
+	    __builtin_types_compatible_p(typeof(&((type *) 0)->member), typeof(ptr)), \
+		((type *) (ptr)), \
+		(void)0)
+#else
+# define GIT_CONTAINER_OF(ptr, type, member) (type *)(ptr)
+#endif
+
 #define GIT_DATE_RFC2822_SZ  32
 
 /**