diff --git a/libsync/include/android/sync.h b/libsync/include/android/sync.h
index 68f74a0b2..32bb878ba 100644
--- a/libsync/include/android/sync.h
+++ b/libsync/include/android/sync.h
@@ -41,28 +41,8 @@
 
 __BEGIN_DECLS
 
-struct sync_fence_info_data {
- uint32_t len;
- char name[32];
- int32_t status;
- uint8_t pt_info[0];
-};
-
-struct sync_pt_info {
- uint32_t len;
- char obj_name[32];
- char driver_name[32];
- int32_t status;
- uint64_t timestamp_ns;
- uint8_t driver_data[0];
-};
-
 /* timeout in msecs */
 int sync_wait(int fd, int timeout);
-struct sync_fence_info_data *sync_fence_info(int fd);
-struct sync_pt_info *sync_pt_info(struct sync_fence_info_data *info,
-                                  struct sync_pt_info *itr);
-void sync_fence_info_free(struct sync_fence_info_data *info);
 
 __END_DECLS
 
diff --git a/libsync/sync.c b/libsync/sync.c
index 6b187faed..b8c48c7f7 100644
--- a/libsync/sync.c
+++ b/libsync/sync.c
@@ -30,6 +30,29 @@
 
 #include <android/sync.h>
 
+/* Prototypes for deprecated functions that used to be declared in the legacy
+ * android/sync.h. They've been moved here to make sure new code does not use
+ * them, but the functions are still defined to avoid breaking existing
+ * binaries. Eventually they can be removed altogether.
+ */
+struct sync_fence_info_data {
+    uint32_t len;
+    char name[32];
+    int32_t status;
+    uint8_t pt_info[0];
+};
+struct sync_pt_info {
+    uint32_t len;
+    char obj_name[32];
+    char driver_name[32];
+    int32_t status;
+    uint64_t timestamp_ns;
+    uint8_t driver_data[0];
+};
+struct sync_fence_info_data* sync_fence_info(int fd);
+struct sync_pt_info* sync_pt_info(struct sync_fence_info_data* info, struct sync_pt_info* itr);
+void sync_fence_info_free(struct sync_fence_info_data* info);
+
 /* Legacy Sync API */
 
 struct sync_legacy_merge_data {
diff --git a/libsync/tests/sync_test.cpp b/libsync/tests/sync_test.cpp
index 0fb86d6fc..011b09d00 100644
--- a/libsync/tests/sync_test.cpp
+++ b/libsync/tests/sync_test.cpp
@@ -15,6 +15,35 @@
 #include <random>
 #include <unordered_map>
 
+/* These deprecated declarations were in the legacy android/sync.h. They've been removed to
+ * encourage code to move to the modern equivalents. But they are still implemented in libsync.so
+ * to avoid breaking existing binaries; as long as that's true we should keep testing them here.
+ * That means making local copies of the declarations.
+ */
+extern "C" {
+
+struct sync_fence_info_data {
+    uint32_t len;
+    char name[32];
+    int32_t status;
+    uint8_t pt_info[0];
+};
+
+struct sync_pt_info {
+    uint32_t len;
+    char obj_name[32];
+    char driver_name[32];
+    int32_t status;
+    uint64_t timestamp_ns;
+    uint8_t driver_data[0];
+};
+
+struct sync_fence_info_data* sync_fence_info(int fd);
+struct sync_pt_info* sync_pt_info(struct sync_fence_info_data* info, struct sync_pt_info* itr);
+void sync_fence_info_free(struct sync_fence_info_data* info);
+
+}  // extern "C"
+
 // TODO: better stress tests?
 // Handle more than 64 fd's simultaneously, i.e. fix sync_fence_info's 4k limit.
 // Handle wraparound in timelines like nvidia.