diff --git a/src/libstore-c/nix_api_store.h b/src/libstore-c/nix_api_store.h
index b56bafbc9..bda5fa7e1 100644
--- a/src/libstore-c/nix_api_store.h
+++ b/src/libstore-c/nix_api_store.h
@@ -259,11 +259,11 @@ nix_err nix_store_get_fs_closure(
  * @param[in] callback The callback to call
  * @param[in] userdata The userdata to pass to the callback
  */
-nix_err nix_store_drv_from_path(
+nix_err nix_store_drv_from_store_path(
     nix_c_context * context,
     Store * store,
     const StorePath * path,
-    void (*callback)(void * userdata, const nix_derivation * drv),
+    nix_err (*callback)(nix_c_context * return_ctx, void * userdata, const nix_derivation * drv),
     void * userdata);
 
 /**
@@ -300,7 +300,7 @@ nix_err nix_store_build_paths(
     Store * store,
     const StorePath ** store_paths,
     unsigned int num_store_paths,
-    void (*callback)(void * userdata, const char * path, const char * result),
+    nix_err (*callback)(nix_c_context * ctx, void * userdata, const char * path, const char * result),
     void * userdata);
 
 // cffi end
diff --git a/src/libstore-c/nix_api_store/store_path.h b/src/libstore-c/nix_api_store/store_path.h
index 140cc4521..9ce6ebb70 100644
--- a/src/libstore-c/nix_api_store/store_path.h
+++ b/src/libstore-c/nix_api_store/store_path.h
@@ -65,7 +65,7 @@ typedef struct nix_store_path_hash_path
  * @param[in] store_path the path to get the hash from
  * @return the decoded hash as 20 raw bytes
  */
-nix_store_path_hash_path nix_store_path_hash(const StorePath * store_path);
+nix_err nix_store_path_hash(const StorePath *store_path, nix_store_path_hash_part * hash_part_out);
 
 /**
  * @brief Create a StorePath from its constituent parts (hash and name)