diff --git a/demo/demo-async-client.c b/demo/demo-async-client.c
index 12250eb..30d227d 100644
--- a/demo/demo-async-client.c
+++ b/demo/demo-async-client.c
@@ -28,7 +28,7 @@ typedef struct {
 
 /* rpc_priv is used by the rpc_client to store information related to
  * this rpc call. */
-static int transport_send(void *arg, const char *fcall_str,
+static int transport_send(void *arg, char *fcall_str,
                           size_t fcall_len, void *rpc_priv)
 {
     TcpTransport *trans = arg;
@@ -49,6 +49,7 @@ static int transport_send(void *arg, const char *fcall_str,
     }
 
     trans->rpc_priv = rpc_priv;
+    g_free (fcall_str);
     
     return 0;
 }
diff --git a/lib/gencode.py b/lib/gencode.py
index 5dc5811..afe75c6 100644
--- a/lib/gencode.py
+++ b/lib/gencode.py
@@ -333,7 +333,7 @@ def gen_dfun_macro_list():
 async_dfun_template = r"""
 #define SEARPC_CLIENT_ASYNC_DEFUN_${RET_TYPE}__${ARG_TYPES}(funcname, gtype)   \
 int                                                                     \
-funcname (SearpcClient *client, ${args},                                \
+funcname (SearpcClient *client, ${args}                                 \
           AsyncCallback callback, void *user_data)                      \
 {                                                                       \
     char *fcall;                                                        \
@@ -358,7 +358,7 @@ def gen_async_dfun_macro(ret_type, arg_types):
 
     args = ""
     for i, arg_type in enumerate(arg_types):
-        args += type_table[arg_type][0] + " param" + str(i+1)
+        args += type_table[arg_type][0] + " param" + str(i+1) + ", "
 
     fcall_args = ""
     for i, arg_type in enumerate(arg_types):
diff --git a/lib/searpc-client.c b/lib/searpc-client.c
index 9123c59..cc0a682 100644
--- a/lib/searpc-client.c
+++ b/lib/searpc-client.c
@@ -74,7 +74,7 @@ searpc_client_generic_callback (char *retstr, size_t len,
 
 int
 searpc_client_async_call (SearpcClient *client,
-                          const gchar *fcall_str,
+                          gchar *fcall_str,
                           size_t fcall_len,
                           AsyncCallback callback,
                           const gchar *ret_type,
diff --git a/lib/searpc-client.h b/lib/searpc-client.h
index 8be753d..8bd843e 100644
--- a/lib/searpc-client.h
+++ b/lib/searpc-client.h
@@ -7,9 +7,13 @@
 typedef char *(*TransportCB)(void *arg, const gchar *fcall_str,
                              size_t fcall_len, size_t *ret_len);
 
-/* rpc_priv is used by the rpc_client to store information related to
- * this rpc call. */
-typedef int (*AsyncTransportSend)(void *arg, const gchar *fcall_str,
+/**
+ * @rpc_priv is used by the rpc_client to store information related to
+ * this rpc call.
+ * @fcall_str is an allocated string, and the sender should free it
+ * when not needed.
+ */
+typedef int (*AsyncTransportSend)(void *arg, gchar *fcall_str,
                                   size_t fcall_len, void *rpc_priv);
 
 typedef void (*AsyncCallback) (void *result, void *user_data, GError *error);
@@ -42,13 +46,15 @@ char* searpc_client_transport_send (SearpcClient *client,
  * @cbdata: the data that will be given to the callback.
  */
 int searpc_client_async_call (SearpcClient *client,
-                              const gchar *fcall_str,
+                              gchar *fcall_str,
                               size_t fcall_len,
                               AsyncCallback callback,
                               const gchar *ret_type,
                               int gtype,
                               void *cbdata);
 
+/* called by the transport layer, the rpc layer should be able to
+ * modify the str, but not take ownership of it */
 int
 searpc_client_generic_callback (char *retstr, size_t len,
                                 void *vdata, const char *errstr);