Add gcc printf attribute to gbuf_printf.

More gbuf tests

include/ucore/ucore_buffer.h

@@ -70,7 +70,8 @@ int uc_gbuf_append(GBuf *buf,const void *data,size_t len);
  * @return The number of chars written excluding the terminating nul
  * character , or negative if an error occurs
  */
-int uc_gbuf_printf(GBuf *buf, const char *fmt, ...);
+int uc_gbuf_printf(GBuf *buf, const char *fmt, ...) 
+                                __attribute__((format(printf, 2, 3)));
 
 /** Expand the capacity of the GBuf.
  *

test/test_buffer.c

@@ -69,6 +69,19 @@ START_TEST (test_gbuf_printf2)
 
 END_TEST
 
+START_TEST (test_gbuf_printf_empty_string)
+    int rc;
+    GBuf *buf = uc_new_gbuf(1);
+    const char *fmt = ""; //tricks gcc to not warn about empty format string
+
+    fail_if(buf == NULL);
+
+    rc = uc_gbuf_printf(buf, fmt);
+    fail_if(rc != 0);
+    fail_if(buf->used != 0);
+
+END_TEST
+
 
 Suite *buffer_suite(void)
 {
@@ -79,6 +92,7 @@ Suite *buffer_suite(void)
     tcase_add_test(tc, test_gbuf_grow);
     tcase_add_test(tc, test_gbuf_printf1);
     tcase_add_test(tc, test_gbuf_printf2);
+    tcase_add_test(tc, test_gbuf_printf_empty_string);
 
     suite_add_tcase(s, tc);