diff --git a/src/include/gnunet_common.h b/src/include/gnunet_common.h
index 2a0ca33..ada13f6 100644
--- a/src/include/gnunet_common.h
+++ b/src/include/gnunet_common.h
@@ -826,9 +826,21 @@ GNUNET_ntoh_double (double d);
  * @param ... data for format string
  * @return number of bytes written to buf or negative value on error
  */
-int
+int __attribute__((format (printf, 3, 4)))
 GNUNET_snprintf (char *buf, size_t size, const char *format, ...);
 
+/**
+ * @ingroup memory
+ * Append to the end of a string. realloc()s its argument to be large enough
+ * to fit the complete string and aborts if insufficient memory.
+ *
+ * @param buf pointer to the string to append to
+ * @param format format string
+ * @param ... data for format string
+ * @return number of bytes writter to end of string excluding 0-terminator.
+ */
+int __attribute__((format (printf, 2, 3)))
+GNUNET_append_sprintf (char **buf, const char *format, ...);
 
 /**
  * @ingroup memory
@@ -839,7 +851,7 @@ GNUNET_snprintf (char *buf, size_t size, const char *format, ...);
  * @param ... data for format string
  * @return number of bytes in "*buf" excluding 0-termination
  */
-int
+int __attribute__((format (printf, 2, 3)))
 GNUNET_asprintf (char **buf, const char *format, ...);
 
 
diff --git a/src/util/common_allocation.c b/src/util/common_allocation.c
index b547367..156774f 100644
--- a/src/util/common_allocation.c
+++ b/src/util/common_allocation.c
@@ -342,6 +342,39 @@ GNUNET_xgrow_ (void **old, size_t elementSize, unsigned int *oldCount,
   *oldCount = newCount;
 }
 
+/**
+ * @ingroup memory
+ * Append to the end of a string. realloc()s its argument to be large enough
+ * to fit the complete string and aborts if insufficient memory.
+ *
+ * @param buf pointer to the string to append to
+ * @param format format string
+ * @param ... data for format string
+ * @return number of bytes writter to end of string excluding 0-terminator.
+ */
+int
+GNUNET_append_sprintf (char **buf, const char *format, ...)
+{
+  int ret;
+  int len;
+  va_list args;
+
+  if (*buf)
+    len = strlen(*buf);
+  else
+    len = 0;
+
+  va_start (args, format);
+  ret = VSNPRINTF (NULL, 0, format, args);
+  va_end (args);
+
+  *buf = GNUNET_realloc (*buf, len + ret + 1);
+  va_start (args, format);
+  VSNPRINTF (*buf + len, ret, format, args);
+  va_end (args);
+
+  return ret;
+}
 
 /**
  * Like asprintf, just portable.