+C_LINKAGE int strutils_append_str(char **str, const char *append);
+
+/*
+ * Like `strutils_append_str`, but the appended string is formatted using
+ * `fmt` and the following arguments.
+ */
+C_LINKAGE ATTR_FORMAT_PRINTF(2, 3)
+int strutils_appendf(char **s, const char *fmt, ...);