very old kernel has no print_fmt field, use "Unknown print_fmt" instead. Signed-off-by: Lai Jiangshan <laijs(a)cn.fujitsu.com&gt; --- diff --git a/extensions/trace.c b/extensions/trace.c index e04c504..714765e 100755 --- a/extensions/trace.c +++ b/extensions/trace.c @@ -1080,8 +1080,10 @@ int ftrace_get_event_type_print_fmt(ulong call, char **print_fmt) fmt_offset = MEMBER_OFFSET("ftrace_event_call", "print_fmt"); } - if (fmt_offset < 0) - return -1; + if (fmt_offset < 0) { + *print_fmt = strdup("Unknown print_fmt"); + return 0; + } if (!readmem(call + fmt_offset, KVADDR, &fmt_addr, sizeof(fmt_addr), "read ftrace_event_call fmt_addr", RETURN_ON_ERROR))