(gdb) p (char*)0x7fffffffe9c8
$16 = 0x7fffffffe9c8 "\363\353\377\377\377\177"
It doesn't look like ascii nor multibyte,what's that?
These are octal character escapes. They are usually used to insert bytes into a string that don't have a meaning as text or need to have a certain binary value. \377 for instance is the hexadecimal value ff or decimal 255 which would be this ÿ in ASCII but most likely has a very different meaning in this context.