Package: libc6-dev Version: 2.3.2-4 Even though the man page for wcwidth and wcswidth() claim that these functions conform to "UNIX98" and "POSIX 1003.1-2001", they're only declared when the macro __USE_XOPEN (via _XOPEN_SOURCE) is defined. I don't know whether these function should be declared by default, but at least they should if _ISOC99_SOURCE is defined - which, however, doesn't set __USE_XOPEN.