MB_LEN_MAX man page
MB_LEN_MAX — maximum multibyte length of a character across all locales
The MB_LEN_MAX macro is the upper bound for the number of bytes needed to represent a single wide character, in any of the supported locales.
A constant integer greater than zero.
POSIX.1-2001, POSIX.1-2008, C99.
The entities MB_LEN_MAX and sizeof(wchar_t) are totally unrelated. In glibc, MB_LEN_MAX is typically 16 (6 in glibc versions earlier than 2.2) while sizeof(wchar_t) is 4.
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