musl/src/multibyte/mbrlen.c

/* 
 * This code was written by Rich Felker in 2010; no copyright is claimed.
 * This code is in the public domain. Attribution is appreciated but
 * unnecessary.
 */

#include <wchar.h>

size_t mbrlen(const char *restrict s, size_t n, mbstate_t *restrict st)
{
	static unsigned internal;
	return mbrtowc(0, s, n, st ? st : (mbstate_t *)&internal);
}