foo.c

#include <string.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <wchar.h>

size_t
nchars(const char *s)
{
	size_t charlen, chars;
	mbstate_t mbs;

	chars = 0;
	memset(&mbs, 0, sizeof(mbs));
	while ((charlen = mbrlen(s, MB_CUR_MAX, &mbs)) != 0 &&
	    charlen != (size_t)-1 && charlen != (size_t)-2) {
		s += charlen;
		chars++;
	}

	return (chars);
}


int
main(int argc, char **argv)
{
	setlocale(LC_ALL, "");

	if (argc == 2) {
		printf("%zu\n", nchars(argv[1]));
		return EXIT_SUCCESS;
	}

	return EXIT_FAILURE;
}