WCWIDTH(3) Library Functions Manual WCWIDTH(3)

wcwidth
number of column positions of a wide-character code

Standard C Library (libc, -lc)

#include <wchar.h>

int
wcwidth(wchar_t wc);

The wcwidth() function determines the number of column positions required to display the wide character wc.

The wcwidth() function returns 0 if the wc argument is a nul wide character (L'\0'), -1 if wc is not printable, otherwise it returns the number of column positions the character occupies.

This code fragment reads text from standard input and breaks lines that are more than 20 column positions wide, similar to the fold(1) utility:
wint_t ch;
int column, w;

column = 0;
while ((ch = getwchar()) != WEOF) {
	w = wcwidth(ch);
	if (w > 0 && column + w >= 20) {
		putwchar(L'\n');
		column = 0;
	}
	putwchar(ch);
	if (ch == L'\n')
		column = 0;
	else if (w > 0)
		column += w;
}

iswprint(3), wcswidth(3)

The wcwidth() function conforms to IEEE Std 1003.1-2001 (“POSIX.1”).
August 17, 2004 NetBSD 9.2