musl/src/stdio/vswprintf.c

#include "stdio_impl.h"

struct cookie {
	wchar_t *ws;
	size_t l;
};

static size_t sw_write(FILE *f, const unsigned char *s, size_t l)
{
	size_t l0 = l;
	int i = 0;
	struct cookie *c = f->cookie;
	while (c->l && l && (i=mbtowc(c->ws, s, l))>=0) {
		s+=i;
		l-=i;
		c->l--;
		c->ws++;
	}
	*c->ws = 0;
	return i<0 ? i : l0;
}

int vswprintf(wchar_t *s, size_t n, const wchar_t *fmt, va_list ap)
{
	int r;
	FILE f;
	unsigned char buf[256];
	struct cookie c = { s, n-1 };

	memset(&f, 0, sizeof(FILE));
	f.lbf = EOF;
	f.write = sw_write;
	f.buf_size = sizeof buf;
	f.buf = buf;
	f.owner = -1;
	f.cookie = &c;
	if (!n) {
		return -1;
	} else if (n > INT_MAX) {
		errno = EOVERFLOW;
		return -1;
	}
	r = vfwprintf(&f, fmt, ap);
	__oflow(&f);
	return r>=n ? -1 : r;
}