fusl/src/stdio/vswprintf.c

#include "stdio_impl.h"
#include <limits.h>
#include <string.h>
#include <errno.h>
#include <stdint.h>
#include <wchar.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;
  if (s != f->wbase && sw_write(f, f->wbase, f->wpos - f->wbase) == -1)
    return -1;
  while (c->l && l && (i = mbtowc(c->ws, (void*)s, l)) >= 0) {
    s += i;
    l -= i;
    c->l--;
    c->ws++;
  }
  *c->ws = 0;
  return i < 0 ? i : l0;
}

int vswprintf(wchar_t* restrict s,
              size_t n,
              const wchar_t* restrict 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.lock = -1;
  f.cookie = &c;
  if (!n) {
    return -1;
  } else if (n > INT_MAX) {
    errno = EOVERFLOW;
    return -1;
  }
  r = vfwprintf(&f, fmt, ap);
  sw_write(&f, 0, 0);
  return r >= n ? -1 : r;
}