fusl/src/stdio/vsnprintf.c

#include "stdio_impl.h"
#include <limits.h>
#include <string.h>
#include <errno.h>
#include <stdint.h>

static size_t sn_write(FILE* f, const unsigned char* s, size_t l) {
  size_t k = f->wend - f->wpos;
  if (k > l)
    k = l;
  memcpy(f->wpos, s, k);
  f->wpos += k;
  /* pretend to succeed, but discard extra data */
  return l;
}

int vsnprintf(char* restrict s,
              size_t n,
              const char* restrict fmt,
              va_list ap) {
  int r;
  char b;
  FILE f = {.lbf = EOF, .write = sn_write, .lock = -1};

  if (n - 1 > INT_MAX - 1) {
    if (n) {
      errno = EOVERFLOW;
      return -1;
    }
    s = &b;
    n = 1;
  }

  /* Ensure pointers don't wrap if "infinite" n is passed in */
  if (n > (char*)0 + SIZE_MAX - s - 1)
    n = (char*)0 + SIZE_MAX - s - 1;
  f.buf_size = n;
  f.buf = f.wpos = (void*)s;
  f.wbase = f.wend = (void*)(s + n);
  r = vfprintf(&f, fmt, ap);

  /* Null-terminate, overwriting last char if dest buffer is full */
  if (n)
    f.wpos[-(f.wpos == f.wend)] = 0;
  return r;
}