strcpy() / strlcpy() / asprintf()

Posted Mar 30, 2012 15:56 UTC (Fri) by nybble41 (subscriber, #55106) [Link]

It seems to me like asprintf() should be easy to implement in terms of two calls to vsnprintf():

int vasprintf(char **strp, const char *fmt, va_list ap)
{
  va_list ap_copy;
  char *str;
  int bytes;
  char nul;

  /* Determine the amount of memory required to format the string */
  /* vsnprintf() returns the space required, but only stores the NUL. */
  va_copy(ap_copy, ap);
  bytes = vsnprintf(&nul, 1, fmt, ap_copy);
  va_end(ap_copy);

  if (bytes <= 0)
    return bytes;

  str = (char*)malloc(bytes);

  if (!str)
    return -1;

  /* Format the string into the destination buffer */
  bytes = vsnprintf(str, bytes, fmt, ap);

  if (bytes <= 0)
  {
    free(str);
    return bytes;
  }

  /* No errors; store pointer to destination buffer and return its size. */
  *strp = str;
  return bytes;
}

int asprintf(char **strp, const char *fmt, ...)
{
  int bytes;
  va_list ap;

  va_start(ap, fmt);
  bytes = vasprintf(strp, fmt, ap);
  va_end(ap);

  return bytes;
}

Is there any reason this implementation wouldn't work? (Ignoring minor issues/typos; I'm making this up as I go.)