printf()函数调用

__swrite
    _write_r
        _write
            HAL_UART_Transmit

__swrite
\newlib-cygwin-cygwin-3_3_2-release\newlib\libc\stdio\stdio.c

_READ_WRITE_RETURN_TYPE
__swrite (struct _reent *ptr, void *cookie,
       char const *buf, _READ_WRITE_BUFSIZE_TYPE n)
{
  register FILE *fp = (FILE *) cookie;
  ssize_t w;
#ifdef __SCLE
  int oldmode=0;
#endif

  if (fp->_flags & __SAPP)
    _lseek_r (ptr, fp->_file, (_off_t) 0, SEEK_END);
  fp->_flags &= ~__SOFF;	/* in case O_APPEND mode is set */

#ifdef __SCLE
  if (fp->_flags & __SCLE)
    oldmode = setmode (fp->_file, O_BINARY);
#endif

  w = _write_r (ptr, fp->_file, buf, n);

#ifdef __SCLE
  if (oldmode)
    setmode (fp->_file, oldmode);
#endif

  return w;
}

_write_r
\newlib-cygwin-cygwin-3_3_2-release\newlib\libc\reent\writer.c

_ssize_t
_write_r (struct _reent *ptr, int fd, const void *buf, size_t cnt)
{
  _ssize_t ret;

  errno = 0;
  if ((ret = (_ssize_t)_write (fd, buf, cnt)) == -1 && errno != 0)
    ptr->_errno = errno;
  return ret;
}