How to compile CH56x applications without the vendor toolchain, using a stock embedded riscv toolchain

syscalls.c

/****************************************************************************
*  Copyright (c) 2009 by Michael Fischer. All rights reserved.
*  Copyright (c) 2022 by Hans Baier
*
*  Redistribution and use in source and binary forms, with or without
*  modification, are permitted provided that the following conditions
*  are met:
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*  1. Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*  2. Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in the
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*  3. Neither the name of the author nor the names of its contributors may
*     be used to endorse or promote products derived from this software
*     without specific prior written permission.
*
*  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
*  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
*  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
*  FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
*  THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
*  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
*  BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
*  OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
*  AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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*  THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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****************************************************************************
*  History:
*
*  28.03.09  mifi   First Version, based on the original syscall.c from
*                   newlib version 1.17.0
*  13.08.22  haba   adapted to CH56x
****************************************************************************/

#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>

#include "CH56x_common.h"

/***************************************************************************/

int _read_r (struct _reent *r, int file, char * ptr, int len)
{
  r = r;
  file = file;
  ptr = ptr;
  len = len;

  return UART1_rx(ptr, len);
}

/***************************************************************************/

int _lseek_r (struct _reent *r, int file, int ptr, int dir)
{
  r = r;
  file = file;
  ptr = ptr;
  dir = dir;

  return 0;
}

/***************************************************************************/

int _write_r (struct _reent *r, int file, char * ptr, int len)
{
  r = r;
  file = file;
  ptr = ptr;

  int index;

  UART1_tx(ptr, (uint16_t) len); /* Serial Multibyte send */

  return len;
}

/***************************************************************************/

int _close_r (struct _reent *r, int file)
{
  return 0;
}

/***************************************************************************/

/* Register name faking - works in collusion with the linker.  */
register char * stack_ptr asm ("sp");

caddr_t _sbrk_r (struct _reent *r, int incr)
{
  extern char   end asm ("end"); /* Defined by the linker.  */
  static char * heap_end;
  char *        prev_heap_end;

  if (heap_end == NULL)
    heap_end = & end;

  prev_heap_end = heap_end;

  if (heap_end + incr > stack_ptr)
  {
      /* Some of the libstdc++-v3 tests rely upon detecting
        out of memory errors, so do not abort here.  */
#if 0
      extern void abort (void);

      _write (1, "_sbrk: Heap and stack collision\n", 32);

      abort ();
#else
      errno = ENOMEM;
      return (caddr_t) -1;
#endif
  }

  heap_end += incr;

  return (caddr_t) prev_heap_end;
}

/***************************************************************************/

int _fstat_r (struct _reent *r, int file, struct stat * st)
{
  r = r;
  file = file;

  memset (st, 0, sizeof (* st));
  st->st_mode = S_IFCHR;
  return 0;
}

/***************************************************************************/

int _isatty_r(struct _reent *r, int fd)
{
  r = r;
  fd = fd;

  return 1;
}