此方案可以一并解决printf与scanf函数的重定向

可参考博客

GCC Arm 11.3rel1, 12.2编译提示 _close is not implemented and will always fail - Milton - 博客园

#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>
#include "usart.h"

__attribute__((weak)) int _close(int fd)
{
    if (fd >= STDIN_FILENO && fd <= STDERR_FILENO)
        return 0;
 
    errno = EBADF;
    return -1;
}

__attribute__((weak)) int _lseek(int fd, int ptr, int dir)
{
    (void)fd;
    (void)ptr;
    (void)dir;
 
    errno = EBADF;
    return -1;
}

__attribute__((weak)) int _isatty_r(int fd)
{
    if (fd >= STDIN_FILENO && fd <= STDERR_FILENO)
        return 1;
 
    errno = EBADF;
    return 0;
}

__attribute__((weak)) int _fstat_r(int fd, struct stat *st)
{
    if (fd >= STDIN_FILENO && fd <= STDERR_FILENO)
    {
        st->st_mode = S_IFCHR;
        return 0;
    }
 
    errno = EBADF;
    return 0;
}

int __io_getchar(void)
{
	uint8_t ch = 0;
	// Clear the Overrun flag just before receiving the first character
	__HAL_UART_CLEAR_OREFLAG(&huart1);
 
	HAL_UART_Receive(&huart1, (uint8_t *)&ch, 1, 0xFFFF);
	return ch;
}

__attribute__((weak)) int _read(int file, char *ptr, int len)
{
    (void)file;
    int DataIdx;
    for (DataIdx = 0; DataIdx < len; DataIdx++)
    {
        *ptr++ = __io_getchar();
    }
    return len;
}

int _write(int file, char *ptr, int len)
{
  HAL_UART_Transmit(&huart1, (uint8_t *)ptr, len, HAL_MAX_DELAY);
  return len;
}