1. 创建工程

用cubemx创建一个工程makefile工程。记得开串口。

2. 重定向

在main.c对printf重定向，增加如下代码：

#ifdef __GNUC__
/* With GCC/RAISONANCE, small printf (option LD Linker->Libraries->Small printf
   set to 'Yes') calls __io_putchar() */
#define PUTCHAR_PROTOTYPE int __io_putchar(int ch)
#else
#define PUTCHAR_PROTOTYPE int fputc(int ch, FILE *f)
#endif /* __GNUC__ */

/**
 * @brief  Retargets the C library printf function to the USART.
 * @param  None
 * @retval None
 */
PUTCHAR_PROTOTYPE
{
    /* Place your implementation of fputc here */
    /* e.g. write a character to the USART1 and Loop until the end of transmission */
    HAL_UART_Transmit(&huart1, (uint8_t *)&ch, 1, 0xFFFF);

    return ch;
}

3. syscalls.c

很多网上的资料只提到了上一步，忽视了需要加入syscalls.c文件。
在cubemx的仓库目录下找到文件"C:\Users\xxx\STM32Cube\Repository\STM32Cube_FW_H7_V1.11.1\Projects\STM32H743I-EVAL\Examples\UART\UART_Printf\STM32CubeIDE\Example\User\syscalls.c"
复制syscalls.c到工程中，并在makefile中加入syscalls.c。

/* Support files for GNU libc.  Files in the system namespace go here.
   Files in the C namespace (ie those that do not start with an
   underscore) go in .c.  */

#include <_ansi.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/fcntl.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#include <sys/times.h>
#include <sys/errno.h>
#include <reent.h>
#include <unistd.h>
#include <sys/wait.h>

#define FreeRTOS
#define MAX_STACK_SIZE 0x200

extern int __io_putchar(int ch) __attribute__((weak));
extern int __io_getchar(void) __attribute__((weak));

#ifndef FreeRTOS
register char *stack_ptr asm("sp");
#endif

caddr_t _sbrk(int incr)
{
    extern char end asm("end");
    static char *heap_end;
    char *prev_heap_end, *min_stack_ptr;

    if (heap_end == 0)
        heap_end = &end;

    prev_heap_end = heap_end;

#ifdef FreeRTOS
    /* Use the NVIC offset register to locate the main stack pointer. */
    min_stack_ptr = (char *)(*(unsigned int *)*(unsigned int *)0xE000ED08);
    /* Locate the STACK bottom address */
    min_stack_ptr -= MAX_STACK_SIZE;

    if (heap_end + incr > min_stack_ptr)
#else
    if (heap_end + incr > stack_ptr)
#endif
    {
        //      write(1, "Heap and stack collision\n", 25);
        //      abort();
        errno = ENOMEM;
        return (caddr_t)-1;
    }

    heap_end += incr;

    return (caddr_t)prev_heap_end;
}

/*
 * _gettimeofday primitive (Stub function)
 * */
int _gettimeofday(struct timeval *tp, struct timezone *tzp)
{
    /* Return fixed data for the timezone.  */
    if (tzp)
    {
        tzp->tz_minuteswest = 0;
        tzp->tz_dsttime = 0;
    }

    return 0;
}
void initialise_monitor_handles()
{
}

int _getpid(void)
{
    return 1;
}

int _kill(int pid, int sig)
{
    errno = EINVAL;
    return -1;
}

void _exit(int status)
{
    _kill(status, -1);
    while (1)
    {
    }
}

int _write(int file, char *ptr, int len)
{
    int DataIdx;

    for (DataIdx = 0; DataIdx < len; DataIdx++)
    {
        __io_putchar(*ptr++);
    }
    return len;
}

int _close(int file)
{
    return -1;
}

int _fstat(int file, struct stat *st)
{
    st->st_mode = S_IFCHR;
    return 0;
}

int _isatty(int file)
{
    return 1;
}

int _lseek(int file, int ptr, int dir)
{
    return 0;
}

int _read(int file, char *ptr, int len)
{
    int DataIdx;

    for (DataIdx = 0; DataIdx < len; DataIdx++)
    {
        *ptr++ = __io_getchar();
    }

    return len;
}

int _open(char *path, int flags, ...)
{
    /* Pretend like we always fail */
    return -1;
}

int _wait(int *status)
{
    errno = ECHILD;
    return -1;
}

int _unlink(char *name)
{
    errno = ENOENT;
    return -1;
}

int _times(struct tms *buf)
{
    return -1;
}

int _stat(char *file, struct stat *st)
{
    st->st_mode = S_IFCHR;
    return 0;
}

int _link(char *old, char *new)
{
    errno = EMLINK;
    return -1;
}

int _fork(void)
{
    errno = EAGAIN;
    return -1;
}

int _execve(char *name, char **argv, char **env)
{
    errno = ENOMEM;
    return -1;
}

我们需要syscalls.c，是因为syscalls.c中实现了int _write(int file, char *ptr, int len)。

4. 补充

如果需要打印浮点型数据，需要在makefile文件中加入-u_printf_float编译参数。

#######################################
# LDFLAGS
#######################################
# link script
LDSCRIPT = STM32H743IITx_FLASH.ld

# libraries
LIBS = -lc -lm -lnosys 
LIBDIR = 
LDFLAGS = $(MCU) -specs=nano.specs -T$(LDSCRIPT) $(LIBDIR) $(LIBS) -Wl,-Map=$(BUILD_DIR)/$(TARGET).map,--cref -Wl,--gc-sections -u_printf_float

# default action: build all
all: $(BUILD_DIR)/$(TARGET).elf $(BUILD_DIR)/$(TARGET).hex $(BUILD_DIR)/$(TARGET).bin