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On a bare-metal MCU there is no terminal, no stdout and no file system. But you can retarget the low-level write function that `printf` calls internally: on GCC/Newlib this is `_write()`, on Keil/armcc it is `fputc()`. Once you override it to transmit via UART HAL, any `printf` call anywhere in the firmware goes out over the serial port. This is faster to set up than a dedicated logging library and uses the standard format specifiers you already know.
/* Newlib _write retarget for STM32 HAL: I redirect stdout to UART2 */
#include <errno.h>
#include <sys/unistd.h>
#include "stm32f4xx_hal.h"
extern UART_HandleTypeDef huart2;
int _write(int file, char *data, int len) {
if (file == STDOUT_FILENO || file == STDERR_FILENO) {
HAL_StatusTypeDef status =
HAL_UART_Transmit(&huart2, (uint8_t *)data, len, HAL_MAX_DELAY);
if (status == HAL_OK) return len;
errno = EIO;
return -1;
}
errno = EBADF;
return -1;
}Drop this into any STM32 Newlib project and printf goes out over UART2
HAL_MAX_DELAY means printf blocks until the UART finishes transmitting. In a FreeRTOS project this holds the scheduler. Replace with a DMA-backed ring buffer or a FreeRTOS queue for non-blocking output in production firmware.
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