Eclipse CDT: Linkerfehler

#include "stm32f4xx_hal.h"

#include "stm32f4xx_hal_gpio.h"

#include "stm32f4xx_hal_uart.h"

static void SystemClock_Config(void);

int main(void) {

  HAL_Init();

  unsigned char message[] = "Hello UART!\n";

  SystemClock_Config();

  __HAL_RCC_GPIOA_CLK_ENABLE();

  GPIO_InitTypeDef GPIOinit;

  GPIOinit.Pin = GPIO_PIN_0 | GPIO_PIN_1;

  GPIOinit.Mode = GPIO_MODE_AF_PP;

  GPIOinit.Pull = GPIO_PULLUP;

  GPIOinit.Speed = GPIO_SPEED_FAST;

  GPIOinit.Alternate = GPIO_AF8_UART4;

  HAL_GPIO_Init(GPIOD, &GPIOinit);

  UART_HandleTypeDef uart4;

  uart4.Instance = UART4;

  uart4.Init.BaudRate = 9600;

  uart4.Init.WordLength = UART_WORDLENGTH_8B;

  uart4.Init.StopBits = UART_STOPBITS_1;

  uart4.Init.Parity = UART_PARITY_NONE;

  uart4.Init.Mode = UART_MODE_TX_RX;

  HAL_UART_Init(&uart4);

  HAL_UART_Transmit(&uart4, message, 12, 100);

  while (1) {

  }

}