CODEWITHSUNDEEP
cprintfstm32
frank
frank

Reputation: 13

How might one retarget printf with a custom callback on STM32 with Eclipse/gcc/ac6/sw4stm32?

I'm programming the stm32g071rb nucleo with Eclipse/gcc/ac6/sw4stm32 and I'm having some difficulty retargetting printf with a custom callback which prints to a SPI-connected display.

For now, my main looks something like this:

void main(void)
{
    /* Declare BOARD typedef. */
    BOARD_TypeDef_t board;

    /* Initialize BOARD typedef. */
    board_init(&board);

    /* Initialize display */
    display_init(&board.display[displayA]);

    /* Writes a "B" to the display */
    display_write(&board.display[displayA], 0x42);
}

What I would like to do is to use my display_write function and the display destination in a retargetted printf.

The options I have seen are:

  1. Using __io_putchar function: This is available to be externed from the syscalls.c file. However, the only parameter it takes is the character to be printed, so I'm not sure and don't think I can get my BOARD_TypeDef_t in there.

  2. Using write function: This is available in the syscalls.c file. However, I would rather use this the canonical way via __io_putchar and not by directly editing the write function.

  3. Using fopencookie: I think this is the way I would like to do this but I'm having some difficulty. I learned that I had to activate _GNU_SOURCE and include stdio.h, so I added this atop main:

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#include <stdio.h> // Needed for ssize_t, cookie_io_functions_t, etc
#endif /* _GNU_SOURCE */

However, I'm still getting an "unknown type name 'cookie_io_functions_t'" error although this struct is defined in the included stdio.h and other calls to types in this stdio.h are referenced just fine, e.g., cookie_write_function_t, cookie_seek_function_t, etc.

I can't quite tell if this is per se a gcc issue or an Eclipse issue so progress has been slow.

Any suggestions on what I might do to clear this up are appreciated.

Upvotes: 0

Views: 1052

Answers (2)

Kashyap Gada
Kashyap Gada

Reputation: 1

I got the printf retargetted on USB CDC VCOM Port adding __io_putchar to main.c - Following Code

/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  * @attention
  *
  * <h2><center>&copy; Copyright (c) 2020 STMicroelectronics.
  * All rights reserved.</center></h2>
  *
  * This software component is licensed by ST under BSD 3-Clause license,
  * the "License"; You may not use this file except in compliance with the
  * License. You may obtain a copy of the License at:
  *                        opensource.org/licenses/BSD-3-Clause
  *
  ******************************************************************************
  */
/* USER CODE END Header */

/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "usb_device.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include "usbd_cdc_if.h"
/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/
/* USER CODE BEGIN PTD */

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */

/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */

/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/

/* USER CODE BEGIN PV */

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
/* USER CODE BEGIN PFP */
int __io_putchar(int ch);
/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */

/* USER CODE END 0 */

/**
  * @brief  The application entry point.
  * @retval int
  */
int main(void)
{
  /* USER CODE BEGIN 1 */
    setvbuf(stdout, NULL, _IONBF, 0);
  /* USER CODE END 1 */


  /* MCU Configuration--------------------------------------------------------*/

  /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
  HAL_Init();

  /* USER CODE BEGIN Init */

  /* USER CODE END Init */

  /* Configure the system clock */
  SystemClock_Config();

  /* USER CODE B                   EGIN SysInit */

  /* USER CODE END SysInit */

  /* Initialize all configured peripherals */
  MX_GPIO_Init();
  MX_USB_DEVICE_Init();
  /* USER CODE BEGIN 2 */
  HAL_Delay(5000);
//  CDC_Transmit_HS("Hello World", strlen("Hello World"));
  printf("Hello World");
  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    /* USER CODE END WHILE */

    /* USER CODE BEGIN 3 */
  }
  /* USER CODE END 3 */
}

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

  /** Configure the main internal regulator output voltage 
  */
  __HAL_RCC_PWR_CLK_ENABLE();
  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE3);
  /** Initializes the CPU, AHB and APB busses clocks 
  */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
  RCC_OscInitStruct.HSEState = RCC_HSE_ON;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
  RCC_OscInitStruct.PLL.PLLM = 4;
  RCC_OscInitStruct.PLL.PLLN = 120;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
  RCC_OscInitStruct.PLL.PLLQ = 5;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    Error_Handler();
  }
  /** Initializes the CPU, AHB and APB busses clocks 
  */
  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
                              |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV4;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV8;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV4;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief GPIO Initialization Function
  * @param None
  * @retval None
  */
static void MX_GPIO_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct = {0};

  /* GPIO Ports Clock Enable */
  __HAL_RCC_GPIOC_CLK_ENABLE();
  __HAL_RCC_GPIOF_CLK_ENABLE();
  __HAL_RCC_GPIOH_CLK_ENABLE();
  __HAL_RCC_GPIOA_CLK_ENABLE();
  __HAL_RCC_GPIOB_CLK_ENABLE();
  __HAL_RCC_GPIOG_CLK_ENABLE();
  __HAL_RCC_GPIOE_CLK_ENABLE();
  __HAL_RCC_GPIOD_CLK_ENABLE();

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOC, NCS_MEMS_SPI_Pin|CSX_Pin|OTG_FS_PSO_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(ACP_RST_GPIO_Port, ACP_RST_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOD, RDX_Pin|WRX_DCX_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOG, LD3_Pin|LD4_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pins : A0_Pin A1_Pin A2_Pin A3_Pin 
                           A4_Pin A5_Pin SDNRAS_Pin A6_Pin 
                           A7_Pin A8_Pin A9_Pin */
  GPIO_InitStruct.Pin = A0_Pin|A1_Pin|A2_Pin|A3_Pin 
                          |A4_Pin|A5_Pin|SDNRAS_Pin|A6_Pin 
                          |A7_Pin|A8_Pin|A9_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  GPIO_InitStruct.Alternate = GPIO_AF12_FMC;
  HAL_GPIO_Init(GPIOF, &GPIO_InitStruct);

  /*Configure GPIO pins : SPI5_SCK_Pin SPI5_MISO_Pin SPI5_MOSI_Pin */
  GPIO_InitStruct.Pin = SPI5_SCK_Pin|SPI5_MISO_Pin|SPI5_MOSI_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF5_SPI5;
  HAL_GPIO_Init(GPIOF, &GPIO_InitStruct);

  /*Configure GPIO pin : ENABLE_Pin */
  GPIO_InitStruct.Pin = ENABLE_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF14_LTDC;
  HAL_GPIO_Init(ENABLE_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pin : SDNWE_Pin */
  GPIO_InitStruct.Pin = SDNWE_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  GPIO_InitStruct.Alternate = GPIO_AF12_FMC;
  HAL_GPIO_Init(SDNWE_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pins : NCS_MEMS_SPI_Pin CSX_Pin OTG_FS_PSO_Pin */
  GPIO_InitStruct.Pin = NCS_MEMS_SPI_Pin|CSX_Pin|OTG_FS_PSO_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

  /*Configure GPIO pins : B1_Pin MEMS_INT1_Pin MEMS_INT2_Pin TP_INT1_Pin */
  GPIO_InitStruct.Pin = B1_Pin|MEMS_INT1_Pin|MEMS_INT2_Pin|TP_INT1_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_EVT_RISING;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*Configure GPIO pins : B5_Pin VSYNC_Pin G2_Pin R4_Pin 
                           R5_Pin */
  GPIO_InitStruct.Pin = B5_Pin|VSYNC_Pin|G2_Pin|R4_Pin 
                          |R5_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF14_LTDC;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*Configure GPIO pin : ACP_RST_Pin */
  GPIO_InitStruct.Pin = ACP_RST_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(ACP_RST_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pin : OTG_FS_OC_Pin */
  GPIO_InitStruct.Pin = OTG_FS_OC_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_EVT_RISING;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(OTG_FS_OC_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pins : R3_Pin R6_Pin */
  GPIO_InitStruct.Pin = R3_Pin|R6_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF9_LTDC;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  /*Configure GPIO pin : BOOT1_Pin */
  GPIO_InitStruct.Pin = BOOT1_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(BOOT1_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pins : A10_Pin A11_Pin BA0_Pin BA1_Pin 
                           SDCLK_Pin SDNCAS_Pin */
  GPIO_InitStruct.Pin = A10_Pin|A11_Pin|BA0_Pin|BA1_Pin 
                          |SDCLK_Pin|SDNCAS_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  GPIO_InitStruct.Alternate = GPIO_AF12_FMC;
  HAL_GPIO_Init(GPIOG, &GPIO_InitStruct);

  /*Configure GPIO pins : D4_Pin D5_Pin D6_Pin D7_Pin 
                           D8_Pin D9_Pin D10_Pin D11_Pin 
                           D12_Pin NBL0_Pin NBL1_Pin */
  GPIO_InitStruct.Pin = D4_Pin|D5_Pin|D6_Pin|D7_Pin 
                          |D8_Pin|D9_Pin|D10_Pin|D11_Pin 
                          |D12_Pin|NBL0_Pin|NBL1_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  GPIO_InitStruct.Alternate = GPIO_AF12_FMC;
  HAL_GPIO_Init(GPIOE, &GPIO_InitStruct);

  /*Configure GPIO pins : G4_Pin G5_Pin B6_Pin B7_Pin */
  GPIO_InitStruct.Pin = G4_Pin|G5_Pin|B6_Pin|B7_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF14_LTDC;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  /*Configure GPIO pins : D13_Pin D14_Pin D15_Pin D0_Pin 
                           D1_Pin D2_Pin D3_Pin */
  GPIO_InitStruct.Pin = D13_Pin|D14_Pin|D15_Pin|D0_Pin 
                          |D1_Pin|D2_Pin|D3_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  GPIO_InitStruct.Alternate = GPIO_AF12_FMC;
  HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);

  /*Configure GPIO pin : TE_Pin */
  GPIO_InitStruct.Pin = TE_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(TE_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pins : RDX_Pin WRX_DCX_Pin */
  GPIO_InitStruct.Pin = RDX_Pin|WRX_DCX_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);

  /*Configure GPIO pins : R7_Pin DOTCLK_Pin B3_Pin */
  GPIO_InitStruct.Pin = R7_Pin|DOTCLK_Pin|B3_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF14_LTDC;
  HAL_GPIO_Init(GPIOG, &GPIO_InitStruct);

  /*Configure GPIO pins : HSYNC_Pin G6_Pin R2_Pin */
  GPIO_InitStruct.Pin = HSYNC_Pin|G6_Pin|R2_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF14_LTDC;
  HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

  /*Configure GPIO pin : I2C3_SDA_Pin */
  GPIO_InitStruct.Pin = I2C3_SDA_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_OD;
  GPIO_InitStruct.Pull = GPIO_PULLUP;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF4_I2C3;
  HAL_GPIO_Init(I2C3_SDA_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pin : I2C3_SCL_Pin */
  GPIO_InitStruct.Pin = I2C3_SCL_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_OD;
  GPIO_InitStruct.Pull = GPIO_PULLUP;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF4_I2C3;
  HAL_GPIO_Init(I2C3_SCL_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pins : STLINK_RX_Pin STLINK_TX_Pin */
  GPIO_InitStruct.Pin = STLINK_RX_Pin|STLINK_TX_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  GPIO_InitStruct.Alternate = GPIO_AF7_USART1;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*Configure GPIO pins : G7_Pin B2_Pin */
  GPIO_InitStruct.Pin = G7_Pin|B2_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF14_LTDC;
  HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);

  /*Configure GPIO pins : G3_Pin B4_Pin */
  GPIO_InitStruct.Pin = G3_Pin|B4_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  GPIO_InitStruct.Alternate = GPIO_AF9_LTDC;
  HAL_GPIO_Init(GPIOG, &GPIO_InitStruct);

  /*Configure GPIO pins : LD3_Pin LD4_Pin */
  GPIO_InitStruct.Pin = LD3_Pin|LD4_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOG, &GPIO_InitStruct);

  /*Configure GPIO pins : SDCKE1_Pin SDNE1_Pin */
  GPIO_InitStruct.Pin = SDCKE1_Pin|SDNE1_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  GPIO_InitStruct.Alternate = GPIO_AF12_FMC;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

}

/* USER CODE BEGIN 4 */
int __io_putchar(int ch) {
    CDC_Transmit_HS(&ch, 1);
    return ch;
}
/* USER CODE END 4 */

/**
  * @brief  This function is executed in case of error occurrence.
  * @retval None
  */
void Error_Handler(void)
{
  /* USER CODE BEGIN Error_Handler_Debug */
  /* User can add his own implementation to report the HAL error return state */

  /* USER CODE END Error_Handler_Debug */
}

#ifdef  USE_FULL_ASSERT
/**
  * @brief  Reports the name of the source file and the source line number
  *         where the assert_param error has occurred.
  * @param  file: pointer to the source file name
  * @param  line: assert_param error line source number
  * @retval None
  */
void assert_failed(uint8_t *file, uint32_t line)
{ 
  /* USER CODE BEGIN 6 */
  /* User can add his own implementation to report the file name and line number,
     tex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
  /* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/

Upvotes: 0

frank
frank

Reputation: 13

After a bit of investigating, I found a workable solution.

The full post is here, including an image of the final product: https://zfembedded.wordpress.com/2020/02/27/redirecting-printf-on-stm32-using-fopencookie/

Starting with the basic program that prints a "B" to the screen, I did the following:

  1. The fopencookie function is in stdio.h so this needs to be included.
  2. In order to activate it, _GNU_SOURCE must be defined before this include.
  3. We create a stream for our display, my_stream0. We can also create one for stdout.
  4. The fopencookie function needs cookie_io_functions so we define them. These don't appear to participate in the action but without them things don't work for me. Hints on why this is so are helpful.
  5. We write out the function for a component of the cookie_io_functions, in this case for .write. For me, this function need not do anything.
  6. We write the stream write function.
  7. We pass the stream write function and the board definition to the streams defined.
  8. We modify the buffering of the streams.
  9. We test out both fprintfs and the printf of the created streams.

`

/* 2 */
#define _GNU_SOURCE

/* 1 */
#include <stdio.h>

/* 5 */
static ssize_t dummy_cookie_write(void *c, const char *buf, size_t size)
{
   return size;
}

/* 6 */
int displayA_stream_write(struct _reent *q1, void *q2, const char *q3, int q4)
{
    (void)q1;
    BOARD_TypeDef_t* q5 = (BOARD_TypeDef_t*)q2;

    int DataIdx;

    for (DataIdx = 0; DataIdx < q4; DataIdx++)
    {
        display_write(&q5->display[displayA], *q3++);
    }

    return q4;
}

void main(void)
{
    /* Declare BOARD typedef. */
    BOARD_TypeDef_t board;

    /* Initialize BOARD typedef. */
    board_init(&board);

    /* Initialize display */
    display_init(&board.display[displayA]);

    /* Writes a "B" to the display */
    display_write(&board.display[displayA], 0x42);

    /* 4 */
    cookie_io_functions_t dummy_cookie_funcs = {
            .read  = 0,
            .write = dummy_cookie_write, /* Setting this to 0 breaks the solution. */
            .seek  = 0,
            .close = 0
        };

    /* 3 */
    FILE *my_stream0 = fopencookie(NULL,"w", dummy_cookie_funcs); /* Replacing dummy_cookie_funcs with NULL breaks the solution. */
    stdout = fopencookie(NULL,"w", dummy_cookie_funcs);

    /* 8 */
    setbuf(my_stream0, NULL);
    setbuf(stdout, NULL);

    /* 7 */
    my_stream0->_write = displayA_stream_write;
    my_stream0->_cookie = (void *)&board;
    stdout->_write = displayA_stream_write;
    stdout->_cookie = (void *)&board;

    /* 9 */
    fprintf(my_stream0, "\nNowhere");
    fprintf(stdout, "\nSomewhere");
    printf("\nHere");

}

`

The key find that made this all possible was finding out that #7 write was more important than #4 write.

Thanks for your suggestions.

Upvotes: 1

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