Hardfault when writing an array - c

Question

I was working on a project when I came across whit this behavior that I can't quite understand
Context
I have a function like:
float SCL_calculate(AVG_struct_type* data)
The inner workings of this function are not relevant, the function output a float number correctly (it has been fully debugged already)
I have a uint8_t global array defined like this:
char output_buff[9] = {0x0};
I'm trying to write a float number from the [1] index of this array
All this in the context of embedded systems
I'm using a STM32F411CEU6
The problem
Originally I had this code:
*( (float *) (&output_buff[1]) ) = SCL_calculate(&Voltage);
but if I tried to use this, then the UC jumped into the HardFault_Handler when trying to write into the array, but if instead I do:
float data;
data = SCL_calculate(&Voltage);
*( (float *) (&output_buff[1]) ) = data;
it works just fine.
My question
Why one way it jumps into the HardFault_Handler and the other way not?
minimal reproducible example
Here is a minimal reproducible example, I left all stm32 device configurations by default, I deleted all the compiler comments and functions to make it easier to read.
float SCL_calculate(void);
int main(void)
{
HAL_Init();
float data = SCL_calculate( );
*( (float *) (&output_buff[1]) ) = data; //NO ERROR
*( (float *) (&output_buff[1]) ) = SCL_calculate( ); //ERROR
while (1)
{
}
}
float SCL_calculate(void){
return 12.34;
}
Here is the full main.c file
/* USER CODE BEGIN Header */
/**
******************************************************************************
* #file : main.c
* #brief : Main program body
******************************************************************************
* #attention
*
* Copyright (c) 2023 STMicroelectronics.
* All rights reserved.
*
* This software is licensed under terms that can be found in the LICENSE file
* in the root directory of this software component.
* If no LICENSE file comes with this software, it is provided AS-IS.
*
******************************************************************************
*/
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "main.h"
/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
/* 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);
/* USER CODE BEGIN PFP */
char output_buff[9] = {0x0};
/* USER CODE END PFP */
/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */
float SCL_calculate( void );
/* USER CODE END 0 */
/**
* #brief The application entry point.
* #retval int
*/
int main(void)
{
/* USER CODE BEGIN 1 */
/* 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 BEGIN SysInit */
/* USER CODE END SysInit */
/* Initialize all configured peripherals */
/* USER CODE BEGIN 2 */
float data = SCL_calculate( );
*( (float *) (&output_buff[1]) ) = data; //NO ERROR
*( (float *) (&output_buff[1]) ) = SCL_calculate( ); //ERROR
/* 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_SCALE1);
/** Initializes the RCC Oscillators according to the specified parameters
* in the RCC_OscInitTypeDef structure.
*/
RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
RCC_OscInitStruct.HSIState = RCC_HSI_ON;
RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
{
Error_Handler();
}
/** Initializes the CPU, AHB and APB buses clocks
*/
RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
|RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSI;
RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK)
{
Error_Handler();
}
}
/* USER CODE BEGIN 4 */
float SCL_calculate( void ){
return 12.34; //random number
}
/* 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 */
__disable_irq();
while (1)
{
}
/* 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,
ex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
/* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

Answer 1

Why one way it jumps into the HardFault_Handler and the other way not?
Let's compile the code:
output_buff:
main:
push {r4, r7, lr}
sub sp, sp, #12
add r7, sp, #0
bl SCL_calculate
vstr.32 s0, [r7, #4]
ldr r2, .L3
ldr r3, [r7, #4] # float
str r3, [r2] # float
ldr r4, .L3
bl SCL_calculate
vmov.f32 s15, s0
vstr.32 s15, [r4]
.L2:
b .L2
.L3:
.word output_buff+1
SCL_calculate:
push {r7}
add r7, sp, #0
ldr r3, .L7
vmov s15, r3
vmov.f32 s0, s15
mov sp, r7
ldr r7, [sp], #4
bx lr
.L7:
.word 1095069860
First store is using str instruction which does not require aligned access.
str r3, [r2] # float
The latter is using FPU instruction vstr.32 (I assume standard Cube settings) and FPU instructions require aligned access.
vstr.32 s15, [r4]
That is the reason why the first one works, and the second does not. It can only happen if you do not enable the optimizations (-O3 version below):
main:
ldr r3, .L4
ldr r2, .L4+4
str r2, [r3, #1] # unaligned
.L2:
b .L2
.L4:
.word .LANCHOR0
.word 1095069860
SCL_calculate:
vldr.32 s0, .L7
bx lr
.L7:
.word 1095069860
output_buff:
How to prevent problems? Simply do not use pointer punning.
#define STORE(dest, src, type) do {type temp; temp = (src); memcpy(&(dest), &(temp), sizeof(temp));}while(0)
float SCL_calculate(void);
uint8_t output_buff[100];
int main(void)
{
float data = SCL_calculate( );
STORE(output_buff[1], data, float); //NO ERROR
STORE(output_buff[1], SCL_calculate(), float);
while (1)
{
}
}
float SCL_calculate(void){
return 12.34;
}
Calls to memcpy will be optimized out even if optimizations are mot enabled.
output_buff:
main:
push {r7, lr}
sub sp, sp, #16
add r7, sp, #0
bl SCL_calculate
vstr.32 s0, [r7, #12]
ldr r3, [r7, #12] # float
str r3, [r7, #8] # float
ldr r3, [r7, #8]
ldr r2, .L3
str r3, [r2, #1] # unaligned
bl SCL_calculate
vmov.f32 s15, s0
vstr.32 s15, [r7, #4]
ldr r3, [r7, #4]
ldr r2, .L3
str r3, [r2, #1] # unaligned
.L2:
b .L2
.L3:
.word output_buff
SCL_calculate:
push {r7}
add r7, sp, #0
ldr r3, .L7
vmov s15, r3
vmov.f32 s0, s15
mov sp, r7
ldr r7, [sp], #4
bx lr
.L7:
.word 1095069860
https://godbolt.org/z/37nY8Wbe9
Using memcpy will prevent another problem. If you port the code to for example Cortex-M0 it will actually call memcpy or will use byte size instructions as this core requires aligned access.
https://godbolt.org/z/9eo89anqa

Answer 2

You're writing a float (32-bit) into the second ([1]) element of char (8-bit) array. So if your array started at address 0x20000000, then you're writing a 32-bit (4-byte) value into 0x20000001. This is an alignment issue. You can write a 32-bit data block only to 32-bit aligned memory address, 0x20000000 or 0x20000004 or 0x20000008 and so on. Similarly, a 16-bit value should be 2-byte aligned and should not be partially in one 32-bit group and partially in another one.