Allocating memory in Flash for user data (STM32F4 HAL) - microcontroller

Question

I'm trying to use the internal flash of an STM32F405 to store a bunch of user settable bytes that remain after rebooting.
I'm using:
uint8_t userConfig[64] __attribute__((at(0x0800C000)));
to allocate memory for the data I want to store.
When the program starts, I check to see if the first byte is set to 0x42, if not, i set it using:
HAL_FLASH_Unlock();
HAL_FLASH_Program(TYPEPROGRAM_BYTE, &userConfig[0], 0x42);
HAL_FLASH_Lock();
After that I check the value in userConfig[0] and I see 0x42... Great!
When I hit reset, however, and look at the location again, it's not 0x42 anymore...
Any idea where I'm going wrong? I've also tried:
#pragma location = 0x0800C00
volatile const uint8_t userConfig[64]
but I get the same result..

Answer 1

Okay I found an answer on the ST forums thanks to clive1. This example works for an STM32F405xG.
First we need to modify the memory layout in the linker script file (.ld file)
Modify the existing FLASH and add a new line for DATA. Here I've allocated all of section 11.
MEMORY
{
FLASH (RX) : ORIGIN = 0x08000000, LENGTH = 1M-128K
DATA (RWX) : ORIGIN = 0x080E0000, LENGTH = 128k
...
...
}
Manual for editing linker files on the sourceware website
In the same file, we need to add:
.user_data :
{
. = ALIGN(4);
*(.user_data)
. = ALIGN(4);
} > DATA
This creates a section called .user_data that we can address in the program code.
Finally, in your .c file add:
__attribute__((__section__(".user_data"))) const uint8_t userConfig[64]
This specifies that we wish to store the userConfig variable in the .user_data section and const makes sure the address of userConfig is kept static.
Now, to write to this area of flash during runtime, you can use the stm32f4 stdlib or HAL flash driver.
Before you can write to the flash, it has to be erased (all bytes set to 0xFF) The instructions for the HAL library say nothing about doing this for some reason...
HAL_FLASH_Unlock();
__HAL_FLASH_CLEAR_FLAG(FLASH_FLAG_EOP | FLASH_FLAG_OPERR | FLASH_FLAG_WRPERR | FLASH_FLAG_PGAERR | FLASH_FLAG_PGSERR );
FLASH_Erase_Sector(FLASH_SECTOR_11, VOLTAGE_RANGE_3);
HAL_FLASH_Program(TYPEPROGRAM_WORD, &userConfig[index], someData);
HAL_FLASH_Lock();