Using section attributes within a union

[NSR]

I've been assigning some of my storage areas into different memory areas as defined within the STM32H743ZITX_RAM.ld file in a project using STM32CubeIDE. This has first required me to insert the appropriate definitions where I've used memory domain D3 as an example:

MEMORY
{
  DTCMRAM    (xrw)    : ORIGIN = 0x20000000,   LENGTH = 128K
  ITCMRAM    (xrw)    : ORIGIN = 0x00000000,   LENGTH = 64K
  RAM_D1    (xrw)    : ORIGIN = 0x24000000,   LENGTH = 512K
  RAM_D3    (xrw)    : ORIGIN = 0x38000000,   LENGTH = 64K
  FLASH    (rx)    : ORIGIN = 0x8000000,   LENGTH = 2048K
}
 
/* Sections */
SECTIONS
/* snip .. */
  .myD3memory (NOLOAD):
  {  
    . = ALIGN(4);
     *(.myD3memory)
    . = ALIGN(4);
  } >RAM_D3
/* etc .. */
}

I can then utilize this in main() as follows:

uint16_t	buffer16[32768] __attribute__ ((section(".myD3memory")));

This works exactly as I intended and Build Analyzer shows the correct allocation. However, I now have a new case where I would like to reference the memory area not just as 16-bit unsigned shorts but also 8-bit unsigned bytes and have tried the following:

union
{
    uint8_t	buffer8[65536] __attribute__ ((section(".myD3memory")));
    uint16_t	buffer16[32768] __attribute__ ((section(".myD3memory")));
}

which unfortunately produces the errors:

../Core/Src/main.c: error: section attribute not allowed for 'buffer8'

 uint8_t buffer8[65536] __attribute__ ((section(".myD3memory")));

      ^~~~~~~~~~~

../Core/Src/main.c: error: section attribute not allowed for 'buffer16'

 uint16_t buffer16[32768] __attribute__ ((section(".myD3memory")));

Hopefully this will be an easy suggestion from someone more (GCC?) compiler savvy and any help in moving this forward would be greatly appreciated.

[waclawek.jan · Accepted solution]

    union
    {
        uint8_t	buffer8[65536];
        uint16_t	buffer16[32768];
    } foo __attribute__ ((section(".myD3memory")));

JW

[Piranha]

All union members by definition represent the same physical memory. Therefore they cannot be placed at different addresses.

[NSR]

Thanks JW, that's exactly what I was trying to achieve....I hadn't think of applying it to the end of the union. I knew it what something silly and an likely an easy remedy....sometimes the woods cannot be seen for the trees!

Piranha: placing the two variables was my intention for them to occupy the same physical memory but referenced differently according to the member type, but thanks for your input.

[Piranha]

What I mean is, allowing to add an individual section attribute for each member is not logical, because they can be placed only at a single address. That's why the section attribute is allowed only for the whole union. 🙂

[NSR]

When put like that then yes, I completely agree - I was really hoping it was something obvious that I hadn't considered. I've not long started using the attribute directive so it's still a little alien to me; although being able to access the whole of the memory map, including DTCMRAM and ITCMRAM has become somewhat liberating!