Runtime linker symbol address resolved as 0x1

[flotter-cs]

Summary
I am getting strange address values for linker symbols, unless the target variable is of volatile type.

0801a064 <intFlashHal_init>:
801a064: b5f0 push {r4, r5, r6, r7, lr}
801a066: b08d sub sp, #0x34
801a068: f643 70f4 movw r0, #0x3ff4
801a06c: f2c2 0000 movt r0, #0x2000
801a070: 2101 movs r1, #0x1 <----------------------------------- Linker symbol address = 0x1
801a072: 7001 strb r1, [r0]
801a074: f643 70f8 movw r0, #0x3ff8
801a078: f647 6144 movw r1, #0x7e44
801a07c: f2c2 0000 movt r0, #0x2000
801a080: f6c0 0102 movt r1, #0x802
801a084: 6001 str r1, [r0]

To Reproduce

intFlashHal.c:

static uint32_t maxFlashPages;

#define RUNTIMESYMBOL static volatile

RUNTIMESYMBOL U32 flashImageEndAddress;

ErrorCode intFlashHal_init(void)
{
ErrorCode result = ERROR_OK;

maxFlashPages           = SYMBOL_VALUE(definesLnk_flashPagesPerBank);   <--- working (destination has volatile)

flashImageEndAddress    = SYMBOL_VALUE(flashRTLnk_imageEndAddress);     <--- not working

:

linker.h:

#define SYMBOL extern uint32_t

SYMBOL definesLnk_flashOrigin; // The origin of the flash bank
SYMBOL definesLnk_flashSize; // The size of the Flash memory
SYMBOL definesLnk_flashPageSize; // The size of a flash page
SYMBOL definesLnk_flashBankSize; // The size of the bank in flash
SYMBOL definesLnk_flashDualBank; // 1 if dual bank is supposed to be active
SYMBOL definesLnk_flashPagesPerBank; // number of flash pages on device

(sofia) flotter@flotter-laptop:~/sofia/DEBUG/mcb-control-mcb-e1$ starm-nm firmware-mcb-control-mcb-e1.elf | grep flash
00000100 A _flash_page_count
00000800 A _flash_page_size
00000100 A _flash_pages
00080000 A _flash_size
00080000 A definesLnk_flashBankSize
00000000 A definesLnk_flashDualBank
08000000 A definesLnk_flashOrigin
00000800 A definesLnk_flashPageSize
00000100 A definesLnk_flashPagesPerBank
00080000 A definesLnk_flashSize
08027e44 D flashRTLnk_imageEndAddress

[CC] [build]/mcb-control-mcb-e1/hal/stm32l4xx-v1.18.2/intFlashHal/intFlashHal.o
/home/flotter/sofia/tools-installed/st-arm-clang-v21.1.1/bin/starm-clang -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard -Wall -Wextra -Werror -fdata-sections -ffunction-sections -ftls-model=local-exec -fstack-usage -g3 -Og -DDEBUG -DOS_freertos -DUSE_HAL_DRIVER -DUSER_VECT_TAB_ADDRESS -DSTM32L452xx -DMAKE_GEN3 -DBUILD_BOARD_SOC= -DBUILD_NODE_TYPE=mcb -DBUILD_NODE_FIRMWARE=control -DBUILD_NODE_VERSION_MAJOR=5 -DBUILD_NODE_VERSION_MINOR=0 -DBUILD_NODE_VERSION_PATCH=9 -DBUILD_SYSTEM_VERSION_MAJOR=5 -DBUILD_SYSTEM_VERSION_MINOR=0 -DBUILD_SYSTEM_VERSION_PATCH=9 -DBUILD_CONFIG_VERSION=1 -DBUILD_COMPILER_NAME= -DBUILD_COMPILER_VERSION= -DBUILD_GIT_HASH=0x5465a69c -DBUILD_GIT_BRANCH=mcb-rover -DBUILD_DATETIME=1776067468U -DBUILD_SHORT_HASH=0x2a27b4a8 -DPICOLIBC -std=gnu11
:
-c /home/flotter/sofia/common/hal/stm32l4xx-v1.18.2/intFlashHal/intFlashHal.c -o DEBUG/mcb-control-mcb-e1/hal/stm32l4xx-v1.18.2/intFlashHal/intFlashHal.o

[LD] [build]/mcb-control-mcb-e1/firmware-mcb-control-mcb-e1.elf
/home/flotter/sofia/tools-installed/st-arm-clang-v21.1.1/bin/starm-clang -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard -Wall -Wextra -Werror -fdata-sections -ffunction-sections -ftls-model=local-exec -fstack-usage -g3 -Og -DDEBUG -DOS_freertos -DUSE_HAL_DRIVER -DUSER_VECT_TAB_ADDRESS -DSTM32L452xx -DMAKE_GEN3 -DBUILD_BOARD_SOC= -DBUILD_NODE_TYPE=mcb -DBUILD_NODE_FIRMWARE=control -DBUILD_NODE_VERSION_MAJOR=5 -DBUILD_NODE_VERSION_MINOR=0 -DBUILD_NODE_VERSION_PATCH=9 -DBUILD_SYSTEM_VERSION_MAJOR=5 -DBUILD_SYSTEM_VERSION_MINOR=0 -DBUILD_SYSTEM_VERSION_PATCH=9 -DBUILD_CONFIG_VERSION=1 -DBUILD_COMPILER_NAME= -DBUILD_COMPILER_VERSION= -DBUILD_GIT_HASH=0x5465a69c -DBUILD_GIT_BRANCH=mcb-rover -DBUILD_DATETIME=1776067468U -DBUILD_SHORT_HASH=0x2a27b4a8 -DPICOLIBC -lcrt0-hosted -z norelro -L /home/flotter/sofia/common/linker -L /home/flotter/sofia/common/linker/stm32l452xx -L /home/flotter/sofia/boards/mcb/e1.x -T /home/flotter/sofia/nodes/mcb/control/node.ld -Wl,-Map=DEBUG/mcb-control-mcb-e1/firmware-mcb-control-mcb-e1.map -Wl,--gc-sections -z noexecstack -Wl,--print-memory-usage
:
-o DEBUG/mcb-control-mcb-e1/firmware-mcb-control-mcb-e1.elf > DEBUG/mcb-control-mcb-e1/firmware-mcb-control-mcb-e1.mem 2>&1

Expected behavior
Should match the address as per the symbol table

Actual behavior
Codegeneration with value stored as 0x1

Environment
STMicroelectronics Arm Clang Toolchain -- clang version 21.1.1 [rel1.20260121-1116]
Target: arm-st-none-eabi
Thread model: posix

[Additional context]
I am using the ST released version based on this. I am asking the question here as I strongly suspect the issue to be unrelated to ST tweaks. Likely something I am not understanding and doing wrong. I do not understand why the variable has to be volatile, and it bugs my doing something that I do not understand. Any help will be hugely appreciated.

[voltur01]

Hi, it appears that this kind of declaration #define SYMBOL extern uint32_t is not correct - a linker defined symbols is an address, thus it must be either declared like a sizeless array , e.g. extern char definesLnk_flashPagesPerBank[]; or otherwise used as (uintptr_t)&definesLnk_flashPagesPerBank.

This link from search seems to have a good summary of how to handle linker defined symbols: https://downloads.ti.com/docs/esd/SPRUI03/using-linker-symbols-in-c-c-applications-slau1318080.html

The volatile workaround just prevents the compiler from doing optimizations - the constant value of 1 seems to be the constant value inferred by the compiler trying to optimize the variable out completely.

[flotter-cs]

Thank you so much for the feedback.

Just to make sure I understand, in both cases (working and not working):

#define SYMBOL_VALUE(symbol) ((uint32_t)(&symbol))

So we are always taking the address of the extern defined symbol.

I have also tested with the proposed change:

diff --git a/common/hal/stm32l4xx-v1.18.2/intFlashHal/intFlashHal.c b/common/hal/stm32l4xx-v1.18.2/intFlashHal/intFlashHal.c
index 6f7590989..6e2629dbe 100755
--- a/common/hal/stm32l4xx-v1.18.2/intFlashHal/intFlashHal.c
+++ b/common/hal/stm32l4xx-v1.18.2/intFlashHal/intFlashHal.c
@@ -87,7 +87,7 @@ PRIVATE FlashHal_BootBank flashBootBank;
  * @brief The maximum number of flash pages that can be read
  * @note this is equal to the number of flash pages per bank
  */
-RUNTIMESYMBOL U32 maxFlashPages;
+PRIVATE U32 maxFlashPages;
 
 /**
  * @brief Address of the last byte of the binary image
@@ -573,7 +573,7 @@ ErrorCode intFlashHal_init(void)
        ErrorCode result = ERROR_OK;
 
        // Linker hooks
-       maxFlashPages                   = SYMBOL_VALUE(definesLnk_flashPagesPerBank);
+       maxFlashPages                   = (U32)definesLnk_flashPagesPerBank;
 
        flashImageEndAddress    = SYMBOL_VALUE(flashRTLnk_imageEndAddress);
 
diff --git a/common/include/linker.h b/common/include/linker.h
index dfc53ffc7..1ee2549b0 100755
--- a/common/include/linker.h
+++ b/common/include/linker.h
@@ -21,7 +21,7 @@ SYMBOL definesLnk_flashSize;                                                  // The size of the Flash memory
 SYMBOL definesLnk_flashPageSize;                                               // The size of a flash page
 SYMBOL definesLnk_flashBankSize;                                               // The size of the bank in flash
 SYMBOL definesLnk_flashDualBank;                                               // 1 if dual bank is supposed to be active
-SYMBOL definesLnk_flashPagesPerBank;                                   // number of flash pages on device
+extern U32 definesLnk_flashPagesPerBank[];                             // number of flash pages on device
 SYMBOL definesLnk_baseblCfgTableAddress;                               // Base bootloader config table address
 SYMBOL definesLnk_baseblCfgTableRedAddress;                            // Base bootloader redundant config table address (CC only)
 SYMBOL definesLnk_baseblFileTableAddress;                              // Base bootloader file table address

Which still appears to give me the same issue:

08019264 <intFlashHal_init>:
 8019264: b5f0          push    {r4, r5, r6, r7, lr}
 8019266: b08d          sub sp, #0x34
 8019268: f643 70a0     movw    r0, #0x3fa0
 801926c: f2c2 0000     movt    r0, #0x2000
 8019270: 2101          movs    r1, #0x1
 8019272: 7001          strb    r1, [r0]

and inspecting the build ELF:

(sofia) flotter@flotter-laptop:~/sofia/RELEASE/mcb-control-mcb-e1$ starm-objdump --syms firmware-mcb-control-mcb-e1.elf | grep definesLnk_flashPagesPerBank
00000100 g       *ABS*	00000000 definesLnk_flashPagesPerBank

So it looks like I am unable to get it right.

[voltur01]

Hi, the fact that without volatile (so without optimizations) the compiler generates a store of just a single byte, not an address, hints that the compiler likely reduces the whole expression to a boolean value of true, so there may be a mistake in the expression or an undefined behavior.

It would be much easier to judge if you could provide a self-contained reproducer that we can build on our side, not just snippets of output. Compiler Explorer example https://godbolt.org/ could do as well.

[flotter-cs]

Thank you for taking the time to help me. I will give you a small reproducer asap.