How to fix linker error with atomic compare_exchange_weak

Question

I have an atomic variable and want to use compare_exchange_weak on it.

But I'm getting the following linker error when compiling with STM32CubeIDE v1.8.0 (arm-none-eabi 9.3.1:(

arm-none-eabi\bin\ld.exe: ./Core/Src/main.o: in function `std::__atomic_base<unsigned char>::compare_exchange_weak(unsigned char&, unsigned char, std::memory_order, std::memory_order)':
arm-none-eabi\include\c++\9.3.1\bits/atomic_base.h:457: undefined reference to `__atomic_compare_exchange_1'

Here is a minimal example to demonstrate the linker error:

/* main.cpp */
 
#include <cstdint>
#include <cstdio>
#include <atomic>
 
int main() {
	std::atomic<uint8_t> some_atomic_var { 0 };
 
	some_atomic_var.store(3U); // atomic store works fine
	printf("%u\n", some_atomic_var.load()); // atomic load works fine
 
	auto val = some_atomic_var.load();
 
	// set second bit
	while (!some_atomic_var.compare_exchange_weak(val, val | (1 << 2U))); // gives linker error
	printf("%u\n", some_atomic_var.load()); 
 
	return 0;
}

Update 6.12.2021:

Platform:Windows version of CubeIDE 1.8.0

STM32 target: STM32F030CC

KnarfB · Accepted Answer

Internet wisdom says that you should link -latomic in that case, but I cannot find it in the STM32 release.

otoh, Cortex-M0 is so simple. Are you using multi tasking? If so, check the implementation in RIOT OS https://github.com/RIOT-OS/RIOT/blob/master/core/atomic_c11.c for adding the missing function. If not, you could further simplify it.

Did you write that code or is it meant to be super portable, e.g. for multi-core, out-of-order execution CPUs?

hth

KnarfB

KnarfB · Answer

Hmm. It links here without errors:10:30:16 **** Incremental Build of configuration Debug for project atomic ****make -j12 all arm-none-eabi-g++ "../Core/Src/main.cpp" -mcpu=cortex-m4 -std=gnu++14 -g3 -DDEBUG -DUSE_HAL_DRIVER -DSTM32L432xx -c -I../Core/Inc -I../Drivers/STM32L4xx_HAL_Driver/Inc -I../Drivers/STM32L4xx_HAL_Driver/Inc/Legacy -I../Drivers/CMSIS/Device/ST/STM32L4xx/Include -I../Drivers/CMSIS/Include -O0 -ffunction-sections -fdata-sections -fno-exceptions -fno-rtti -fno-use-cxa-atexit -Wall -fstack-usage -MMD -MP -MF"Core/Src/main.d" -MT"Core/Src/main.o" --specs=nano.specs -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mthumb -o "Core/Src/main.o"arm-none-eabi-g++ -o "atomic.elf" @"objects.list" -mcpu=cortex-m4 -T"STM32_workspace\atomic\STM32L432KCUX_FLASH.ld" --specs=nosys.specs -Wl,-Map="atomic.map" -Wl,--gc-sections -static --specs=nano.specs -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mthumb -Wl,--start-group -lc -lm -lstdc++ -lsupc++ -Wl,--end-groupFinished building target: atomic.elf arm-none-eabi-size atomic.elf arm-none-eabi-objdump -h -S atomic.elf > "atomic.list"arm-none-eabi-objcopy -O binary atomic.elf "atomic.bin" text data bss dec hexfilename 5848 116 1600 7564 1d8catomic.elfFinished building: default.size.stdout Finished building: atomic.binFinished building: atomic.list

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