Compiler warning in `subsys/logging/backends/log_backend_mqtt.c` when `CONFIG_LOG_BACKEND_MQTT=y`

[raphael-fritz]

Describe the bug

I'm getting a compiler warning when using CONFIG_LOG_BACKEND_MQTT=y:

<ws-path>/zephyr/subsys/logging/backends/log_backend_mqtt.c:112:20: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]

Steps to reproduce

• create a basic project
  • CMakeLists.txt:

   cmake_minimum_required(VERSION 3.20.0)
  
   find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
   project(log_mqtt_warning)
  
   FILE(GLOB app_sources src/*.c)
   target_sources(app PRIVATE ${app_sources})

  • main.c:

  int main()
  {
       return 0;
  }

• Add these configs to prj.conf:

CONFIG_LOG=y
CONFIG_NETWORKING=y
CONFIG_MQTT_LIB=y
CONFIG_LOG_BACKEND_MQTT=y

• west build -b nucleo_f429zi

Expected behaviour

Build suceeds without any warnings.

Impact

Annoying but not much more but can be a problem if you're trying to build with CONFIG_COMPILER_OPT="-Wall -Werror".

Relevant log output

-- west build: generating a build system
Loading Zephyr default modules (Zephyr base).
-- CMake version: 3.28.1
-- Zephyr version: 4.2.0-rc2 (<ws-path>/zephyr)
-- Found west (found suitable version "1.4.0", minimum required is "0.14.0")
-- Board: nucleo_f429zi, qualifiers: stm32f429xx
-- Found host-tools: zephyr 0.17.1 (/home/<user>/zephyr-sdk-0.17.1)
-- Found toolchain: zephyr 0.17.1 (/home/<user>/zephyr-sdk-0.17.1)
-- Found GnuLd: /home/<user>/zephyr-sdk-0.17.1/arm-zephyr-eabi/arm-zephyr-eabi/bin/ld.bfd (found version "2.38") 
-- The C compiler identification is GNU 12.2.0
-- The CXX compiler identification is GNU 12.2.0
-- The ASM compiler identification is GNU
-- Found assembler: /home/<user>/zephyr-sdk-0.17.1/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc
-- Zephyr version: 4.2.0-rc2 (<ws-path>/zephyr), build: v4.2.0-rc2-69-g7a8bd3dbe752
[53/212] Building C object zephyr/CMakeFiles/zephyr.dir/subsys/logging/backends/log_backend_mqtt.c.obj
<ws-path>/zephyr/subsys/logging/backends/log_backend_mqtt.c: In function 'log_backend_mqtt_topic_set':
<ws-path>/zephyr/subsys/logging/backends/log_backend_mqtt.c:112:20: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  112 |         mqtt_topic = topic;
      |                    ^
[212/212] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       87576 B         2 MB      4.18%
             RAM:       42656 B       192 KB     21.70%
             CCM:          0 GB        64 KB      0.00%
        IDT_LIST:          0 GB        32 KB      0.00%
Generating files from <ws-path>/zephyr/dev/log_mqtt_error/build/zephyr/zephyr.elf for board: nucleo_f429zi

Impact

Annoyance – Minor irritation; no significant impact on usability or functionality.

Environment

• Debian GNU/Linux 12
• Zephyr SDK 0.17.1
• Zephyr OS build v4.2.0-rc2-69-g7a8bd3dbe752

Additional Context

No response

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