'Packed' attribute support for STM32F4 (Cortex-M4)

Hi.

My target is a STM32 MCU with a Cortex-M4.

I have downloaded the arm-none-eabi toolchain for Windows 10 (gcc-arm-none-eabi-8-2018-q4-major).

I am compiling using Makefile and Cygwin.

In my software I need to use packed structures but when I compile my source code I have this warning message:

 warning: 'packed' attribute ignored [-Wattributes]

To be sure I tested the feature with this code

typedef __packed struct
{
	uint8_t field1;
	uint32_t field2;
} test_pack;

int main(void)
{
  test_pack t1;
  
  printf("%d", sizeof(t1));
  
  return 0;
}

The expected result is 5 but the output is 8.

I found that '__packed' is defined in "gcc-arm-none-eabi-8-2018\arm-none-eabi\include\sys".

I have included these paths to compile command:

gcc-arm-none-eabi-8-2018/arm-none-eabi/include
gcc-arm-none-eabi-8-2018/lib/gcc/arm-none-eabi/8.2.1/include
gcc-arm-none-eabi-8-2018/lib/gcc/arm-none-eabi/8.2.1/include-fixed

Is the 'packed' attribute supported/working? Am I missing something or doing something wrong?

Thanks.

  • Obviously I found the solution just after posting the question.

    As described in GNU GCC Documentation, the attribute should go after the struct's fields.

    So moving the attribute after the closing curly brace like:

    typedef struct
    {
    	uint8_t field1;
    	uint32_t field2;
    } __packed test_pack;
    
    int main(void)
    {
      test_pack t1;
      
      printf("%d", sizeof(t1));
      
      return 0;
    }

    removed the warning and now the size is correct.

    Hope this will help someone else.