ARMCLANG: Migration: L6242E: Clash with Different Sized Enum and Wchar

Information in this knowledgebase article applies to:

• Keil MDK v5.23 or later

SYMPTOM

While migrating a project from Arm Compiler 5 to Arm Compiler 6, I receive the following linker error:

L6242E: Cannot link object fs_fat.o as its attributes
           are incompatible with the image attributes.
   ... wchart-16 clashes with wchart-32.
   ... packed-enum clashes with enum_is_int.

L6242E: Cannot link object rtx_kernel.o as its attributes
               are incompatible with the image attributes.
   ... wchart-16 clashes with wchart-32.
   ... packed-enum clashes with enum_is_int.

CAUSE

In Arm Compiler 5, the default size of the wchar_t type is unsigned short, which is two bytes. In Arm Compiler 6, the default size of the wchar_t type is unsigned int, which is four bytes. The difference can be represented by the following default options:

Arm Compiler 5:  --wchar16             //unsigned short
Arm Compiler 6:  -fno-short-wchar      //unsigned int

The wchar_t type must be the same in all source and libraries to successfully link.

For enums in Arm Compiler 5, if no option is specified, the smallest data type that can hold the values of all enumerators is used. This is why prebuild libraries can have a minimum size, for enums. The lower default was also used to generate the CMSIS RTOS RTXv4 libraries with Arm Compiler 5. It is lower than the option "--enum_is_int", which is 32-bit. Arm Compiler 6, by default, uses -fno-short-enums to set the minimum size of an enumeration type to 32-bit.

RESOLUTION

Go to Project => Options for Target => C/C++ (AC6) tab. Make sure short enums/wchar is enabled. This is equivalent to the command line option, -fshort-enums -fshort-wchar. Using the smaller sizes will help conserve memory in the application, and the last option agrees with the size of wchar_t in the prebuild libraries.

MORE INFORMATION

• Refer to Application Note 298: Migrate from ARM C/C++ Compiler 5 to ARM Compiler 6.
• Refer to Arm Compiler Migration and Compatibility Guide Version.
• Refer to -fshort-wchar and -fno-short-wchar in the ARM Compiler armclang Reference Guide.
• Refer to the -fno-short-enums and -fshort-enums in the ARM Compiler armclang Reference Guide.
• Refer to --enum_is_int in the Compiler User Guide.
• Refer to List of the armlink error and warning messages in the Errors and Warnings Reference Guide.

Last Reviewed: Friday, January 8, 2021