Enforce 32-bit enums with GCC linker

arm, c, enums, gcc, ld

Solution

After a while I got it working. I rebuilt the whole toolchain including the compiler with this flag. Here is the way I did:

Get the toolchain source from https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads

Add 3 lines to some sections of the buildscript `build-toolchain.sh`:

saveenv
saveenvvar CFLAGS_FOR_TARGET '-fno-short-enums'
  [...build commands...]
restoreenv

Modified sections:

- `Task [III-1] /$HOST_NATIVE/gcc-first/`

- `Task [III-2] /$HOST_NATIVE/newlib/`

- `Task [III-4] /$HOST_NATIVE/gcc-final/`

- `Task [IV-3] /$HOST_MINGW/gcc-final/`

I Skipped building `newlib-nano` and `gcc-size-libstdcxx`.

Run the modified Scripts `build-prerequisites.sh` and `build-toolchain.sh` to build everything.

After that, the compiler is using the large-enum-mode and the linker is fine with my objects. But now, I get the opposit warning for some objects of the newlib (`lib_a-mbtowc_r.o`, `lib_a-svfiprintf.o`, `lib_a-svfprintf.o`, `lib_a-vfprintf.o` and `lib_a-vfiprintf.o`):

c:/gcc-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/6.2.1/../../../../arm-none-eabi/bin/ld.exe: warning: c:/gcc-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/6.2.1/../../../../arm-none-eabi/lib\libc.a(lib_a-mbtowc_r.o) uses variable-size enums yet the output is to use 32-bit enums; use of enum values across objects may fail

I looked into the makefiles of these object and they are explicitly set to variable-size-enums, sadly. The only "solution" for this, was to add a linker flag to mute this warning:

-Xlinker -no-enum-size-warning

That's it.

Problem

I'm writing a bare metal application for an ARM device (no OS). I need 32-bit enums, so I compiled the application with the `-fno-short-enums` compiler flag. Without this flag, I get variable enums (and enforcing the size by adding an additional `0xFFFFFFFF` value to each enum is not an option). Now I get the following linker warning for every object: ``` c:/gcc-arm-none-eabi/bin/../lib/gcc/arm-none-eabi/6.2.1/../../../../arm-none-eabi/bin/ld.exe: warning: ./src/test.o uses 32-bit enums yet the output is to use variable-size enums; use of enum values across objects may fail ``` It's just a warning, no error. But what does it mean exactly? How can I specify the "output"? I tried to recompile the newlib with the above flag to ensure that all objects use the same enum size, but I'm still getting the warning. Is there something I missed?

Original source