First, Bfile_GetFileSize_OS has a single parameter and not two (discard the last one, it always starts counting from the beginning of the file).

Second, Bfile_OpenFile_OS has a third parameter which must be set to zero (as things are now, its value is random, which may lead to a crash).

I will update this post/thread as I know about more things which are wrong, according to Simon and to my own testing, in libfxcg...

Fixed in git. Thanks!

Today I learned Bfile_WriteFile_OS has a problem with writing a small number of bytes, independently from that number being odd or even (word-aligned seemed to cause even more problems!).
It was this discovery that allowed me to finally fix the nasty bug that was delaying the release of the new Utilities add-in.

For the Bfile_OpenFile_OS, do you have to change the definition in file.h to get it to compile without error? It says too many arguments for me.

Yes you need, add something like a "int mustBeZero" as a third parameter to the definition.