Support for Bit Field about fiddle

[kou]

Could you share your use case?

It seems that memory layout of bit field is implementation defined: https://wiki.sei.cmu.edu/confluence/display/c/EXP11-C.+Do+not+make+assumptions+regarding+the+layout+of+structures+with+bit-fields

It means that we can't create portable memory layout with Fiddle/Ruby-FFI.

[lukejahnke]

Ah, good point about it being implementation defined.

My use case is accessing shared memory (shmat) to communicate with another (not written by me) process. They're using bit fields for space efficiency. I guess for my use case I don't care about portability, but I can understand it being important for this project.

[kou]

OK. For your use case, it's better that you use bit operations such as Integer#& against a byte that includes the target data. The memory layout (which bit has your data) depends on your use case, you may need to do some try & errors. You may need to change your program when you run your program in another environment.

Can we close this because we can't implement bit field support in portable way?
If we find a way to implement it in portable way, we can reopen this.

[lukejahnke]

Yes, happy to close it. Thank you for considering this and your feedback.