There is, but it's part of the platform ABI and not the C language standard. The latter specifies syntax and behavior, the former is what's concerned with inte

There is, but it's part of the platform ABI and not the C language standard. The latter specifies syntax and behavior, the former is what's concerned with interoperability details like memory layout. I happen to have a copy of AAPCS64 in front of me, and you can find the specification of bit packing in section 10.1.8. The sysv ABI for x86/x86_64 has its own wording (but I'm pretty sure is compatible). MSVC does something else I believe, etc...