I don't believe C standardizes the ABI across all OS/hardware combinations. But some OS/hardware combinations are standardized; for example, there's the AMD64 ABI[1], which I think just about every operating system that runs on amd64 uses … except Windows.

[1]: http://www.x86-64.org/documentation/abi.pdf

Yeah, but that's a bit of a long bow... That's not a C ABI :-)