3 ms·

If we can't say anything about the system at time T, that's a failure of the execution model. I'm not trying to implicate Rice's theorem here or other undecide

If we can't say anything about the system at time T, that's a failure of the execution model. I'm not trying to implicate Rice's theorem here or other undecideable problems. C makes it difficult to correctly implement any code (rather than all code) that has a property we want and show that it holds solely with the tools available to mortals. Divine intervention is helpful, but notoriously unreliable.

shadowgovt 3y ago
This is probably my bias just from all of the distributed work I've had to do, but in general being able to say anything about the state of the system at a discrete time T is an expensive luxury in my ecosystem, If not actually infeasible. You can say things about the expected input and output, you can reason about how it's supposed to get there, and you can do traces of a discrete path that was taken in hindsight. But being able to pause the universe and say "What is the current state of the machine" isn't possible when the 'machine' is spread across data centers in multiple geographic locations and it's barely, if we turn our head and squint and lie to ourselves, possible on a modern CPU architecture with SIMD instructions and branch prediction.
- AlotOfReading 3y ago
  Totally agree, but it's a bit outside C. The C execution model is essentially built around the idea of a single-thread computer with a flat memory space, with very minor considerations for exceptions hidden in UB throughout the standard. Most of the difficult and expensive consistency stuff is punted outside the language or to other standards like Cilk and UPC, though recent versions have at least recognized that threads and parallel processors exist.
  - jstimpfle 3y ago
    Other languages aren't much different in this regard (other than often being less well specified), because it's the reasonable thing to do. You want to give threads a lot of independence for performance reasons. But people just love to shit on C.
    - AlotOfReading 3y ago
      Yes, many popular languages are even worse. Thankfully most of those languages (C++ excepted) aren't used for systems programming like C is and hopefully we can agree that the safety story with C++ is mixed at best. Those that have some history as real systems languages (e.g. Java) tend to have pretty decent execution semantics. But just to provide a real counterargument, Ada/SPARK meets the bars I set above. Rust is also a significant improvement over C in practical terms, even if there isn't an official standard you can point to in the same way.

shadowgovt 3y ago

This is probably my bias just from all of the distributed work I've had to do, but in general being able to say anything about the state of the system at a discrete time T is an expensive luxury in my ecosystem, If not actually infeasible. You can say things about the expected input and output, you can reason about how it's supposed to get there, and you can do traces of a discrete path that was taken in hindsight. But being able to pause the universe and say "What is the current state of the machine" isn't possible when the 'machine' is spread across data centers in multiple geographic locations and it's barely, if we turn our head and squint and lie to ourselves, possible on a modern CPU architecture with SIMD instructions and branch prediction.

AlotOfReading 3y ago
Totally agree, but it's a bit outside C. The C execution model is essentially built around the idea of a single-thread computer with a flat memory space, with very minor considerations for exceptions hidden in UB throughout the standard. Most of the difficult and expensive consistency stuff is punted outside the language or to other standards like Cilk and UPC, though recent versions have at least recognized that threads and parallel processors exist.
- jstimpfle 3y ago
  Other languages aren't much different in this regard (other than often being less well specified), because it's the reasonable thing to do. You want to give threads a lot of independence for performance reasons. But people just love to shit on C.
  - AlotOfReading 3y ago
    Yes, many popular languages are even worse. Thankfully most of those languages (C++ excepted) aren't used for systems programming like C is and hopefully we can agree that the safety story with C++ is mixed at best. Those that have some history as real systems languages (e.g. Java) tend to have pretty decent execution semantics. But just to provide a real counterargument, Ada/SPARK meets the bars I set above. Rust is also a significant improvement over C in practical terms, even if there isn't an official standard you can point to in the same way.