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Undefined behavior means something specific in the standard, it's not just an operation that might do different things on different compilers and machines. It m

by remram 5y ago

Undefined behavior means something specific in the standard, it's not just an operation that might do different things on different compilers and machines. It means that if it happens, the program is allowed to do anything and everything, even before the UB is reached. Undefined behavior is breaking an assumption that the compiler is allowed to make. It is probably impossible to make a low-level language with no implementation-defined behavior, but it is certainly possible to make one with no undefined behavior. For example, you can put in your spec that overflowing an unsigned integer can give any value; that is different that putting in your spec that it doesn't happen and if you write it the variable might have no value, multiple values, or burn your socks off. https://en.wikipedia.org/wiki/Undefined_behavior https://en.wikipedia.org/wiki/Undefined_behavior

comp54321 5y ago
It's actually impossible as far as I can see as long as you have unsafe memory access via pointers and memory is used to hold information about control flow and local variables, because overwriting memory can then do weird and wonderful things to the state of the program. E.g. local variables can magically change values, but worse, the control flow of your program can be hijacked in pretty arbitrary ways. It would be great if this wasn't possible, because then you wouldn't have a whole class of security vulnerabilities. On older systems without memory protection, the situation is even worse - you could scribble all over the OS memory as well and who knows what happens then.
- spc476 5y ago
  It's been attempted. Intel tried doing this with the Intel 432, but it never got beyond the prototype stage as it was very slow, even by the standards of the day (1982).
- remram 5y ago
  This happens in C because compilers are allowed a wide range of optimisations. I'm just saying it's probably possible to build a low-level language without UB. Of course no one would use it because optimizing compilers are desirable and skipping optimizations just so broken code is handled to spec won't happen.

by remram 5y ago

comp54321 5y ago

It's actually impossible as far as I can see as long as you have unsafe memory access via pointers and memory is used to hold information about control flow and local variables, because overwriting memory can then do weird and wonderful things to the state of the program. E.g. local variables can magically change values, but worse, the control flow of your program can be hijacked in pretty arbitrary ways. It would be great if this wasn't possible, because then you wouldn't have a whole class of security vulnerabilities. On older systems without memory protection, the situation is even worse - you could scribble all over the OS memory as well and who knows what happens then.

spc476 5y ago
It's been attempted. Intel tried doing this with the Intel 432, but it never got beyond the prototype stage as it was very slow, even by the standards of the day (1982).
remram 5y ago
This happens in C because compilers are allowed a wide range of optimisations. I'm just saying it's probably possible to build a low-level language without UB. Of course no one would use it because optimizing compilers are desirable and skipping optimizations just so broken code is handled to spec won't happen.