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What do you expect a language to do if you index an array using a runtime-calculated value? Rust checks at runtime and panics if your program exceeds the bound

What do you expect a language to do if you index an array using a runtime-calculated value? Rust checks at runtime and panics if your program exceeds the bounds. You can opt-in to asking if the bounds are exceeded and fail gracefully if you like, or if you want to promise the compiler you know for sure your bounds are tight, you can use unsafe blocks and act like C. Opt-in to danger. C lets you do it with no checks. You have to opt in to the safe path of checking and failing. You don't automatically segfault if you exceed the bounds; instead you read arbitrary memory. Welcome to the land of undefined behavior. You may crash, but more likely, you will read some value from an unexpected place, and carry on executing incorrectly for who knows how long. Opt-in to safety. That's what people mean by Rust is safe by default. And that's just bounds checks. Carry that notion over to pointers, references, threads, lifetimes, ... What ever made you think "Safe Rust" meant "compile time checks of runtime values are possible" or "C is just as safe because it lets you index outside an array"?

MaulingMonkey 10y ago
> What do you expect a language to do if you index an array using a runtime-calculated value? Optimizers already try to prove index bounds to eliminate unnecessary checks, and static analysis tools to demand necessary checks. Turning the latter into compile time errors is a reasonable approach if your language can provide sufficient information to deal with false positives - likely by forcing you to add your own bounds checking to explicitly handle out-of-bounds cases. A language John Carmack was using or researching at one point comes to mind, which had this kind of thing going on IIRC. I'm afraid I can't find it off hand, so I might recall in error.
- jjnoakes 10y ago
  Sure, but this can be the same in Rust or C, which was the context of this discussion. New languages may provide more tools to eliminate more checks, but in general, if you have a runtime value and you index an array with it, someone has to check somewhere. And the point of this thread is that Rust's default (check index bounds and fail at runtime unless the check can be proven unnecessary by the compiler or optimizer) is safer than C's default (don't check anything by default and hit UB if the index is out of bounds).
ue_ 10y ago
>You don't automatically segfault if you exceed the bounds; instead you read arbitrary memory. GCC and clang both have sanitizers either built in or available for them. Sure, it's not default, but let's not act like there is no choice in C but to account for every OOB access while programming or to read memory you don't want to. Furthermore, I never said that compile time checks of variables are possible, but rather we could move to using dependent typing, or at least a way to judge whether a variable would work as a subscript based on the type of the array and variable. The Rust designers didn't do that. Instead they put in a feature common in the two most popular C compilers to "panic" at runtime instead of accessing memory. That's nothing. It's rubbish. And if you know to "catch" the panic, why don't you check the value of what you're subscripting with? Saying you can catch the panic is missing the point of unintentional OOB accesses, which is that they're unintentional. C is just as safe with regard to OOB accessing, and to be honest that's pretty poor in 2017.
- jjnoakes 10y ago
  > let's not act like there is no choice in C If you are using gcc or clang, you have more options. True. But not all C compilers give you those options. However, the point is moot, since I never said you can't catch these things in C; I said it wasn't the default. Which you agree with. > I never said that compile time checks of variables are possible, but rather we could move to using dependent typing You didn't say anything about dependent typing. You said "Rust is no better than C". And I'm pointing out that it is. Dependent typing may be even better in some cases; I'm not arguing otherwise. > Saying you can catch the panic is missing the point of unintentional OOB accesses, which is that they're unintentional. No one said you should catch the panic. You can use Vec::get() for example if you are using runtime-derived indices and want bounds checking in an ergonomic fashion. And saying a panic for unintentional OOB is the same as in C is not true, since you get a panic by default in Rust, and to get one in C, not only must you be using a specific compiler or two, you must have the sanitizers enabled for every source file in your program. Not "by default" by any stretch. > C is just as safe with regard to OOB accessing, and to be honest that's pretty poor in 2017. It is nowhere close, and saying it is is pretty poor in 2017 as well. And you are still ignoring pointers, references, lifetimes, threads, ... you know, the other things that also help Rust make "Safe by default" and C "dangerous by default".

MaulingMonkey 10y ago

> What do you expect a language to do if you index an array using a runtime-calculated value? Optimizers already try to prove index bounds to eliminate unnecessary checks, and static analysis tools to demand necessary checks. Turning the latter into compile time errors is a reasonable approach if your language can provide sufficient information to deal with false positives - likely by forcing you to add your own bounds checking to explicitly handle out-of-bounds cases. A language John Carmack was using or researching at one point comes to mind, which had this kind of thing going on IIRC. I'm afraid I can't find it off hand, so I might recall in error.

jjnoakes 10y ago
Sure, but this can be the same in Rust or C, which was the context of this discussion. New languages may provide more tools to eliminate more checks, but in general, if you have a runtime value and you index an array with it, someone has to check somewhere. And the point of this thread is that Rust's default (check index bounds and fail at runtime unless the check can be proven unnecessary by the compiler or optimizer) is safer than C's default (don't check anything by default and hit UB if the index is out of bounds).

ue_ 10y ago

>You don't automatically segfault if you exceed the bounds; instead you read arbitrary memory. GCC and clang both have sanitizers either built in or available for them. Sure, it's not default, but let's not act like there is no choice in C but to account for every OOB access while programming or to read memory you don't want to. Furthermore, I never said that compile time checks of variables are possible, but rather we could move to using dependent typing, or at least a way to judge whether a variable would work as a subscript based on the type of the array and variable. The Rust designers didn't do that. Instead they put in a feature common in the two most popular C compilers to "panic" at runtime instead of accessing memory. That's nothing. It's rubbish. And if you know to "catch" the panic, why don't you check the value of what you're subscripting with? Saying you can catch the panic is missing the point of unintentional OOB accesses, which is that they're unintentional. C is just as safe with regard to OOB accessing, and to be honest that's pretty poor in 2017.

jjnoakes 10y ago
> let's not act like there is no choice in C If you are using gcc or clang, you have more options. True. But not all C compilers give you those options. However, the point is moot, since I never said you can't catch these things in C; I said it wasn't the default. Which you agree with. > I never said that compile time checks of variables are possible, but rather we could move to using dependent typing You didn't say anything about dependent typing. You said "Rust is no better than C". And I'm pointing out that it is. Dependent typing may be even better in some cases; I'm not arguing otherwise. > Saying you can catch the panic is missing the point of unintentional OOB accesses, which is that they're unintentional. No one said you should catch the panic. You can use Vec::get() for example if you are using runtime-derived indices and want bounds checking in an ergonomic fashion. And saying a panic for unintentional OOB is the same as in C is not true, since you get a panic by default in Rust, and to get one in C, not only must you be using a specific compiler or two, you must have the sanitizers enabled for every source file in your program. Not "by default" by any stretch. > C is just as safe with regard to OOB accessing, and to be honest that's pretty poor in 2017. It is nowhere close, and saying it is is pretty poor in 2017 as well. And you are still ignoring pointers, references, lifetimes, threads, ... you know, the other things that also help Rust make "Safe by default" and C "dangerous by default".