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It is just the opposite. UB is a clarification to tell programmers what the language considers to be undesired behavior. If they didn't say anything, it would b

It is just the opposite. UB is a clarification to tell programmers what the language considers to be undesired behavior. If they didn't say anything, it would be always a mystery if a certain construct was allowed or not, effectively making it compiler dependent. Compilers would also have less avenue for creating optimizations. In the next iterations of the C standard we may see more constructs classified as UB.

adgjlsfhk1 4y ago
That sounds good in theory, but many things that are UB in C/C++ are UB because they are really hard to verify at compile time which makes them almost impossible to program around. Any signed addition in C is potential UB unless you have a proof that all numbers that will ever be input to the addition won't cause overflow (which is made harder because C doesn't define the size of the default integer types). Furthermore, no progress is UB which means that as a programmer, you have to solve the halting problem for your program before knowing whether it has a bug.
- chongli 4y ago
  See my other comment [1] which addresses the exact things you brought up here. Safe checked arithmetic is a new standard feature in C23. If no progress were not UB, then tons of loop optimizations would be impossible and then we couldn’t have nice things, like numpy. [1] https://news.ycombinator.com/item?id=35406554 https://news.ycombinator.com/item?id=35406554
- coliveira 4y ago
  > Any signed addition in C is potential UB unless you have a proof that all numbers that will ever be input to the addition won't cause overflow This has always been the case. Standard C has always operated with the possibility that addition can overflow. The programmer or library writer is responsible to check if the used types are large enough. If you want to be perfectly sure you need to check for overflow. Making this UB has not changed the nature of the issue. > is made harder because C doesn't define the size of the default integer types They correctly made this implementation defined. But C now has different byte sized integer types if you want to be sure.
- jcranmer 4y ago
  > many things that are UB in C/C++ are UB because they are really hard to verify at compile time which makes them almost impossible to program around The second half of the sentence doesn't follow from the first. Take everyone's favorite example, signed integer overflow: all you have to do to avoid UB on signed integer overflow is check for overflow before doing the operation (and C23 finally adds features to do that for you). Taking a step back, the fundamental thing about UB is that it is very nearly always a bug in your code (and this includes especially integer overflow!). Even if you gave well-defined semantics to UB, the semantics you'd give would very rarely make the program not buggy. Complaining that we can't prove programs free of UB is tantamount to complaining that we can't prove programs free of bugs. It actually turns out that UB is actually extremely helpful for tools that try to help programmers find bugs in their code. Since UB is automatically a bug, any tool that finds UB knows that it found a bug; if you give it well-defined semantics instead, it's a lot trickier to assert that it's a bug. In a real-world example, the infamous buffer overflow vulnerability Heartbleed stymied most (all?) static analyzers for the simple reason that, due to how OpenSSL did memory management, it wasn't actually undefined behavior by C's definition. Unsigned integer overflow also falls into this bucket--it's very hard to distinguish between intentional cases of unsigned integer overflow (e.g., hashing algorithms) from unintentional cases (e.g., calculating buffer sizes).
  - the_why_of_y 4y ago
    My complaint here is that it took C more than 30 years between defining signed integer overflow as UB and providing programmers with standard library facilities to check if a signed integer operation would result in overflow. I much prefer Rust's approach to arithmetic, where overflow with plain arithmetic operators is defined as a bug, and panics on debug-enabled builds, plus special operations in the standard library like wrapping_add and saturating_add for the special cases where overflow is expected.
    - chongli 4y ago
      My complaint here is that it took C more than 30 years ... I much prefer Rust's approach That's an odd complaint. Rust didn't spring forth fully formed from the ether, it stands on the shoulders of C (and other giants of PL history). 30 years ago you couldn't use Rust at all because it didn't exist. The reason the committee doesn't just radically change C in all these nice ways to catch up to Rust is because it would be incompatible. Then you wouldn't have fixed C, you'd just have two languages: "old C", which all of the existing C code in the world is written in, and "new C", which nothing is written in. At that point why not just start over from scratch, like they did with Rust?
CJefferson 4y ago
Is the improved performance of C over say Java, or Rust (which both have much less undefined behaviour -- Java almost none) worth the pain and bugs which have been caused by UB? Honestly, I don't think so, and as computers get more powerful and the amount of the world which relies on their correct functioning grows, I feel the arguments for UB become increasingly difficult to justify.
- chongli 4y ago
  I went to look up undefined behaviour in Rust and I got this scary warning: Warning: The following list is not exhaustive. There is no formal model of Rust's semantics for what is and is not allowed in unsafe code, so there may be more behavior considered unsafe. The following list is just what we know for sure is undefined behavior. Please read the Rustonomicon before writing unsafe code. After the warning was a list of many of the same types of things that are undefined behaviour in C. In addition, there’s a bunch more undefined behaviour related to improper usage of the unsafe keyword. So I don’t think you get a free lunch with Rust here. What you get is a “safe” playground if you stay within the guard rails and avoid using the unsafe keyword. But then you are limited to writing programs which can be expressed in safe Rust, a proper subset of all programs you might want to write. Furthermore, the lack of a formal specification for Rust is one area where it lags behind C, a standardized language. All of the undefined behaviour in C is decreed and documented by the standard, having been decided by the committee. Rust, on the other hand, may have weird and unpredictable behaviour that you just have to debug yourself, which may or may not be compiler bugs.
  - CJefferson 4y ago
    I agree rust isn’t perfect, but I think you underestimate the value of “safe” code. I often write programs that have unsafe code. However, the unsafe code is never more than 100 lines, which means I have a very small amount of code to reason about — Rust users expect (of course, you as a programmer has to enforce) that it should be possible to cause UB from safe code, so my “safe interface” to my unsafe code ensures my code can’t cause UB, no matter what I call. On problem with Rust is generally when you mess up it panics — I think that’s better than buffer overflows and the like, but still not a good user experience. This means there is a very small amount of code I have to really think about, while in C or C++, basically any place x[i] appears (regardless of if x is a pointer or a std::vector). You can of course write safe C code, people do, but it’s hard, and it only takes one slip up anywhere in your program to blow it.

adgjlsfhk1 4y ago

That sounds good in theory, but many things that are UB in C/C++ are UB because they are really hard to verify at compile time which makes them almost impossible to program around. Any signed addition in C is potential UB unless you have a proof that all numbers that will ever be input to the addition won't cause overflow (which is made harder because C doesn't define the size of the default integer types). Furthermore, no progress is UB which means that as a programmer, you have to solve the halting problem for your program before knowing whether it has a bug.

chongli 4y ago
See my other comment [1] which addresses the exact things you brought up here. Safe checked arithmetic is a new standard feature in C23. If no progress were not UB, then tons of loop optimizations would be impossible and then we couldn’t have nice things, like numpy. [1] https://news.ycombinator.com/item?id=35406554 https://news.ycombinator.com/item?id=35406554
coliveira 4y ago
> Any signed addition in C is potential UB unless you have a proof that all numbers that will ever be input to the addition won't cause overflow This has always been the case. Standard C has always operated with the possibility that addition can overflow. The programmer or library writer is responsible to check if the used types are large enough. If you want to be perfectly sure you need to check for overflow. Making this UB has not changed the nature of the issue. > is made harder because C doesn't define the size of the default integer types They correctly made this implementation defined. But C now has different byte sized integer types if you want to be sure.
jcranmer 4y ago
> many things that are UB in C/C++ are UB because they are really hard to verify at compile time which makes them almost impossible to program around The second half of the sentence doesn't follow from the first. Take everyone's favorite example, signed integer overflow: all you have to do to avoid UB on signed integer overflow is check for overflow before doing the operation (and C23 finally adds features to do that for you). Taking a step back, the fundamental thing about UB is that it is very nearly always a bug in your code (and this includes especially integer overflow!). Even if you gave well-defined semantics to UB, the semantics you'd give would very rarely make the program not buggy. Complaining that we can't prove programs free of UB is tantamount to complaining that we can't prove programs free of bugs. It actually turns out that UB is actually extremely helpful for tools that try to help programmers find bugs in their code. Since UB is automatically a bug, any tool that finds UB knows that it found a bug; if you give it well-defined semantics instead, it's a lot trickier to assert that it's a bug. In a real-world example, the infamous buffer overflow vulnerability Heartbleed stymied most (all?) static analyzers for the simple reason that, due to how OpenSSL did memory management, it wasn't actually undefined behavior by C's definition. Unsigned integer overflow also falls into this bucket--it's very hard to distinguish between intentional cases of unsigned integer overflow (e.g., hashing algorithms) from unintentional cases (e.g., calculating buffer sizes).
- the_why_of_y 4y ago
  My complaint here is that it took C more than 30 years between defining signed integer overflow as UB and providing programmers with standard library facilities to check if a signed integer operation would result in overflow. I much prefer Rust's approach to arithmetic, where overflow with plain arithmetic operators is defined as a bug, and panics on debug-enabled builds, plus special operations in the standard library like wrapping_add and saturating_add for the special cases where overflow is expected.
  - chongli 4y ago
    My complaint here is that it took C more than 30 years ... I much prefer Rust's approach That's an odd complaint. Rust didn't spring forth fully formed from the ether, it stands on the shoulders of C (and other giants of PL history). 30 years ago you couldn't use Rust at all because it didn't exist. The reason the committee doesn't just radically change C in all these nice ways to catch up to Rust is because it would be incompatible. Then you wouldn't have fixed C, you'd just have two languages: "old C", which all of the existing C code in the world is written in, and "new C", which nothing is written in. At that point why not just start over from scratch, like they did with Rust?

CJefferson 4y ago

Is the improved performance of C over say Java, or Rust (which both have much less undefined behaviour -- Java almost none) worth the pain and bugs which have been caused by UB? Honestly, I don't think so, and as computers get more powerful and the amount of the world which relies on their correct functioning grows, I feel the arguments for UB become increasingly difficult to justify.

chongli 4y ago
I went to look up undefined behaviour in Rust and I got this scary warning: Warning: The following list is not exhaustive. There is no formal model of Rust's semantics for what is and is not allowed in unsafe code, so there may be more behavior considered unsafe. The following list is just what we know for sure is undefined behavior. Please read the Rustonomicon before writing unsafe code. After the warning was a list of many of the same types of things that are undefined behaviour in C. In addition, there’s a bunch more undefined behaviour related to improper usage of the unsafe keyword. So I don’t think you get a free lunch with Rust here. What you get is a “safe” playground if you stay within the guard rails and avoid using the unsafe keyword. But then you are limited to writing programs which can be expressed in safe Rust, a proper subset of all programs you might want to write. Furthermore, the lack of a formal specification for Rust is one area where it lags behind C, a standardized language. All of the undefined behaviour in C is decreed and documented by the standard, having been decided by the committee. Rust, on the other hand, may have weird and unpredictable behaviour that you just have to debug yourself, which may or may not be compiler bugs.
- CJefferson 4y ago
  I agree rust isn’t perfect, but I think you underestimate the value of “safe” code. I often write programs that have unsafe code. However, the unsafe code is never more than 100 lines, which means I have a very small amount of code to reason about — Rust users expect (of course, you as a programmer has to enforce) that it should be possible to cause UB from safe code, so my “safe interface” to my unsafe code ensures my code can’t cause UB, no matter what I call. On problem with Rust is generally when you mess up it panics — I think that’s better than buffer overflows and the like, but still not a good user experience. This means there is a very small amount of code I have to really think about, while in C or C++, basically any place x[i] appears (regardless of if x is a pointer or a std::vector). You can of course write safe C code, people do, but it’s hard, and it only takes one slip up anywhere in your program to blow it.