6 ms·

I guess people overreact to "undefined behavior". If your software runs always on the same platform then you can know what the behavior would be. It might also

by scoutt 6y ago

I guess people overreact to "undefined behavior". If your software runs always on the same platform then you can know what the behavior would be. It might also be intentional and should not crash your program. Undefined behavior is used when doing a specification for a compiler that will be used in many architectures and they cannot clearly specify what would happen on ALL architectures for a given operation, because the result might change among these architectures. Undefined behavior doesn't mean that your CPU has either a chance to suddenly blow away or that it would gain self-awareness and destroy the world. You can actually predict what would happen if you throw away a rock on earth. You cannot predict it for every planet in the universe, so theoretically "throwing a rock" produces undefined behavior.

jjnoakes 6y ago
> If your software runs always on the same platform then you can know what the behavior would be. This is not necessarily true, and I consider it to be a harmful misconception. I think people under-react to "undefined behavior". What you think the platform would do if you overflowed an integer using machine instructions and what your C or C++ code ends up executing can be wildly different things because C and C++ compilers do everything in their power to optimize your code assuming your signed integers will never overflow. If they do, who knows what state your program is in or which machine instructions the compiler would have scheduled.
- scoutt 6y ago
  You are right. I might correct the sentence with "you can often know". Undefined behavior should be avoided but really is not that terrible.
  - jjnoakes 6y ago
    We will have to agree to disagree. Fully understanding what a program might do if there is undefined behavior is not trivial. It requires basically reviewing and fully understanding the output assembler for every single build. Any change in system headers, compiler version, compiler flags, or source code might invalidate previous reviews. And programs with lurking undefined behavior may not seem dangerous but sufficiently clever exploits can do quite amazing things with the smallest of attack surfaces.
    - scoutt 6y ago
      Sure but in 17 years writing C for a living, with lines of code by the millions, and (embedded) devices running my code all over the world I still have to know about an integer overflow bug. Sure, I don't always write bug-free code (as anyone), but when I say it's not so terrible I mean that UD should be considered a rare case, and it's not worth even a single night of quiet sleep, let alone switching languages or overreacting. Hey, I am more afraid of a flipping bit due to power supply noises. And I don't want to say either that my code is perfect, or since it never happened to me it will never happen to you and that you can go around triggering UD everywhere since it's not a big deal. But I also like to warn young players that "boooo UD!" is just part of a FUD tactic for them to adopt some shiny new language.
      - pjmlp 6y ago
        Would you feel the same way when we finally manage to get liability laws for security exploits, like we are now discussing in Germany?
        scoutt 6y ago
        If some of my systems fail, people might die. Should I still be worried for a liability?
        pjmlp 6y ago
        As an professional Engineer I would be.
      - jjnoakes 6y ago
        There is a ton of software out there that is riddled with bugs. Much of it is run behind closed doors, isn't exposed to attack vectors like the internet, and happily churns along because no one is trying to attack it. Lots of other software might be exposed to the internet or to malicious users but no one has stumbled across it and tried to exploit it yet. Code like that may be ok for now simply because of the circumstances, but I don't think that's a pass to ignore UB or to discourage folks from trying to improve the status quo. Because some day either the developer of that code, who is used to ignoring this kind of thing, might write some code that has to be more correct or it will get exploited. Or the code itself might get changed, or exposed to attackers, or copied into some other program. These things happen and they are preventable. And you don't even have to switch languages - turning on sanitizers when you compile and fixing issues as they are found, and encouraging others to do the same, is better (in my opinion) than categorizing folks worrying about and working on this issue - in C and in other languages - as FUD spreaders.
- asddubs 6y ago
  yup, classic example: https://lwn.net/Articles/278137/ https://lwn.net/Articles/278137/
fbkr 6y ago
This is not correct, what you are thinking is implementation defined or maybe unspecified behavior. Please do explain the behavior of calling an uninitialized function pointer on x86 if you insist.
MaxBarraclough 6y ago
As jjnoakes and fbkr already commented, this account of undefined behaviour is not correct. You've conflated it with unspecified behavior and implementation-defined behavior, and they're not the same beast. [0] Undefined behaviour does not mean that the behaviour simply depends on your platform and is otherwise harmless. That's a widespread misconception, and a rather harmful one. You may see that a signed integer overflow appears to wrap harmlessly on your platform, but the compiler is under no obligation to consistently handle signed integer overflow in this way, unless its own documentation makes this guarantee. > people overreact to "undefined behavior" I have to agree with jjnoakes that people more commonly underreact to it. > If your software runs always on the same platform then you can know what the behavior would be. You cannot infer what non-standard guarantees your compiler is committed to. Unless the compiler guarantees to handle UB in a certain way, it is free to surprise you at any time. > It might also be intentional As mentioned above, intentional UB only makes sense if the compiler guarantees to give you the expected behaviour. If that's not the case, it's a bug. Even then, it's probably best to write portable standards-compliant code. Either way I'd hope to see a static-assert checking that the expected compiler is being used, loudly breaking the build if this check fails. > should not crash your program. This isn't the case. Dereferencing NULL may cause a segfault, terminating your process. Dividing by zero might cause a similar kind of explosion. Depending on platform, freeing a pointer twice might also immediately crash the program (the most helpful way for a platform to handle a double-free). > Undefined behavior is used when doing a specification for a compiler that will be used in many architectures and they cannot clearly specify what would happen on ALL architectures for a given operation, because the result might change among these architectures. Not always. The C rule about data-races being undefined behaviour, is intended to enable compiler optimisation, by permitting compilers to assume the absence of data-races. This rule would make sense even if C weren't intended to be portable across different hardware architectures. > Undefined behavior doesn't mean that your CPU has either a chance to suddenly blow away In principle, undefined behaviour could result in hardware damage, but this seems pretty unlikely unless you're writing power-management code for a motherboard, or something like that. Of course, if you're writing code for a medical system, industrial-control system, or avionics, then undefined behaviour might have terrible consequences. Self-awareness would be permitted by the C standard, but, again, rather unlikely. > You can actually predict what would happen if you throw away a rock on earth. You cannot predict it for every planet in the universe, so theoretically "throwing a rock" produces undefined behavior. I don't see much value in this analogy. It's possible to build a fully deterministic programming language, with no undefined behaviour, and no platform-specific variation. C isn't designed that way, but this is a property of C, not a general fact of computation. It's also possible to write C code entirely devoid of undefined behaviour. [0] https://stackoverflow.com/a/4105123/ https://stackoverflow.com/a/4105123/

by scoutt 6y ago

jjnoakes 6y ago

> If your software runs always on the same platform then you can know what the behavior would be. This is not necessarily true, and I consider it to be a harmful misconception. I think people under-react to "undefined behavior". What you think the platform would do if you overflowed an integer using machine instructions and what your C or C++ code ends up executing can be wildly different things because C and C++ compilers do everything in their power to optimize your code assuming your signed integers will never overflow. If they do, who knows what state your program is in or which machine instructions the compiler would have scheduled.

scoutt 6y ago
You are right. I might correct the sentence with "you can often know". Undefined behavior should be avoided but really is not that terrible.
- jjnoakes 6y ago
  We will have to agree to disagree. Fully understanding what a program might do if there is undefined behavior is not trivial. It requires basically reviewing and fully understanding the output assembler for every single build. Any change in system headers, compiler version, compiler flags, or source code might invalidate previous reviews. And programs with lurking undefined behavior may not seem dangerous but sufficiently clever exploits can do quite amazing things with the smallest of attack surfaces.
  - scoutt 6y ago
    Sure but in 17 years writing C for a living, with lines of code by the millions, and (embedded) devices running my code all over the world I still have to know about an integer overflow bug. Sure, I don't always write bug-free code (as anyone), but when I say it's not so terrible I mean that UD should be considered a rare case, and it's not worth even a single night of quiet sleep, let alone switching languages or overreacting. Hey, I am more afraid of a flipping bit due to power supply noises. And I don't want to say either that my code is perfect, or since it never happened to me it will never happen to you and that you can go around triggering UD everywhere since it's not a big deal. But I also like to warn young players that "boooo UD!" is just part of a FUD tactic for them to adopt some shiny new language.
    - pjmlp 6y ago
      Would you feel the same way when we finally manage to get liability laws for security exploits, like we are now discussing in Germany?
      - scoutt 6y ago
        If some of my systems fail, people might die. Should I still be worried for a liability?
        pjmlp 6y ago
        As an professional Engineer I would be.
    - jjnoakes 6y ago
      There is a ton of software out there that is riddled with bugs. Much of it is run behind closed doors, isn't exposed to attack vectors like the internet, and happily churns along because no one is trying to attack it. Lots of other software might be exposed to the internet or to malicious users but no one has stumbled across it and tried to exploit it yet. Code like that may be ok for now simply because of the circumstances, but I don't think that's a pass to ignore UB or to discourage folks from trying to improve the status quo. Because some day either the developer of that code, who is used to ignoring this kind of thing, might write some code that has to be more correct or it will get exploited. Or the code itself might get changed, or exposed to attackers, or copied into some other program. These things happen and they are preventable. And you don't even have to switch languages - turning on sanitizers when you compile and fixing issues as they are found, and encouraging others to do the same, is better (in my opinion) than categorizing folks worrying about and working on this issue - in C and in other languages - as FUD spreaders.
asddubs 6y ago
yup, classic example: https://lwn.net/Articles/278137/ https://lwn.net/Articles/278137/

fbkr 6y ago

This is not correct, what you are thinking is implementation defined or maybe unspecified behavior. Please do explain the behavior of calling an uninitialized function pointer on x86 if you insist.

MaxBarraclough 6y ago

As jjnoakes and fbkr already commented, this account of undefined behaviour is not correct. You've conflated it with unspecified behavior and implementation-defined behavior, and they're not the same beast. [0] Undefined behaviour does not mean that the behaviour simply depends on your platform and is otherwise harmless. That's a widespread misconception, and a rather harmful one. You may see that a signed integer overflow appears to wrap harmlessly on your platform, but the compiler is under no obligation to consistently handle signed integer overflow in this way, unless its own documentation makes this guarantee. > people overreact to "undefined behavior" I have to agree with jjnoakes that people more commonly underreact to it. > If your software runs always on the same platform then you can know what the behavior would be. You cannot infer what non-standard guarantees your compiler is committed to. Unless the compiler guarantees to handle UB in a certain way, it is free to surprise you at any time. > It might also be intentional As mentioned above, intentional UB only makes sense if the compiler guarantees to give you the expected behaviour. If that's not the case, it's a bug. Even then, it's probably best to write portable standards-compliant code. Either way I'd hope to see a static-assert checking that the expected compiler is being used, loudly breaking the build if this check fails. > should not crash your program. This isn't the case. Dereferencing NULL may cause a segfault, terminating your process. Dividing by zero might cause a similar kind of explosion. Depending on platform, freeing a pointer twice might also immediately crash the program (the most helpful way for a platform to handle a double-free). > Undefined behavior is used when doing a specification for a compiler that will be used in many architectures and they cannot clearly specify what would happen on ALL architectures for a given operation, because the result might change among these architectures. Not always. The C rule about data-races being undefined behaviour, is intended to enable compiler optimisation, by permitting compilers to assume the absence of data-races. This rule would make sense even if C weren't intended to be portable across different hardware architectures. > Undefined behavior doesn't mean that your CPU has either a chance to suddenly blow away In principle, undefined behaviour could result in hardware damage, but this seems pretty unlikely unless you're writing power-management code for a motherboard, or something like that. Of course, if you're writing code for a medical system, industrial-control system, or avionics, then undefined behaviour might have terrible consequences. Self-awareness would be permitted by the C standard, but, again, rather unlikely. > You can actually predict what would happen if you throw away a rock on earth. You cannot predict it for every planet in the universe, so theoretically "throwing a rock" produces undefined behavior. I don't see much value in this analogy. It's possible to build a fully deterministic programming language, with no undefined behaviour, and no platform-specific variation. C isn't designed that way, but this is a property of C, not a general fact of computation. It's also possible to write C code entirely devoid of undefined behaviour. [0] https://stackoverflow.com/a/4105123/ https://stackoverflow.com/a/4105123/