5 ms·

Undefined behavior is not a compiler specific guarantee. UB can change based on almost random factors, especially between newer releases of the same compiler. T

Undefined behavior is not a compiler specific guarantee. UB can change based on almost random factors, especially between newer releases of the same compiler. They are bugs, they were just masked.

anfilt 8y ago
This honestly depends on what undefined behavior we are talking about. Sometimes it will be guaranteed to behave a certain way on a compiler. A few will also be the same across compilers if your compiling for the same architecture. However, I do agree that cross compiling is good for finding bugs like this. And really if we are letting the compiler or architecture define undefined behavior, I find it better to break out the inline assembly. It's explicit that this code is platform dependent, and avoids any issues that a subtle change in the future may cause it to break. Although, it's usually possible to define what your attempting in C without issue, and I only ever find I am doing such a thing if there is a good reason to use a platform specific feature. Generally, relying on how compiler handles uninitialized memory and similar is not what I call a compelling platform specific feature. Cross compiling is good in the regard because it forces everyone working on a project to avoid those things.
- steveklabnik 8y ago
  > This honestly depends on what undefined behavior we are talking about. Sometimes it will be guaranteed to behave a certain way on a compiler. That is implementation defined, not undefined, behavior.
  - anfilt 8y ago
    Undefined as per the spec.
  - anfilt 8y ago
    Undefined as per the spec. Does not mean it does not have a certain behavior on a given implementation. The spec also does mention implemtation defined behavior. However, undefined things still need to be handled.
    - doughboy23 8y ago
      Not really. Undefined means that no purposeful explicit behavior for handling has to occur even within a specific implementation, which means things can blow up randomly just changing some compiler settings or minor things in the environment (or even randomly at runtime).. eg running out of bounds of an array in C is a perfect example of undefined behavior.. no guarantee on what occurs from run to run. Yes obviously time doesn’t stop dead and something happens, but I think that stretches any meaningful definition of “handled”. True, undefined behavior can be implementation defined but that is not a requirement, and it usually is not.
  - atq2119 8y ago
    That's at least unnecessarily splitting hairs and possibly missing the point, considering that some compilers allow you to turn undefined behaviour into implementation-defined behaviour using an option. -fwrapv comes to mind.
mehrdadn 8y ago
If the compiler defines a behavior for some UB, then it's no longer UB. It's been defined for your implementation. It might still be undefined for another implementation but that doesn't mean your code is buggy on the first one.
- pertymcpert 8y ago
  No, it does not. It's still UB. UB is defined by the standard, not by your compiler's implementation. Certain behaviors may be implementation defined by the standard, those can be defined by your compiler. But if the standard says it's UB, it's UB. End of story.
  - mehrdadn 8y ago
    Where/how do you obtain such confidence in something so wrong? The standard not only doesn't prohibit the implementation from defining something it leaves undefined (surely you don't think even possible behavior becomes invalid as soon as it is documented??), it in fact explicitly permits this possibility to occur -- I suppose to emphasize just how nuts the notion of some kind of 'enforced unpredictability' is: > Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner...

Undefined behavior is not a compiler specific guarantee. UB can change based on almost random factors, especially between newer releases of the same compiler. They are bugs, they were just masked.

anfilt 8y ago

This honestly depends on what undefined behavior we are talking about. Sometimes it will be guaranteed to behave a certain way on a compiler. A few will also be the same across compilers if your compiling for the same architecture. However, I do agree that cross compiling is good for finding bugs like this. And really if we are letting the compiler or architecture define undefined behavior, I find it better to break out the inline assembly. It's explicit that this code is platform dependent, and avoids any issues that a subtle change in the future may cause it to break. Although, it's usually possible to define what your attempting in C without issue, and I only ever find I am doing such a thing if there is a good reason to use a platform specific feature. Generally, relying on how compiler handles uninitialized memory and similar is not what I call a compelling platform specific feature. Cross compiling is good in the regard because it forces everyone working on a project to avoid those things.

steveklabnik 8y ago
> This honestly depends on what undefined behavior we are talking about. Sometimes it will be guaranteed to behave a certain way on a compiler. That is implementation defined, not undefined, behavior.
- anfilt 8y ago
  Undefined as per the spec.
- anfilt 8y ago
  Undefined as per the spec. Does not mean it does not have a certain behavior on a given implementation. The spec also does mention implemtation defined behavior. However, undefined things still need to be handled.
  - doughboy23 8y ago
    Not really. Undefined means that no purposeful explicit behavior for handling has to occur even within a specific implementation, which means things can blow up randomly just changing some compiler settings or minor things in the environment (or even randomly at runtime).. eg running out of bounds of an array in C is a perfect example of undefined behavior.. no guarantee on what occurs from run to run. Yes obviously time doesn’t stop dead and something happens, but I think that stretches any meaningful definition of “handled”. True, undefined behavior can be implementation defined but that is not a requirement, and it usually is not.
- atq2119 8y ago
  That's at least unnecessarily splitting hairs and possibly missing the point, considering that some compilers allow you to turn undefined behaviour into implementation-defined behaviour using an option. -fwrapv comes to mind.

mehrdadn 8y ago

If the compiler defines a behavior for some UB, then it's no longer UB. It's been defined for your implementation. It might still be undefined for another implementation but that doesn't mean your code is buggy on the first one.

pertymcpert 8y ago
No, it does not. It's still UB. UB is defined by the standard, not by your compiler's implementation. Certain behaviors may be implementation defined by the standard, those can be defined by your compiler. But if the standard says it's UB, it's UB. End of story.
- mehrdadn 8y ago
  Where/how do you obtain such confidence in something so wrong? The standard not only doesn't prohibit the implementation from defining something it leaves undefined (surely you don't think even possible behavior becomes invalid as soon as it is documented??), it in fact explicitly permits this possibility to occur -- I suppose to emphasize just how nuts the notion of some kind of 'enforced unpredictability' is: > Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner...