4 ms·

What do you think UB is if not something where the behaviour is not defined?

by hvdijk 5y ago

tsimionescu 5y ago
UB, in this context, is very explicitly used in the standard: it is undefined behavior related to a construct that the standard describes.
- hvdijk 5y ago
  > behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements That is literally the definition of UB from the C standard. It is explicitly also about constructs that the standard does not describe. That makes sense: the standard does not and cannot define the behaviour for any construct not in the standard, so cannot impose any requirements for such constructs, and that is all UB is: something where the standard imposes no requirements.
  - tsimionescu 5y ago
    The relevant discussion about UB is restricted to constructa that the standard describes. For example, writing past the end of object is UB - the construct is described in the standard, but is given no semantics by the standard. The standard does not describe pattern matching, so using pattern matching is also undefined behavior, but there is nothing to be talked about here.
    - hvdijk 5y ago
      The comment I replied to did talk about something not described by the standard though, namely syscalls. If you want to argue that we should not be talking about syscalls here, your issue should be with the original comment that brought them up (https://news.ycombinator.com/item?id=27222325 https://news.ycombinator.com/item?id=27222325), not with my reply, I think. However, that comment looks perfectly fine to me. Also, depending on how the syscalls are made, it actually may be explicitly described as UB by the standard, see my comment https://news.ycombinator.com/item?id=27228701 https://news.ycombinator.com/item?id=27228701 too.
      - tsimionescu 5y ago
        Syscalls are not any more UB than any other function call, though. Whether talking about write(2) or my_foo(), the call has the semantics given by the function signature visible in the current translation unit. Sure, the C standard doesn't define what write(2)'s effects will be, but that does not mean that calling it is UB according to the standard. If the function has not been declared by the time it is first used, even then calling it is not UB - it is defined to be a compilation error (in versions earlier than C99 it was actually valid, but UB if the call did not match the actual function definition).
        hvdijk 5y ago
        > Sure, the C standard doesn't define what write(2)'s effects will be, but that does not mean that calling it is UB according to the standard. Yes, it does. I already explained exactly why it needs to be UB, but let me quote where the standard says so: C99 6.9 External definitions: > Semantics: > An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one. If your program provides a declaration of write() and uses it without also providing a definition, the program does not have "exactly one external definition for the identifier", it has zero definitions for the identifier. This violates a "shall" that appears outside of a constraint, for which we turn to: C99 4 Conformance: > If a "shall" or "shall not" requirement that appears outside of a constraint is violated, the behavior is undefined.
        aw1621107 5y ago
        > let me quote where the standard says so: Wouldn't this hinge on what precisely "entire program" means? A definition for write(2) may not appear in the source code you wrote, but if "entire program" includes e.g., libraries dynamically linked in then it's quite feasible for the end result to be fully defined. For example, 5.2.2 Paragraph 2 starts with (emphasis added): > In the set of translation units and libraries that constitutes an entire program
Google234 5y ago
So is everything UB since all hardware isn’t perfect?
hvdijk 5y ago
About your edit: > "ISO C does not say anything about write(2), hence using write(2) is undefined behavior according to the C standard", which seems like a rather tautological and useless statement to me. That is actually not so useless at all: if you try to compile and link a program that declares and calls a function but does not define it, you will typically get a linker error about an unresolved reference. If the name matches a non-ISO C library function, however, the implementation cannot know whether your program is in error or whether you want to use that library function, and will usually accept it. For this reason, the C standard does actually make it clear that using write(2) is UB to make it clear that implementations are not required to diagnose that as an error.

What do you think UB is if not something where the behaviour is not defined?

by hvdijk 5y ago

What do you think UB is if not something where the behaviour is not defined?

UB, in this context, is very explicitly used in the standard: it is undefined behavior related to a construct that the standard describes.

hvdijk 5y ago
> behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements That is literally the definition of UB from the C standard. It is explicitly also about constructs that the standard does not describe. That makes sense: the standard does not and cannot define the behaviour for any construct not in the standard, so cannot impose any requirements for such constructs, and that is all UB is: something where the standard imposes no requirements.
- tsimionescu 5y ago
  The relevant discussion about UB is restricted to constructa that the standard describes. For example, writing past the end of object is UB - the construct is described in the standard, but is given no semantics by the standard. The standard does not describe pattern matching, so using pattern matching is also undefined behavior, but there is nothing to be talked about here.
  - hvdijk 5y ago
    The comment I replied to did talk about something not described by the standard though, namely syscalls. If you want to argue that we should not be talking about syscalls here, your issue should be with the original comment that brought them up (https://news.ycombinator.com/item?id=27222325 https://news.ycombinator.com/item?id=27222325), not with my reply, I think. However, that comment looks perfectly fine to me. Also, depending on how the syscalls are made, it actually may be explicitly described as UB by the standard, see my comment https://news.ycombinator.com/item?id=27228701 https://news.ycombinator.com/item?id=27228701 too.
    - tsimionescu 5y ago
      Syscalls are not any more UB than any other function call, though. Whether talking about write(2) or my_foo(), the call has the semantics given by the function signature visible in the current translation unit. Sure, the C standard doesn't define what write(2)'s effects will be, but that does not mean that calling it is UB according to the standard. If the function has not been declared by the time it is first used, even then calling it is not UB - it is defined to be a compilation error (in versions earlier than C99 it was actually valid, but UB if the call did not match the actual function definition).
      - hvdijk 5y ago
        > Sure, the C standard doesn't define what write(2)'s effects will be, but that does not mean that calling it is UB according to the standard. Yes, it does. I already explained exactly why it needs to be UB, but let me quote where the standard says so: C99 6.9 External definitions: > Semantics: > An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one. If your program provides a declaration of write() and uses it without also providing a definition, the program does not have "exactly one external definition for the identifier", it has zero definitions for the identifier. This violates a "shall" that appears outside of a constraint, for which we turn to: C99 4 Conformance: > If a "shall" or "shall not" requirement that appears outside of a constraint is violated, the behavior is undefined.
        aw1621107 5y ago
        > let me quote where the standard says so: Wouldn't this hinge on what precisely "entire program" means? A definition for write(2) may not appear in the source code you wrote, but if "entire program" includes e.g., libraries dynamically linked in then it's quite feasible for the end result to be fully defined. For example, 5.2.2 Paragraph 2 starts with (emphasis added): > In the set of translation units and libraries that constitutes an entire program

Google234 5y ago

So is everything UB since all hardware isn’t perfect?

hvdijk 5y ago

About your edit: > "ISO C does not say anything about write(2), hence using write(2) is undefined behavior according to the C standard", which seems like a rather tautological and useless statement to me. That is actually not so useless at all: if you try to compile and link a program that declares and calls a function but does not define it, you will typically get a linker error about an unresolved reference. If the name matches a non-ISO C library function, however, the implementation cannot know whether your program is in error or whether you want to use that library function, and will usually accept it. For this reason, the C standard does actually make it clear that using write(2) is UB to make it clear that implementations are not required to diagnose that as an error.