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> According to your definition, is there any language that doesn't have undefined behavior? After all, even the hardware can have flaws and undefined behavior,

> According to your definition, is there any language that doesn't have undefined behavior? After all, even the hardware can have flaws and undefined behavior, so to mix metaphors, it's all a house of cards built upon sand. That's correct, and it's not pedantic to point that out. It proves that undefined behavior is not a useful criteria for choosing which language to use. But even if we accept your more restricted definition of what constitutes undefined behavior, we still must accept that any language that is implemented in C could possibly exhibit any of C's undefined behavior. Therefore, writing a Python library in Python instead of C cannot help you avoid C's undefined behavior. It might prevent you from introducing more undefined behavior, but that's not the same as avoiding it entirely -- if the implementation is written in C, then that ship has sailed.

dalke 10y ago
I now realize I did not understand your original comment. Thank you for the clarification. I don't agree with how you use "undefined behavior", but it's appropriate for the OP's context.
- twblalock 10y ago
  I don't think my definition is different from the common one. I agree that undefined behavior is behavior that the spec for the language says is undefined. Pretty much every language has some undefined behavior. That undefined behavior can be invoked intentionally, or because of bugs. C has a lot of undefined behaviors. Even simple addition can result in an undefined behavior because int overflow is undefined. Any C program that takes numbers as input (via the shell, or FFI, or whatever) and adds them together can exhibit undefined behavior. Null pointer dereferences are also undefined, and pretty much any C program of reasonable complexity can encounter those, in the form of bugs. A bug in hardware can also cause these undefined behaviors to be invoked -- for example, a friend of mine who does embedded programming had some null pointer dereferences or something like that because the hardware did not set a value when he told it to set the value. Null pointer dereferences are undefined, and if they happen because of hardware bugs, that is a case of hardware causing undefined behaviors to happen. It would not be reasonable to say they were not undefined behavior simply because they were caused by hardware instead of programmer error. The fact that it happened at all is what matters, not what caused it. Even if a language does not have any undefined behaviors whatsoever, that's not the end of it. If a program in that language interacts with other programs that do have undefined behavior, such as C programs, it could potentially trigger or be affected by undefined behaviors in those other programs. If that happens, it's not reasonable to say that it was not undefined behavior simply because it happened in a piece of code in a different language. (So, if the result of running a Python program is affected by undefined behavior in the C implementation of Python, that program has undefined behavior -- even if the Python code does not. The undefined behavior is C's, not Python's -- but it happens because of running the Python code, which caused the C code to run.) In other words, any invocation of code that causes undefined behavior to happen -- whether it is intentional, or caused by a software bug, or even caused by a hardware bug -- counts.
  - dalke 10y ago
    Where I disagree with you is your inclusion of language implementation errors as a type of undefined behavior. Those are compliance errors. That is, if the spec allows it (as "undefined behavior") then it's not a bug. If the spec doesn't allow it, then there is a defined behavior and it is a bug. For example, a hardware bug is one that does not comply with the specification. Either the spec must be changed (perhaps allowing the existing behavior), or the bug fixed.
    - twblalock 10y ago
      Are you saying that when a C program, which happens to be an interpreter for another language, does something that results in undefined behavior according to the C spec, it doesn't count as undefined behavior? I suspect that is where we differ. It seems to me that if a C program does something that results in undefined behavior according to the C spec, it is undefined behavior period, regardless of any other factors, because the spec says it's undefined and that's all that matters.
      - dalke 10y ago
        No, I am not saying that. I'm saying that when the Python language specification prescribes a behavior, and the Python implementation in C has a different behavior because it does something which the C language specification considers undefined, then the Python implementation in C does not comply with the Python language specification. I'm saying that it's incorrect to say that non-compliant behavior with respect to the Python specification is the same thing as undefined behavior. It makes no sense to say something is undefined when the specification defines what is supposed to happen.
pjc50 10y ago
> That's correct, and it's not pedantic to point that out. It proves that undefined behavior is not a useful criteria for choosing which language to use No, it means that you've warped the definition of "undefined" to the point where it's no longer useful. It's entirely possible to have fully-defined languages where every possible string of symbols is either a valid program with single well-defined behaviour or not a valid program. And no, writing the implementation in C does not introduce undefined behaviour, although it may require a number of compile and run time checks to ensure that you're never invoking it.
- twblalock 10y ago
  > And no, writing the implementation in C does not introduce undefined behaviour, although it may require a number of compile and run time checks to ensure that you're never invoking it. This is a valid C program that contains an undefined behavior, from http://blog.regehr.org/archives/213 http://blog.regehr.org/archives/213: #include <limits.h> #include <stdio.h> int main (void) { printf ("%d\n", (INT_MAX+1) < 0); return 0; } It is a perfectly valid program. The outcome is undefined (integer overflow is considered undefined behavior in C by every authoritative source I've ever seen). GCC does not emit any warnings when this program is compiled, even with all warnings turned on. There certainly aren't any runtime checks for it. Any program that adds numbers which are passed into it can run into undefined operations. Here is another example: i = i++ + 1; The result of that operation is undefined. These examples demonstrate that undefined behavior is not something you "invoke" in a special way -- it's often the result of a mistake. It is not restricted to certain operations -- these examples use simple addition! The compiler and runtime checks can't help you avoid it in many cases -- you sometimes won't even get a compiler warning. Here is an example of a bug in the C implementation of Python that caused undefined behavior because of integer handling: https://bugs.python.org/issue23999 https://bugs.python.org/issue23999 It's basically impossible to avoid. Even if your code is perfect, the compiler might optimize it into something that might contain undefined behavior. I suppose that if you never did any math, or anything with strings, or any pointer dereferences of any kind, or any casting, or any recursion, and you made sure the compiler didn't try to optimize anything, you could end up with a C program that could not have undefined behavior. But it's clearly impossible to implement Python without doing all of those things.

dalke 10y ago

I now realize I did not understand your original comment. Thank you for the clarification. I don't agree with how you use "undefined behavior", but it's appropriate for the OP's context.

twblalock 10y ago
I don't think my definition is different from the common one. I agree that undefined behavior is behavior that the spec for the language says is undefined. Pretty much every language has some undefined behavior. That undefined behavior can be invoked intentionally, or because of bugs. C has a lot of undefined behaviors. Even simple addition can result in an undefined behavior because int overflow is undefined. Any C program that takes numbers as input (via the shell, or FFI, or whatever) and adds them together can exhibit undefined behavior. Null pointer dereferences are also undefined, and pretty much any C program of reasonable complexity can encounter those, in the form of bugs. A bug in hardware can also cause these undefined behaviors to be invoked -- for example, a friend of mine who does embedded programming had some null pointer dereferences or something like that because the hardware did not set a value when he told it to set the value. Null pointer dereferences are undefined, and if they happen because of hardware bugs, that is a case of hardware causing undefined behaviors to happen. It would not be reasonable to say they were not undefined behavior simply because they were caused by hardware instead of programmer error. The fact that it happened at all is what matters, not what caused it. Even if a language does not have any undefined behaviors whatsoever, that's not the end of it. If a program in that language interacts with other programs that do have undefined behavior, such as C programs, it could potentially trigger or be affected by undefined behaviors in those other programs. If that happens, it's not reasonable to say that it was not undefined behavior simply because it happened in a piece of code in a different language. (So, if the result of running a Python program is affected by undefined behavior in the C implementation of Python, that program has undefined behavior -- even if the Python code does not. The undefined behavior is C's, not Python's -- but it happens because of running the Python code, which caused the C code to run.) In other words, any invocation of code that causes undefined behavior to happen -- whether it is intentional, or caused by a software bug, or even caused by a hardware bug -- counts.
- dalke 10y ago
  Where I disagree with you is your inclusion of language implementation errors as a type of undefined behavior. Those are compliance errors. That is, if the spec allows it (as "undefined behavior") then it's not a bug. If the spec doesn't allow it, then there is a defined behavior and it is a bug. For example, a hardware bug is one that does not comply with the specification. Either the spec must be changed (perhaps allowing the existing behavior), or the bug fixed.
  - twblalock 10y ago
    Are you saying that when a C program, which happens to be an interpreter for another language, does something that results in undefined behavior according to the C spec, it doesn't count as undefined behavior? I suspect that is where we differ. It seems to me that if a C program does something that results in undefined behavior according to the C spec, it is undefined behavior period, regardless of any other factors, because the spec says it's undefined and that's all that matters.
    - dalke 10y ago
      No, I am not saying that. I'm saying that when the Python language specification prescribes a behavior, and the Python implementation in C has a different behavior because it does something which the C language specification considers undefined, then the Python implementation in C does not comply with the Python language specification. I'm saying that it's incorrect to say that non-compliant behavior with respect to the Python specification is the same thing as undefined behavior. It makes no sense to say something is undefined when the specification defines what is supposed to happen.

pjc50 10y ago

> That's correct, and it's not pedantic to point that out. It proves that undefined behavior is not a useful criteria for choosing which language to use No, it means that you've warped the definition of "undefined" to the point where it's no longer useful. It's entirely possible to have fully-defined languages where every possible string of symbols is either a valid program with single well-defined behaviour or not a valid program. And no, writing the implementation in C does not introduce undefined behaviour, although it may require a number of compile and run time checks to ensure that you're never invoking it.

twblalock 10y ago
> And no, writing the implementation in C does not introduce undefined behaviour, although it may require a number of compile and run time checks to ensure that you're never invoking it. This is a valid C program that contains an undefined behavior, from http://blog.regehr.org/archives/213 http://blog.regehr.org/archives/213: #include <limits.h> #include <stdio.h> int main (void) { printf ("%d\n", (INT_MAX+1) < 0); return 0; } It is a perfectly valid program. The outcome is undefined (integer overflow is considered undefined behavior in C by every authoritative source I've ever seen). GCC does not emit any warnings when this program is compiled, even with all warnings turned on. There certainly aren't any runtime checks for it. Any program that adds numbers which are passed into it can run into undefined operations. Here is another example: i = i++ + 1; The result of that operation is undefined. These examples demonstrate that undefined behavior is not something you "invoke" in a special way -- it's often the result of a mistake. It is not restricted to certain operations -- these examples use simple addition! The compiler and runtime checks can't help you avoid it in many cases -- you sometimes won't even get a compiler warning. Here is an example of a bug in the C implementation of Python that caused undefined behavior because of integer handling: https://bugs.python.org/issue23999 https://bugs.python.org/issue23999 It's basically impossible to avoid. Even if your code is perfect, the compiler might optimize it into something that might contain undefined behavior. I suppose that if you never did any math, or anything with strings, or any pointer dereferences of any kind, or any casting, or any recursion, and you made sure the compiler didn't try to optimize anything, you could end up with a C program that could not have undefined behavior. But it's clearly impossible to implement Python without doing all of those things.