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You wrote: > 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/

by dalke 10y ago

You wrote: > 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 The comments in that bug report suggest it was a false positive in Coverity. In any case, the Python language defines how left and right shift are supposed to work. https://docs.python.org/3/reference/expressions.html?highlight=shift#shifting-operations https://docs.python.org/3/reference/expressions.html?highlig... . What you pointed to, if it were a true positive, would be an example of where the implementation didn't comply with the specification. It wouldn't be an example of undefined behavior as given at https://en.wikipedia.org/wiki/Undefined_behavior https://en.wikipedia.org/wiki/Undefined_behavior : "undefined behavior (UB) is the result of executing computer code that does not have a prescribed behavior by the language specification" because Python prescribes that behavior.

twblalock 10y ago
> It wouldn't be an example of undefined behavior as given at https://en.wikipedia.org/wiki/Undefined_behavior https://en.wikipedia.org/wiki/Undefined_behavior : "undefined behavior (UB) is the result of executing computer code that does not have a prescribed behavior by the language specification" because Python prescribes that behavior. But the C spec doesn't prescribe any behavior in this case, and the code that is running in this case is C, not Python! It was written in C, it was compiled by a C compiler -- it's C! Whether or not the behavior is undefined for anything a C program does is determined by the C spec only. The Python spec is not relevant. Any C program that overflows an int has done something undefined, whether or not that C program happens to be a Python implementation.
- dalke 10y ago
  The C specification is irrelevant. The Python specification defines what the implementation is supposed to do. It happens that the implementation doesn't comply with the specification. That doesn't mean the Python specification is undefined, it means the implementation is in error. Remember too that the actual implementation is a binary. The C compiler converted the C code into that binary, but in theory it could have been generated manually, as a byte-for-byte equivalent, with C completely out of the picture. Would you still say it's "undefined behavior" if there were no C compiler? How do you tell the difference in the binaries?
  - twblalock 10y ago
    By that logic, you couldn't say anything is ever undefined when it happens in binary machine code, or what spec should apply -- you couldn't say it is C or that it is Python. That level of reductionism doesn't get you anywhere.
    - dalke 10y ago
      Do you not realize that that reductionism is exactly why I don't like your definition of "undefined behavior"? You are using a non-standard definition of "undefined behavior". In common use, "undefined behavior" is only meaningful relative to a language specification, not an implementation. Why do you think you are using the common definition when you include implementation bugs as part of UB?
      - twblalock 10y ago
        Your definition leads to a serious contradiction: Case 1: A C program overflows an int (which is listed as an undefined behavior in the C spec). Under your definition, and pretty much any other, that program has executed code that results in undefined behavior. Case 2: A C program overflows an int. That C program happens to be the Python interpreter. Under your definition, based on your responses to previous comments, that did not result in undefined behavior, but was an implementation bug instead. There is no difference between case 1 and case 2. They are both C programs, so the spec for C determines what is undefined behavior for them. They both did the same thing. Either both are executing code that results in undefined behavior, or neither is. Case 2 is also an example of an implementation bug if it affects the result of the Python code running in the interpreter.
        dalke 10y ago
        There is no contradiction. There are two specifications, the C language specification and the Python language specification. Programs written in Python follow the Python language specification. When run in CPython, the implementation follows the C language specification. If the implementation uses C UB, which causes it to be out of compliance with the Python specification, then it is both undefined behavior for C and a failure to follow defined behavior for Python. It is not undefined behavior for Python. I have said multiple times that I don't like how your non-standard definition mixed the two together. It is no longer interesting to come up with new ways to restate my statement.

by dalke 10y ago

twblalock 10y ago

> It wouldn't be an example of undefined behavior as given at https://en.wikipedia.org/wiki/Undefined_behavior https://en.wikipedia.org/wiki/Undefined_behavior : "undefined behavior (UB) is the result of executing computer code that does not have a prescribed behavior by the language specification" because Python prescribes that behavior. But the C spec doesn't prescribe any behavior in this case, and the code that is running in this case is C, not Python! It was written in C, it was compiled by a C compiler -- it's C! Whether or not the behavior is undefined for anything a C program does is determined by the C spec only. The Python spec is not relevant. Any C program that overflows an int has done something undefined, whether or not that C program happens to be a Python implementation.

dalke 10y ago
The C specification is irrelevant. The Python specification defines what the implementation is supposed to do. It happens that the implementation doesn't comply with the specification. That doesn't mean the Python specification is undefined, it means the implementation is in error. Remember too that the actual implementation is a binary. The C compiler converted the C code into that binary, but in theory it could have been generated manually, as a byte-for-byte equivalent, with C completely out of the picture. Would you still say it's "undefined behavior" if there were no C compiler? How do you tell the difference in the binaries?
- twblalock 10y ago
  By that logic, you couldn't say anything is ever undefined when it happens in binary machine code, or what spec should apply -- you couldn't say it is C or that it is Python. That level of reductionism doesn't get you anywhere.
  - dalke 10y ago
    Do you not realize that that reductionism is exactly why I don't like your definition of "undefined behavior"? You are using a non-standard definition of "undefined behavior". In common use, "undefined behavior" is only meaningful relative to a language specification, not an implementation. Why do you think you are using the common definition when you include implementation bugs as part of UB?
    - twblalock 10y ago
      Your definition leads to a serious contradiction: Case 1: A C program overflows an int (which is listed as an undefined behavior in the C spec). Under your definition, and pretty much any other, that program has executed code that results in undefined behavior. Case 2: A C program overflows an int. That C program happens to be the Python interpreter. Under your definition, based on your responses to previous comments, that did not result in undefined behavior, but was an implementation bug instead. There is no difference between case 1 and case 2. They are both C programs, so the spec for C determines what is undefined behavior for them. They both did the same thing. Either both are executing code that results in undefined behavior, or neither is. Case 2 is also an example of an implementation bug if it affects the result of the Python code running in the interpreter.
      - dalke 10y ago
        There is no contradiction. There are two specifications, the C language specification and the Python language specification. Programs written in Python follow the Python language specification. When run in CPython, the implementation follows the C language specification. If the implementation uses C UB, which causes it to be out of compliance with the Python specification, then it is both undefined behavior for C and a failure to follow defined behavior for Python. It is not undefined behavior for Python. I have said multiple times that I don't like how your non-standard definition mixed the two together. It is no longer interesting to come up with new ways to restate my statement.