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by Farmadupe 873 days ago
The author is making a deliberate point about undefined behaviour in the article. Hence them not executing worked examples.

In fact, by not doing so they are making a subtle implicit statement that it is uninteresting to consider actually attempting to execute these snippets.

The third paragraph of the "P.S" of the article (you have to press submit to see it) is the one that really gives the game away.
2 comments
ryao 873 days ago
Most of these things are implementation defined rather than undefined. Only the 5th is undefined.
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1000100_1000101 873 days ago
More than implementation defined, for some you need context that simply isn't given. On the ones with mixed-type structs, even if you know what system it's compiled for you don't know if someone has used pragma pack 1 to byte pack the data instead of standard packing. Just seeing the struct, you still don't know.
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ryao 873 days ago
Good point, although that is not part of standard C.
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ksherlock 873 days ago
'#pragma pack' isn't part of the C standard, but #pragma is and "causes the implementation to behave in an implementation-defined manner."
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LatticeAnimal 873 days ago
I agree that in theory it would be cool to have C code that uses only defined behavior and works on all platforms for all eternity. However, I think most programs have a fairly clear understanding of what platforms (OS+arch) they are targeting and what compilers they are using to target those platforms.

If the compiler has defined behavior (and you have unit tests for that behavior) on all of these platforms, I don't think it is a huge deal. (Ideally you wouldn't... but sometimes its an accident or unavoidable)

As an example, while struct padding (problem 1) might not technically be in the spec, it is a cornerstone of FFI and every new compiler (that supports C FFI) has a way to compile structs with the same padding.

To my original point, if the article had instead given examples of compilers + architectures that produced different answers, I might feel differently. However, just saying mentioning that these weird edge cases are undefined (in the spec) doesn't mean much to me.

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