undefined | Better HN

0 points10000truths4y ago0 comments

I don't see any undefined behavior here. As I mentioned below, gcc explicitly documents type punning via unions as being well defined. But yes, this is compiler specific and is not guaranteed to work elsewhere.

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formerly_proven4y ago

Accessing packed struct members works fine on x86, but will blow up at runtime or do weird things on platforms which don't support unaligned loads or stores.

The correct way to access packed structs is through memcpy, just like you'd access any other potentially unaligned object.

10000truthsOP4y ago

For architectures where unaligned accesses are illegal, gcc will generate multiple load/store instructions when accessing packed struct fields by name. The main caveat to look out for is taking the address of a packed struct member and then dereferencing it.

formerly_proven4y ago

Ah, right. Thanks for the correction.

midjji4y ago

There is absolutely undefined behaviour there. Undefined behaviour is defined not as nasal daemons but as: The compiler implementer does not guarantee that this behaviour will be hardware, circumstance, compiler version, or os consistent, nor that we will warn if we change this.

Packed is technically not a undefined behaviour, but it is certainly a trap. Especially because the compiler macros leads people to make defines which select packed by compiler automatically. Then the special case of didn't recognize compiler is just left empty, meaning compiles but no longer does what you think.

fifjdynb4y ago

You don't get to decide what UB means. It really does mean nasal demons are a possibility: all bets are off when you run that executable. Use of the term "undefined behaviour" to mean something else may be on the increase, unfortunately (https://mars.nasa.gov/technology/helicopter/status/298/what-...), but if we're talking about C, it's meaning is fixed.

midjji4y ago

I didnt, the deterministic nature of computer programs guarantees that what I wrote is the actual outcome.

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formerly_proven4y ago

Accessing packed struct members works fine on x86, but will blow up at runtime or do weird things on platforms which don't support unaligned loads or stores.

The correct way to access packed structs is through memcpy, just like you'd access any other potentially unaligned object.

10000truthsOP4y ago

formerly_proven4y ago

Ah, right. Thanks for the correction.

midjji4y ago

fifjdynb4y ago

midjji4y ago

I didnt, the deterministic nature of computer programs guarantees that what I wrote is the actual outcome.

j / k navigate · click thread line to collapse