undefined | Better HN

0 pointsamluto24d ago0 comments

Lots of reasons:

The code is not portable between architectures.

You can’t actually define your data structure. You can pretend with your compiler’s version of “pack” with regrettable results.

You probably have multiple kinds of undefined behavior.

Dealing with compatibility between versions of your software is awkward at best.

You might not even get amazing performance. mmap is not a panacea. Page faults and TLB flushing are not free.

You can’t use any sort of advanced data types — you get exactly what C gives you.

Forget about enforcing any sort of invariant at the language level.

0 comments

Negitivefrags24d ago

I've written a lot of code using that method, and never had any portability issues. You use types with number of bits in them.

Hell, I've slung C structs across the network between 3 CPU architectures. And I didn't even use htons!

Maybe it's not portable to some ancient architecture, but none that I have experienced.

If there is undefined behavior, it's certainly never been a problem either.

And I've seen a lot of talk about TLB shootdown, so I tried to reproduce those problems but even with over 32 threads, mmap was still faster than fread into memory in the tests I ran.

Look, obviously there are use cases for libraries like that, but a lot of the time you just need something simple, and writing some structs to disk can go a long way.

pjmlp24d ago

Some people also don't use protective gear when going downhill biking, it is a matter of feeling lucky.

benj11124d ago

On the other hand some people have things to ward off evil demons, and aren't bothered by evil demons.

The parent has actually done the thing, and found no issues, I don't think you can hand wave that away with a biased metaphor.

Otherwise you get 'Goto considered harmful' and people not using it even when it fits.

1 more reply

ddtaylor24d ago

C allows most of this, whereas C++ doesn't allow pointer aliasing without a compiler flag, tricks and problems.

I agree you can certainly just use bytes of the correct sizes, but often to get the coverage you need for the data structure you end up writing some form of wrapper or fixup code, which is still easier and gives you the control versus most of the protobuf like stuff that introduces a lot of complexity and tons of code.

nly24d ago

__attribute__((may_alias, packed)) right on the struct.

2 more replies

lionkor24d ago

That seems highly unlikely. Let's assume that all compilers use the exact same padding in C structs, that all architectures use the same alignment, and that endianness is made up, that types are the same size across 64 and 32 bit platforms, and also pretend that pointers inside a struct will work fine when sent across the network; the question remains still: Why? Is THIS your bottleneck? Will a couple memcpy() operations that are likely no-op if your structs happen to line up kill your perf?

zadikian23d ago

I guess to not have to set up protobuf or asn1. Those preconditions of both platforms using the same padding and endianness aren't that hard to satisfy if you own it all.

But do you really have such a complex struct where everything inside is fixed-size? I wouldn't be surprised if it happens, but this isn't so general-purpose like the article suggests.

1 more reply

direwolf2023d ago

"Portable" has originally meant "able to be ported" and not "is already ported"

j / k navigate · click thread line to collapse

0 pointsamluto24d ago0 comments

Lots of reasons:

The code is not portable between architectures.

You can’t actually define your data structure. You can pretend with your compiler’s version of “pack” with regrettable results.

You probably have multiple kinds of undefined behavior.

Dealing with compatibility between versions of your software is awkward at best.

You might not even get amazing performance. mmap is not a panacea. Page faults and TLB flushing are not free.

You can’t use any sort of advanced data types — you get exactly what C gives you.

Forget about enforcing any sort of invariant at the language level.

0 comments

Negitivefrags24d ago

I've written a lot of code using that method, and never had any portability issues. You use types with number of bits in them.

Hell, I've slung C structs across the network between 3 CPU architectures. And I didn't even use htons!

Maybe it's not portable to some ancient architecture, but none that I have experienced.

If there is undefined behavior, it's certainly never been a problem either.

And I've seen a lot of talk about TLB shootdown, so I tried to reproduce those problems but even with over 32 threads, mmap was still faster than fread into memory in the tests I ran.

Look, obviously there are use cases for libraries like that, but a lot of the time you just need something simple, and writing some structs to disk can go a long way.

pjmlp24d ago

Some people also don't use protective gear when going downhill biking, it is a matter of feeling lucky.

benj11124d ago

On the other hand some people have things to ward off evil demons, and aren't bothered by evil demons.

The parent has actually done the thing, and found no issues, I don't think you can hand wave that away with a biased metaphor.

Otherwise you get 'Goto considered harmful' and people not using it even when it fits.

1 more reply

ddtaylor24d ago

C allows most of this, whereas C++ doesn't allow pointer aliasing without a compiler flag, tricks and problems.

nly24d ago

__attribute__((may_alias, packed)) right on the struct.

2 more replies

lionkor24d ago

zadikian23d ago

I guess to not have to set up protobuf or asn1. Those preconditions of both platforms using the same padding and endianness aren't that hard to satisfy if you own it all.

But do you really have such a complex struct where everything inside is fixed-size? I wouldn't be surprised if it happens, but this isn't so general-purpose like the article suggests.

1 more reply

direwolf2023d ago

"Portable" has originally meant "able to be ported" and not "is already ported"

j / k navigate · click thread line to collapse