alt Hacker NewsLots 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.
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.