How do you guarantee that you won't get the same value from a PRNG without global state? A PRNG is a state-updating machine that spits out a value based on that state.
You don't need it global
Seed it once per thread from /dev/random and you are done
I'm saying that if you have one PRNG, then you have global state no matter how it's designed. This is true whether you write it so that you get decent statistics, or you write it so you get tons of duplicate values.
And many of the fixes remove global state. Per-thread PRNGs are one option, but so are PRNGs that are used by specific batches of objects.
So, the straightforward broken option has global state, and the non-broken options might or might not have global state.
Which means I have no idea what you're talking about when you use the phrase "need to have global state". What is the PRNG API where that need exists, and what does the version without global state look like?
The only way to make a PRNG that does not share state between its consumers is to shard per-consumer. The PRNG in libc doesn't do that, it just uses a lock internal to the PRNG to keep things safe.
You could attempt to make a concurrent PRNG that has atomic-sized state and use CAS loops when generating numbers to keep the consumers in sync, but that's probably worse than using a lock.
Right, but again why did you say it needs to have global state "if you want to get decent statistics"?
What is the alternative to having global state that doesn't have decent statistics?
That "if" is the part I'm confused about.
The part of your comment directly after that sounds like you're describing the situation where clients are data-racing the PRNG and generating repeat numbers and other problems. But that's a problem specifically because of global state being used incorrectly, not something that happens as an alternative to global state.
I was considering "fairly broken PRNG" as its own significant group, and those need state too if you treat them as separate from "this literally gives the same number every time". But if those all go in the same bucket, then the comparison you made makes sense.