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by lmm 718 days ago
> surely it doesn't touch any global (or per-thread) state

Not necessarily. An implementation might choose to e.g. use some kind of cache similar to what the JVM does with interned strings, and then a function like strcmp() might behave badly if it happened to run while that cache was halfway through being rebuilt.
2 comments
lilyball 717 days ago
A function like strcmp() cannot assume that if it sees the same pointer multiple times that this pointer contains the same data, so there's no opportunity for doing any sort of caching of results. The JVM has a lot more flexibility here in that it's working with objects, not raw pointers to arbitrary memory.
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lmm 717 days ago
> A function like strcmp() cannot assume that if it sees the same pointer multiple times that this pointer contains the same data

For arbitrary pointers no. But it could special-case e.g. string constants in the source code and/or pointers returned by some intern function (which is also how the JVM does it - for arbitrary strings, even though they're objects, it's always possible that the object has been GCed and another string allocated at the same location).

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SomeoneFromCA 718 days ago
Contrived example, never seen in reality.
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jcul 718 days ago
You could be surprised.

For example, recently I wanted to call `gettid()` in a signal handler. Which I guessed was just a simple wrapper around the syscall.

However, it seems this can cache the thread ID in thread local storage (can't remember exact details).

I switched to making a syscall instead.

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SomeoneFromCA 717 days ago
Well, this is possible for sure, but not for primitive functions, such as strcmp. It just does not happen in practice.
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jcul 716 days ago
For functions like strcmp, I think they must be signal safe, to be POSIX compliant.

https://man7.org/linux/man-pages/man7/signal-safety.7.html

If it's on this list I generally trust it is safe.

I guess my point is, that if it's not, even a simple function may appear safe, but could do surprising things.

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lmm 717 days ago
The JVM does it in reality, I can't see why a C runtime wouldn't.
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SomeoneFromCA 717 days ago
It in theory may, but no one does.
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