Support threads in the new crt1-command.c ctor check. #339
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Use an atomic compare-and-swap for checking whether constructors have been run, when threads are enabled.
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cc: @haraldh, this might be helpful for you. |
libc-bottom-half/crt/crt1-command.c
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| // ensures that the `_start` function isn't started more than once. | ||
| static volatile int started = 0; | ||
| #ifdef _REENTRANT | ||
| if (__sync_val_compare_and_swap(&started, 0, 1) != 0) { |
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What about using C11 atomics here? (https://www.ibm.com/docs/en/zos/2.1.0?topic=c11-atomic-compare-exchange-strong). Maybe we don't want to depend on C11?
| __builtin_trap(); | ||
| } | ||
| #else | ||
| static volatile int started = 0; |
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No need for volatile here I think? (or above actually?).
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I've now added a comment about volatile. It's admittedly a very theoretical concern. But also, this variable is read and written to exactly once, so there's nothing to optimize about it. So I'm inclined to keep the volatile.
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Persisting an in-memory variable past the point of an _exit call isn't something that's even possible to think about in C, and yet that's what this code is doing, so volatile helps make sure that the compiler just does what we're telling it to do, because it won't understand what we're actually doing.
Use an atomic compare-and-swap for checking whether constructors have been run, when threads are enabled.
Use an atomic compare-and-swap for checking whether constructors have been run, when threads are enabled.