[swift-corelibs-dev] libdispatch import order / nondeterminism
phausler at apple.com
Wed Feb 10 20:08:04 CST 2016
> On Feb 10, 2016, at 6:05 PM, Drew Crawford via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
> I have built the Feb 8th snapshot into /usr/local on Linux 64. I have enabled libdispatch, and it's installed to /usr/local as well. Additionally, I cherry-picked the PR https://github.com/apple/swift/pull/1212 <https://github.com/apple/swift/pull/1212>. It compiles, installs, everything's fine.
> My /usr/local/include/dispatch directory has
> # ls /usr/local/include/dispatch/
> Dispatch.swiftdoc base.h data.h group.h io.h object.h queue.h source.h
> Dispatch.swiftmodule block.h dispatch.h introspection.h module.map once.h semaphore.h time.h
> Now I attempt to use Dispatch via
> swift -Xcc -fblocks -L/usr/local/lib -I /usr/local/include/dispatch -lBlocksRuntime -L/usr/lib/x86_64-linux-gnu
> This is because:
> -Xcc -fblocks to pick up blocks support
> -I /usr/local/include/dispatch to pick up /usr/local/include/dispatch/Dispatch.swiftmodule (as well as module.modulemap etc.)
> -lBlocksRuntime -L/usr/lib/x86_64-linux-gnu to pick up blocks runtime
> Okay. So now I import Dispatch:
> Welcome to Swift version 3.0-dev (LLVM a7663bb722, Clang 4ca3c7fa28, Swift 319a912638). Type :help for assistance.
> 1> import Dispatch
> What I find is about half the time, this works just fine. The other half the time, I hit an import error:
>> <module-includes>:1:10: note: in file included from <module-includes>:1:
>> #include "/usr/local/include/dispatch/dispatch.h"
>> /usr/local/include/dispatch/dispatch.h:34:10: note: in file included from /usr/local/include/dispatch/dispatch.h:34:
>> #include <fcntl.h>
>> /usr/include/fcntl.h:67:11: note: in file included from /usr/include/fcntl.h:67:
>> # include <time.h>
>> /usr/local/include/dispatch/time.h:25:2: error: "Please #include <dispatch/dispatch.h> instead of this file directly."
>> #error "Please #include <dispatch/dispatch.h> instead of this file directly."
> What seems to be happening here is that we interpret /usr/include/fcntl.h's #include <time.h> to be /usr/local/include/dispatch/time.h instead of /usr/include/time.h.
> This behavior is puzzling to me.
> I don't understand why the behavior is nondeterministic. Shouldn't an import failure be deterministic behavior? Or are we just trying imports in random order? That seems like a bad implementation.
> I don't understand how to make sure fcntl's `time.h` goes to the right time.h without also losing Dispatch.swiftmodule and module.modulemap in the exchange.
The -I/usr/local/include/dispatch probably sees time.h in there and thinks that is part of a system root header? Just a speculation.
> Thanks in advance.
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