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Declare Windows imports jobserver depends on #106

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Sep 17, 2024
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Declare Windows imports jobserver depends on #106

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5 changes: 5 additions & 0 deletions src/windows.rs
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Thanks for the patch!

As a bonus this is mildly more efficient.

Out of curiosity, how does it get more efficient with this change?

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@ChrisDenton ChrisDenton Sep 17, 2024 •
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I wrote a bit about something similar on zulip: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Link.20attribute.20on.20Windows/near/470532686

It's a super minor thing and almost certainly doesn't affect the performance of jobserver. In short, an extern block defaults to static linkage whereas #[link] defaults to dynamic linkage. Since this function is from a DLL we want dynamic linkage because the location of a function is stored at an address in the import table rather than a static function you can call directly.

Static linkage wouldn't work at all except import libraries add a "jump stub". That is a kinda of dummy function that just jumps to the real function. It's the equivalent to this:

fn CloseHandle(handle: HANDLE) -> BOOL {
    (*__imp_CloseHandle)(handle)
}

So there's an extra level of indirection. Like, I said it's super minor and only rarely matters.

Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,7 @@ const WAIT_FAILED: DWORD = 4294967295u32;
const WAIT_OBJECT_0: DWORD = 0u32;
const WAIT_TIMEOUT: DWORD = 258u32;

#[link(name = "kernel32")]
extern "system" {
fn CloseHandle(handle: HANDLE) -> BOOL;
fn SetEvent(hEvent: HANDLE) -> BOOL;
Expand Down Expand Up @@ -64,6 +65,10 @@ extern "system" {
) -> HANDLE;
fn OpenSemaphoreA(dwDesiredAccess: DWORD, bInheritHandle: BOOL, lpName: *const i8) -> HANDLE;
fn WaitForSingleObject(hHandle: HANDLE, dwMilliseconds: DWORD) -> DWORD;
}

#[link(name = "advapi32")]
extern "system" {
#[link_name = "SystemFunction036"]
fn RtlGenRandom(RandomBuffer: *mut u8, RandomBufferLength: u32) -> u8;
}
Expand Down