syscall: cannot call Win32 API functions that spawn new threads

[dgolub]

It is not possible to call Win32 API functions that spawn new threads because the callback wrapper generated by syscall.NewCallback does not perform the needed steps to set up the call stack as needed by the Go runtime for a new thread. Attempting to call such functions will cause the new thread to hang forever. This can be easily reproduced with the following simple test program:

package main

import (
    "fmt"
    "syscall"
)

func ThreadProc(p uintptr) uintptr {
    fmt.Println("FOO")
    return 0
}

func main() {
    modkernel32 := syscall.MustLoadDLL("kernel32.dll")
    procCreateThread := modkernel32.MustFindProc("CreateThread")
    r1, _, _ := procCreateThread.Call(0, 0, syscall.NewCallback(ThreadProc), 0, 0, 0)
    h := syscall.Handle(r1)
    syscall.WaitForSingleObject(h, syscall.INFINITE)
    syscall.CloseHandle(h)
}

This will hang forever in Go. Translating this program into C/C++ and compiling it with an adequate compiler such as Microsoft Visual C++ will give the expected output of printing FOO and terminating.

[ianlancetaylor]

You can't arbitrarily create new threads on Unix systems either. The Go runtime more or less has to be in charge of threading.

Calling CreateThread in a Go program is not obviously useful. Is there a problem in more obviously useful code?

[dgolub]

If you try to implement a Windows service in Go, calling StartServiceCtrlDispatcher will invoke the service main callback in a new thread. That's the motivating case for this issue. I used CreateThread in the example code because it's the simplest way to reproduce the issue.

[ianlancetaylor]

Is this the Windows version of issue #227?

[dgolub]

Not exactly. It looks like #227 involves forking a separate process, which you can do on Windows without an issue by calling CreateProcess. This is an issue with using Win32 API calls that spawn a new thread within the current process.

[alexbrainman]

I think this is a dup of #6751.

Alex

[mattn]

Sorry about previous mistake comment.
I figure out pattern to reproduce. Below should work. Having import "C" seems initialize C runtime?

package main

import (
    "C"
    "fmt"
    "syscall"
)

func ThreadProc(p uintptr) uintptr {
    fmt.Println("FOO")
    return 0
}

func main() {
    modkernel32 := syscall.MustLoadDLL("kernel32.dll")
    procCreateThread := modkernel32.MustFindProc("CreateThread")
    r1, _, _ := procCreateThread.Call(0, 0, syscall.NewCallback(ThreadProc), 0, 0, 0)
    h := syscall.Handle(r1)
    syscall.WaitForSingleObject(h, syscall.INFINITE)
    syscall.CloseHandle(h)
}

[rsc]

We actually do allow calls from non-Go-created threads, provided cgo is in use. We should probably assume cgo is in use all the time on Windows, since it effectively is.

[alexbrainman]

Dup of #6751.

Alex