syscall GetLastError() doesn't return error

Question

In golang syscall.GetLastError() doesn't return the last error. See the following example

if handle := _OpenSCManager(machineNamePtr, databaseNamePtr, desiredAccess); handle == nil {
    if err := syscall.GetLastError(); err != nil {
        return InvalidServiceDatabaseHandleHandle, ServiceErrno(err.(syscall.Errno))
    }
}

err is always nil . Assume machineNamePtr is a non exsiting machine. Tested the same code with c++ and GetLastError() throws RPC server is not available . So why not on go ?

EDIT

_OpenSCManager is generated with go generate . //sys _OpenSCManager(machineName *uint16, databaseName *uint16, desiredAcces ServiceAccessRight) (handle ServiceDatabaseHandle) = advapi32.OpenSCManagerW

func _OpenSCManager(machineName *uint16, databaseName *uint16, desiredAcces ServiceAccessRight) (handle ServiceDatabaseHandle) {
r0, _, _ := syscall.Syscall(procOpenSCManagerW.Addr(), 3, uintptr(unsafe.Pointer(machineName)), uintptr(unsafe.Pointer(databaseName)), uintptr(desiredAcces))
handle = ServiceDatabaseHandle(r0)
return

}

Answer 1

So finally i've got this working. First i have tried to return an error variable with the following signature //sys _OpenSCManager(machineName *uint16, databaseName *uint16, desiredAcces ServiceAccessRight) (handle ServiceDatabaseHandle, lasterror error) = advapi32.OpenSCManagerW . But go generate throws always Only last windows error is allowed as second return value... . But if you change it to //sys ... (handle ServiceDatabaseHandle, err error) = advapi32.OpenSCManagerW it successfully generates the code. So you explicit have to write err error . Someone knows why? So now the function looks like

func _OpenSCManager(machineName *uint16, databaseName *uint16, desiredAcces ServiceAccessRight) (handle ServiceDatabaseHandle, err error) {
r0, _, e1 := syscall.Syscall(procOpenSCManagerW.Addr(), 3, uintptr(unsafe.Pointer(machineName)), uintptr(unsafe.Pointer(databaseName)), uintptr(desiredAcces))
handle = ServiceDatabaseHandle(r0)
if handle == 0 {
    if e1 != 0 {
        err = errnoErr(e1)
    } else {
        err = syscall.EINVAL
    }
}
return
}

and it returns an error. So there is no need to call GetLastError() .

Answer 2

IIUC, the syscall.Syscall() on Windows automatically and atomically calls GetLastError() after the actual syscall finishes. That should be understandable once you consider that as soon as a goroutine exits from a syscall, the Go runtime scheduler is free to preempt it and run another goroutine on the very thread the just-preempted goroutine had just been running on.

Since GetLastError() accesses per-thread state, if the second goroutine makes another syscall, it would likely thrash that last error value, so in the context of Go, each syscall should be accompanied with the following call to GetLastError() done in the context of a single syscall invocation — as seen from the Go side.