Calling EnumServicesStatusEx in Go, memory allocation?

I'm writing an app that interacts with the Windows API from a Windows Service.

After loads of help from @chowey here, I sort of got the hang of things and started a basic library which I've put on GitHub here.

I've now moved on to "Services", with the requirement to list all Windows Services on a machine, start, stop, restart them. The start/stop/restart look pretty straight forward once you've got a service handle to work with, but I'm struggling with getting a list of installed services.

EnumServicesStatusEx in Advapi32.dll is the function I need to call, but it requires a pointer to pre-allocated memory for an array of ENUM_SERVICE_STATUS_PROCESS structs.

You can call the function with a null pointer and it will return the memory allocation size required, but I don't believe there is a way to directly allocate memory in Go.

At first I thought I could get the memory allocation requirement, divide it by the SizeOf the struct using the unsafe package, create a slice containing that number of elements, then pass a pointer to the first element to the function, but it says the memory needs to include space for the string data, which this wouldn't.

Does anyone know how this could be accomplished, pretty please? :).

Solution

After the suggestions from @alex, I've got the following example code working.

Looks like we create a byte slice of the right size then use the unsafe class to cast to our struct type.

    _, _, _ = svcEnumServicesStatusEx.Call(
        uintptr(handle),
        uintptr(uint32(SVC_SC_ENUM_PROCESS_INFO)),
        uintptr(uint32(SVC_SERVICE_WIN32)),
        uintptr(uint32(SVC_SERVICE_STATE_ALL)),
        uintptr(0),
        0,
        uintptr(unsafe.Pointer(&bytesReq)),
        uintptr(unsafe.Pointer(&numReturned)),
        uintptr(unsafe.Pointer(&resumeHandle)),
        uintptr(0),
    )

    if bytesReq > 0 {
        var buf []byte = make([]byte, bytesReq)

        ret, _, _ := svcEnumServicesStatusEx.Call(
            uintptr(handle),
            uintptr(uint32(SVC_SC_ENUM_PROCESS_INFO)),
            uintptr(uint32(SVC_SERVICE_WIN32)),
            uintptr(uint32(SVC_SERVICE_STATE_ALL)),
            uintptr(unsafe.Pointer(&buf[0])),
            uintptr(bytesReq),
            uintptr(unsafe.Pointer(&bytesReq)),
            uintptr(unsafe.Pointer(&numReturned)),
            uintptr(unsafe.Pointer(&resumeHandle)),
            uintptr(0),
        )

        if ret > 0 {
            var sizeTest ENUM_SERVICE_STATUS_PROCESS
            iter := uintptr(unsafe.Pointer(&buf[0]))

            for i := uint32(0); i < numReturned; i++ {
                var data *ENUM_SERVICE_STATUS_PROCESS = (*ENUM_SERVICE_STATUS_PROCESS)(unsafe.Pointer(iter))

                fmt.Printf("Service Name: %s - Display Name: %s - %#v\r\n", syscall.UTF16ToString((*[4096]uint16)(unsafe.Pointer(data.lpServiceName))[:]), syscall.UTF16ToString((*[4096]uint16)(unsafe.Pointer(data.lpDisplayName))[:]), data.ServiceStatusProcess)

                iter = uintptr(unsafe.Pointer(iter + unsafe.Sizeof(sizeTest)))
            }
        } else {
            return nil, fmt.Errorf("Failed to get Service List even with allocated memory.")
        }
    } else {
        return nil, fmt.Errorf("Unable to get size of required memory allocation.")
    }