lxd/storage_cgo.go
func prepareLoopDev(source string, flags int) (*os.File, error) {
	cLoopDev := C.malloc(C.size_t(C.LO_NAME_SIZE))
	if cLoopDev == nil {
		return nil, fmt.Errorf("Failed to allocate memory in C")
	}
	defer C.free(cLoopDev)

	cSource := C.CString(source)
	defer C.free(unsafe.Pointer(cSource))
	loopFd, _ := C.find_associated_loop_device(cSource, (*C.char)(cLoopDev))
	if loopFd >= 0 {
		return os.NewFile(uintptr(loopFd), C.GoString((*C.char)(cLoopDev))), nil
	}

	loopFd, err := C.prepare_loop_dev_retry(cSource, (*C.char)(cLoopDev), C.int(flags))
	if loopFd < 0 {
		if err != nil {
			return nil, errors.Wrapf(err, "Failed to prepare loop device for %q", source)
		}

		return nil, fmt.Errorf("Failed to prepare loop device for %q", source)
	}

	return os.NewFile(uintptr(loopFd), C.GoString((*C.char)(cLoopDev))), nil
}
func prepareLoopDev(source string, flags int) (*os.File, error) {
	cLoopDev := C.malloc(C.size_t(C.LO_NAME_SIZE))
	if cLoopDev == nil {
		return nil, fmt.Errorf("Failed to allocate memory in C")
	}
	defer C.free(cLoopDev)

	cSource := C.CString(source)
	defer C.free(unsafe.Pointer(cSource))
	loopFd, _ := C.find_associated_loop_device(cSource, (*C.char)(cLoopDev))
	if loopFd >= 0 {
		return os.NewFile(uintptr(loopFd), C.GoString((*C.char)(cLoopDev))), nil
	}

	loopFd, err := C.prepare_loop_dev_retry(cSource, (*C.char)(cLoopDev), C.int(flags))
	if loopFd < 0 {
		if err != nil {
			return nil, errors.Wrapf(err, "Failed to prepare loop device for %q", source)
		}

		return nil, fmt.Errorf("Failed to prepare loop device for %q", source)
	}

	return os.NewFile(uintptr(loopFd), C.GoString((*C.char)(cLoopDev))), nil
}
// prepareLoopDev() detects and sets up a loop device for source. It returns an
// open file descriptor to the free loop device and the path of the free loop
// device. It's the callers responsibility to close the open file descriptor.