diff --git a/include/libdivecomputer/context.h b/include/libdivecomputer/context.h
index 757245b..f8fa9ea 100644
--- a/include/libdivecomputer/context.h
+++ b/include/libdivecomputer/context.h
@@ -31,6 +31,9 @@ extern "C" {
 
 typedef struct dc_context_t dc_context_t;
 
+/* Opaque libdivecomputer *user* data structure */
+typedef struct dc_user_device_t dc_user_device_t;
+
 typedef enum dc_loglevel_t {
 	DC_LOGLEVEL_NONE,
 	DC_LOGLEVEL_ERROR,
@@ -49,7 +52,7 @@ dc_status_t
 dc_context_free (dc_context_t *context);
 
 dc_status_t
-dc_context_set_custom_io (dc_context_t *context, dc_custom_io_t *custom_io);
+dc_context_set_custom_io (dc_context_t *context, dc_custom_io_t *custom_io, dc_user_device_t *);
 
 dc_status_t
 dc_context_set_loglevel (dc_context_t *context, dc_loglevel_t loglevel);
diff --git a/include/libdivecomputer/custom_io.h b/include/libdivecomputer/custom_io.h
index 6cbf766..14a66ca 100644
--- a/include/libdivecomputer/custom_io.h
+++ b/include/libdivecomputer/custom_io.h
@@ -63,24 +63,47 @@ typedef enum dc_line_t {
 #endif /* __SERIAL_TYPES__ */
 
 struct dc_context_t;
+struct dc_user_device_t;
 
+/*
+ * Two different pointers to user-supplied data.
+ *
+ * The 'userdata' pointer is for the IO routines themselves,
+ * generally filled in by the 'xyz_open()' routine with whatever
+ * file descriptor etc information.
+ *
+ * The 'user_device' pointer is set when registering the
+ * custom IO with the download context, and has whatever
+ * data the downloader needs.
+ *
+ * The two are very different. The userdata is "per instance",
+ * and when nesting custom IO handlers, each level would
+ * generally have its own userdata, that would be specific
+ * to that particular set of IO routines.
+ *
+ * In contrast, the user_device is filled in when the
+ * download context is created, before open() is even called,
+ * and isn't specific to the IO routines, but to the download
+ * as a whole.
+ */
 typedef struct dc_custom_io_t
 {
 	void *userdata;
+	struct dc_user_device_t *user_device;
 
 	// Custom serial (generally BT rfcomm)
-	dc_status_t (*serial_open) (void **userdata, const char *name);
-	dc_status_t (*serial_close) (void **userdata);
-	dc_status_t (*serial_read) (void **userdata, void* data, size_t size, size_t *actual);
-	dc_status_t (*serial_write) (void **userdata, const void* data, size_t size, size_t *actual);
-	dc_status_t (*serial_purge) (void **userdata, dc_direction_t);
-	dc_status_t (*serial_get_available) (void **userdata, size_t *value);
-	dc_status_t (*serial_set_timeout) (void **userdata, long timeout);
-	dc_status_t (*serial_configure) (void **userdata, unsigned int baudrate, unsigned int databits, dc_parity_t parity, dc_stopbits_t stopbits, dc_flowcontrol_t flowcontrol);
-	dc_status_t (*serial_set_dtr) (void **userdata, int level);
-	dc_status_t (*serial_set_rts) (void **userdata, int level);
-	dc_status_t (*serial_set_halfduplex) (void **userdata, unsigned int value);
-	dc_status_t (*serial_set_break) (void **userdata, unsigned int level);
+	dc_status_t (*serial_open) (struct dc_custom_io_t *io, struct dc_context_t *, const char *name);
+	dc_status_t (*serial_close) (struct dc_custom_io_t *io);
+	dc_status_t (*serial_read) (struct dc_custom_io_t *io, void* data, size_t size, size_t *actual);
+	dc_status_t (*serial_write) (struct dc_custom_io_t *io, const void* data, size_t size, size_t *actual);
+	dc_status_t (*serial_purge) (struct dc_custom_io_t *io, dc_direction_t);
+	dc_status_t (*serial_get_available) (struct dc_custom_io_t *io, size_t *value);
+	dc_status_t (*serial_set_timeout) (struct dc_custom_io_t *io, long timeout);
+	dc_status_t (*serial_configure) (struct dc_custom_io_t *io, unsigned int baudrate, unsigned int databits, dc_parity_t parity, dc_stopbits_t stopbits, dc_flowcontrol_t flowcontrol);
+	dc_status_t (*serial_set_dtr) (struct dc_custom_io_t *io, int level);
+	dc_status_t (*serial_set_rts) (struct dc_custom_io_t *io, int level);
+	dc_status_t (*serial_set_halfduplex) (struct dc_custom_io_t *io, unsigned int value);
+	dc_status_t (*serial_set_break) (struct dc_custom_io_t *io, unsigned int level);
 	//dc_serial_set_latency (dc_serial_t *device, unsigned int milliseconds) - Unused
 	//dc_serial_get_lines (dc_serial_t *device, unsigned int *value) - Unused
 	//dc_serial_flush (dc_serial_t *device) - No device interaction
diff --git a/include/libdivecomputer/version.h.in b/include/libdivecomputer/version.h.in
index 0f46298..6404821 100644
--- a/include/libdivecomputer/version.h.in
+++ b/include/libdivecomputer/version.h.in
@@ -28,7 +28,7 @@ extern "C" {
 
 /* use these defines to detect Subsurface specific features */
 #define SSRF_LIBDC_VERSION 2
-#define SSRF_CUSTOM_IO 1
+#define SSRF_CUSTOM_IO 2
 
 #define DC_VERSION "@DC_VERSION@"
 #define DC_VERSION_MAJOR @DC_VERSION_MAJOR@
diff --git a/src/context-private.h b/src/context-private.h
index 96fc0af..800ef24 100644
--- a/src/context-private.h
+++ b/src/context-private.h
@@ -81,7 +81,7 @@ _dc_context_custom_io (dc_context_t *context);
 		dc_status_t _rc; \
 		if (c) { \
 			if (c->serial_##function) \
-				_rc = c->serial_##function(&c->userdata, ##__VA_ARGS__); \
+				_rc = c->serial_##function(c, ##__VA_ARGS__); \
 			else \
 				_rc = DC_STATUS_SUCCESS; \
 			block ;\
diff --git a/src/context.c b/src/context.c
index b15a9fc..f731184 100644
--- a/src/context.c
+++ b/src/context.c
@@ -47,6 +47,7 @@ struct dc_context_t {
 #endif
 #endif
 	dc_custom_io_t *custom_io;
+	dc_user_device_t *user_device;
 };
 
 #ifdef ENABLE_LOGGING
@@ -213,12 +214,13 @@ dc_context_free (dc_context_t *context)
 }
 
 dc_status_t
-dc_context_set_custom_io (dc_context_t *context, dc_custom_io_t *custom_io)
+dc_context_set_custom_io (dc_context_t *context, dc_custom_io_t *custom_io, dc_user_device_t *user_device)
 {
 	if (context == NULL)
 		return DC_STATUS_INVALIDARGS;
 
 	context->custom_io = custom_io;
+	custom_io->user_device = user_device;
 
 	return DC_STATUS_SUCCESS;
 }
diff --git a/src/serial_posix.c b/src/serial_posix.c
index 1b4a4d6..9327d96 100644
--- a/src/serial_posix.c
+++ b/src/serial_posix.c
@@ -167,7 +167,7 @@ dc_serial_open (dc_serial_t **out, dc_context_t *context, const char *name)
 	device->baudrate = 0;
 	device->nbits = 0;
 
-	RETURN_IF_CUSTOM_SERIAL(context, *out = device, open, name);
+	RETURN_IF_CUSTOM_SERIAL(context, *out = device, open, context, name);
 
 	// Open the device in non-blocking mode, to return immediately
 	// without waiting for the modem connection to complete.
diff --git a/src/serial_win32.c b/src/serial_win32.c
index 406d0a2..40cddf9 100644
--- a/src/serial_win32.c
+++ b/src/serial_win32.c
@@ -157,7 +157,7 @@ dc_serial_open (dc_serial_t **out, dc_context_t *context, const char *name)
 	device->baudrate = 0;
 	device->nbits = 0;
 
-	RETURN_IF_CUSTOM_SERIAL(context, *out = device, open, name);
+	RETURN_IF_CUSTOM_SERIAL(context, *out = device, open, context, name);
 
 	// Open the device.
 	device->hFile = CreateFileA (devname,
diff --git a/src/usbhid.c b/src/usbhid.c
index 0512830..784b53c 100644
--- a/src/usbhid.c
+++ b/src/usbhid.c
@@ -119,7 +119,7 @@ dc_usbhid_custom_io (dc_context_t *context, unsigned int vid, unsigned int pid)
 		return status;
 
 	custom.userdata = (void *)usbhid;
-	dc_context_set_custom_io(context, &custom);
+	dc_context_set_custom_io(context, &custom, NULL);
 
 	dc_usbhid_set_timeout(usbhid, 10);