Refine libusb_set_configuration() semantics

Applications will generally want to set a configuration before claiming
interfaces. The problem is that the interface may already be set, and
someone else may have claimed an interface (meaning that all calls to
set_configuration will fail, even if it's for the same configuration).

There are now 2 options:
1. Use the new libusb_get_configuration() to determine active
   configuration before calling libusb_set_configuration()
2. Or just call libusb_set_configuration() as usual, which will do
   nothing if that configuration is already active.

1 files changed, 19 insertions, 0 deletions

diff --git a/libusb/libusbi.h b/libusb/libusbi.h
index 41c8097..5c00a63 100644
--- a/libusb/libusbi.h
+++ b/libusb/libusbi.h
@@ -436,6 +436,25 @@ struct usbi_os_backend {
 		uint8_t config_index, unsigned char *buffer, size_t len,
 		int *host_endian);
 
+	/* Get the bConfigurationValue for the active configuration for a device.
+	 * Optional. This should only be implemented if you can retrieve it from
+	 * cache (don't generate I/O).
+	 *
+	 * If you cannot retrieve this from cache, either do not implement this
+	 * function, or return LIBUSB_ERROR_NOT_SUPPORTED. This will cause
+	 * libusb to retrieve the information through a standard control transfer.
+	 *
+	 * This function must be non-blocking.
+	 * Return:
+	 * - 0 on success
+	 * - LIBUSB_ERROR_NO_DEVICE if the device has been disconnected since it
+	 *   was opened
+	 * - LIBUSB_ERROR_NOT_SUPPORTED if the value cannot be retrieved without
+	 *   blocking
+	 * - another LIBUSB_ERROR code on other failure.
+	 */
+	int (*get_configuration)(struct libusb_device_handle *handle, int *config);
+
 	/* Set the active configuration for a device.
 	 *
 	 * A configuration value of -1 should put the device in unconfigured state.