libusb: Device handling and enumeration

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Detailed Description

The functionality documented below is designed to help with the following operations:

Enumerating the USB devices currently attached to the system

Choosing a device to operate from your software

Opening and closing the chosen device

In a nutshell...

The description below really makes things sound more complicated than they actually are. The following sequence of function calls will be suitable for almost all scenarios and does not require you to have such a deep understanding of the resource management issues:

    
     // discover devices
    
     libusb_device
    
    **list;
   
     libusb_device
    
    *found = NULL;
   
    ssize_t cnt =
    
     libusb_get_device_list
    
    (NULL, &list);
   
    ssize_t i = 0;
   
     int
    
    err = 0;
   
     if
    
    (cnt < 0)
   
    error();
   
     for
    
    (i = 0; i < cnt; i++) {
   
     libusb_device
    
    *device = list[i];
   
     if
    
    (is_interesting(device)) {
   
    found = device;
   
     break
    
    ;
   
     if
    
    (found) {
   
     libusb_device_handle
    
    *handle;
   
    err =
    
     libusb_open
    
    (found, &handle);
   
     if
    
    (err)
   
    error();
   
     // etc
    
     libusb_free_device_list
    
    (list, 1);

The two important points:

You asked libusb_free_device_list() to unreference the devices (2nd parameter)
You opened the device before freeing the list and unreferencing the devices

If you ended up with a handle, you can now proceed to perform I/O on the device.

libusb has a concept of a USB device, represented by the libusb_device opaque type. A device represents a USB device that is currently or was previously connected to the system. Using a reference to a device, you can determine certain information about the device (e.g. you can read the descriptor data).

The libusb_get_device_list() function can be used to obtain a list of devices currently connected to the system. This is known as device discovery. Devices can also be discovered with the hotplug mechanism, whereby a callback function registered with libusb_hotplug_register_callback() will be called when a device of interest is connected or disconnected.

Just because you have a reference to a device does not mean it is necessarily usable. The device may have been unplugged, you may not have permission to operate such device, or another program or driver may be using the device.

When you've found a device that you'd like to operate, you must ask libusb to open the device using the libusb_open() function. Assuming success, libusb then returns you a device handle (a libusb_device_handle pointer). All "real" I/O operations then operate on the handle rather than the original device pointer.

Device discovery (i.e. calling libusb_get_device_list() ) returns a freshly-allocated list of devices. The list itself must be freed when you are done with it. libusb also needs to know when it is OK to free the contents of the list - the devices themselves.

To handle these issues, libusb provides you with two separate items:

A function to free the list itself
A reference counting system for the devices inside

New devices presented by the libusb_get_device_list() function all have a reference count of 1. You can increase and decrease reference count using libusb_ref_device() and libusb_unref_device() . A device is destroyed when its reference count reaches 0.

With the above information in mind, the process of opening a device can be viewed as follows:

Discover devices using libusb_get_device_list() or libusb_hotplug_register_callback() .
Choose the device that you want to operate, and call libusb_open() .
Unref all devices in the discovered device list.
Free the discovered device list.

The order is important - you must not unreference the device before attempting to open it, because unreferencing it may destroy the device.

For convenience, the libusb_free_device_list() function includes a parameter to optionally unreference all the devices in the list before freeing the list itself. This combines steps 3 and 4 above.

As an implementation detail, libusb_open() actually adds a reference to the device in question. This is because the device remains available through the handle via libusb_get_device() . The reference is deleted during libusb_close() .

Typedef Documentation

◆ libusb_device

Structure representing a USB device detected on the system.

This is an opaque type for which you are only ever provided with a pointer, usually originating from libusb_get_device_list() or libusb_hotplug_register_callback() .

Certain operations can be performed on a device, but in order to do any I/O you will have to first obtain a device handle using libusb_open() .

Devices are reference counted with libusb_ref_device() and libusb_unref_device() , and are freed when the reference count reaches 0. New devices presented by libusb_get_device_list() have a reference count of 1, and libusb_free_device_list() can optionally decrease the reference count on all devices in the list. libusb_open() adds another reference which is later destroyed by libusb_close() .

Structure representing a handle on a USB device.

This is an opaque type for which you are only ever provided with a pointer, usually originating from libusb_open() .

A device handle is used to perform I/O and other operations. When finished with a device handle, you should call libusb_close() .

Enumeration Type Documentation

◆ libusb_speed

Indicates the speed at which the device is operating.

The OS doesn't report or know the device speed.

The device is operating at low speed (1.5MBit/s).

The device is operating at full speed (12MBit/s).

The device is operating at high speed (480MBit/s).

The device is operating at super speed (5000MBit/s).

The device is operating at super speed plus (10000MBit/s).

Returns a list of USB devices currently attached to the system.

This is your entry point into finding a USB device to operate.

You are expected to unreference all the devices when you are done with them, and then free the list with libusb_free_device_list() . Note that libusb_free_device_list() can unref all the devices for you. Be careful not to unreference a device you are about to open until after you have opened it.

This return value of this function indicates the number of devices in the resultant list. The list is actually one element larger, as it is NULL-terminated.

Parameters

ctxthe context to operate on, or NULL for the default context listoutput location for a list of devices. Must be later freed with libusb_free_device_list() .

Returns: the number of devices in the outputted list, or any libusb_error according to errors encountered by the backend.

Frees a list of devices previously discovered using libusb_get_device_list() .

If the unref_devices parameter is set, the reference count of each device in the list is decremented by 1.

Parameters

listthe list to free unref_deviceswhether to unref the devices in the list

Get the number of the port that a device is connected to.

Unless the OS does something funky, or you are hot-plugging USB extension cards, the port number returned by this call is usually guaranteed to be uniquely tied to a physical port, meaning that different devices plugged on the same physical port should return the same port number.

But outside of this, there is no guarantee that the port number returned by this call will remain the same, or even match the order in which ports have been numbered by the HUB/HCD manufacturer.

Parameters

deva device

Returns: the port number (0 if not available)

Get the list of all port numbers from root for the specified device.

Since version 1.0.16, LIBUSB_API_VERSION >= 0x01000102

Parameters

deva device port_numbersthe array that should contain the port numbers port_numbers_lenthe maximum length of the array. As per the USB 3.0 specs, the current maximum limit for the depth is 7.

Returns: the number of elements filled

Returns: the device parent or NULL if not available You should issue a libusb_get_device_list() before calling this function and make sure that you only access the parent before issuing libusb_free_device_list() . The reason is that libusb currently does not maintain a permanent list of device instances, and therefore can only guarantee that parents are fully instantiated within a libusb_get_device_list() - libusb_free_device_list() block.

Convenience function to retrieve the wMaxPacketSize value for a particular endpoint in the active device configuration.

This function was originally intended to be of assistance when setting up isochronous transfers, but a design mistake resulted in this function instead. It simply returns the wMaxPacketSize value without considering its contents. If you're dealing with isochronous transfers, you probably want libusb_get_max_iso_packet_size() instead.

Parameters

deva device endpointaddress of the endpoint in question

Returns: the wMaxPacketSize value

LIBUSB_ERROR_OTHER

Calculate the maximum packet size which a specific endpoint is capable is sending or receiving in the duration of 1 microframe.

Only the active configuration is examined. The calculation is based on the wMaxPacketSize field in the endpoint descriptor as described in section 9.6.6 in the USB 2.0 specifications.

If acting on an isochronous or interrupt endpoint, this function will multiply the value found in bits 0:10 by the number of transactions per microframe (determined by bits 11:12). Otherwise, this function just returns the numeric value found in bits 0:10. For USB 3.0 device, it will attempts to retrieve the Endpoint Companion Descriptor to return wBytesPerInterval.

This function is useful for setting up isochronous transfers, for example you might pass the return value from this function to libusb_set_iso_packet_lengths() in order to set the length field of every isochronous packet in a transfer.

Since v1.0.3.

Parameters

deva device endpointaddress of the endpoint in question

Returns: the maximum packet size which can be sent/received on this endpoint

LIBUSB_ERROR_OTHER

Decrement the reference count of a device.

If the decrement operation causes the reference count to reach zero, the device shall be destroyed.

Parameters

devthe device to unreference

Wrap a platform-specific system device handle and obtain a libusb device handle for the underlying device.

The handle allows you to use libusb to perform I/O on the device in question.

Call libusb_set_option() with the parameters (NULL, LIBUSB_OPTION_NO_DEVICE_DISCOVERY ) before libusb_init() if you want to skip enumeration of USB devices. In particular, this might be needed on Android if you don't have authority to access USB devices in general.

On Linux, the system device handle must be a valid file descriptor opened on the device node.

The system device handle must remain open until libusb_close() is called. The system device handle will not be closed by libusb_close() .

Internally, this function creates a temporary device and makes it available to you through libusb_get_device() . This device is destroyed during libusb_close() . The device shall not be opened through libusb_open() .

This is a non-blocking function; no requests are sent over the bus.

Since version 1.0.23, LIBUSB_API_VERSION >= 0x01000107

Parameters

ctxthe context to operate on, or NULL for the default context sys_devthe platform-specific system device handle dev_handleoutput location for the returned device handle pointer. Only populated when the return code is 0.

Returns: 0 on success

LIBUSB_ERROR_ACCESS

LIBUSB_ERROR_NOT_SUPPORTED

Open a device and obtain a device handle.

A handle allows you to perform I/O on the device in question.

Internally, this function adds a reference to the device and makes it available to you through libusb_get_device() . This reference is removed during libusb_close() .

This is a non-blocking function; no requests are sent over the bus.

Parameters

devthe device to open dev_handleoutput location for the returned device handle pointer. Only populated when the return code is 0.

Returns: 0 on success

LIBUSB_ERROR_ACCESS LIBUSB_ERROR_NO_DEVICE

Convenience function for finding a device with a particular idVendor / idProduct combination.

This function is intended for those scenarios where you are using libusb to knock up a quick test application - it allows you to avoid calling libusb_get_device_list() and worrying about traversing/freeing the list.

This function has limitations and is hence not intended for use in real applications: if multiple devices have the same IDs it will only give you the first one, etc.

Parameters

ctxthe context to operate on, or NULL for the default context vendor_idthe idVendor value to search for product_idthe idProduct value to search for

Returns: a device handle for the first found device, or NULL on error or if the device could not be found.

Close a device handle.

Should be called on all open handles before your application exits.

Internally, this function destroys the reference that was added by libusb_open() on the given device.

This is a non-blocking function; no requests are sent over the bus.

Parameters

dev_handlethe device handle to close

Get the underlying device for a device handle.

This function does not modify the reference count of the returned device, so do not feel compelled to unreference it when you are done.

Parameters

dev_handlea device handle

Returns: the underlying device

Determine the bConfigurationValue of the currently active configuration.

You could formulate your own control request to obtain this information, but this function has the advantage that it may be able to retrieve the information from operating system caches (no I/O involved).

If the OS does not cache this information, then this function will block while a control transfer is submitted to retrieve the information.

This function will return a value of 0 in the config output parameter if the device is in unconfigured state.

Parameters

dev_handlea device handle configoutput location for the bConfigurationValue of the active configuration (only valid for return code 0)

Returns: 0 on success

Set the active configuration for a device.

The operating system may or may not have already set an active configuration on the device. It is up to your application to ensure the correct configuration is selected before you attempt to claim interfaces and perform other operations.

If you call this function on a device already configured with the selected configuration, then this function will act as a lightweight device reset: it will issue a SET_CONFIGURATION request using the current configuration, causing most USB-related device state to be reset (altsetting reset to zero, endpoint halts cleared, toggles reset).

Not all backends support setting the configuration from user space, which will be indicated by the return code LIBUSB_ERROR_NOT_SUPPORTED . As this suggests that the platform is handling the device configuration itself, this error should generally be safe to ignore.

You cannot change/reset configuration if your application has claimed interfaces. It is advised to set the desired configuration before claiming interfaces.

Alternatively you can call libusb_release_interface() first. Note if you do things this way you must ensure that auto_detach_kernel_driver for dev is 0, otherwise the kernel driver will be re-attached when you release the interface(s).

You cannot change/reset configuration if other applications or drivers have claimed interfaces.

A configuration value of -1 will put the device in unconfigured state. The USB specifications state that a configuration value of 0 does this, however buggy devices exist which actually have a configuration 0.

You should always use this function rather than formulating your own SET_CONFIGURATION control request. This is because the underlying operating system needs to know when such changes happen.

This is a blocking function.

Parameters

dev_handlea device handle configurationthe bConfigurationValue of the configuration you wish to activate, or -1 if you wish to put the device in an unconfigured state

Returns: 0 on success

LIBUSB_ERROR_BUSY

LIBUSB_ERROR_NOT_SUPPORTED

LIBUSB_ERROR_NO_DEVICE

See also: libusb_set_auto_detach_kernel_driver()

Claim an interface on a given device handle.

You must claim the interface you wish to use before you can perform I/O on any of its endpoints.

It is legal to attempt to claim an already-claimed interface, in which case libusb just returns 0 without doing anything.

If auto_detach_kernel_driver is set to 1 for dev , the kernel driver will be detached if necessary, on failure the detach error is returned.

Claiming of interfaces is a purely logical operation; it does not cause any requests to be sent over the bus. Interface claiming is used to instruct the underlying operating system that your application wishes to take ownership of the interface.

This is a non-blocking function.

Parameters

dev_handlea device handle interface_numberthe


        bInterfaceNumber

of the interface you wish to claim

Returns: 0 on success

LIBUSB_ERROR_BUSY LIBUSB_ERROR_NO_DEVICE

See also: libusb_set_auto_detach_kernel_driver()

Release an interface previously claimed with libusb_claim_interface() .

You should release all claimed interfaces before closing a device handle.

This is a blocking function. A SET_INTERFACE control request will be sent to the device, resetting interface state to the first alternate setting.

If auto_detach_kernel_driver is set to 1 for dev , the kernel driver will be re-attached after releasing the interface.

Parameters

dev_handlea device handle interface_numberthe


        bInterfaceNumber

of the previously-claimed interface

Returns: 0 on success

LIBUSB_ERROR_NO_DEVICE

See also: libusb_set_auto_detach_kernel_driver()

Activate an alternate setting for an interface.

The interface must have been previously claimed with libusb_claim_interface() .

You should always use this function rather than formulating your own SET_INTERFACE control request. This is because the underlying operating system needs to know when such changes happen.

This is a blocking function.

Parameters

dev_handlea device handle interface_numberthe


        bInterfaceNumber

of the previously-claimed interface alternate_settingthe


        bAlternateSetting

of the alternate setting to activate

Returns: 0 on success

LIBUSB_ERROR_NO_DEVICE

Clear the halt/stall condition for an endpoint.

Endpoints with halt status are unable to receive or transmit data until the halt condition is stalled.

You should cancel all pending transfers before attempting to clear the halt condition.

This is a blocking function.

Parameters

dev_handlea device handle endpointthe endpoint to clear halt status

Returns: 0 on success

LIBUSB_ERROR_NO_DEVICE

Perform a USB port reset to reinitialize a device.

The system will attempt to restore the previous configuration and alternate settings after the reset has completed.

If the reset fails, the descriptors change, or the previous state cannot be restored, the device will appear to be disconnected and reconnected. This means that the device handle is no longer valid (you should close it) and rediscover the device. A return code of LIBUSB_ERROR_NOT_FOUND indicates when this is the case.

This is a blocking function which usually incurs a noticeable delay.

Parameters

dev_handlea handle of the device to reset

Returns: 0 on success

Determine if a kernel driver is active on an interface.

If a kernel driver is active, you cannot claim the interface, and libusb will be unable to perform I/O.

This functionality is not available on Windows.

Parameters

dev_handlea device handle interface_numberthe interface to check

Returns: 0 if no kernel driver is active

LIBUSB_ERROR_NO_DEVICE LIBUSB_ERROR_NOT_SUPPORTED

See also: libusb_detach_kernel_driver()

Detach a kernel driver from an interface.

If successful, you will then be able to claim the interface and perform I/O.

This functionality is not available on Windows.

Note that libusb itself also talks to the device through a special kernel driver, if this driver is already attached to the device, this call will not detach it and return LIBUSB_ERROR_NOT_FOUND .

Parameters

dev_handlea device handle interface_numberthe interface to detach the driver from

Returns: 0 on success

LIBUSB_ERROR_INVALID_PARAM LIBUSB_ERROR_NO_DEVICE LIBUSB_ERROR_NOT_SUPPORTED

See also: libusb_kernel_driver_active()

Re-attach an interface's kernel driver, which was previously detached using libusb_detach_kernel_driver() .

This functionality is not available on Windows.

Parameters

dev_handlea device handle interface_numberthe interface to attach the driver from

Returns: 0 on success

LIBUSB_ERROR_INVALID_PARAM LIBUSB_ERROR_NO_DEVICE LIBUSB_ERROR_NOT_SUPPORTED LIBUSB_ERROR_BUSY

See also: libusb_kernel_driver_active()

Enable/disable libusb's automatic kernel driver detachment.

When this is enabled libusb will automatically detach the kernel driver on an interface when claiming the interface, and attach it when releasing the interface.

Automatic kernel driver detachment is disabled on newly opened device handles by default.

On platforms which do not have LIBUSB_CAP_SUPPORTS_DETACH_KERNEL_DRIVER this function will return LIBUSB_ERROR_NOT_SUPPORTED , and libusb will continue as if this function was never called.

Parameters

dev_handlea device handle enablewhether to enable or disable auto kernel driver detachment

Returns: LIBUSB_SUCCESS on success

See also: libusb_claim_interface()