Asynchronous device I/O

Classes
struct	libusb_control_setup
struct	libusb_iso_packet_descriptor
struct	libusb_transfer

Enumerations
enum	libusb_transfer_type {   LIBUSB_TRANSFER_TYPE_CONTROL = 0U, LIBUSB_TRANSFER_TYPE_ISOCHRONOUS = 1U, LIBUSB_TRANSFER_TYPE_BULK = 2U, LIBUSB_TRANSFER_TYPE_INTERRUPT = 3U,   LIBUSB_TRANSFER_TYPE_BULK_STREAM = 4U }
enum	libusb_transfer_status {   LIBUSB_TRANSFER_COMPLETED, LIBUSB_TRANSFER_ERROR, LIBUSB_TRANSFER_TIMED_OUT, LIBUSB_TRANSFER_CANCELLED,   LIBUSB_TRANSFER_STALL, LIBUSB_TRANSFER_NO_DEVICE, LIBUSB_TRANSFER_OVERFLOW }
enum	libusb_transfer_flags { LIBUSB_TRANSFER_SHORT_NOT_OK = (1U << 0), LIBUSB_TRANSFER_FREE_BUFFER = (1U << 1), LIBUSB_TRANSFER_FREE_TRANSFER = (1U << 2), LIBUSB_TRANSFER_ADD_ZERO_PACKET = (1U << 3) }

Functions
int API_EXPORTED	libusb_alloc_streams (libusb_device_handle *dev_handle, uint32_t num_streams, unsigned char *endpoints, int num_endpoints)
int API_EXPORTED	libusb_free_streams (libusb_device_handle *dev_handle, unsigned char *endpoints, int num_endpoints)
DEFAULT_VISIBILITY unsigned char *LIBUSB_CALL	libusb_dev_mem_alloc (libusb_device_handle *dev_handle, size_t length)
int API_EXPORTED	libusb_dev_mem_free (libusb_device_handle *dev_handle, unsigned char *buffer, size_t length)
DEFAULT_VISIBILITY struct libusb_transfer *LIBUSB_CALL	libusb_alloc_transfer (int iso_packets)
void API_EXPORTED	libusb_free_transfer (struct libusb_transfer *transfer)
int API_EXPORTED	libusb_submit_transfer (struct libusb_transfer *transfer)
int API_EXPORTED	libusb_cancel_transfer (struct libusb_transfer *transfer)
void API_EXPORTED	libusb_transfer_set_stream_id (struct libusb_transfer *transfer, uint32_t stream_id)
uint32_t API_EXPORTED	libusb_transfer_get_stream_id (struct libusb_transfer *transfer)
typedef	void (LIBUSB_CALL *libusb_transfer_cb_fn)(struct libusb_transfer *transfer)

Variables
struct libusb_control_setup	LIBUSB_PACKED

Detailed Description

This page details libusb's asynchronous (non-blocking) API for USB device I/O. This interface is very powerful but is also quite complex - you will need to read this page carefully to understand the necessary considerations and issues surrounding use of this interface. Simplistic applications may wish to consider the synchronous I/O API instead.

The asynchronous interface is built around the idea of separating transfer submission and handling of transfer completion (the synchronous model combines both of these into one). There may be a long delay between submission and completion, however the asynchronous submission function is non-blocking so will return control to your application during that potentially long delay.

Transfer abstraction

For the asynchronous I/O, libusb implements the concept of a generic transfer entity for all types of I/O (control, bulk, interrupt, isochronous). The generic transfer object must be treated slightly differently depending on which type of I/O you are performing with it.

This is represented by the public libusb_transfer structure type.

Asynchronous transfers

We can view asynchronous I/O as a 5 step process:

1. Allocation: allocate a libusb_transfer
2. Filling: populate the libusb_transfer instance with information about the transfer you wish to perform
3. Submission: ask libusb to submit the transfer
4. Completion handling: examine transfer results in the libusb_transfer structure
5. Deallocation: clean up resources

Allocation

This step involves allocating memory for a USB transfer. This is the generic transfer object mentioned above. At this stage, the transfer is "blank" with no details about what type of I/O it will be used for.

Allocation is done with the libusb_alloc_transfer() function. You must use this function rather than allocating your own transfers.

Filling

This step is where you take a previously allocated transfer and fill it with information to determine the message type and direction, data buffer, callback function, etc.

You can either fill the required fields yourself or you can use the helper functions: libusb_fill_control_transfer(), libusb_fill_bulk_transfer() and libusb_fill_interrupt_transfer().

Submission

When you have allocated a transfer and filled it, you can submit it using libusb_submit_transfer(). This function returns immediately but can be regarded as firing off the I/O request in the background.

Completion handling

After a transfer has been submitted, one of four things can happen to it:

• The transfer completes (i.e. some data was transferred)
• The transfer has a timeout and the timeout expires before all data is transferred
• The transfer fails due to an error
• The transfer is cancelled

Each of these will cause the user-specified transfer callback function to be invoked. It is up to the callback function to determine which of the above actually happened and to act accordingly.

The user-specified callback is passed a pointer to the libusb_transfer structure which was used to setup and submit the transfer. At completion time, libusb has populated this structure with results of the transfer: success or failure reason, number of bytes of data transferred, etc. See the libusb_transfer structure documentation for more information.

Important Note: The user-specified callback is called from an event handling context. It is therefore important that no calls are made into libusb that will attempt to perform any event handling. Examples of such functions are any listed in the synchronous API and any of the blocking functions that retrieve USB descriptors.

Deallocation

When a transfer has completed (i.e. the callback function has been invoked), you are advised to free the transfer (unless you wish to resubmit it, see below). Transfers are deallocated with libusb_free_transfer().

It is undefined behaviour to free a transfer which has not completed.

Resubmission

You may be wondering why allocation, filling, and submission are all separated above where they could reasonably be combined into a single operation.

The reason for separation is to allow you to resubmit transfers without having to allocate new ones every time. This is especially useful for common situations dealing with interrupt endpoints - you allocate one transfer, fill and submit it, and when it returns with results you just resubmit it for the next interrupt.

Cancellation

Another advantage of using the asynchronous interface is that you have the ability to cancel transfers which have not yet completed. This is done by calling the libusb_cancel_transfer() function.

libusb_cancel_transfer() is asynchronous/non-blocking in itself. When the cancellation actually completes, the transfer's callback function will be invoked, and the callback function should check the transfer status to determine that it was cancelled.

Freeing the transfer after it has been cancelled but before cancellation has completed will result in undefined behaviour.

Attention

When a transfer is cancelled, some of the data may have been transferred. libusb will communicate this to you in the transfer callback. Do not assume that no data was transferred.

Partial data transfer resulting from cancellation

As noted above, some of the data may have been transferred at the time a transfer is cancelled. It is helpful to see how this is possible if you consider a bulk transfer to an endpoint with a packet size of 64 bytes. Supposing you submit a 512-byte transfer to this endpoint, the operating system will divide this transfer up into 8 separate 64-byte frames that the host controller will schedule for the device to transfer data. If this transfer is cancelled while the device is transferring data, a subset of these frames may be descheduled from the host controller before the device has the opportunity to finish transferring data to the host.

What your application should do with a partial data transfer is a policy decision; there is no single answer that satisfies the needs of every application. The data that was successfully transferred should be considered entirely valid, but your application must decide what to do with the remaining data that was not transferred. Some possible actions to take are:

• Resubmit another transfer for the remaining data, possibly with a shorter timeout
• Discard the partially transferred data and report an error

Timeouts

When a transfer times out, libusb internally notes this and attempts to cancel the transfer. As noted in above, it is possible that some of the data may actually have been transferred. Your application should always check how much data was actually transferred once the transfer completes and act accordingly.

Overflows on device-to-host bulk/interrupt endpoints

If your device does not have predictable transfer sizes (or it misbehaves), your application may submit a request for data on an IN endpoint which is smaller than the data that the device wishes to send. In some circumstances this will cause an overflow, which is a nasty condition to deal with. See the Packets and overflows page for discussion.

Considerations for control transfers

The libusb_transfer structure is generic and hence does not include specific fields for the control-specific setup packet structure.

In order to perform a control transfer, you must place the 8-byte setup packet at the start of the data buffer. To simplify this, you could cast the buffer pointer to type struct libusb_control_setup, or you can use the helper function libusb_fill_control_setup().

The wLength field placed in the setup packet must be the length you would expect to be sent in the setup packet: the length of the payload that follows (or the expected maximum number of bytes to receive). However, the length field of the libusb_transfer object must be the length of the data buffer - i.e. it should be wLength plus the size of the setup packet (LIBUSB_CONTROL_SETUP_SIZE).

If you use the helper functions, this is simplified for you:

1. Allocate a buffer of size LIBUSB_CONTROL_SETUP_SIZE plus the size of the data you are sending/requesting.
2. Call libusb_fill_control_setup() on the data buffer, using the transfer request size as the wLength value (i.e. do not include the extra space you allocated for the control setup).
3. If this is a host-to-device transfer, place the data to be transferred in the data buffer, starting at offset LIBUSB_CONTROL_SETUP_SIZE.
4. Call libusb_fill_control_transfer() to associate the data buffer with the transfer (and to set the remaining details such as callback and timeout).
   • Note that there is no parameter to set the length field of the transfer. The length is automatically inferred from the wLength field of the setup packet.
5. Submit the transfer.

The multi-byte control setup fields (wValue, wIndex and wLength) must be given in little-endian byte order (the endianness of the USB bus). Endianness conversion is transparently handled by libusb_fill_control_setup() which is documented to accept host-endian values.

Further considerations are needed when handling transfer completion in your callback function:

• As you might expect, the setup packet will still be sitting at the start of the data buffer.
• If this was a device-to-host transfer, the received data will be sitting at offset LIBUSB_CONTROL_SETUP_SIZE into the buffer.
• The actual_length field of the transfer structure is relative to the wLength of the setup packet, rather than the size of the data buffer. So, if your wLength was 4, your transfer's length was 12, then you should expect an actual_length of 4 to indicate that the data was transferred in entirety.

To simplify parsing of setup packets and obtaining the data from the correct offset, you may wish to use the libusb_control_transfer_get_data() and libusb_control_transfer_get_setup() functions within your transfer callback.

Even though control endpoints do not halt, a completed control transfer may have a LIBUSB_TRANSFER_STALL status code. This indicates the control request was not supported.

Considerations for interrupt transfers

All interrupt transfers are performed using the polling interval presented by the bInterval value of the endpoint descriptor.

Considerations for isochronous transfers

Isochronous transfers are more complicated than transfers to non-isochronous endpoints.

To perform I/O to an isochronous endpoint, allocate the transfer by calling libusb_alloc_transfer() with an appropriate number of isochronous packets.

During filling, set type to LIBUSB_TRANSFER_TYPE_ISOCHRONOUS, and set num_iso_packets to a value less than or equal to the number of packets you requested during allocation. libusb_alloc_transfer() does not set either of these fields for you, given that you might not even use the transfer on an isochronous endpoint.

Next, populate the length field for the first num_iso_packets entries in the iso_packet_desc array. Section 5.6.3 of the USB2 specifications describe how the maximum isochronous packet length is determined by the wMaxPacketSize field in the endpoint descriptor. Two functions can help you here:

• libusb_get_max_iso_packet_size() is an easy way to determine the max packet size for an isochronous endpoint. Note that the maximum packet size is actually the maximum number of bytes that can be transmitted in a single microframe, therefore this function multiplies the maximum number of bytes per transaction by the number of transaction opportunities per microframe.
• libusb_set_iso_packet_lengths() assigns the same length to all packets within a transfer, which is usually what you want.

For outgoing transfers, you'll obviously fill the buffer and populate the packet descriptors in hope that all the data gets transferred. For incoming transfers, you must ensure the buffer has sufficient capacity for the situation where all packets transfer the full amount of requested data.

Completion handling requires some extra consideration. The actual_length field of the transfer is meaningless and should not be examined; instead you must refer to the actual_length field of each individual packet.

The status field of the transfer is also a little misleading:

• If the packets were submitted and the isochronous data microframes completed normally, status will have value LIBUSB_TRANSFER_COMPLETED. Note that bus errors and software-incurred delays are not counted as transfer errors; the transfer.status field may indicate COMPLETED even if some or all of the packets failed. Refer to the status field of each individual packet to determine packet failures.
• The status field will have value LIBUSB_TRANSFER_ERROR only when serious errors were encountered.
• Other transfer status codes occur with normal behaviour.

The data for each packet will be found at an offset into the buffer that can be calculated as if each prior packet completed in full. The libusb_get_iso_packet_buffer() and libusb_get_iso_packet_buffer_simple() functions may help you here.

Transfer length limitations

Some operating systems may impose limits on the length of the transfer data buffer or, in the case of isochronous transfers, the length of individual isochronous packets. Such limits can be difficult for libusb to detect, so in most cases the library will simply try and submit the transfer as set up by you. If the transfer fails to submit because it is too large, libusb_submit_transfer() will return LIBUSB_ERROR_INVALID_PARAM.

The following are known limits for control transfer lengths. Note that this length includes the 8-byte setup packet.

• Linux (4,096 bytes)
• Windows (4,096 bytes)

Memory caveats

In most circumstances, it is not safe to use stack memory for transfer buffers. This is because the function that fired off the asynchronous transfer may return before libusb has finished using the buffer, and when the function returns it's stack gets destroyed. This is true for both host-to-device and device-to-host transfers.

The only case in which it is safe to use stack memory is where you can guarantee that the function owning the stack space for the buffer does not return until after the transfer's callback function has completed. In every other case, you need to use heap memory instead.

Fine control

Through using this asynchronous interface, you may find yourself repeating a few simple operations many times. You can apply a bitwise OR of certain flags to a transfer to simplify certain things:

• LIBUSB_TRANSFER_SHORT_NOT_OK results in transfers which transferred less than the requested amount of data being marked with status LIBUSB_TRANSFER_ERROR (they would normally be regarded as COMPLETED)
• LIBUSB_TRANSFER_FREE_BUFFER allows you to ask libusb to free the transfer buffer when freeing the transfer.
• LIBUSB_TRANSFER_FREE_TRANSFER causes libusb to automatically free the transfer after the transfer callback returns.

Event handling

An asynchronous model requires that libusb perform work at various points in time - namely processing the results of previously-submitted transfers and invoking the user-supplied callback function.

This gives rise to the libusb_handle_events() function which your application must call into when libusb has work do to. This gives libusb the opportunity to reap pending transfers, invoke callbacks, etc.

Note

All event handling is performed by whichever thread calls the libusb_handle_events() function. libusb does not invoke any callbacks outside of this context. Consequently, any callbacks will be run on the thread that calls the libusb_handle_events() function.

When to call the libusb_handle_events() function depends on which model your application decides to use. The 2 different approaches:

1. Repeatedly call libusb_handle_events() in blocking mode from a dedicated thread.
2. Integrate libusb with your application's main event loop. libusb exposes a set of file descriptors which allow you to do this.

The first approach has the big advantage that it will also work on Windows were libusb' poll API for select / poll integration is not available. So if you want to support Windows and use the async API, you must use this approach, see the Using an event handling thread section below for details.

If you prefer a single threaded approach with a single central event loop, see the polling and timing section for how to integrate libusb into your application's main event loop.

Using an event handling thread

Lets begin with stating the obvious: If you're going to use a separate thread for libusb event handling, your callback functions MUST be thread-safe.

Other then that doing event handling from a separate thread, is mostly simple. You can use an event thread function as follows:

void *event_thread_func(void *ctx)
{
    while (event_thread_run)
        libusb_handle_events(ctx);

    return NULL;
}

There is one caveat though, stopping this thread requires setting the event_thread_run variable to 0, and after that libusb_handle_events() needs to return control to event_thread_func. But unless some event happens, libusb_handle_events() will not return.

There are 2 different ways of dealing with this, depending on if your application uses libusb' hotplug support or not.

Applications which do not use hotplug support, should not start the event thread until after their first call to libusb_open(), and should stop the thread when closing the last open device as follows:

void my_close_handle(libusb_device_handle *dev_handle)
{
    if (open_devs == 1)
        event_thread_run = 0;

    libusb_close(dev_handle); // This wakes up libusb_handle_events()

    if (open_devs == 1)
        pthread_join(event_thread);

    open_devs--;
}

Applications using hotplug support should start the thread at program init, after having successfully called libusb_hotplug_register_callback(), and should stop the thread at program exit as follows:

void my_libusb_exit(void)
{
    event_thread_run = 0;
    libusb_hotplug_deregister_callback(ctx, hotplug_cb_handle); // This wakes up libusb_handle_events()
    pthread_join(event_thread);
    libusb_exit(ctx);
}

Enumeration Type Documentation

libusb_transfer_flags

enum libusb_transfer_flags

libusb_transfer.flags values

Enumerator
LIBUSB_TRANSFER_SHORT_NOT_OK	Report short frames as errors
LIBUSB_TRANSFER_FREE_BUFFER	Automatically free() transfer buffer during libusb_free_transfer(). Note that buffers allocated with libusb_dev_mem_alloc() should not be attempted freed in this way, since free() is not an appropriate way to release such memory.
LIBUSB_TRANSFER_FREE_TRANSFER	Automatically call libusb_free_transfer() after callback returns. If this flag is set, it is illegal to call libusb_free_transfer() from your transfer callback, as this will result in a double-free when this flag is acted upon.
LIBUSB_TRANSFER_ADD_ZERO_PACKET	Terminate transfers that are a multiple of the endpoint's wMaxPacketSize with an extra zero length packet. This is useful when a device protocol mandates that each logical request is terminated by an incomplete packet (i.e. the logical requests are not separated by other means). This flag only affects host-to-device transfers to bulk and interrupt endpoints. In other situations, it is ignored. This flag only affects transfers with a length that is a multiple of the endpoint's wMaxPacketSize. On transfers of other lengths, this flag has no effect. Therefore, if you are working with a device that needs a ZLP whenever the end of the logical request falls on a packet boundary, then it is sensible to set this flag on every transfer (you do not have to worry about only setting it on transfers that end on the boundary). This flag is currently only supported on Linux. On other systems, libusb_submit_transfer() will return LIBUSB_ERROR_NOT_SUPPORTED for every transfer where this flag is set. Available since libusb-1.0.9.

libusb_transfer_status

enum libusb_transfer_status

Transfer status codes

Enumerator
LIBUSB_TRANSFER_COMPLETED	Transfer completed without error. Note that this does not indicate that the entire amount of requested data was transferred.
LIBUSB_TRANSFER_ERROR	Transfer failed
LIBUSB_TRANSFER_TIMED_OUT	Transfer timed out
LIBUSB_TRANSFER_CANCELLED	Transfer was cancelled
LIBUSB_TRANSFER_STALL	For bulk/interrupt endpoints: halt condition detected (endpoint stalled). For control endpoints: control request not supported.
LIBUSB_TRANSFER_NO_DEVICE	Device was disconnected
LIBUSB_TRANSFER_OVERFLOW	Device sent more data than requested

libusb_transfer_type

enum libusb_transfer_type

Transfer type

Enumerator
LIBUSB_TRANSFER_TYPE_CONTROL	Control transfer
LIBUSB_TRANSFER_TYPE_ISOCHRONOUS	Isochronous transfer
LIBUSB_TRANSFER_TYPE_BULK	Bulk transfer
LIBUSB_TRANSFER_TYPE_INTERRUPT	Interrupt transfer
LIBUSB_TRANSFER_TYPE_BULK_STREAM	Bulk stream transfer

Function Documentation

libusb_alloc_streams()

int API_EXPORTED libusb_alloc_streams	(	libusb_device_handle *	dev_handle,
		uint32_t	num_streams,
		unsigned char *	endpoints,
		int	num_endpoints
	)

Allocate up to num_streams usb bulk streams on the specified endpoints. This function takes an array of endpoints rather then a single endpoint because some protocols require that endpoints are setup with similar stream ids. All endpoints passed in must belong to the same interface.

Note this function may return less streams then requested. Also note that the same number of streams are allocated for each endpoint in the endpoint array.

Stream id 0 is reserved, and should not be used to communicate with devices. If libusb_alloc_streams() returns with a value of N, you may use stream ids 1 to N.

Since version 1.0.19, LIBUSB_API_VERSION >= 0x01000103

Parameters

dev_handle	a device handle
num_streams	number of streams to try to allocate
endpoints	array of endpoints to allocate streams on
num_endpoints	length of the endpoints array

Returns

number of streams allocated, or a LIBUSB_ERROR code on failure

libusb_alloc_transfer()

DEFAULT_VISIBILITY struct libusb_transfer* LIBUSB_CALL libusb_alloc_transfer

(

int

iso_packets

)

Allocate a libusb transfer with a specified number of isochronous packet descriptors. The returned transfer is pre-initialized for you. When the new transfer is no longer needed, it should be freed with libusb_free_transfer().

Transfers intended for non-isochronous endpoints (e.g. control, bulk, interrupt) should specify an iso_packets count of zero.

For transfers intended for isochronous endpoints, specify an appropriate number of packet descriptors to be allocated as part of the transfer. The returned transfer is not specially initialized for isochronous I/O; you are still required to set the num_iso_packets and type fields accordingly.

It is safe to allocate a transfer with some isochronous packets and then use it on a non-isochronous endpoint. If you do this, ensure that at time of submission, num_iso_packets is 0 and that type is set appropriately.

Parameters

iso_packets

number of isochronous packet descriptors to allocate. Must be non-negative.

Returns

a newly allocated transfer, or NULL on error

libusb_cancel_transfer()

int API_EXPORTED libusb_cancel_transfer

(

struct libusb_transfer *

transfer

)

Asynchronously cancel a previously submitted transfer. This function returns immediately, but this does not indicate cancellation is complete. Your callback function will be invoked at some later time with a transfer status of LIBUSB_TRANSFER_CANCELLED.

Parameters

transfer

the transfer to cancel

Returns

0 on success

LIBUSB_ERROR_NOT_FOUND if the transfer is not in progress, already complete, or already cancelled.

a LIBUSB_ERROR code on failure

libusb_dev_mem_alloc()

DEFAULT_VISIBILITY unsigned char* LIBUSB_CALL libusb_dev_mem_alloc	(	libusb_device_handle *	dev_handle,
		size_t	length
	)

Attempts to allocate a block of persistent DMA memory suitable for transfers against the given device. If successful, will return a block of memory that is suitable for use as "buffer" in libusb_transfer against this device. Using this memory instead of regular memory means that the host controller can use DMA directly into the buffer to increase performance, and also that transfers can no longer fail due to kernel memory fragmentation.

Note that this means you should not modify this memory (or even data on the same cache lines) when a transfer is in progress, although it is legal to have several transfers going on within the same memory block.

Will return NULL on failure. Many systems do not support such zero-copy and will always return NULL. Memory allocated with this function must be freed with libusb_dev_mem_free. Specifically, this means that the flag LIBUSB_TRANSFER_FREE_BUFFER cannot be used to free memory allocated with this function.

Since version 1.0.21, LIBUSB_API_VERSION >= 0x01000105

Parameters

dev_handle	a device handle
length	size of desired data buffer

Returns

a pointer to the newly allocated memory, or NULL on failure

libusb_dev_mem_free()

int API_EXPORTED libusb_dev_mem_free	(	libusb_device_handle *	dev_handle,
		unsigned char *	buffer,
		size_t	length
	)

Free device memory allocated with libusb_dev_mem_alloc().

Parameters

dev_handle	a device handle
buffer	pointer to the previously allocated memory
length	size of previously allocated memory

Returns

LIBUSB_SUCCESS, or a LIBUSB_ERROR code on failure

libusb_free_streams()

int API_EXPORTED libusb_free_streams	(	libusb_device_handle *	dev_handle,
		unsigned char *	endpoints,
		int	num_endpoints
	)

Free usb bulk streams allocated with libusb_alloc_streams().

Note streams are automatically free-ed when releasing an interface.

Since version 1.0.19, LIBUSB_API_VERSION >= 0x01000103

Parameters

dev_handle	a device handle
endpoints	array of endpoints to free streams on
num_endpoints	length of the endpoints array

Returns

LIBUSB_SUCCESS, or a LIBUSB_ERROR code on failure

libusb_free_transfer()

void API_EXPORTED libusb_free_transfer

(

struct libusb_transfer *

transfer

)

Free a transfer structure. This should be called for all transfers allocated with libusb_alloc_transfer().

If the LIBUSB_TRANSFER_FREE_BUFFER flag is set and the transfer buffer is non-NULL, this function will also free the transfer buffer using the standard system memory allocator (e.g. free()).

It is legal to call this function with a NULL transfer. In this case, the function will simply return safely.

It is not legal to free an active transfer (one which has been submitted and has not yet completed).

Parameters

transfer

the transfer to free

libusb_submit_transfer()

int API_EXPORTED libusb_submit_transfer

(

struct libusb_transfer *

transfer

)

Submit a transfer. This function will fire off the USB transfer and then return immediately.

Parameters

transfer

the transfer to submit

Returns

0 on success

LIBUSB_ERROR_NO_DEVICE if the device has been disconnected

LIBUSB_ERROR_BUSY if the transfer has already been submitted.

LIBUSB_ERROR_NOT_SUPPORTED if the transfer flags are not supported by the operating system.

LIBUSB_ERROR_INVALID_PARAM if the transfer size is larger than the operating system and/or hardware can support (see Transfer length limitations)

another LIBUSB_ERROR code on other failure

libusb_transfer_get_stream_id()

uint32_t API_EXPORTED libusb_transfer_get_stream_id

(

struct libusb_transfer *

transfer

)

Get a transfers bulk stream id.

Since version 1.0.19, LIBUSB_API_VERSION >= 0x01000103

Parameters

transfer

the transfer to get the stream id for

Returns

the stream id for the transfer

libusb_transfer_set_stream_id()

void API_EXPORTED libusb_transfer_set_stream_id	(	struct libusb_transfer *	transfer,
		uint32_t	stream_id
	)

Set a transfers bulk stream id. Note users are advised to use libusb_fill_bulk_stream_transfer() instead of calling this function directly.

Since version 1.0.19, LIBUSB_API_VERSION >= 0x01000103

Parameters

transfer	the transfer to set the stream id for
stream_id	the stream id to set

See also

libusb_alloc_streams()

void()

typedef void

(

LIBUSB_CALL *

libusb_transfer_cb_fn

)

Asynchronous transfer callback function type. When submitting asynchronous transfers, you pass a pointer to a callback function of this type via the callback member of the libusb_transfer structure. libusb will call this function later, when the transfer has completed or failed. See Asynchronous device I/O for more information.

Parameters

transfer

The libusb_transfer struct the callback function is being notified about.