Data Structures | |
struct | libusb_control_setup |
Setup packet for control transfers. More... | |
struct | libusb_iso_packet_descriptor |
Isochronous packet descriptor. More... | |
struct | libusb_transfer |
The generic USB transfer structure. More... | |
Typedefs | |
typedef void(* | libusb_transfer_cb_fn )(struct libusb_transfer *transfer) |
Asynchronous transfer callback function type. | |
Enumerations | |
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 } |
Transfer status codes. More... | |
enum | libusb_transfer_flags { LIBUSB_TRANSFER_SHORT_NOT_OK = 1<<0, LIBUSB_TRANSFER_FREE_BUFFER = 1<<1, LIBUSB_TRANSFER_FREE_TRANSFER = 1<<2 } |
libusb_transfer.flags values More... | |
Functions | |
struct libusb_transfer * | libusb_alloc_transfer (int iso_packets) |
Allocate a libusb transfer with a specified number of isochronous packet descriptors. | |
void | libusb_free_transfer (struct libusb_transfer *transfer) |
Free a transfer structure. | |
int | libusb_submit_transfer (struct libusb_transfer *transfer) |
Submit a transfer. | |
int | libusb_cancel_transfer (struct libusb_transfer *transfer) |
Asynchronously cancel a previously submitted transfer. | |
static unsigned char * | libusb_control_transfer_get_data (struct libusb_transfer *transfer) |
Get the data section of a control transfer. | |
static struct libusb_control_setup * | libusb_control_transfer_get_setup (struct libusb_transfer *transfer) |
Get the control setup packet of a control transfer. | |
static void | libusb_fill_control_setup (unsigned char *buffer, uint8_t bmRequestType, uint8_t bRequest, uint16_t wValue, uint16_t wIndex, uint16_t wLength) |
Helper function to populate the setup packet (first 8 bytes of the data buffer) for a control transfer. | |
static void | libusb_fill_control_transfer (struct libusb_transfer *transfer, libusb_device_handle *dev_handle, unsigned char *buffer, libusb_transfer_cb_fn callback, void *user_data, unsigned int timeout) |
Helper function to populate the required libusb_transfer fields for a control transfer. | |
static void | libusb_fill_bulk_transfer (struct libusb_transfer *transfer, libusb_device_handle *dev_handle, unsigned char endpoint, unsigned char *buffer, int length, libusb_transfer_cb_fn callback, void *user_data, unsigned int timeout) |
Helper function to populate the required libusb_transfer fields for a bulk transfer. | |
static void | libusb_fill_interrupt_transfer (struct libusb_transfer *transfer, libusb_device_handle *dev_handle, unsigned char endpoint, unsigned char *buffer, int length, libusb_transfer_cb_fn callback, void *user_data, unsigned int timeout) |
Helper function to populate the required libusb_transfer fields for an interrupt transfer. | |
static void | libusb_fill_iso_transfer (struct libusb_transfer *transfer, libusb_device_handle *dev_handle, unsigned char endpoint, unsigned char *buffer, int length, int num_iso_packets, libusb_transfer_cb_fn callback, void *user_data, unsigned int timeout) |
Helper function to populate the required libusb_transfer fields for an isochronous transfer. | |
static void | libusb_set_iso_packet_lengths (struct libusb_transfer *transfer, unsigned int length) |
Convenience function to set the length of all packets in an isochronous transfer, based on the num_iso_packets field in the transfer structure. | |
static unsigned char * | libusb_get_iso_packet_buffer (struct libusb_transfer *transfer, unsigned int packet) |
Convenience function to locate the position of an isochronous packet within the buffer of an isochronous transfer. | |
static unsigned char * | libusb_get_iso_packet_buffer_simple (struct libusb_transfer *transfer, unsigned int packet) |
Convenience function to locate the position of an isochronous packet within the buffer of an isochronous transfer, for transfers where each packet is of identical size. |
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.
This is represented by the public libusb_transfer structure type.
Allocation is done with the libusb_alloc_transfer() function. You must use this function rather than allocating your own transfers.
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().
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.
It is undefined behaviour to free a transfer which has not completed.
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.
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.
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.
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:
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:
length
was 12, then you should expect an actual_length
of 4 to indicate that the data was transferred in entirity.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.
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:
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:
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.
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.
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.
The first issue to discuss here is how your application can figure out when libusb has work to do. In fact, there are two naive options which do not actually require your application to know this:
The first option is plainly not very nice, and will cause unnecessary CPU wakeups leading to increased power usage and decreased battery life. The second option is not very nice either, but may be the nicest option available to you if the "proper" approach can not be applied to your application (read on...).
The recommended option is to integrate libusb with your application main event loop. libusb exposes a set of file descriptors which allow you to do this. Your main loop is probably already calling poll() or select() or a variant on a set of file descriptors for other event sources (e.g. keyboard button presses, mouse movements, network sockets, etc). You then add libusb's file descriptors to your poll()/select() calls, and when activity is detected on such descriptors you know it is time to call libusb_handle_events().
There is one final event handling complication. libusb supports asynchronous transfers which time out after a specified time period, and this requires that libusb is called into at or after the timeout so that the timeout can be handled. So, in addition to considering libusb's file descriptors in your main event loop, you must also consider that libusb sometimes needs to be called into at fixed points in time even when there is no file descriptor activity.
For the details on retrieving the set of file descriptors and determining the next timeout, see the polling and timing API documentation.
typedef void(* libusb_transfer_cb_fn)(struct libusb_transfer *transfer) |
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.
transfer | The libusb_transfer struct the callback function is being notified about. |
Transfer status codes.
libusb_transfer.flags values
LIBUSB_TRANSFER_SHORT_NOT_OK | Report short frames as errors. |
LIBUSB_TRANSFER_FREE_BUFFER | Automatically free() transfer buffer during libusb_free_transfer(). |
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. |
struct libusb_transfer* libusb_alloc_transfer | ( | int | iso_packets | ) | [read] |
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.
iso_packets | number of isochronous packet descriptors to allocate |
void 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).
transfer | the transfer to free |
int libusb_submit_transfer | ( | struct libusb_transfer * | transfer | ) |
Submit a transfer.
This function will fire off the USB transfer and then return immediately.
transfer | the transfer to submit |
LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
LIBUSB_ERROR_BUSY if the transfer has already been submitted.
another LIBUSB_ERROR code on other failure
int 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.
transfer | the transfer to cancel |
LIBUSB_ERROR_NOT_FOUND if the transfer is already complete or cancelled.
a LIBUSB_ERROR code on failure
static unsigned char* libusb_control_transfer_get_data | ( | struct libusb_transfer * | transfer | ) | [inline, static] |
Get the data section of a control transfer.
This convenience function is here to remind you that the data does not start until 8 bytes into the actual buffer, as the setup packet comes first.
Calling this function only makes sense from a transfer callback function, or situations where you have already allocated a suitably sized buffer at transfer->buffer.
transfer | a transfer |
static struct libusb_control_setup* libusb_control_transfer_get_setup | ( | struct libusb_transfer * | transfer | ) | [static, read] |
Get the control setup packet of a control transfer.
This convenience function is here to remind you that the control setup occupies the first 8 bytes of the transfer data buffer.
Calling this function only makes sense from a transfer callback function, or situations where you have already allocated a suitably sized buffer at transfer->buffer.
transfer | a transfer |
static void libusb_fill_control_setup | ( | unsigned char * | buffer, | |
uint8_t | bmRequestType, | |||
uint8_t | bRequest, | |||
uint16_t | wValue, | |||
uint16_t | wIndex, | |||
uint16_t | wLength | |||
) | [inline, static] |
Helper function to populate the setup packet (first 8 bytes of the data buffer) for a control transfer.
The wIndex, wValue and wLength values should be given in host-endian byte order.
buffer | buffer to output the setup packet into | |
bmRequestType | see the bmRequestType field of libusb_control_setup | |
bRequest | see the bRequest field of libusb_control_setup | |
wValue | see the wValue field of libusb_control_setup | |
wIndex | see the wIndex field of libusb_control_setup | |
wLength | see the wLength field of libusb_control_setup |
static void libusb_fill_control_transfer | ( | struct libusb_transfer * | transfer, | |
libusb_device_handle * | dev_handle, | |||
unsigned char * | buffer, | |||
libusb_transfer_cb_fn | callback, | |||
void * | user_data, | |||
unsigned int | timeout | |||
) | [inline, static] |
Helper function to populate the required libusb_transfer fields for a control transfer.
If you pass a transfer buffer to this function, the first 8 bytes will be interpreted as a control setup packet, and the wLength field will be used to automatically populate the length field of the transfer. Therefore the recommended approach is:
It is also legal to pass a NULL buffer to this function, in which case this function will not attempt to populate the length field. Remember that you must then populate the buffer and length fields later.
transfer | the transfer to populate | |
dev_handle | handle of the device that will handle the transfer | |
buffer | data buffer. If provided, this function will interpret the first 8 bytes as a setup packet and infer the transfer length from that. | |
callback | callback function to be invoked on transfer completion | |
user_data | user data to pass to callback function | |
timeout | timeout for the transfer in milliseconds |
static void libusb_fill_bulk_transfer | ( | struct libusb_transfer * | transfer, | |
libusb_device_handle * | dev_handle, | |||
unsigned char | endpoint, | |||
unsigned char * | buffer, | |||
int | length, | |||
libusb_transfer_cb_fn | callback, | |||
void * | user_data, | |||
unsigned int | timeout | |||
) | [inline, static] |
Helper function to populate the required libusb_transfer fields for a bulk transfer.
transfer | the transfer to populate | |
dev_handle | handle of the device that will handle the transfer | |
endpoint | address of the endpoint where this transfer will be sent | |
buffer | data buffer | |
length | length of data buffer | |
callback | callback function to be invoked on transfer completion | |
user_data | user data to pass to callback function | |
timeout | timeout for the transfer in milliseconds |
static void libusb_fill_interrupt_transfer | ( | struct libusb_transfer * | transfer, | |
libusb_device_handle * | dev_handle, | |||
unsigned char | endpoint, | |||
unsigned char * | buffer, | |||
int | length, | |||
libusb_transfer_cb_fn | callback, | |||
void * | user_data, | |||
unsigned int | timeout | |||
) | [inline, static] |
Helper function to populate the required libusb_transfer fields for an interrupt transfer.
transfer | the transfer to populate | |
dev_handle | handle of the device that will handle the transfer | |
endpoint | address of the endpoint where this transfer will be sent | |
buffer | data buffer | |
length | length of data buffer | |
callback | callback function to be invoked on transfer completion | |
user_data | user data to pass to callback function | |
timeout | timeout for the transfer in milliseconds |
static void libusb_fill_iso_transfer | ( | struct libusb_transfer * | transfer, | |
libusb_device_handle * | dev_handle, | |||
unsigned char | endpoint, | |||
unsigned char * | buffer, | |||
int | length, | |||
int | num_iso_packets, | |||
libusb_transfer_cb_fn | callback, | |||
void * | user_data, | |||
unsigned int | timeout | |||
) | [inline, static] |
Helper function to populate the required libusb_transfer fields for an isochronous transfer.
transfer | the transfer to populate | |
dev_handle | handle of the device that will handle the transfer | |
endpoint | address of the endpoint where this transfer will be sent | |
buffer | data buffer | |
length | length of data buffer | |
num_iso_packets | the number of isochronous packets | |
callback | callback function to be invoked on transfer completion | |
user_data | user data to pass to callback function | |
timeout | timeout for the transfer in milliseconds |
static void libusb_set_iso_packet_lengths | ( | struct libusb_transfer * | transfer, | |
unsigned int | length | |||
) | [inline, static] |
Convenience function to set the length of all packets in an isochronous transfer, based on the num_iso_packets field in the transfer structure.
transfer | a transfer | |
length | the length to set in each isochronous packet descriptor |
static unsigned char* libusb_get_iso_packet_buffer | ( | struct libusb_transfer * | transfer, | |
unsigned int | packet | |||
) | [inline, static] |
Convenience function to locate the position of an isochronous packet within the buffer of an isochronous transfer.
This is a thorough function which loops through all preceding packets, accumulating their lengths to find the position of the specified packet. Typically you will assign equal lengths to each packet in the transfer, and hence the above method is sub-optimal. You may wish to use libusb_get_iso_packet_buffer_simple() instead.
transfer | a transfer | |
packet | the packet to return the address of |
static unsigned char* libusb_get_iso_packet_buffer_simple | ( | struct libusb_transfer * | transfer, | |
unsigned int | packet | |||
) | [inline, static] |
Convenience function to locate the position of an isochronous packet within the buffer of an isochronous transfer, for transfers where each packet is of identical size.
This function relies on the assumption that every packet within the transfer is of identical size to the first packet. Calculating the location of the packet buffer is then just a simple calculation: buffer + (packet_size * packet)
Do not use this function on transfers other than those that have identical packet lengths for each packet.
transfer | a transfer | |
packet | the packet to return the address of |