Man page 9 : usb_alloc_urb

usb_alloc_urb - creates a new urb for a USB driver to use

struct urb * usb_alloc_urb(int  iso_packets , gfp_t  mem_flags );

iso_packets
    number of iso packets for this urb

mem_flags
    the type of memory to allocate, see kmalloc for a list of valid options for this.

Creates an urb for the USB driver to use, initializes a few internal structures, incrementes the usage counter, and returns a pointer to it.

If no memory is available, NULL is returned.

If the driver want to use this urb for interrupt, control, or bulk endpoints, pass Aq0Aq as the number of iso packets.

The driver must call usb_free_urb when it is finished with the urb.