The Printer function channels data between the USB host and a
userspace program driving the print engine. The user space
program reads and writes the device file /dev/g_printer<X> to
receive or send printer data. It can use ioctl calls to
the device file to get or set printer status.

For more information, see <a href="https://www.kernel.org/doc/Documentation/usb/gadget_printer.txt
which">Documentation/usb/gadget_printer.txt
which</a> includes sample code for accessing the device file.