I built the code from the STM32CubeF4 for the USB CDC example. I added the missing receive code for CDC_Receive_FS() in usbd_cdc_if.c. I loaded this into my STM32F4 Discovery and it works. A character typed on Tera Term returns and is displayed on Tera Term.
I am hoping that someone here, could give me some knowledge about how this USB CDC firmware works, specifically, is this being driven by an interrupt that is generated when there is a level shift in voltage on the USB -D and +D pins, or is there an infinite while loop that was launched somewhere, and it's just polling waiting for some data to appear? What prompted my question is that I see that one can blink the LEDs on this board by toggling the state of the GPIO pins within an infinite while loop in main.c. However, there is nothing within this while loop at all within main.c for USB. So how does this USB CDC firmware get and send a character from/to Tera Term.
I will take the 2 minutes to answer you instead of lecturing you. Receive is done through interrupts. Very, very simply, the hardware sees the voltage change on the D+/D- and flags an interrupt based on the intialization functions. The interrupt calls HAL_PCD_IRQHandler, which calls USBD_LL_DataInStage in the usbd_conf.c file. That ends up calling the function USBD_CDC_DataIn in the usbd_cdc.c file. There is your starting point, but it is not simple. To do what you want you might have to stop the output to UART and just handle it in the main loop.