This is the second time that I have forgotten how NxTerm keyboard input is handled and was inspired to write a Wiki page. The first was when I analyzed CNxConsole Keyboard Input. This time, I am focused on the NxTerm example at apps/examples/nxterm.

The Normal Case

Keyboard and mouse inputs are received by the application through window callbacks, just like with the Xorg X server. Some listener needs to inject the keyboard input via nx_kbdin() (libs/libnx/nx_kbdin.c) which sends a message containing the keyboard input to the NX server. The NX server will forward that keyboard input to the window that has focus.

The Window application listens for NX server events by calling nx_eventhandler() (libs/libnx/nx_eventhandler) on another listener thread. If the window has focus when the key press is entered, nx_eventhandler()will forward the key press information to the registered window event handler.

In apps/examples/nxterm, nxterm_listener.c is the thread that drives nx_eventhandler(). The normal window NX keyboard callback is the function nxwndo_kbdin() in nxterm_wndo.c. That callback function just writes the keyboard data to stdout.

The apps/examples/nxterm/ Kludge

apps/examples/nxterm does not do things in the normal way, however. NSH does not receive keyboard input from NX; it gets keyboard input directly from the default stdin which is probably not a keyboard at all but more likely the host PC serial console. This is okay because only the background window is used and that example does not need the help of NX to select the window that has focus.

Re-direction of stdout and stderr

stdin in and stderr are re-directed to the NxTerm character driver in nxterm_main.c:

/* Now re-direct stdout and stderr so that they use the NX console driver.
* Note that stdin is retained (file descriptor 0, probably the serial
* console).
*/

printf("nxterm_main: Starting the console task\n");

fflush(stdout);
fflush(stderr);

fclose(stdout);
fclose(stderr);

dup2(fd, 1);
dup2(fd, 2);

Note that stdin is not re-directed in this example.

What Is BOARDIOC_NXTERM_IOCTL and Where Is It Used?

The boardctl() command BOARDIOC_NXTERM_IOCTL allows an application to inject keyboard data into NX for forwarding to the window with focus. In apps/examples/nxterm, the BOARDIOC_NXTERM_IOCTL is only called for the case of a redraw event. A redraw event may happen when a window above the current window is moved and the text is exposed.

If you use only the background window for NxTerm, then the background window will always have focus. It will always have focus and will never be redrawn and the BOARDIOC_NXTERM_IOCTL will never be used (Unless, perhaps, you choose to implement pop-up error messages or menus on top of the NxTerm background).

Keyboard Data Flow in apps/examples/nxterm

Character input driver receives input
NSH receives input on stdin and processes it (stdin is not redirected)
Data is output to stdout (redirected to the NxTerm driver)

In this simple, background-window-only case, BOARDIOC_NXTERM_IOCTL will never be used.

Keyboard Data Flow in the Generic Window Case

See, for an example, apps/graphics/nxwm/src/cnxterm.cxx. In this case, the behavior will change, depending on the selection of CONFIG_NXTERM_NXKBDIN: If CONFIG_NXTERM_NXKBDIN is not selected, then the behavior will be similar to apps/examples/nxterm; stdin will not be redirected an keyboard input will come directly from the the system console.

But is CONFIG_NXTERM_NXKBDIN is select, NSH's stdin will be re-redirected to the to the NxTerm character driver. Keyboard input will arrive on stdin from the NxTerm driver rather than from the system console. The following sequence describes the keyboard input in this latter case:

Character input driver receives input,
Some keyboard listener thread receives input and injects it into NX via a call to nx_kbdin(),
NX sends an event to the registered kbdin() method of the window that has focus, providing the keyboard input to the window application. In this case, the window application of interest is the NxTerm character driver,
If the NxTerm is the window with focus, the NxTerm character driver receives, buffers, and provides that keyboard input for the next read operation,
NSH receives input on stdin which was re-directed to the NxTerm character driver. NSH processes the input, and
NSH outputs data to stdout which was re-directed to the NxTerm character driver.

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NxTerm Example