Is there a way to check if QNEthernet library is used

[sstaub]

I want to check if the QNEthernet library is used.
For example I want to do following code:

#ifdef QNETHERNET
// code
#endif

Is this possible or is there another way?

[ssilverman]

Can you tell me more about your use case?

[sstaub]

I write a library for sACN. I need to see if the user takes the original Ethernet library for WIZnet or Teensy 4.1 with QNEthernet

[ssilverman]

So is this code that is supposed to run on multiple platforms?

[sstaub]

YES
Maybe you add in QNEthernet.h following lines:

#ifndef QNETHERNET_H
#define QNETHERNET_H

This is a standard procedure when writing libraries.

[ssilverman]

I use #pragma once.

[ssilverman]

Why not check for the platform itself? Eg. ARDUINO_TEENSY41.

[sstaub]

Because you can use Teensy also with external Wiznet chips.

[ssilverman]

Can you tell me more about what you would do differently for every case for which you need to tell the difference? For example, what’s different about Teensy 4.1 with built-in Ethernet vs. Teensy 4.1 with Wiznet vs. some other platform with Wiznet?

[ssilverman]

I’ll add that I have lots of experience with sACN.

[sstaub]

My library is using Multicast which works very different in LWIP than with WIZnet.

[ssilverman]

Note that my Wiznet driver for QNEthernet operates the chip in raw mode, so it behaves the same as the native Ethernet driver does.

I haven’t really played with the Wiznet chip in non-raw mode. Can you tell me more about what’s different about it?

[ssilverman]

…and how you’d do multicast differently? I’m curious.

[sstaub]

Using my own sACN library https://github.com/sstaub/sACN and a user makes following issue sstaub/sACN#12
With the original Ethernet library I open multiple UDP instances like

EthernetUDP sacn1;
EthernetUDP sacn2;
EthernetUDP sacn3;
EthernetUDP sacn4;

Then I use beginMulticast(mcastIP, ACN_SDT_MULTICAST_PORT) for each instance.

[ssilverman]

What about using C++’s __has_include feature? Might that work for you?

[sstaub]

Never used the feature, thank you for that hint.

[ssilverman]

Eg. #if __has_include(<QNEthernet.h>)

However, that may not work with the Arduino IDE; I’m not sure…

[ssilverman]

Are you using the Arduino IDE or something else, such as PlatformIO?

[sstaub]

I'm using PIO but also the Arduino IDE to check if it works there also.

[ssilverman]

I’m going to think more about this. I’m trying to think of ways to make this more helpful, and what I’m coming up with is an externally-set #define for the project. This is easy in PlatformIO (-D option), but less easy with Arduino IDE and Teensy (platform.local.txt).

[ssilverman]

It’s easier with non-Teensy plus the Arduino IDE because there already exists a platform.local.txt.

See: https://github.com/ssilverman/QNEthernet#configuring-macros-using-the-arduino-ide

[ssilverman]

The more I think about this, the more I’m thinking that the way to go is this: the way to affect what changes at compile time is by changing the compile options.

I don’t love defines from a header because users can get confused if the expected define doesn’t exist. This is because they often don’t realize you must include that header everywhere the define is referenced.

[sstaub]

The defines at the beginning for the .h file comes from the Arduino docs https://docs.arduino.cc/learn/contributions/arduino-creating-library-guide/

[ssilverman]

That's not a guaranteed way to determine existence — its purpose is to prevent multiple inclusions, not to detect presence because the mechanism to actually do this will change between libraries and headers. For example, I use #pragma once instead of #define guards. (See also https://google.github.io/styleguide/cppguide.html#The__define_Guard.) Including <QNEthernet.h> just to find out if the QNEthernet library is being used doesn't make sense to me. The whole point is to not include it if you don't want to use the library. At that point, you're going to need something external telling the compiler what to include and what to not include. That's the whole purpose of conditional compilation using compiler defines. i.e. -Dvariable=value.

[ssilverman]

If you don't want to use conditional compilation based on your own compiler defines, you could check ARDUINO_TEENSY41 or even something that LWIP itself defines. But remember that detecting a define doesn't work unless the file that looks at that define also includes the header that defines that define.

I'm stating the following for future readers; you may already know this: It's a misconception with some Arduino IDE users that if you include a file, then #defines are global to a project. Now, the IDE itself may provide some preprocessing magic to make that true, I'm not sure, but it's not correct to assume this is the case. That's not how defines work. The only way to guarantee that a define can be detected by everything is either by giving the appropriate compiler option on every file the define is needed, or by including some header that defines the macro in every file that needs it.