CAN.write() return-code does not comply to API spec

[samuelsadok]

CAN.write() in this core returns 0 for errors but should return a negative error code.

Details

The docstring for the Arduino CAN API states that HardwareCAN::write() returns:

1 if the message was enqueued, an implementation defined error code < 0 if there was an error

This core implements HardwareCAN::write() by calling mbed::CAN::write() and directly returning the result (code, header).

However mbed::CAN::write() has a different spec for returning errors:

0 if write failed, 1 if write was successful

Discussion

This was brought up by @bthacher-crabi in odriverobotics/ODriveArduino#4 and I'd be interested what the maintainers of this repo think. Technically the meaning of return 0 is unspecified in the API, so it's unclear what client code should do with it. It's conceivable that other cores use 0 to mean "success" (which is a fairly common convention). Should we add a preprocessor special case for the mbed core specifically?

Separately, going forward it would be cool if either the code or the API specs are changed so that they are consistent.

[samuelsadok]

Any comment on this? (perhaps @facchinm?)

[aentinger]

Hi @samuelsadok ☕ 👋 Thank you for reporting this, you are indeed correct. I'll prepare a PR with a fix.