Update Variable via Arduino to Arduino I2C

Question

I am attempting to send a simple 1 byte value from one arduino to another using I2C with the Wire library. From reading I see that this is typically done using the event monitor supplied by the Wire library, but this has caused problems for me because the handler function may only have one argument and it must be a void function or else the library throws errors during compilation. So I have a couple of questions about what is or is not possible with the Wire library short of modifying the source:

1. Should the Wire.available() function respond to attempted transmissions if the event handler referenced above does not exist? My first idea was to simply write my own function that would be called whenever this value is not 0, but it appears that this value never changes - unless the event handler is used.

2. Can you update variables which reside outside of a function from inside of the function WITHOUT passing those variables as pointer arguments? Because recall from above that I cannot pass any arguments besides the one specified for the number of bytes received.

I cannot understand how this library could be useful for Arduino to Arduino communication if it is literally not possible to use the transmitted values anywhere other than inside of the event handler. My next step is start modifying the source to suit my needs, but I would imagine what I'm describing is a basic thing and would be shocked to find out that it's not possible without modification.

Answer 1

I see now what the issue was. But to answer my questions:

1. Wire.available(); will not work if called outside of an event handler. I confirmed this while reading through the Wire.cpp source code and it explicitly states that the function should only be called from an event handler.

2. It is as simple as only a matter of updating global variables. I feel quite silly for not realizing that simply declaring variables outside of a function gives ANY function access to it rather than only the main function.

I see that the other answer mentions applying the volatile modification to the variables, but this is only necessary when the function in question is attached to an interrupt handler.

Answer 2

Can you update variables which reside outside of a function from inside of the function WITHOUT passing those variables as pointer arguments?

Yes, that's exactly what global variables are for. Make them volatile as they will be updated inside an ISR (or more exactly, a function called from an ISR).