Using object from library within different *.c file in project

Hello together,
I am trying to add a library mcp4725 to a grbl Arduino project from GitHub.
My question is how do I declare an object of the dac library which then can be used in.a.different sub .c file within the project to let's say set a dacvoltage.
Everything works fine within one sketch but as soon as I want to use a defined object in a different sketch file it is unknown.
Can anyone give me some support on the matter?

Why is the separate file a .c? Arduino uses a C++ compiler. While it’s possible to include a .c file, the C language doesn’t know anything about objects and OOP. Using a .cpp file will make your life easier. For the proper technique to share a global object across files, see my Reply #3 here.

thank you for your input. I might have to be more specific about my current problem:

If you look at the picture: I declare a member od an I2C sensor in grbl_Laser:

#include <Adafruit_MCP4725.h>
Adafruit_MCP4725 dacx, dacy;

and want to use that member in lets say gcode.c file. If I call that member by e.g.

dacx.setVoltage(1,false);

I get the following failure from compiler:
‘dacx’ undeclared (first use in this function); did you mean ‘max’?
How am I able to succesfully implmement that member into gcode.c???

Add:

#include <Adafruit_MCP4725.h>
extern Adafruit_MCP4725 dacx, dacy;

I won't be going to some random web site to look at a picture. Especially if it's a picture of code -- that's useless. To see how to properly post code, see Item #6 here: Read this before posting a programming question .... As the title says, you should have read that before posting your question.

Julianchen:
and want to use that member in lets say gcode.c file. If I call that member by e.g.

dacx.setVoltage(1,false);

I get the following failure from compiler:
'dacx' undeclared (first use in this function); did you mean 'max'?
How am I able to succesfully implmement that member into gcode.c???

As I already told you, the C language doesn't know about C++ objects. So you can't do this. That's why I said to use a .cpp file.