I saw so many topics about this issue: Error: L6218E: Undefined symbol
But I don't understand the solution for it.
In my program, the symbol is a function from other file (simplicity studio file). so why?
anyone please!
If you didn't know, then the warning level may not have been cranked up high enough. The compiler should be able to issue a warning if you define a globally visible function without first having seen a matching global declaration.
But generally speaking, that's just one of those aspects where writing code differs from writing prose: compilers are way pickier than humans about exactly the right spelling of things. "Almost the same" does not cut it.