Your information is scanty, so I'll hazard a guess, which is a dangerous thing to do.
When your compiler sees a call to a function, it has to generate machine code to call that function and it has to place the arguments in a place (which has been agreed upon before) where the function may find them.
Obviously, it can't do that without knowing the number and form of the arguments. This means that the function's definition, or a description of it, must occur before the compiler encounters the call. The compiler can be appeased by writing the function before you write the part that calls it, or it can be appeased if you provide a pattern (prototype) before you call it.
If you fail to meet any of these requirements in C, the compiler will assume that the function takes an int argument and returns an int. This presumption can lead to problems if the presumption is not true. C++ will not presume anything; it will merely run off into the weeds and puke on its shoes.
You are the architect and designer; C/C++ is just the hourly paid slave. Do your job.