How to separate functions between .h and .c files - c++

Question

I am attempting to separate my function prototypes and the actual implementantions between .h and .c files to make it easier to see what functions I have.
my compiler, visual studio, has removed the curly green underline from under the function prototypes in the header files implying it is seeing the implementations in the .c file, but when I try to compile it I get 100+ errors all saying stuff like "syntax error: missing ';' before '*'" or "syntax error: missing ')' before '*'" or "syntax error: missing '}' before '*'" both in the .h and .c files. They are mostly pointing to the parameters in the prototypes, but I could not go trough all to know for sure.
Is there some other special way to do this?
I tried:
Making them extern
Changing guards to a name i know for sure isnt already used
I do not know what else I can try.

Answer 1

The first thing you could try is posting your code so we can see it :-)
However, the error syntax error: missing ';' before ... almost always shows up because you have an unknown type, as per the following:
int main() {
NOTYPE *x;
return 0;
}
Since you're moving function prototypes into a header, it may be that they're using types that still exist in the C file. Those type declarations must exist before use, or you will get that error.
That (function prototypes with unknown types) is not the only reason that error occurs but I can almost guarantee it'll be a similar issue, trying the use a type not yet declared.
As per other information posted here, you can also get this problem with circular dependencies, such as with:
typedef struct { TWO *two; } ONE; // don't know anything about TWO here.
typedef struct { ONE *one; } TWO;
If the dependency was only one way, then you could just ensure the order of the two definitions was correct. With a circular dependency, you have to introduce an incomplete definition (really a declaration) to allow it to work:
typedef struct TWO; // declare that TWO exists.
typedef struct { TWO *two; } ONE; // define ONE, we know TWO exists.
typedef struct { ONE *one; } TWO; // define ONE, we know all about ONE.
Note that this only works if you declare a pointer to the incomplete type, you won't be able to do:
typedef struct TWO;
typedef struct { TWO two; } ONE;
because the definition of ONE requires a complete type.

Answer 2

I have figured out the error.
The problem was that I had a circular dependency with 2 structures and I tried to fix it by defining them both before implementing them, but that obviously did not work.
this was my code:
typedef struct A;
typedef struct B;
typedef struct {
B* blist;
}A;
typedef struct {
A* a_parent;
}B;
The following stackoverflow answer helped me fix my errors:
Resolve circular typedef dependency?