How to call C functions from CPP file

I have somу C source files and some CPP source files.
I'm trying to call function, written in C from CPP file.
For example

filea.c

int apb(int a, int b)
{
 return a+b;
}

filea.h

int apb(int a, int b);

fileb.cpp

#include "filea.h"
int main(void)
{
 return apb(5,1);
}

compiler tells me error:

Error: L6218E: Undefined symbol apb(int, int) (referred from fileb.o).