In order to link C procedures into C++ programs, you need to declare your C functions in your C++ programs by declaring them in your C++ file. An example is
which declares sqrt to be a function that takes a double as an argument and returns a double. This information is needed so that C++ can check that your uses of this function have the correct type.
If you're using makefiles, you can have the files combined automatically. You'd could use a Makefile like:
to link together "foo.C" (the c++ file) and "bar.c" (ordinary C) to make the executable program "prog".