"undefined reference to" many (all?) of the functions in shared library (newbie)

I'm a novice at C++. Be patient if this is incoherent. I'm called upon to build a large system on linux that was originally built on OS X, where it works fine. The original authors are no longer with the company. The build system makes use of autotools, but there are also some hand made Makefiles which walk through the system calling the auto-made Makefiles. I've managed to get all of the c++ code compiled. The build system also uses libtools, and shared libraries are produced and deposited in /usr/local/lib.

So now I'd like to use these libraries. I've written a short program that simply instantiates an object of class ds_dictionary and calls one of its methods. Here it is:

#include <iostream>
#include <DSUtils/DSUtils.h>

int main (int argc, char * const argv[]) {

    int32_t integer_data=123;
    char key_alice_integer[] = "alice_integer"; 

    ds_dictionary my_dict;
    my_dict.add_int(key_alice_integer, integer_data);

    return 0;
}

I compile this with

g++ -lDSUtils -o main my_test_code.cpp

With the result:

//usr/local/lib/libDSUtils.so: undefined reference to `ds_breakdown_from_time_interval'
//usr/local/lib/libDSUtils.so: undefined reference to `ds_date_breakdown_with_string'
//usr/local/lib/libDSUtils.so: undefined reference to `ds_seconds_duration_of_interval'
...  (about 25 lines like these)
collect2: ld returned 1 exit status

Let's look inside the library:

garyp@VM:/usr/local/lib$ nm libDSUtils.so | grep ds_breakdown_from_time
    U ds_breakdown_from_time_interval

1. The "U" in the line above ... does that mean that the library wasn't built correctly?
2. Am I calling g++ correctly?
3. Do I have to put something in the code to tell it that I'm using functions found in that library?
4. What are possible errors? Where should I start poking around?

EDIT: Aha. The library DSUtils is built from several c++ sources. There is one c program in the source, and it contains all of the problem functions. The Makefile system doesn't deal at all with that one c file. That c program compiles. Ideally I suppose I'd figure out how to modify the Makefile to compile that file and add it to the library, but I'm not to the point where I can figure out how to do that.

Can I add the .o file to the existing library? How? Create a library with one file? etc?

EDIT_2: I simply did

g+开发者_开发问答+ -o main -lDSUtils main.o my_new_objectfile.o

and the thing compiles, links, and runs without error. Should that work? After fixing a logic bug, it does work.

This

U ds_breakdown_from_time_interval

tells me that ds_breakdown_from_time_interval will be resolved by another library during runtime. So I am guessing you need to link to the library that defines ds_breakdown_from_time_interval.