How do I create a library in linux from c++ code with external references

Am having a problem with external references of libraries. I need to access the ogg libraries from xiph.org. I have simplified it down to the following. This works: in main.cpp

   #include "OpusInfoTest.h"

   int main (int arggc, char *argv[]) {
       OpusInfoTest *o = new OpusInfoTest("sampletest.opus");
       delete (o);
       return 0;
   }

in OpusInfoTest.cpp

#include "OpusInfoTest.h"

    OpusInfoTest::OpusInfoTest (char *qualifiedOpusFile) {
        printf ("File %s\n", qualifiedOpusFile);
        ogg_sync_state ogsync;
        ogg_sync_init(&ogsync);
    
    }
    
    OpusInfoTest::~OpusInfoTest () {
        printf ("Gone\n");
    }

and OpusInfoTest.h

#pragma once

#include <string>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <ogg.h>



class OpusInfoTest {
    public:
        OpusInfoTest(char *qualifiedOpusFile);
        ~OpusInfoTest(void);

};

Compiled with

g++ main.cpp OpusInfoTest.cpp -I ../ogg/buildl/include/ogg -I ../ogg/buildl/include -I ../opustools/src -I ../ogg/include/ogg -I ../ogg/include/ -Wno-write-strings -L ../ogg/buildl -l:libogg.a

Works fine. I then created a library OIT.a with

g++ -c -o OIT.a OpusInfoTest.cpp -I ../ogg/buildl/include/ogg -I ../ogg/buildl/include -I ../opustools/src -I ../ogg/include/ogg -I ../ogg/include/ -Wno-write-strings -L ../ogg/buildl -l:libogg.a

And then tried to use it with main.cpp

 g++ main.cpp -I ../ogg/buildl/include/ogg -I ../ogg/buildl/include -I ../opustools/src -I ../ogg/include/ogg -I ../ogg/include/ -Wno-write-strings -L ../ogg/buildl -l:libogg.a -L . -l:OIT.a

returns link error

buildl -l:libogg.a -L . -l:OIT.a
/usr/bin/ld: ./OIT.a: in function `OpusInfoTest::OpusInfoTest(char*)':
OpusInfoTest.cpp:(.text+0x33): undefined reference to `ogg_sync_init'
collect2: error: ld returned 1 exit status

Probably missing something simple. New to building libraries on linux

Solution

Compile to object files like:

g++ -c OpusInfoTest.cpp -I ../ogg/buildl/include/ogg [other include paths]

You should get a file called OpusInfoTest.o (not a library yet).

Then you can use the ar command to create libOIT.a , which is a static library.

ar rcs libOIT.a OpusInfoTest.o

Lastly, link everything in the correct order:

g++ main.cpp [include paths] -L. -lOIT -L../ogg/buildl -logg

You need to make sure your libOIT.a library shows up first in the linking above, since it depends on the other libogg.a library.