how to remove pthread undefined reference while building single thread library

Question

I am getting undefined reference for pthread API's and I don't know how to resolve them?

Here is the scenario:

libA.a -- this is 3rd party library [it contains lots of API's which are pthread dependent]

libB.a -- This is my own library. I am using few API's of 3rd party library[libA.a] and created my own library.[I myself havn't used any pthread API in libB.a]

I am giving libA.a + libB.a + headers of (A + B) to my client exe. -- say MAIN.exe

MAIN.cpp -- will be using API's provided by my library.

When, I am trying to run MAIN.exe, I am getting undefined reference errors.

Below is the source code:

libA.a: It only contains A.h and A.cpp

A.h

class A { public: void dispA(); void printNumber(); };

A.cpp:

#include "iostream"
#include "A.h"
#include<stdlib.h>
#include "pthread.h"
using namespace std;

void* printNum(void*)
{
    sleep(1);

    for(int i = 1; i<= 10; i++)
    {
       cout<<"i: "<<i<<endl;
    }
    pthread_exit(NULL);
    return NULL;
}
void A::dispA()
{
    cout<<"A::disp()"<<endl;
}
void A::printNumber()
{
    pthread_t t1;
    pthread_create(&t1, NULL, &printNum, NULL);
    pthread_exit(NULL);
}

Command to create libA.a:

cd /practise/A

g++ -c A.cpp

ar -cvq libA.a *.o

libB.a: It only contains B.h and B.cpp

B.h:

class B
{
public:
    void funB();
    void dispB();
};

B.cpp:

#include "B.h"
#include "iostream"
#include "A.h"
using namespace std;

void B::funB()
{
    cout<<"B::funB()"<<endl;
}

void B::dispB()
{
    A a;
    a.dispA();
    a.printNumber();
}

Command to create libB.a:

cd /practise/B

g++ -c B.cpp -I../A

ar -cvq libB.a *.o

Main.cpp:

#include "iostream"
#include "B.h"
using namespace std;

int main()
{
    B b;
    b.dispB();
    b.funB(); 
    return 0;
}

Command to create main.exe:

cd /practise/MAIN

g++ -o noThread MAIN.cpp -I../A -I../B -L../A -L../B -lB -lA

Error I am getting: ../A/libA.a(A.o): In function A::printNumber()': A.cpp:(.text+0x8c): undefined reference topthread_create' collect2: ld returned 1 exit status

NOTE: I know, if I try to use -lrt flag, it will not give any error. But the problem is that my client [MAIN.cpp] cannot use -lrt flag or -lpthread or any thread related library. and hence, he has suggested me to provide SINGLE THREAD LIBRARY.

So, How to provide SINGLE THREAD LIBRARY ????

libA.a is third party and I cant change its code. libB.a is my own library [and I have to use API's from libA.a]

Is there any specific flag which I can use to make main.cpp run properly??

Another Doubt:

Why Main.cpp is giving me error, even when client is only calling thread independent function :

    int main()
    {
        B b;
        //b.dispB(); <== commented thread dependent function
        b.funB(); <== this doesn't depend on pthread. but still main.cpp is failing. Don't know WHY !!
        return 0;
    }

If you're certain that no pthread code gets called by your application's execution path then, I suppose` you could create dummy pthread symbols/calls` and link against that? — Galik
@Galik: Can you tell me how to do what you're saying? How to create dummy pthread symbols ? — Jatin
Just make dummy versions of the pthread calls in your own .cpp file of everything libA.a uses and include that when you link your program. — Galik

Galik Galik · Accepted Answer · 2016-03-15T07:44:18

If you are certain no actual pthread code gets called from the code path your library uses then you could try making dummy versions of the ptherad calls like this:

DummyPThreads.c (note c not c++)

int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void *), void*)
{
    return 0;
}

void pthread_exit(void*)
{
}

// etc...

Compile with:

gcc -c -o DummyPThreads.o DummyPThreads.c

Add to your application:

g++ -o noThread MAIN.cpp -I../A -I../B DummyPThreads.o -L../A -L../B -lB -lA