ns3-create your own module

 I need compile some C code to in my self defined module. And my code use malloc function.But I always got this error, undefined symbol `malloc’

relocation R_X86_64_PC32 against undefined symbol `malloc@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC

 I add the “-fPIC” option to get the problem solved.
 The following is the codes about my created module.
 The first step is to create a folder name “zsy-pacer” under src folder. The zsy-pacer has a folder and a wscript file.

zsy-pacer
├── model
│   ├── pacer.cc
│   ├── pacer.h
│   ├── pacer_sender.c
│   └── pacer_sender.h
└── wscript

pacer.h

#ifndef PACING_H
#define PACING_H
#include "pacer_sender.h"
#include "ns3/timer.h"
#include "ns3/data-rate.h"

namespace ns3
{
class Pacing
{
public:
    Pacing();
    ~Pacing();
    void ChangePacingRate(uint64_t bitrate);
    void NotifyPacingPerformed (void);
    void Start(void);
    static void DataSend(void* handler, uint32_t packet_id, int retrans, size_t size);
    private:
    Timer  m_pacingTimer{Timer::REMOVE_ON_DESTROY};
    DataRate m_minPacingRate;
    DataRate m_curentPacingRate;
    pace_sender_t *m_paceSender;

};  
}

#endif

pacer.cc

#include"pacer.h"
#include<stdio.h>
#include<string>
#include<sstream>
#include"ns3/log.h"
namespace ns3
{
NS_LOG_COMPONENT_DEFINE ("PacingModel");
/*12000bps=1500*8bit/s*/
Pacing::Pacing():m_minPacingRate("12000bps"),
m_curentPacingRate(m_minPacingRate)
{
    m_paceSender=pace_create((void*)this,(pace_send_func)&Pacing::DataSend);
    m_pacingTimer.SetFunction(&Pacing::NotifyPacingPerformed,this);
}
Pacing::~Pacing()
{
    if(m_paceSender!=nullptr)
    {
        pace_destroy(m_paceSender);
        m_paceSender=nullptr;
    }
}
void Pacing::ChangePacingRate(uint64_t bitrate)
{
    std::stringstream ss;
    ss<<bitrate<<"bps";
    std::string s=ss.str();
    DataRate newval(s);
    NS_LOG_INFO("change pacing rate from"<<m_curentPacingRate<<"to"<<newval);
    m_curentPacingRate=newval;

}
void Pacing::NotifyPacingPerformed()
{
    pace_send(this->m_paceSender);
}
void Pacing::DataSend(void* handler, uint32_t packet_id, int retrans, size_t size)
{
    Pacing *obj=static_cast<Pacing*>(handler);
    if(obj->m_pacingTimer.IsExpired ())
    {
        NS_LOG_DEBUG("pacer timer in expire state"<<"packet seq"<<packet_id);
        obj->m_pacingTimer.Schedule(obj->m_curentPacingRate.CalculateBytesTxTime(size));
    }

}
void Pacing::Start()
{
  m_pacingTimer.Schedule();
}   
}

pacer_sender.h

#ifndef __pace_sender_h_
#define __pace_sender_h_
#include<stdint.h>
#include<stddef.h>
#include <stdlib.h>
#ifdef __cplusplus
extern "C" {
#endif
typedef void(*pace_send_func)(void* handler, uint32_t packet_id, int retrans, size_t size);  
typedef struct
{
    void*       handler;
    pace_send_func      send_cb;    
}pace_sender_t;

pace_sender_t*          pace_create(void* handler, pace_send_func send_cb);
void                    pace_destroy(pace_sender_t* pace);
int pace_send(pace_sender_t* pace);
#ifdef __cplusplus
}
#endif
#endif

pacer_sender.h

#include"pacer_sender.h"
pace_sender_t* pace_create(void* handler, pace_send_func send_cb)
{
    pace_sender_t *sender=(pace_sender_t*)malloc(sizeof(pace_sender_t));
    sender->handler=handler;
    sender->send_cb=send_cb;
    return sender;  
}
void pace_destroy(pace_sender_t* pace)
{
    free(pace); 
}
static uint32_t seq=0;
int pace_send(pace_sender_t* pace)
{
    if(pace->send_cb)
    {
        pace->send_cb(pace->handler,seq,0,1500);
        seq++;
    }
return 0;
}

wscript

## -*- Mode: python; py-indent-offset: 4; indent-tabs-mode: nil; coding: utf-8; -*-

def build(bld):
    pacer = bld.create_ns3_module('zsy-pacer', ['core','network'])
    pacer.source = [
        'model/pacer.cc',
        'model/pacer_sender.c',
        ]
    headers = bld(features='ns3header')
    headers.module = 'zsy-pacer'
    headers.source = [
        'model/pacer.h',
        'model/pacer_sender.h',
        ]
    pacer.env.append_value("CFLAGS","-fPIC")

 And the test Code in folder Scratch:
pacer_example.cc

#include "ns3/core-module.h"
#include "ns3/zsy-pacer-module.h"

NS_LOG_COMPONENT_DEFINE("PACER_EXAMPLE");
using namespace ns3;
using namespace std;

static double simDuration=10;
int main(int argc, char *argv[])
{
    LogComponentEnable("PACER_EXAMPLE", LOG_LEVEL_ALL); 
    LogComponentEnable("PacingModel", LOG_LEVEL_ALL);

    Pacing pacer;
//pacer.ChangePacingRate(2*1500*8);
    pacer.Start();
    Simulator::Stop (Seconds(simDuration + 10.0));
    Simulator::Run ();
    Simulator::Destroy();   
}

wscript

## -*- Mode: python; py-indent-offset: 4; indent-tabs-mode: nil; coding: utf-8; -*-

def build(bld):
    obj = bld.create_ns3_program('pacer_example',
                                 ['core','zsy-pacer'])
    obj.source = 'pacer_example.cc'

[1]Compile and link NS3 program with external library
[2]Mixing c code in NS3