C++ linking error with multiple definition on linux

Question

I am trying to compile a code and it’s getting a linking error with multiple definitions. Unfortunately, I am not able to fix it and any help would be appreciated.

I have following files: Header files: CEST.h; CEST_UI.h; GlobalVariable.h;
Source file: CEST.cpp; CEST_UI.cpp; GlobalVariable.cpp

All parameters, which are claimed to have multiple definitions, are defined in “GlobalVariable.h” and initialized in “GlobalVariable.cpp”.

I am including “GlobalVariable.h” twice: once in CEST.cpp and second time in CEST_UI.cpp.

I was thinking that the following guard in “GlobalVariable.h” would have protected me aginst multiple definition linking error:

# ifndef GLOBALVARIABLE_H
#define GLOBALVARIABLE_H
………….
………….
#endif

I am also attaching “GlobalVariable.h” and “GlobalVariable.cpp” so that you can have a look.

In “GlobalVariable.h”

# ifndef GLOBALVARIABLE_H
#define GLOBALVARIABLE_H

    #include <vector>
    ////////////////////////////////////////
    extern long lFA_MTPulse;
    extern long lNoOfMTPulses;
    extern long ltDK_MTPulse_Duration_us;
    //extern long ltDK_MTPulse_Delay_us;
    extern long ltDK_Wait_After_MT_us;
    extern long ltDK_Wait_After_MTSpoil_us;
    extern long lNoOfMTPulses_PerRTEB;
    extern long ltDK_PreAcqCESTPulseTime_ms;
    extern long ltDK_PreAcqCESTPulseTime_us;
    extern long lTest_XgradStrength;
    //double TR_MTPulse_Remain = 0.0;                                                                   // CEST This will be calculated later 
    long ltDK_TR_MTPulse_us;
    long ltDK_TimeNeeded_for_sMTCSpoilerOnly;
    long ltDK_MTBlockTime_DK;

    ////////////////////////////////////////
    extern double dBWTimeProd;
    extern double dSpoilerCTRL;
    extern double dOffResonance_Rel;
    ////////////////////////////////////////
    long No_of_Samples = (long)512;                                                                     // CEST
    long lNo_of_MTPulses_PreAcq;
    //static sRF_PULSE  sRfMSat1("sRfMSat");                                                        // CEST("sRfMSat")

    extern long lNoOfKSpaceAcq_PerCEST_ArrayValues[];


#endif

In GlobalVariable.cpp

// NOTE: usually name of any parameters is prefixed by type e.g. 
// I am introducing another parameter with prefix "ltDK_" for "long-time" parameter 
//
long lFA_MTPulse = 100;
long lNoOfMTPulses = 1;
long ltDK_MTPulse_Duration_us = 10000;
//long ltDK_MTPulse_Delay_us = 10000;
long ltDK_Wait_After_MT_us = 0;
long ltDK_Wait_After_MTSpoil_us = 0;
long ltDK_PreAcqCESTPulseTime_ms = 3500;                                                // in micro sec 
long ltDK_PreAcqCESTPulseTime_us = (long)((double)ltDK_PreAcqCESTPulseTime_ms*1000);    // in milli sec
long lTest_XgradStrength = 0;

long lNoOfMTPulses_PerRTEB = 30;
double dBWTimeProd = 1.79;
double dSpoilerCTRL = 1.0;
double dOffResonance_Rel = 0.0;
long lNoOfKSpaceAcq_PerCEST_ArrayValues[5] = {1, 3, 5, 7, 9};

Answer 1

Include guards prevent one source file from including the same header more than once. They do not prevent multiple definition errors. Move all your definitions in the header to a source file and declare them as extern in the header. Separate sources files will include that header and you will get multiple definition errors.