Multiple definition and file management - c

Question

I'm writing a program for vocabulary training, for myself. And the program itself should be available in different languages, atm in German and English.
What I want is to have a main file which manage all and two separate files for the functions in the right language.
I compile all the files with:
gcc vocTrainer.c german_menue.c english_menue.c -o v.exe
But I get an error of multiple definition even though I only include one of the language files depending on your input.
Multiple defintion of 'orderOfVoc'
First defined here: collect2.exe error: ld returned 1 exit status
My code:
vocTrainer.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "german_menue.h"
#include "english_menue.h"
int main(void)
{
char selectLang[1]; //store 1 for English or 2 for German
system("cls"); //clear screen
memset(selectLang,0,1); //set all fields in the array to 0
while(selectLang[1] != 1 && selectLang[1] != 2)
{
//select your language
printf("Choose language - Sprache auswaehlen:\n(1) - Englisch/English\n(2) - Deutsch/German\n");
scanf("%d",&selectLang[1]);
system("cls");
}
//language query
if(selectLang[1] == 2)
{
#include "german_menue.c"
}
else
{
#include "english_menue.c"
}
printf("Test of select Order: %d",orderOfVoc());
return 0;
}
german_menue.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "german_menue.h"
int orderOfVoc()
{
char selectOrder[1]; /*store the choosen order of vocabulary.
1 for one after another 2 for a random sequence of words*/
printf("Wie sollen die Vokabeln abgefragt werden?\n(1) - Der Reihe nach\n(2) - Zufaellig\n");
scanf("%d",&selectOrder[1]);
return selectOrder[1];
}
english_menue.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "german_menue.h"
int orderOfVoc()
{
char selectOrder[1]; /*store the choosen order of vocabulary.
1 for one after another 2 for a random sequence of words*/
printf("How do you want to learn the vocabulary?\n(1) - Vocabulary in order\n(2) - Random order\n");
scanf("%d",&selectOrder[1]);
return selectOrder[1];
}
german_menue.h
#ifndef GERMAN_MENUE_H //include guards
#define GERMAN_MENUE_H
extern int orderOfVoc();
#endif //GERMAN_MENUE_H
english_menue.h
#ifndef ENGLISH_MENUE_H //include guards
#define ENGLISH_MENUE_H
extern int orderOfVoc();
#endif //ENGLISH_MENUE_H

Answer 1

Primary Issue: In your code,
if(selectLang[1] == 2)
{
#include "german_menue.c"
}
else
{
#include "english_menue.c"
}
is not doing what you're thinking. There are may issues, like
#include is compile time operation (during preprocessing state) and essentially cannot be controlled at runtime.
You don't include source files. You compile and link them together. Your compilation statement looks correct. Just leave out the above mentioned code snippet from your code.
Just to add a bit detail regarding the reason behind the error you received, is because, you have #includeed the source files (which is essentially adding the source code of that .c file in vocTrainer.c file itself) and again, at compile time, you're putting the .c files. Thus, after compilation, at linking state, compiler sees more than one occurrences of orderOfVoc() which is why compiler is complaining.
Solution:
You remove different definition of orderOfVoc() function. Make use of the user selected value. Pass the value to the orderOfVoc() as an argument, and execute accordingly.
Secondary Issue(s): Apart from above issue(s), in your code, with a definition like
char selectLang[1];
writing
scanf("%d",&selectLang[1]);
is wrong, because
selectLang[1] is out of bound access. Array index in C starts from 0.
%d is not the correct formart specifier for char.
FWIW, char selectLang[1]; is functionally equivalent with char selectLang;
A modified version (not tested) for aforesaid approach:
select_menue.h
#ifndef SELECT_MENUE_H //include guards
#define SELECT_MENUE_H
//according to {store 1 for English or 2 for German}
#define ENGLISH 1
#define GERMAN 2
extern int orderOfVoc(int);
#endif //SELECT_MENUE_H
select_menue.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "select_menue.h"
int orderOfVoc(int lang)
{
int selectOrder = 0;
switch (lang)
{
case ENGLISH:
printf("How do you want to learn the vocabulary?\n(1) - Vocabulary in order\n(2) - Random order\n");
scanf("%d",&selectOrder); //add possible error check
break;
case GERMAN:
printf("Wie sollen die Vokabeln abgefragt werden?\n(1) - Der Reihe nach\n(2) - Zufaellig\n");
scanf("%d",&selectOrder); //add possible error check
break;
}
return selectOrder;
}
vocTrainer.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "select_menu.h"
int main(void)
{
int selectLang = 0; //array not required, initialize in single statement
//store 1 for English or 2 for German
while(selectLang != 1 && selectLang != 2)
{
//select your language
printf("Choose language - Sprache auswaehlen:\n(1) - Englisch/English\n(2) - Deutsch/German\n");
scanf("%d",&selectLang);
}
printf("Test of select Order: %d",orderOfVoc(selectLang));
return 0;
}

Answer 2

#include is a preprocessor directive that includes the contents of the file named at compile time.
The code that conditionally includes stuff is executed at run time...not compile time. So both files are being compiled in. ( You're also including each file twice, once in the main function and once above it, which is just confusing and probably wrong, but we'll ignore that for now. )
You can't really conditionally include stuff at run time. You can use other preprocessor directives (#ifdef, etc. ) to conditionally include one or the other file at compile time, but for your purposes you really need to have some sort of global flag that each function in the included files uses to determine if it should display english or german, etc.
Internationalization of strings is a whole topic in itself. There are lots of ways to handle it, and some libraries to make it easier depending on your platform.
Here's one way you could handle the same scenari:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "german_menue.h"
char *getLocalizedString(int stringId)
{
// Pseudo-Code, not real C++
// Also ignores memory issues and deallocating strings when done
char *localizedString = LoadGermanOrEnglishStringBasedOnGlobalVarForLanguage(stringId);
return localizedString ;
}
int orderOfVoc()
{
int stringId = 1;//should be constant for this message
char *localizedString = getLocalizedString(stringId);
printf("%s", localizedString);
scanf("%d",&selectOrder[1]);
return selectOrder[1];
}