Linker error - undefined reference -gcc

Question

After running,

 Cygwin-PC ~/code_practice/Computing/list
  $ gcc -c arrayImpl.c -o arrayImpl.o

Code directory structure, in windows(cygwin)

Cygwin-PC ~/code_practice/Computing/stack
$ ls
main.c  stack.h  stackImpl.c

Cygwin-PC ~/code_practice/Computing/list
$ ls
arrayImpl.c  arrayImpl.o  linkedListImpl.c  list.h  main.c

Stack abstraction depends on List abstraction, for data representation & usage.

List abstraction is as shown below,

/************ list.h ************/


/***************** Usage-start ************/
typedef enum{false, true}bool;
typedef enum {CREATE_NEW_LIST, DOUBLE_THE_LIST, HALF_THE_LIST}Op;

#if defined(ARRAY)

  /* To ensure Encapsulation(i.e., maintain invariants of array) */
  typedef struct List List;

#elif defined(LINKED_LIST)

  /* To ensure Encapsulation(i.e., maintain invariants of linked list) */
  /* User will not get access to node*/
  typedef struct List List;


#else
  #error "Wrong list implementation macro name !!!"
#endif


void insertItem(List *, void *newItem);
void *deleteItem(List *, int listIndex);
void *deleteLastItem(List *);


List* createList(List *, Op opType);

/***************** Usage-end ***************/

/***************** arrayImple.c **************/

#if defined(ARRAY)

#include"list.h"


/************ Representation - start ************/
typedef struct List{
  void **array;

  /* Following members for Housekeeping - Array enhancement*/
  int lastItemPosition;
  int size;
}List;

#define INITIAL_LIST_SIZE 50
/********************* Representation - end ************/




/************* Usage - start ***************/
List *createList(List *list, Op opType){

       ....
}

void insertItem(List *arrayList, void *newItem){
            ...
}

void *deleteItem(List *arrayList, int listIndex){
     ....
}

void * deleteLastItem(List *arrayList){
     ...
}
/******************** Usage - end *******************/

#endif

Stack abstraction, is shown below,

/********* stack.h *********/

#include"../list/list.h"

typedef struct Stack Stack;

Stack *createStack();
void push(Stack *, void *item);
void*pop(Stack *);

/*********** stackImpl.c *******/

#include"../list/list.h"

typedef struct Stack{
  List *stack;
}Stack;

Stack* createStack(){

  Stack *s = malloc(sizeof(Stack));
  s->stack = createList((void *)0, CREATE_NEW_LIST);

  return s;
}

void push(Stack *s, void *item){
  insertItem(s->stack, item);
}

void *pop(Stack *s){
  void *item = deleteLastItem(s->stack);
  return item;
}

Below compilation says, with given below message, which does not involve linker,

Cygwin-PC ~/code_practice/Computing/stack
   $ gcc -c -DARRAY main.c stackImpl.c ../list/arrayImpl.o
gcc: warning: ../list/arrayImpl.o: linker input file unused because linking  
      not done

Below compilation fails, with given below error,

Cygwin-PC ~/code_practice/Computing/stack
   $ gcc -DARRAY main.c stackImpl.c ../list/arrayImpl.o
/tmp/ccapgYQI.o:stackImpl.c:(.text+0x25): undefined reference to 
   `createList'
/tmp/ccapgYQI.o:stackImpl.c:(.text+0x4b): undefined reference to 
   `insertItem'
/tmp/ccapgYQI.o:stackImpl.c:(.text+0x61): undefined reference to 
   `deleteLastItem'
collect2: error: ld returned 1 exit status

Question,

From above compilation command, Why GNU linker(ld) does not accept ../list/arrayImpl.o file to find the definitions of createList(), insertItem() & deleteLastItem()

Below compilation works,

  Cygwin-PC ~/code_practice/Computing/stack
    $  gcc -DARRAY main.c stackImpl.c ../list/arrayImpl.c

Because gcc -c is compiling only without linking. Giving it *.o files is useless as the warning is friendly suggesting. — Eugene Sh.
– Eugene Sh., Commented Dec 20, 2016 at 22:02
-c means compile only (don't link). So a .o file cannot be provided as it cannot be compiled (it's already compiled and needs to be linked). — kaylum
– kaylum, Commented Dec 20, 2016 at 22:02
Also note that <stdbool.h> from C99 provides bool, true, false, so you don't need the definition typedef enum { false, true } bool;, though it does no great harm. None of the four headers that are included in stack.h are actually needed by stack.h; you should not clutter your headers with unnecessary other headers. The Op typedef is needed; the typedef struct List List; should be unconditional (the consumers of your abstraction shouldn't need to care how your list type is implemented; that is purely an implementation concern). You should radically simplify your header. — Jonathan Leffler
– Jonathan Leffler, Commented Dec 20, 2016 at 22:31
Tons of useless code, when the reason was an incomplete command line. — Jens
– Jens, Commented Dec 20, 2016 at 22:32
the file: stackimpl.c seems to be missing the statement: #include "stack.h" Strongly suggest a header file for a *.c file have the same name as the *.c file with the only difference being the name extension is .h rather than .c — user3629249
– user3629249, Commented Dec 21, 2016 at 12:22

kaylum · Accepted Answer · 2016-12-20 22:18:43Z

1

gcc -c arrayImpl.c -o arrayImpl.o

That is missing -DARRAY. Without that the code in arrayImpl.c is conditionally removed:

/***************** arrayImple.c **************/

#if defined(ARRAY)

answered Dec 20, 2016 at 22:18

kaylum

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Comments

pm100 · Accepted Answer · 2016-12-20 22:25:16Z

0

this

Cygwin-PC ~/code_practice/Computing/stack
   $ gcc -DARRAY main.c stackImpl.c ../list/arrayImpl.o
/tmp/ccapgYQI.o:stackImpl.c:(.text+0x25): undefined reference to 
   `createList'
/tmp/ccapgYQI.o:stackImpl.c:(.text+0x4b): undefined reference to

failed becuase you missed off arrayImpl.c

kaylum has the right answer, when you compiled arrayImpl.c you forgot -DARRAY

edited Dec 20, 2016 at 22:25

answered Dec 20, 2016 at 22:06

pm100

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11 Comments

overexchange Over a year ago

But linker accepts arrayImpl.o files for linking right?

pm100 Over a year ago

seems not, why not just put arrayImpl.c

overexchange Over a year ago

I did use linker to link main.c with multiple .o files in the past. If gcc actually run ld after preprocessing(cpp), compilation(cc) & assembling(as), Does gcc take .o or .c to link? This is why I gave ../list/arrayImpl.o

nos Over a year ago

@overexchange It's quite unusual to compile more than 1 source code file at a time, If you do it the traditional way and compile stackImpl.c also , and then do gcc -DARRAY main.c stackImpl.o ../list/arrayImpl.o it will work.

kaylum Over a year ago

Please add that info to question. It's wrong and is the cause of the problem. Missing -DARRAY.

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