Spliting a c file and using a Makefile

Question

my goal is to split my program up into few files and use a Makefile in order to run it. However, I keep getting this error: I used to initilize "int top" in my main file as well but I figured that might be the issue, however still no results and I can't seem to find anything online that would help me with this error, if anyone knows the issues or a helpful resource they could reccomend to me that would be great :)

gcc -o hw5 hw5.o pushnpop.o
pushnpop.o:(.bss+0x0): multiple definition of `top'
hw5.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status
Makefile:3: recipe for target 'hw5' failed
make: *** [hw5] Error 1

********Here is my codes*********** *******hw5.c*********

#include <stdio.h>
#include <curses.h>
#include <stdlib.h>
#include "pushnpop.h"
#define MAX 100


int emptyCheck(char *stack)
{
  if(stack[0] == '\0')
    return 1;
  else
    return 0;
}

int main()
{
  /*int top = 0;*/
  char c;
  char line[10000];
  FILE *file;
  char stack[MAX];
  char fileName;
  printf("Please enter a file name with a .c extension\n");
  scanf("%s", &fileName);
  /*printf("testing the name:%s\n", &fileName);*/
  file = fopen(&fileName, "r");

  /*error checking*/
  if (file == NULL)
  {
    printf("Cannot access file\n");
    exit(0);
  }

  for(int i = 0; i < MAX; i++ )
  {
    stack[i] = '\0';
  }


/*getting the data from the file*/
int lineCounter = 1;
while(fgets(line, 10000, file) != NULL)
  {
    int j = 0;
    while(line[j] != '\n')
    {
      if(line[j] == '{')
      {
        push(stack);
      }
      else if(line[j] == '}')
      {
        if(emptyCheck(stack) == 1)
        {
        printf("too many } on line %d\n", lineCounter);
        break;
        }
        else
         pop(stack);
      }

    j++;

    }

    if(emptyCheck(stack)==0)
    {
      printf("too many { on line %d\n", lineCounter);
    }

    if(fgets(line, 10000, file) == NULL)
    {
      printf("END of FILE\n");
      exit(0);
    }

    for(int i = 0; i<MAX; i++)
      stack[i] = '\0';

    lineCounter++;
    top = 0;

  }

  fclose(file);
  return 0;
}

****my second file***** *****pushnpop.c******

#include "pushnpop.h"


char pop(char *stack)
{
  char *temp;

    temp = stack;
    top--;
    temp[top] = '\0';

  return *temp;
}

char push(char *stack)
{
  char *temp;

  temp = stack;
  temp[top] = '{';
  top++;

  return *temp;
}

******my third file****** ****pushnpop.h*******

#ifndef pnp
#define pnp

int top = 0;
char pop(char *stack);
char push(char *stack);

#endif

********MAKEFILE********

all: hw5
hw5: hw5.o pushnpop.o
    gcc -o hw5 hw5.o pushnpop.o

pushnpop.o: pushnpop.c
    gcc -o pushnpop.o -c pushnpop.c -W -Wall -pedantic

hw5.o: hw5.c pushnpop.h
    gcc -o hw5.o -c hw5.c -W -Wall -pedantic

clean:
    rm -rf *.o

mrproper: clean
    rm -rf pushnpop

Possible duplicate of [Multiple definition of ... linker error](https://stackoverflow.com/questions/17764661/multiple-definition-of-linker-error) — Mike Kinghan, Oct 04 '18 at 07:37

score 1 · Accepted Answer · answered Oct 04 '18 at 03:25

1

Step 1, move the int top to pushnpop and don't access top from main. Like,

// pushnpop.c
#include "pushnpop.h"

int top = 0;

char pop(char *stack)
{

Then modify pushnpop.h to declare the functions extern. Like,

// pushnpop.h
#ifndef pnp
#define pnp

extern char pop(char *stack);
extern char push(char *stack);

#endif

And don't forget to comment out

// top = 0;

in main.

answered Oct 04 '18 at 03:25

Elliott Frisch

198,278
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`top` is accessed in `main.c`. This approach will give a compile error. What is required is to make `top` as an extern in the header file. – Rishikesh Raje Oct 04 '18 at 03:46
thank you that cleared a lot of confusion up, However, i was wondering why couldn't i use top = 0; in main? – Greasy Chicken Oct 04 '18 at 18:44

score 0 · Answer 2 · answered Oct 04 '18 at 03:52

The issue is in the header file pushnpop.h

int top = 0;

You need to mark this as extern. As a general rule, variables should not be defined in header files. If required in two or more files, they should be marked as extern in the header.

extern int top;

Also, in one of the .c files (possibly pushnpop.c) you need to define the variable in global space.

int top = 0; // outside the functions

Spliting a c file and using a Makefile

2 Answers2