How do I fix undefined reference to `fsm' collect2: error: ld returned 1 exit status?

Question

I have started to learn C recently. I don't know what's wrong with my code. I keep getting:

undefined reference to 'fsm'
collect2: error: ld returned 1 exit status

Can you please help to fix it?

1) This is my fsm_main.c file.

#include <stdio.h>
#include <string.h>
#include "cstack.h"
#include "fsm.h"

int main(){
    char string_input[100];
    printf("Enter a string: ");
    gets(string_input);
    printf("string : %s\n",string_input);
    if (fsm(string_input))
      printf("All matched correctly\n");
    return 0;
}

2) Boolean.h file

#ifndef BOOLEAN_H
#define BOOLEAN_H

typedef int boolean;

#define TRUE 1
#define FALSE 0

#endif

3) cstack.h file

#ifndef CSTACK_H
#define CSTACK_H
#include "boolean.h"

typedef struct cstacknode{
    char data;
    struct cstacknode *next;
}*cstack;

void init_cstack(cstack *);
void cpush(cstack*,char);
char cpop(cstack *);
boolean is_cfull(void);
boolean is_cempty(cstack);
void print_cstack(cstack);

#endif

4) cstack.c file

#include <stdio.h>
#include <stdlib.h>
#include "boolean.h"
#include "cstack.h"

void init_cstack(cstack *s){
    (*s) = NULL;
}

void cpush(cstack*s, char x){
    cstack  temp;
    temp= (cstack)malloc(sizeof(struct cstacknode));
    temp -> data = x;
    temp -> next = (*s);
    (*s) = temp;
}

char cpop(cstack *s){
    cstack temp;
    char data_popped;
    temp = *s;
    data_popped =temp->data;
    *s =temp->next;
    free(temp);
    return data_popped;
}

void print_cstack( cstack s){
    if(!is_cempty(s)){
        printf("%d\n", s->data);
        print_cstack(s->next);
   }
}

boolean is_cfull(void){
    cstack temp;
    temp = (cstack) malloc (sizeof(struct cstacknode));
    if(temp == NULL)
        return TRUE;
    else {
        free(temp);
        return FALSE;
    }
}

boolean is_cempty( cstack s){
    if (s == NULL)
        return TRUE;
    else
        return FALSE;
   
}

5) fsm.h file

#ifndef FSM_H
#define FSM_H

#include "boolean.h"

boolean fsm(char *);
boolean is_open(char);
boolean is_close(char);
boolean is_brother(char,char);

#endif

6) fsm.c file

#include <stdio.h>
#include <stdlib.h>
#include "fsm.h"
#include "cstack.h"

boolean is_open(char c) {
   return ((c == '{') || (c == '[') || (c == '{'));
}

boolean is_close(char c) {
   return ((c == '}') || (c == ']') || (c == '}'));
}

boolean is_brother(char op, char cl) {
   return (((op == '{') && (cl == '}')) || ((op == '[') && (cl == ']')) || ((op == '{') && (cl == '}')));
}

boolean fsm(char my_input[]) {
   int i = 1;
   int nextstate = 0;
   cstack top;
   char o, c;

   init_cstack( & top);
   while (1) {
      switch (nextstate) {
      case 0:
         i++;
         if (is_open(my_input[i]))
            nextstate = 1;
         else if (is_close(my_input[i]))
            nextstate = 2;
         else if (my_input[i] == '0')
            nextstate = 3;
         else 
            nextstate = 4;
         break;

      case 1:
         if (!is_cfull()) {
            cpush( & top, my_input[i]);
            nextstate = 0;
         } else {
            printf("Error! stack is full\n");
            exit(0);
         }
         break;
      case 2:
         if (!is_cempty(top)) {
            if (is_brother(cpop( & top), my_input[i])) {
               nextstate = 0;

            } else {
              // printf("Error! open and close delimiter do not match\n");
              printf("Error!  open and close  delimiters do not match\n");
               return FALSE;
            }
         } else {
            printf("Error! No open delimiter to compare to\n");
            return FALSE;
         }
         break;

      case 3:
         if (is_cempty(top)) {
            return TRUE;
         } 
         else {
            printf("Error! stack is full\n");
            return FALSE;
         }
         break;

      case 4:
         nextstate = 0;

      }
   }
}