Segmentation fault on separate chaining hashtable - c

Question

So i implemented a hashtable with separate chaining for a struct called Objective, so that i could perform some operations on said Objectives. Currently i have this:
Hashtable.h:
#ifndef HASHTABLE_H
#define HASHTABLE_H
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
/*Using separate chaining to store the obejctives*/
typedef struct Objective{
char name [8000];
unsigned long id, duration, deps [9000];
int hasDeps;
}*pObjective;
typedef struct nodehash{ /*Node of list*/
pObjective obj;
struct nodehash*next;
}*link;
void Init(int M);
int search(unsigned long id);
void insert(pObjective o);
void delete(unsigned long id);
link insertBegin(link h, pObjective obj);
int searchList(link h, unsigned long id);
link removeList(link h, unsigned long id);
pObjective searchObj(unsigned long id);
pObjective searchObjAux(link h, unsigned long id);
#endif
Objectives.c:
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include "OBJECTIVES.h"
/*Checks if all inserted dependencies already exist*/
int existDeps(unsigned long dep[9000]){
int i, count = 0;
for(i = 0; i < 9000; i++){
if(search(dep[i]) != 0)
count++;
}
return count;
}
/ *Adds objective with dependencies*/
void addObj(unsigned long id, char name [8000], unsigned long duration,
unsigned long dep[9000]){
int i;
pObjective obj = malloc(sizeof(pObjective));
obj->id = id;
obj->duration = duration;
obj->hasDeps = 1;
strcpy(name, obj->name);
for(i = 0; i < 9000; i++){
obj->deps[i] = dep[i];
}
if(search(id) != 0)
printf("id already exists\n");
else if(existDeps(dep) != 0)
printf("no such task\n");
else
insert(obj);
free(obj);
}
/*Adds objective with no dependencies*/
void addNoDeps(unsigned long id, char name [8000], unsigned long
duration){
pObjective obj = malloc(sizeof(pObjective));
obj->id = id;
obj->duration = duration;
obj->hasDeps = 1;
strcpy(name, obj->name);
if(search(id) != 0)
printf("id already exists\n");
else
insert(obj);
free(obj);
}
/*Removes objective with no dependencies*/
void removeObj(unsigned long id){
int res = search(id);
pObjective obj = searchObj(id);
if(res == 0)
printf("no such task\n");
else if(obj->hasDeps == 1)
printf("task with dependencies\n");
else
delete(id);
}
Objectives.h:
#ifndef OBJECTIVES_H
#define OBJECTIVES_H
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "HASHTABLE.h"
/*Functions to work with objectives*/
int existDeps(unsigned long dep[9000]);
void addObj(unsigned long id, char name [8000], unsigned long duration,
unsigned long dep[9000]);
void addNoDeps(unsigned long id, char name [8000], unsigned long
duration);
void removeObj(unsigned long id);
#endif
Hashtable.c:
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include "HASHTABLE.h"
#define hash(A,B) (A%B) /*Hash function*/
static link *heads;
static int M;
/*Initiates hashtable with size m*/
void Init(int m){
int i;
M = m;
heads = (link*)malloc(M*sizeof(link));
for(i = 0; i < M; i++)
heads[i] = NULL;
}
/*Searches objective with said id*/
int search(unsigned long id){
int i = hash(id, M);
return searchList(heads[i], id);
}
/*Inserts objective into hashtable*/
void insert(pObjective o){
int i = hash(o->id, M);
heads[i] = insertBegin(heads[i], o);
}
/*Deletes objective using it's id*/
void delete(unsigned long id){
int i = hash(id, M);
heads[i] = removeList(heads[i], id);
}
/*Returns objective with said id*/
pObjective searchObj(unsigned long id){
int i = hash(id, M);
return searchObjAux(heads[i], id);
}
/*Inserts objective into list*/
link insertBegin(link h, pObjective obj){
link new = (link)malloc(sizeof(struct nodehash));
new->obj = obj;
new->next = h;
return new;
}
/*Searches objective by id in a list*/
int searchList(link h, unsigned long id){
link t;
int count = 0;
for(t = h; t != NULL; t = t->next){
if(t->obj->id == id)
count++;
}
return count++;
}
/*Removes objective from list*/
link removeList(link h, unsigned long id){
link t, x, z;
for(t = h; t != NULL; t = t->next){
if(t->next->obj->id == id)
x = t;
}
z = x->next;
x->next = z->next;
free(z);
return h;
}
/*Returns objetive from said id from list*/
pObjective searchObjAux(link h, unsigned long id){
link t, x;
for(t = h; t != NULL; t = t->next){
if(t->obj->id == id)
x = t;
}
return x->obj;
}
I'm quick testing the funcions addObj (adds an objective with dependencies) and addNoDeps (adds an objective with no dependencies) on my main:
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include "OBJECTIVES.h"
int main(){
unsigned long array [1] = {3};
Init(11);
addNoDeps(1, "tarefa1", 20);
addObj(2, "tarefa2", 20, array);
return 0;
}
But i keep getting segmentation fault(core dumped) and i can't figure out why. Is my implementation wrong? Are the functions wrong? I can't get to the problem, can someone help me?

Answer 1

I can't run your code right now so I can't analyze the core dump, but I believe what is happening is that you are trying to access memory that has already been freed. At the end of addNoDeps, you free the pObjective after putting it into the list. Then, when you addObj after, you search the list and check to make sure that the links object associated with it is not null. Specifically this code:
for(t = h; t != NULL; t = t->next){
if(t->obj->id == id)
count++;
You only check to see that the t (links pointer) is not null, but since you freed the previous object, the t->obj pointer is not pointing to initialized memroy. Therefore trying to access it via t->obj->id is accessing uninitialized memory. If you remove the free(obj) at the end of your addNoDeps and addObj functions you should be fine. You may also want to add checks to make sure that t->obj is not null as well. In general segmentation faults are caused by accessing uninitialized memory, so when debugging check for accessing pointers after a free, double frees, and other things. Also learning to use GDB can help a lot in these situations.