Segmentation Fault (singal 11 sigsegv) with linked list - c

Question

Was writing a program to practice before with linked lists and pointers before pset5 and am left with two memory errors that i have not been able to remedy.
#include <stdio.h>
#include <stdlib.h>
//define struct for Nodes
typedef struct list
{
int data;
int key;
struct list* next;
}Node;
//function declarations
Node* create(int a, int *counter);
void insert(int a, int *counter);
void delete_list();
void printlist();
//global pointers
Node* Head = NULL;
Node* Current = NULL;
int main()
{
int *keycounter =(int*)malloc(sizeof(int));
int value = 20;
keycounter = 0;
Head=create(value, keycounter);
value = 30;
insert(value, keycounter);
value = 40;
insert(value, keycounter);
printlist();
delete_list();
free(keycounter);
return 0;
}
// VV functions VV
void delete_list()
{
free(Head);
free(Current);
}
Node* create(int a, int *counter)
{
Node* ptr=malloc(sizeof(Node));
if(!ptr)
{
printf("ERROR-NOT ENOUGH MEMORY\n");
free(ptr);
return 0;
}
ptr->data=a;
ptr->key=*counter;
counter++;
return ptr;
}
void insert(int a, int *counter)
{
Node* ptr=malloc(sizeof(Node));
if(!ptr) {
printf("ERROR-NOT ENOUGH MEMORY\n");
free(ptr);
}
ptr->data=a;
ptr->key=*counter;
//point next field to old head
ptr->next=Head;
//assign current node as head of singly linked list
Head=ptr;
counter++;
}
//Thank you guys over at tutorialspoint for this neat idea for testing this.
//https://www.tutorialspoint.com/data_structures_algorithms/linked_list_program_in_c.htm
void printlist()
{
Node* ptr=Head;
printf("TESTING\n");
while(ptr != NULL) {
printf("%p*NODE* KEY:%i VALUE:%i PTR NEXT:%p\n \n", ptr, ptr->key, ptr->data, ptr->next);
ptr=ptr->next;
}
}
Here is my valgrind output:
Still learning so alot of the valgrind output is pretty arcane to me and threads on stack exchange regarding the "signal 11 (SIGSEGV)" error are difficult to comprehend as well.
Also, any tips or advice on my code would be appreciated.

Answer 1

There is a problem in your code. See the below lines:
int main()
{
int *keycounter =(int*)malloc(sizeof(int));
int value = 20;
keycounter = 0; ===> You are setting the pointer to NULL effectively nullifying the effect of your malloc call above
So, in your create function, when you try to access counter, it is leading to NULL pointer dereference
Node* create(int a, int *counter)
{
Node* ptr=malloc(sizeof(Node));
if(!ptr)
{
printf("ERROR-NOT ENOUGH MEMORY\n");
free(ptr);
return 0;
}
ptr->data=a;
ptr->key=*counter; ==> Here it will lead to NULL pointer dereference
If your key member in the struct is just an integer, then no need to pass a pointer (counter is a pointer), you can as well pass an integer and set it.