segmentation fault when trying to deference pointer : C

I was trying to implement circular queue functionality. I am a C++ coder and I found it surprising that in C, struct cannot have member functions. Anyway this is my implementation:-

#include <stdio.h>
#include <stdlib.h>



struct node
{
    int nvalue;
    struct node *next;
};

struct CLlist
{
    struct node* head;
    struct node* tail;
    int size;
};

void insert(struct CLlist *l,int num)
{
    struct node *n=malloc(sizeof(struct node));
    n->nvalue=num;
    n->next=NULL;

    if((l->head==l->tail)==NULL)
    {
        l->head=l->tail=n;
    }
    else if(l->head==l->tail && l->head!=NULL)
    {
        l->head->next=n;
        l->tail=n;
        l->tail->next=l->head;
    }
    else
    {
        l->tail->next=n;
        l->tail=n;
        l->tail->next=l->head;
    }
    l->size++;
}

void print(struct CLlist *l)
{
    int idno=1;
    printf("printing the linked list with size as %d\n",l->size);
    struct node *cptr;
    for(cptr=(l->head);cptr!=(l->tail);cptr=cptr->next)
    {
        printf("The idno is %d and the number is %d\n",idno,cptr->nvalue);
        idno++;
    }
    //this is to print the last node in circular list : the tail node
    idno++;
    cptr=cptr->next;
    printf("The idno is %d and the number is %d\n",idno,cptr->nvalue);
}


int main()
{
    struct CLlist a;
    struct CLlist *l;
    l=&a;

    insert(l,2);
    insert(l,5);
    insert(l,7);
    insert(l,10);
    insert(l,12);
    print(l);


    return 0;
}

I get segmentation fault in the line

printf("The idno is %d and the number is %d\n",idno,cptr->nvalue);

why does the error occur? I guess I am not passing l by pointer by value (passing pointers as by value) properly. could somebody help me in pointing out where I am going wrong?

Thanks