I don't know why you attached conditional statements in the example code of the dual link list program. (feat. C easily written data structure book)

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

typedef char element[100];
typedef struct DListNode {
    element data;
    struct DListNode *llink;
    struct DListNode *rlink;
}DListNode;

DListNode* current;

// Initialize dual connection list
void init(DListNode* phead)
{
    phead->llink = phead;
    phead->rlink = phead;
}

// Output a node in a dual connection list
void print_dlist(DListNode* phead)
{
    DListNode* p;
    for (p = phead->rlink; p != phead; p = p->rlink)
    {
        if(p == current)
            printf("<-| #%s# |->",p->data);
        else
            printf("<-| %s |->",p->data);
    }
    printf("\n");
}

// Insert a node newnode to the right of the node before.
void dinsert(DListNode *before, element data)
{
    DListNode *newnode = (DListNode *)malloc(sizeof(DListNode));
    strcpy_s(newnode->data, data,sizeof(data));
    // I don't understand why you use strcpy() instead of '= operator'.
    // I posted a question about this, too! 
    newnode->llink = before;
    newnode->rlink = before->rlink;
    before->rlink->llink = newnode;
    before->rlink = newnode;
}

// Delete the node removed.
void ddelete(DListNode* head, DListNode* removed)
{
    if (removed == head) return;
    removed->llink->rlink = removed->rlink;
    removed->rlink->llink = removed->llink;
    free(removed);
}

int main(void)
{
    char; // operation key variable
    DListNode *head = (DListNode *)malloc(sizeof(DListNode))); // head node declaration
    init(head); // head node initialization

    // Insertion
    dinsert(head, "Mamamia");
    dinsert(head, "Dancing Queen");
    dinsert(head, "Fernando");

    current = head->rlink;      

    print_dlist(head);

    do {
        printf("\nEnter the command (<,>,q): ");
        ch = getchar();
        if (ch == '<') {
            current = current->llink;
            if (current == head)
                current = current->llink;
        }
        else if (ch == '>') {
            current = current->rlink;
            if (current == head)
                current = current->rlink;
        }
        print_dlist(head);
        getchar();
    } while (ch! = 'q'); // loop until the operation key variable is q
    // Dynamic memory release code here
}

    do {
        printf("\nEnter the command (<,>,q): ");
        ch = getchar();
        if (ch == '<') {
            current = current->llink;
            if (current == head)
                current = current->llink;
        }
        else if (ch == '>') {
            current = current->rlink;
            if (current == head)
                current = current->rlink;
        }
        print_dlist(head);
        getchar();
    } } while (ch != 'q');