Library management

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

struct book{
    int num;
    char title[20];
    char author[20];
    int year;
};
typedef struct book book;

void add_book(book *ptr, char t[20], char a[20], int y);
void show(book *ptr);
int count(book *ptr);
void books_of(book *ptr, char a[20]);

int main(){
    book b_arr[100] = {
        1, "Wonder_of_universe", "Divyansh", 2005,
        2, "Science_behind_us", "Yuvansh", 2004,
        3, "Eternity", "Jatin", 2007,
    };

    book *ptr;
    ptr = &b_arr[0];

    int response=-1;
    char book_title[20];
    char book_author[20];
    int book_year;
    char w_author[20];
    printf("\n\t----------Welcome to library----------\n");
    printf("\tEnter (1) to Add book to the library\n");
    printf("\tEnter (2) to Display all the book in library\n");
    printf("\tEnter (3) to List all the book of a author\n");
    printf("\tEnter (4) to Know number of book in library\n");
    printf("\tEnter (0) to Exit the library.\n");

    while(response != 0){
        printf("\nEnter you response:");
        scanf("%d", &response);
        if(response == 1){
            printf("****Please avoid using spaces, instread use '_' and enter only first name of author****\n");
            printf("Enter book title: ");
            scanf("%s", &book_title);

            printf("Enter author name: ");
            scanf("%s", &book_author);

            printf("Enter year of publication: ");
            scanf("%d", &book_year);

            add_book(ptr, book_title, book_author, book_year);
            printf("Book added sucessfully\n------------------------------\n");
        }
        else if(response == 2){
            printf("\nFollwing are the books in library\n");
            show(ptr);
            printf("------------------------------\n");
        }
        else if(response == 3){
            printf("Enter name of author: ");
            scanf("%s", &w_author);
            books_of(ptr, w_author);
            printf("------------------------------\n");
        }
        else if(response == 4){
            printf("There are total %d books in library\n", count(ptr));
            printf("------------------------------\n");
        }
    }

    return 0;
}

void show(book *ptr){
    int n = count(ptr);
    for(int i=0; i<n; i++){
    printf("Book Number: %d\n", ptr->num);
    printf("Book Title: %s\n", ptr->title);
    printf("Author of book: %s\n", ptr->author);
    printf("Year of publication: %d\n", ptr->year);
    printf("\n");
    ptr++;
    }

}

void add_book(book *ptr, char t[20], char a[20], int y){
    int n = count(ptr);
    n++;
    ptr = ptr + (n-1);
    ptr->num = n;
    strcpy(ptr->title, t);
    strcpy(ptr->author, a);
    ptr->year = y;

}

int count(book *ptr){
    int c=0;
    for(int i=0; i<100; i++){
        if(ptr->num != 0){
            c++;
        }
        ptr++;
    }
    return c;
}

void books_of(book *ptr, char au[20]){
    int c, n=0;
    for(int i=0; i<100; i++){
        c = strcmp(au, ptr->author);
        if(c==0){
            printf("Avalible book: %s\n", ptr->title);
            n++;
        }
        ptr++;

    }
    if(n == 0){
        printf("There are no such books avalible.\n");
    }
}