C Programming

hi,
I am new to c programming. The following codes are compilable but there are a lot of errors. My main concern is that the display _all_infomration function would not work at all.
Can someone kindly look at my codesand explain to me why? I have been buring midnight oil to look at them but i still cannot find the errors!

Thanks a lot!

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



//initialisation
struct book_record
{
char ISBN[11];
char title[31];
char author[31];
char category[3];
char date[11];
float price;
};

struct BkArray
{
struct book_record BookArray[10];
};

//function prototypes
void add_records(BkArray *library);
void display_all_records(BkArray *library);
void display_information(char *ISBN, BkArray *library);
void display_books(BkArray *library);
void delete_records(char *title, BkArray *library);
void BarLine(int num);

main()
{

//variables
int quit,choice;
char ISBN[11]; /*This ISBN refers to the input by the finder*/
char title[31];
char menu;


BkArray library;
BkArray *lib;

for (int i = 0; i != 9; i++)
{
strcpy(library.BookArray[i].title, "NULL" );
strcpy(library.BookArray[i].author, "NULL" );
strcpy(library.BookArray[i].category, "NULL" );
strcpy(library.BookArray[i].date, "NULL" );
strcpy(library.BookArray[i].ISBN, "NULL" );
library.BookArray[i].price = 0.00;
}


do{

printf("\n");

BarLine(30);
printf("\tMENU\n");
BarLine(30);

printf("1 - Add new records.\n");
printf("2 - Display all book records.\n");
printf("3 - Find and display book information given the ISBN.\n");
printf("4 - Find all books written by a particular author.\n");
printf("5 - Delete book record given the title.\n");
printf("6 - Quit\n");
printf("\nEnter your choice: ");
scanf("%d", &choice);

lib = &library;


switch(choice)
{
case 1: add_records(lib);
break;
case 2: display_all_records(lib);
break;
case 3: printf("\nEnter ISBN: ");
scanf("%s", ISBN);
display_information(ISBN, lib);
break;
case 4: display_books(lib);
break;
case 5: printf("\nEnter title: ");
gets( title);
delete_records(title,lib);
break;
case 6: quit = 1;
break;
default: printf("\nThere is no such choice.\n");
}

}while(quit== 0);

return 0;
}


void BarLine(int num)
{
for(int l = 0; l < num; l++)
printf("=");
printf("\n");
}//BarLine


//Add book records in Book Array
void add_records(BkArray* library)
{
//variables
int number;
char buf[10];
printf("\nEnter the number of records that you are going to enter: ");
scanf("%d", &number);

// clscr();

for( int k = 0; k < number; k++)
{
printf("\n#%d Record:\n", k+1);
BarLine(30);
printf("Enter ISBN: ");
gets( library->BookArray[k].ISBN);
gets( library->BookArray[k].ISBN);

printf("Enter the title of the book: ");
gets( library->BookArray[k].title);
printf("Enter the author of the book: ");
gets(library->BookArray[k].author);
printf("Enter the price of the book: ");
gets(buf);
library->BookArray[k].price = (float) atof(buf);
printf("Enter the category of the book: ");
gets( library->BookArray[k].category);
printf("Enter the published date in the format of dd/mm/yyyy: ");
gets (library->BookArray[k].date);
}//for
}//add_records


//Display all book records
void display_all_records(BkArray *library)
{
for (int p = 0; p < sizeof(library); p++)
{
printf("\n#%d Record:\n", p+1);
BarLine(30);
printf("ISBN: %s.\n", library->BookArray[p].ISBN);
printf("Title: %s.\n", library->BookArray[p].title);
printf("Author: %s.\n", library->BookArray[p].author);
printf("Price: %.2f.\n", library->BookArray[p].price);
printf("Category: %s.\n", library->BookArray[p].category);
printf("Published Date: %s.\n", library->BookArray[p].date);
}//for

}//display_records


// Find and display book information given the IBSN
void display_information(char *ISBN, BkArray *library)
{
int cnt = 0;

for (int q = 0; q < sizeof(library); q++)
{
if (strcmp(library->BookArray[q].ISBN,ISBN)== 0)
{
printf("Book Found!\n");
printf("ISBN: %s.\n", ISBN);
printf("Title: %s.\n", library->BookArray[q].title);
printf("Author: %s.\n", library->BookArray[q].author);
printf("Price: %.2f.\n", library->BookArray[q].price);
printf("Category: %s.\n", library->BookArray[q].category);
printf("Published Date: %s.\n", library->BookArray[q].date);
cnt++;
}//if
}//for

if(cnt == 0)
printf("Book cannot be found!");
}//display_information


//find all books written by a particular author
void display_books(BkArray *library)
{
//variable
char author1[31];
int cnt = 0;

printf("\nEnter author: ");
gets(author1);

for(int r = 0; r < sizeof(library); r++)
{
if(strcmp(library->BookArray[r].author, author1)== 0)
{
printf("The title of the book is %s.\n", library->BookArray[r].title);
cnt++;
break;
}
}//for

if(cnt != 1)
printf("No Book is found!");

}//display_book


//Delete book record given the title
void delete_records(char *title, BkArray *library)
{
for (int i = 0; i < sizeof(library->BookArray); i++)
{
if(strcmp(library->BookArray[i].title, title)== 0)
{
for (int j = i; j < sizeof(library->BookArray); j++)
strcpy(library->BookArray[j].title, library->BookArray[j++].title);
strcpy(library->BookArray[j].ISBN, library->BookArray[j++].ISBN);
strcpy(library->BookArray[j].author, library->BookArray[j++].author);
strcpy(library->BookArray[j].date, library->BookArray[j++].date);
strcpy(library->BookArray[j].category, library->BookArray[j++].category);
library->BookArray[j].price = library->BookArray[j++].price;
}//if
}//for
}//delete_record