Question

Line 9 of the below code generates undefined behavior. Is this due to the fact that title1[] is outside of main() and is global? Or is it because something else that I’m missing?

1. char title1[]="The Name of the Rose";
2. Book book1={title1,900,0};
3. int main(){
4.   Book book2={"Foucault's Pendulum",1000,0};
5.   Book* book3=(Book*)malloc(sizeof(Book));
6.   *book3=book2;
7.   book1.next=&book2;
8.   book2.next=book3;
9.   book1.title[0]='B';
10.  book2.title[0]='A';
11.  {
12.    Book list[2];
13.    list[0]=book1;
14.    list[1]=book2;
15.    list[1].next->next=&book2;
16.    {
17.      Book* p=&list[0];
18.      while (p!=0) {
19.        p=p->next;
20.    }
21.  }
22. return 0;
23.}

EDIT:

Added the Book definition:

‫;‪struct Book‬‬
‪typedef struct Book‬ {
‫;‪  char* title‬‬
  int pages;‬‬
‫;‪  struct Book* next‬‬
‫;‪} Book‬‬

Answer 1

No, line 9 is not undefined behavior. It writes to this array:

char title1[]="The Name of the Rose";

which is not a string literal (but initialized by one). Such a plain array can be modified to your liking. It would have been different if you would have declared it like this:

char *title1="The Name of the Rose";

The undefined behavior is in line 10, here you are writing into a string literal, which is not allowed.

BTW, when asking a question here, please cook it down to a minimal example that shows your point. Most of the code you posted is completely useless for your question.