I found something that looked like a flaw in the program.
In the beginning of main function, size and idx are defined without NULL-initialization. And they has 64bit size because their type is size_t.

size_t size;
pthread_t thread_t;
size_t idx;
char leave_comment[32];

However, when it assigns the values from stdin to these variables, it uses "%d" as format string.

scanf("%d", &idx);

scanf("%d", &size);

In my environment, the initial value of size and idx is not NULL and, they have Non-Null value in upper 32 bit. So it can't handle user's input correctly. If this is not intentional, I think it should be fixed.