Writing to and Reading back from typed (struct) file that struct has String field

[LukyGuyLucky]

As an exercise,I wrote a tiny program trying to understand something related to typed files and String vs. fixed length char array.
In below program,only the first run can print the only record just added with write_struct() from the file,not a second chance.While I changed the String field to fixed length char array in the struct Student,then it seems everything is fine.I don't quite understand why String field failed on such case.

Another issue is ,when reading back record from the file using while(!file.eof()) and io::read_any(&file,&alice) as marked Option B in the source, after the while loop finished,an unexpected fault happened but I don't know why.

ERROR: 'Unexpected fault 'io::UNEXPECTED_EOF' was unwrapped!'

module demo2;

import libc;
import std::io;

alias printf=io::printf;

struct Student
{
    String name;
    String major;
    int age;
}
const FILENAME="demo.dat";

fn void main()
{
    //write_struct();
    io::printn("-------------------");
    read_struct();
}
fn void write_struct()
{
    Student alice;

    printf("Enter a name: ");
    alice.name=io::treadline()!!;
    printf("Enter a major: ");
    alice.major=io::treadline()!!;
    printf("Enter an age: ");
    alice.age=io::treadline().to_int()!!;

    File file=file::open(FILENAME,"ab+")!!;
    defer file.close()!!;
    
    io::write_any(&file,&alice)!!;
    //file.flush()!!;
    
    printf("Student added successfully.\n");
}

fn void read_struct()
{
    File file=file::open(FILENAME,"rb")!!;
    defer file.close()!!;

    Student alice;

    while(libc::fread(&alice,Student.sizeof,1,file.file))//Option B-(!file.eof())
    {
        //Option B- usz len=io::read_any(&file,&alice)!!;
        printf("Name:\t%s\nTitle:\t%s\nAge:\t%d\n",
            alice.name,
            alice.major,
            alice.age);
    }    
}

[lerno]

If you use write_any, then you store the struct by value, the struct contains two strings and one int. The strings contain a pointer value and a length each. In other words you are saving the addresses rather than the actual strings. The fact that it works the first time around is simply because you are loading back pointer values that are still valid. What you need in order to properly serialize and deserialize is a custom function for it.

[LukyGuyLucky]

I am thinking~
Something like json marshal before saving to file?

[lerno]

You could make such a library easily yes.