Is reading from a file with multiple threads considered undefined behavior in rust?

user_17283697 0
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Example seen below. It seems like this might by definition be ub, but it remains unclear to me.

fn main(){
  let mut threads = Vec::new();

  for _ in 0..100 {
    let thread = thread::spawn(move || {
      fs::read_to_string("./config/init.json")
     .unwrap()
      .trim()
      .to_string()
    });
    threads.push(thread);
  }

  for handler in threads {
   handler.join().unwrap();
  }
}
rust multithreading
1 Answers

On most operating systems only individual read operations are guaranteed to be atomic. read_to_string may perform multiple distinct reads, which means that it's not guaranteed to be atomic between multiple threads/processes. If another process is modifying this file concurrently, read_to_string could return a mixture of data from before and after the modification. In other words, each read_to_string operation is not guaranteed to return an identical result, and some may even fail while others succeed if another process deletes the file while the program is running.

However, none of this behavior is classified as "undefined." Absent hardware problems, you are guaranteed to get back a std::io::Result<String> in a valid state, which is something you can reason about. Once UB is invoked, you can no longer reason about the state of the program.

By way of analogy, consider a choose your own adventure book. At the end of each segment you'll have some instructions like "If you choose to go into the cave, go to page 53. If you choose to take the path by the river, go to page 20." Then you turn to the appropriate page and keep reading. This is a bit like Result -- if you have an Ok you do one thing, but if you have an Err you do another thing.

Once undefined behavior is invoked, this kind of choice no longer makes sense because the program is in a state where the rules of the language no longer apply. The program could do anything, including deleting random files from your hard drive. In the book analogy, the book caught fire. Trying to follow the rules of the book no longer makes any sense, and you hope the book doesn't burn your house down with it.

In Rust you're not supposed to be able to invoke UB without using the unsafe keyword, so if you don't see that keyword anywhere then UB isn't on the table.

user_5012500
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