-// Rust-101, Part 12: Concurrency (WIP)
-// =================
+// Rust-101, Part 12: Concurrency, Send
+// ====================================
use std::io::prelude::*;
use std::{io, fs, thread};
use std::sync::mpsc::{sync_channel, SyncSender, Receiver};
use std::sync::Arc;
-//@ This part is introducing the concurrency features of Rust. We are going to write our own small version of "grep",
-//@ called *rgrep*, and it is going to make use of multiple cores: One thread reads the input files, one thread does
-//@ the actual matching, and one thread writes the output.
+//@ Our next stop are the concurrency features of Rust. We are going to write our own small version of "grep",
+//@ called *rgrep*, and it is going to make use of concurrency: One thread reads the input files, one thread does
+//@ the actual matching, and one thread writes the output. I already mentioned in the beginning of the course that
+//@ Rust's type system (more precisely, the discipline of ownership and borrowing) will help us to avoid a common
+//@ pitfall of concurrent programming: data races.
// Before we come to the actual code, we define a data-structure `Options` to store all the information we need
// to complete the job: Which files to work on, which pattern to look for, and how to output. <br/>
-// Besides just printing all the matching lines, we will also offer to count them, or alternatively to sort them.
+//@ Besides just printing all the matching lines, we will also offer to count them, or alternatively to sort them.
#[derive(Clone,Copy)]
pub enum OutputMode {
Print,
//@ Now we can write three functions to do the actual job of reading, matching, and printing, respectively.
//@ To get the data from one thread to the next, we will use *message passing*: We will establish communication
-//@ channels between the threads, with one thread *sending* data, and the other one receiving it. `SyncSender<T>`
+//@ channels between the threads, with one thread *sending* data, and the other one *receiving* it. `SyncSender<T>`
//@ is the type of the sending end of a synchronous channel transmitting data of type `T`. *Synchronous* here
//@ means that the `send` operation could block, waiting for the other side to make progress. We don't want to
-//@ end up with the entire files being stored in the buffer of the channels, and the output not being fast enough
+//@ end up with the entire file being stored in the buffer of the channels, and the output not being fast enough
//@ to keep up with the speed of input.
//@
//@ We also need all the threads to have access to the options of the job they are supposed to do. Since it would
//@ be rather unnecessary to actually copy these options around, we will use reference-counting to share them between
-//@ all threads. `Arc` is the thread-safe version of `Rc, using atomic operations to keep the reference count up-to-date.
-//@ You can also think of this as saying that *all* threads own the `Options` "a bit" - and since there could be other
-//@ owners, `Arc` (just like `Rc`) only permits read-only access to its content. That's good enough for the options, though.
+//@ all threads. `Arc` is the thread-safe version of `Rc`, using atomic operations to keep the reference count up-to-date.
-// The first functions reads the files, and sends every line over the `out_channel`.
+// The first function reads the files, and sends every line over the `out_channel`.
fn read_files(options: Arc<Options>, out_channel: SyncSender<String>) {
for file in options.files.iter() {
// First, we open the file, ignoring any errors.
// The second function filters the lines it receives through `in_channel` with the pattern, and sends
// matches via `out_channel`.
-fn filter_lines(options: Arc<Options>, in_channel: Receiver<String>, out_channel: SyncSender<String>) {
+fn filter_lines(options: Arc<Options>,
+ in_channel: Receiver<String>,
+ out_channel: SyncSender<String>) {
// We can simply iterate over the channel, which will stop when the channel is closed.
for line in in_channel.iter() {
// `contains` works on lots of types of patterns, but in particular, we can use it to test whether
- // one string is contained in another.
+ // one string is contained in another. This is another example of Rust using traits as substitute for overloading.
if line.contains(&options.pattern) {
out_channel.send(line).unwrap(); /*@*/
}
// We move the `options` into an `Arc`, as that's what the thread workers expect.
let options = Arc::new(options);
- // Set up the channels. Use `sync_channel` with buffer-size of 16 to avoid needlessly filling RAM.
+ // This sets up the channels. We use a `sync_channel` with buffer-size of 16 to avoid needlessly filling RAM.
let (line_sender, line_receiver) = sync_channel(16);
let (filtered_sender, filtered_receiver) = sync_channel(16);
// Same with the filter thread.
let options2 = options.clone();
- let handle2 = thread::spawn(move || filter_lines(options2, line_receiver, filtered_sender));
+ let handle2 = thread::spawn(move || {
+ filter_lines(options2, line_receiver, filtered_sender)
+ });
// And the output thread.
let options3 = options.clone();
//@ We need to call `to_string` on string literals to convert them to a fully-owned `String`.
pub fn main() {
let options = Options {
- files: vec!["src/part10.rs".to_string(), "src/part11.rs".to_string(), "src/part12.rs".to_string()],
+ files: vec!["src/part10.rs".to_string(),
+ "src/part11.rs".to_string(),
+ "src/part12.rs".to_string()],
pattern: "let".to_string(),
output_mode: Print
};
run(options);
}
-// **Exercise 12.1**: Change rgrep such that it prints now only the matching lines, but also the name of the file
+// **Exercise 12.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file
// and the number of the line in the file. You will have to change the type of the channels from `String` to something
// that records this extra information.
//@ ## Ownership, Borrowing, and Concurrency
-//@ The little demo above showed that concurrency in Rust has a fairly simple API. However, considering Rust has closures,
-//@ that should not be entirely surprising. However, as I mentioned in the beginning, Rust ensures that well-typed programs
-//@ do not have data races. How can that be? A data race is typically defined as having two concurrent, unsynchronized
+//@ The little demo above showed that concurrency in Rust has a fairly simple API. Considering Rust has closures,
+//@ that should not be entirely surprising. However, as it turns out, Rust goes well beyond this and actually ensures
+//@ the absence of data races. <br/>
+//@ A data race is typically defined as having two concurrent, unsynchronized
//@ accesses to the same memory location, at least one of which is a write. In other words, a data race is mutation in
//@ the presence of aliasing, which Rust reliably rules out! It turns out that the same mechanism that makes our single-threaded
//@ programs memory safe, and that prevents us from invalidating iterators, also helps secure our multi-threaded code against
//@ data races. For example, notice how `read_files` sends a `String` to `filter_lines`. At run-time, only the pointer to
-//@ the string will actually be moved around (just like when a `String` is passed to a function with full ownership). However,
+//@ the character data will actually be moved around (just like when a `String` is passed to a function with full ownership). However,
//@ `read_files` has to *give up* ownership of the string to perform `send`, to it is impossible for an outstanding borrow to
-//@ still be around. After it sent the string to the other side, `read_files` has no way to race on the data with someone else.
+//@ still be around. After it sent the string to the other side, `read_files` has no pointer into the string content
+//@ anymore, and hence no way to race on the data with someone else.
//@
-//@ However, there is more to this. Remember the `'static` bound we had to add to `register` in the previous part, to make
-//@ sure that the callbacks to not reference any pointers that might become invalid? This is just as crucial for spawning
+//@ There is a little more to this. Remember the `'static` bound we had to add to `register` in the previous part, to make
+//@ sure that the callbacks do not reference any pointers that might become invalid? This is just as crucial for spawning
//@ a thread: In general, that thread could last for much longer than the current stack frame. Thus, it must not use
//@ any pointers to data in that stack frame. This is achieved by requiring the `FnOnce` closure passed to `thread::spawn`
//@ to be valid for lifetime `'static`, as you can see in [its documentation](http://doc.rust-lang.org/stable/std/thread/fn.spawn.html).
//@ This avoids another kind of data race, where the thread's access races with the callee deallocating its stack frame.
+//@ It is only thanks to the concept of lifetimes that this can be expressed as part of the type of `spawn`.
//@ ## Send
-//@ However, the story goes further. I said above that `Arc` is a thread-safe version of `Rc`, which uses atomic operations
-//@ to manipulate the reference count. It is thus crucial that we don't use `Rc` above, or the reference count may become invalid.
-//@ And indeed, if you replace `Arc` by `Rc` (and add the appropriate imports), Rust will tell you that something is wrong.
-//@ That's great, of course, but how did it do that?
+//@ However, the story goes even further. I said above that `Arc` is a thread-safe version of `Rc`, which uses atomic operations
+//@ to manipulate the reference count. It is thus crucial that we don't use `Rc` across multiple threads, or the reference count may
+//@ become invalid. And indeed, if you replace `Arc` by `Rc` (and add the appropriate imports), Rust will tell you that something
+//@ is wrong. That's great, of course, but how did it do that?
//@
//@ The answer is already hinted at in the error: It will say something about `Send`. You may have noticed that the closure in
//@ `thread::spawn` does not just have a `'static` bound, but also has to satisfy `Send`. `Send` is a trait, and just like `Copy`,
-//@ it's just a marker - there are no functions provided by `Send` What the trait says is that types which are `Send`, can be
+//@ it's just a marker - there are no functions provided by `Send`. What the trait says is that types which are `Send`, can be
//@ safely sent to another thread without causing trouble. Of course, all the primitive data-types are `Send`. So is `Arc`,
//@ which is why Rust accepted our code. But `Rc` is not `Send`, and for a good reason!
//@
//@ So if the environment of your closure contains an `Rc`, it won't be `Send`, preventing it from causing trouble. If however every
//@ captured variable *is* `Send`, then so is the entire environment, and you are good.
-//@ [index](main.html) | [previous](part11.html) | [next](main.html)
+//@ [index](main.html) | [previous](part11.html) | [next](part13.html)
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