6 releases (breaking)

✓ Uses Rust 2018 edition

new 0.6.0 Feb 14, 2020
0.5.0 Feb 12, 2020
0.4.0 Feb 11, 2020
0.3.0 Feb 11, 2020
0.1.0 Feb 8, 2020

#3 in #proc

26 downloads per month


1.5K SLoC


This crate provides the ability to spawn processes with a function similar to thread::spawn.

Unlike thread::spawn data cannot be passed in closures but must be explicitly passed as single argument which must be serde serializable. The return value from the spawned closure also must be serializable and can then be unwrapped from the returned join handle. If your function has data enclosed it will panic at runtime.

// call this early in your main() function.  This is where all spawned
// functions will be invoked.

let data = vec![1, 2, 3, 4];
let handle = procspawn::spawn(data, |data| {
    println!("Received data {:?}", &data);
let result = handle.join().unwrap();

Because procspawn will invoke a subprocess and there is currently no reliable way to intercept main in Rust it's necessary for you to call procspawn::init at an early time in the program. The place where this will be called is the entrypoint for the subprocesses spawned. The subprocess is invoked with the same arguments and environment variables by default.

spawn can pass arbitrary serializable data, including IPC senders and receivers from the ipc-channel crate, down to the new process.


The default way to spawn processes will start and stop processes constantly. For more uses it's a better idea to spawn a Pool which will keep processes around for reuse. Between calls the processes will stay around which also means the can keep state between calls if needed.


By default panics are captured and serialized across process boundaries. This requires that the backtrace crate is used with serialization support. If you do not need this feature you can disable the backtrace crate and disable panic handling through the ProcConfig object.

Feature Flags

The following feature flags exist:

  • safe-shared-libraries: this feature is enabled by default. When this feature is disable then no validation about shared library load status is performed around IPC calls. This is highly unsafe if shared libraries are being used and a function from a shared library is spawned.
  • backtrace: this feature is enabled by default. When in use then backtraces are captured with the backtrace-rs crate and serialized across process boundaries.
  • test-support: when this feature is enabled procspawn can be used with rusttest. See testing for more information.
  • json: enables optional JSON serialization. For more information see Bincode Limitations.

Bincode Limitations

This crate uses bincode internally for inter process communication. Bincode currently has some limitations which make some serde features incompatible with it. Most notably if you use #[serde(flatten)] data cannot be sent across the processes. To work around this you can enable the json feature and wrap affected objects in the Json wrapper to force JSON serialization.


Due to limitations of the rusttest testing system there are some restrictions to how this crate operates. First of all you need to enable the test-support feature for procspawn to work with rusttest at all. Secondly your tests need to invoke the enable_test_support! macro once top-level.

With this done the following behavior applies:

  • Tests behave as if procspawn::init was called (that means with the default arguments). Other configuration is not supported.
  • procspawn will register a dummy test (named procspawn_test_helper) which doesn't do anything when called directly, but acts as the spawning helper for all spawn calls.
  • stdout is silenced by default unless --show-output or --nocapture is passed to tests.
  • when trying to spawn with intercepted stdout be aware that there is extra noise that will be emitted by rusttest.

Shared Libraries

procspawn uses the findshlibs crate to determine where a function is located in memory in both processes. If a shared library is not loaded in the subprocess (because for instance it is loaded at runtime) then the call will fail. Because this adds quite some overhead over every call you can also disable the safe-shared-libraries feature (which is on by default) in which case you are not allowed to invoke functions from shared libraries and no validation is performed.

This in normal circumstances should be okay but you need to validate this. Spawning processes will be disabled if the feature is not enabled until you call the assert_spawn_is_safe function.

Platform Support

Currently this crate only supports macOS and Linux because ipc-channel itself does not support Windows yet. Additionally the findshlibs which is used for the safe-shared-libraries feature also does not yet support Windows.


Here are some examples of procspawn in action:

  • simple.rs: a very simple example showing the basics.
  • args.rs: shows how arguments are available to the subprocess as well.
  • timeout.rs: shows how you can wait on a process with timeouts.
  • bad-serialization.rs: shows JSON based workarounds for bincode limitations.

More examples can be found in the example folder: examples

License: MIT/Apache-2.0


~69K SLoC