RS.CLIPPY.ZOMBIE_PROCESSES

Not waiting on a spawned child process

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: zombie_processes. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Looks for code that spawns a process but never calls wait() on the child.

Why is this bad?

As explained in the standard library documentation, calling wait() is necessary on Unix platforms to properly release all OS resources associated with the process. Not doing so will effectively leak process IDs and/or other limited global resources, which can eventually lead to resource exhaustion, so it's recommended to call wait() in long-running applications. Such processes are called "zombie processes".

To reduce the rate of false positives, if the spawned process is assigned to a binding, the lint actually works the other way around; it conservatively checks that all uses of a variable definitely don't call wait() and only then emits a warning. For that reason, a seemingly unrelated use can get called out as calling wait() in help messages.

Control flow

If a wait() call exists in an if/then block but not in the else block (or there is no else block), then this still gets linted as not calling wait() in all code paths. Likewise, when early-returning from the function, wait() calls that appear after the return expression are also not accepted. In other words, the wait() call must be unconditionally reachable after the spawn expression.

Example

use std::process::Command;

let _child = Command::new("ls").spawn().expect("failed to execute child");

Use instead:

use std::process::Command;

let mut child = Command::new("ls").spawn().expect("failed to execute child");
child.wait().expect("failed to wait on child");