test_spawn.rs

use std::os::unix::process::CommandExt;
use std::process::Command;

// Compile with rustc test_spawn.rs
fn main() {
    println!("About to spawn /bin/sh with pre_exec");

    let mut cmd = Command::new("/bin/sh");
    cmd.arg("-c").arg("echo hello");

    // Comment/uncomment this and run 'strace -kk -e socketpair ./test_spawn'.
    // It will run socketpair with pre_exec callback (aka custom spawn) but not without (aka posix_spawn).
    unsafe {
        cmd.pre_exec(|| {
            eprintln!("Inside pre_exec");
            Ok(())
        });
    }

    match cmd.spawn() {
        Ok(mut child) => {
            println!("Spawned successfully");
            match child.wait() {
                Ok(status) => println!("Exit status: {:?}", status),
                Err(e) => eprintln!("Wait failed: {:?}", e),
            }
        }
        Err(e) => {
            eprintln!("Spawn failed: {:?}", e);
            eprintln!("OS error: {:?}", e.raw_os_error());
        }
    }
}