Application Completion Codes

TRD: 106
Working Group: Kernel
Type: Documentary
Status: Draft
Author: Alyssa Haroldsen
Draft-Created: December 6, 2021
Draft-Modified: January 25, 2022
Draft-Version: 1


This advisory document describes the expected behavior of application completion codes when terminating via the exit syscall, as described in TRD 104.

1 Introduction

When an application exits via the exit syscall, it can specify a completion code, an unsigned 32-bit number which indicates status. This information can be stored in the kernel and used in management or policy decisions.

This number is called an "exit status", "exit code", or "result code" on other platforms.

2 Design Considerations

When possible, Tock applications should follow existing conventions and terminology from other major platforms. This assists in helping the project be more understandable to newcomers by following the principle of least astonishment.

This advisory document provides guidance for the ecosystem of Tock applications using the exit syscall, and does not define the behavior of the syscall itself.

3 Design

A completion code of 0 passed to the exit syscall MUST indicate normal app termination. A non-zero completion code SHOULD be used to indicate abnormal termination. This distinction is useful so that a Tock kernel can handle success/failure cases differently, e.g. by printing error messages, and so that kernel extensions (such as process exit handlers defined by a board) or external tools (such as a tool designed to parse the output from a kernel with trace_syscalls enabled) can match on these two cases. This behavior also matches the convention for Unix exit codes, such that it likely matches the expectations for users coming from that domain.

A completion code between 1 and 1024 inclusive SHOULD be the same value as one of the error codes specified in TRD 104. This requirement is a SHOULD rather than a MUST because it is useful in the common case (it allows software to infer something about the cause of an error that led to an exit, and possibly print a useful message) but also allows a process to do something else if needed (e.g. for compatibility with some other standard of exit codes).

Accordingly, the core kernel MUST NOT assume any semantic meaning for completion codes or take actions based on their values besides printing error messages unless

  • there is a specification of a particular application's completion code space written in a TRD, and

  • the kernel can reliably identify that application and associate it with this specification. While there are common and conventional uses of certain values, applications are not required to follow these and may assign their own semantic meanings to values.

Completion CodeMeaning
1-1024SHOULD be a TRD 104 error code
1025-u32::MAXNot defined

4 Implementation

As of writing, libtock currently implements this TRD via the Termination trait.

fn main() {
pub trait Termination {
    fn complete<S: Syscalls>(self) -> !;

impl Termination for () {
    fn complete<S: Syscalls>(self) -> ! {

impl Termination for Result<(), ErrorCode> {
    fn complete<S: Syscalls>(self) -> ! {
        let exit_code = match self {
            Ok(()) => 0,
            Err(ec) => ec as u32,

5 Author's Address

Alyssa Haroldsen <>
Hudson Ayers <>