A source code library with the µOS++ portable startup code for bare-metal platforms

This project provides the startup source library as an xpm package dependency and includes a portable startup code for bare-metal platforms.

The myth that startup code must be written in assembly because 'the C environment is not ready' is not exactly right, and, with some care to avoid undefined behaviour, can be overcome.

The 'C environment' is mainly the stack (and maybe other architecture ABI registers, like GP on RISC-V), and this can be set in a short assembly entry code and then the standard _start() function can be safely called.

The project is hosted on GitHub as micro-os-plus/startup-xpack.

Maintainer info

This page is addressed to developers who plan to include this source library into their own projects.

For maintainer info, please see the README-MAINTAINER file.

Install

As a source library xpm package, the easiest way to add it to a project is via xpm, but it can also be used as any Git project, for example as a submodule.

Prerequisites

A recent xpm, which is a portable Node.js command line application.

It is recommended to update to the latest version with:

npm install --global xpm@latest

For details please follow the instructions in the xPack install page.

xpm

This package is available as @micro-os-plus/startup from the npmjs.com registry:

cd my-project
xpm init # Unless a package.json is already present

xpm install @micro-os-plus/startup@latest

ls -l xpacks/@micro-os-plus/startup

Git submodule

If, for any reason, xpm is not available, the next recommended solution is to link it as a Git submodule below an xpacks folder.

cd my-project
git init # Unless already a Git project
mkdir -p xpacks

git submodule add https://github.com/micro-os-plus/startup-xpack.git \
  xpacks/@micro-os-plus/startup

Branches

Apart from the unused master branch, there are two active branches:

• xpack, with the latest stable version (default)
• xpack-development, with the current development version

All development is done in the xpack-development branch, and contributions via Pull Requests should be directed to this branch.

When new releases are published, the xpack-development branch is merged into xpack.

Developer info

Overview

This source code library provides the _startup() routine, as a replacement for the newlib code in crt0.o or GCC code in crtbegin.o.

It is responsible for clearing the BSS, copying the DATA section, and running the static constructors.

Specific for bare-metal applications, the code also perform hardware initialisations, by calling custom hooks.

Status

The startup source library is fully functional.

C++ API

• none

C API

This library defines a C function that can be called from the reset handler.

[[noreturn, gnu::weak]]
void
_start (void);

Optionally it can define:

#if defined(MICRO_OS_PLUS_STARTUP_EXIT_ENABLED)

void __attribute__ ((weak, noreturn))
abort (void);

void __attribute__ ((noreturn))
exit (int code);

void __attribute__ ((weak, noreturn))
_Exit (int code);

void __attribute__ ((weak, noreturn, alias ("_Exit")))
_exit (int status);

#endif // defined(MICRO_OS_PLUS_STARTUP_EXIT_ENABLED)

and also:

#if defined(MICRO_OS_PLUS_STARTUP_SBRK_ENABLED)

void*
_sbrk (ptrdiff_t incr);

#endif // defined(MICRO_OS_PLUS_STARTUP_SBRK_ENABLED)

Hooks

The

#if defined(MICRO_OS_PLUS_STARTUP_INITIALISE_HARDWARE_EARLY_ENABLED)

int
micro_os_plus_startup_initialise_hardware_early_hook (void);

#endif // defined(MICRO_OS_PLUS_STARTUP_INITIALISE_HARDWARE_EARLY_ENABLED)

#if defined(MICRO_OS_PLUS_STARTUP_INITIALISE_HARDWARE_ENABLED)

int
micro_os_plus_startup_initialise_hardware_hook (void);

#endif // defined(MICRO_OS_PLUS_STARTUP_INITIALISE_HARDWARE_ENABLED)

// A weak definition is provided here.
void
micro_os_plus_startup_initialise_free_store_hook (void* heap_address,
                                              size_t heap_size_bytes);

// A weak definition is provided here. The RTOS redefines it.
// Called from _Exit().
void
micro_os_plus_startup_exit_goodbye_hook (void);

// Must be defined by the device package.
void __attribute__ ((noreturn))
micro_os_plus_startup_exit_terminate_hook (int code);

Build & integration info

The project is written in C++, and it is expected to be used in C and C++ projects.

The source code was compiled with GCC 11, clang 12, clang 13 arm-none-eabi-gcc 11, riscv-none-elf-gcc 12, and should be warning free.

To ease the integration of this package into user projects, there are already made CMake and meson configuration files (see below).

For other build systems, consider the following details:

Include folders

The following folders should be passed to the compiler during the build:

• include

The header files to be included in user projects are:

#include "micro-os-plus/startup.h"

Source files

The source files to be added to the build are:

• src/startup.cpp

Preprocessor definitions

• MICRO_OS_PLUS_STARTUP_ENABLED - to include the startup code
• MICRO_OS_PLUS_STARTUP_INITIALISE_HARDWARE_EARLY_ENABLED
• MICRO_OS_PLUS_STARTUP_INITIALISE_HARDWARE_ENABLED
• MICRO_OS_PLUS_STARTUP_INIT_MULTIPLE_RAM_SECTIONS_ENABLED
• MICRO_OS_PLUS_STARTUP_SBRK_ENABLED
• MICRO_OS_PLUS_STARTUP_EXIT_ENABLED
• MICRO_OS_PLUS_STARTUP_GUARD_CHECKS_ENABLED

Compiler options

• -std=c++20 or higher for C++ sources
• -std=c11 for C sources

C++ Namespaces

• none

C++ Classes

• none

Dependencies

• the @micro-os-plus/diag/trace package, for the tracing features

CMake

To integrate the startup source library into a CMake application, add this folder to the build:

add_subdirectory("xpacks/@micro-os-plus/startup")`

The result is an interface library that can be added as an application dependency with:

target_link_libraries(your-target PRIVATE

  micro-os-plus::startup
)

meson

To integrate the startup source library into a meson application, add this folder to the build:

subdir('xpacks/@micro-os-plus/startup')

The result is a dependency object that can be added to an application with:

exe = executable(
  your-target,
  link_with: [
    # Nothing, not static.
  ],
  dependencies: [
    micro_os_plus_startup_dependency,
  ]
)

Examples

TBD

Known problems

• none

Tests

TBD

Change log - incompatible changes

According to semver rules:

Major version X (X.y.z | X > 0) MUST be incremented if any backwards incompatible changes are introduced to the public API.

The incompatible changes, in reverse chronological order, are:

• v9.x: add micro_os_plus_startup_exit() and exit hooks
• v8.x: split sections.ld with separate section-interrupt-vectors.ld
• v7.x: add sections.ld here and rename startup hooks that return values
• v6.x: prefer project-config.h, split micro_os_plus_startup_run_main()
• v5.x: rework guard macros
• v4.x: use newlib linker script definitions
• v3.x: rename MICRO_OS_PLUS_DEBUG
• v2.x: rename namespaces, use MICRO_OS_PLUS_ prefix
• v1.x: initial release

License

Unless otherwise stated, the content is released under the terms of the MIT License, with all rights reserved to Liviu Ionescu.