4.2.1 • Public • Published

GitHub package.json version GitHub tag (latest by date) npm (scoped) license CI on Push

A source code library with the µOS++ trace::printf() tracing infrastructure

This project provides support for a tracing channel, separate from the standard output or error streams. The API is similar to the standard functions.

The code is written in C++, but there are C wrappers which can be called from plain C source files as well.

The project is hosted on GitHub as micro-os-plus/diag-trace-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.


As a source library xPack, 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.


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.


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

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

xpm install @micro-os-plus/diag-trace@latest

ls -l xpacks/@micro-os-plus/diag-trace

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/diag-trace-xpack.git \


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

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

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

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

Developer info


Temporarily adding printf() statements is probably one of the oldest method to debug programs.

Modern debuggers, which allow to single step and inspect variables, are very efficient tools, but, in some cases, a carefully crafted sequence of messages can tell more about how a program runs than a long debug session.

For embedded applications, it is even more preferable for the trace device to be a separate output channel, as fast as possible, to minimise the impact on the debugged target.

One of the fastest solutions is the SEGGER RTT (Real Time Transfer), available with the J-Link probes.

Other solution, for Cortex-M devices that support it, is the Arm ITM (Instrumentation Trace Macrocell).

A slower solution, but still functional, is semihosting, either via the DEBUG channel, or the OUTPUT channel.

µOS++ provides implementations for all those channels, in separate packages.


The diag-trace source library is fully functional.


The following C++ functions are available:

// Write a formatted string to the trace device.
int micro_os_plus::trace::printf (const char *format, ...);

// Write the single character to the trace device.
int micro_os_plus::trace::putchar (int c);

// Write the string and a line terminator to the trace device.
int micro_os_plus::trace::puts (const char *s);

// Write a formatted variable arguments list to the trace device.
int micro_os_plus::trace::vprintf (const char *format, std::va_list arguments);

// Write the `argv[]` array to the trace device.
void dump_args (int argc, char* argv[], const char* name = "main");

// Flush the output.
void flush(void);

// Initialize the trace device; called by the startup code.
void initialize (void);

// Write the given number of bytes to the trace output channel.
ssize_t write (const void* buf, std::size_t nbyte);


The following C functions are available:

// Write a formatted string to the trace device.
int micro_os_plus_trace_printf (const char *format,...);

// Write the single character to the trace device.
int micro_os_plus_trace_putchar (int c);

// Write the string and a line terminator to the trace device.
int micro_os_plus_trace_puts (const char *s);

// Write a formatted variable arguments list to the trace device.
int micro_os_plus_trace_vprintf (const char *format, std::va_list arguments);

// Write the argv[] array to the trace device.
void micro_os_plus_dump_args (int argc, char* argv[]);

// Flush the output.
void micro_os_plus_flush(void);

// Initialize the trace device.
void micro_os_plus_initialize (void);

// Write the given number of bytes to the trace output channel.
ssize_t micro_os_plus_write (const void* buf, std::size_t nbyte);


The application must provide implementations for the following basic functions:

namespace micro_os_plus::trace
  // --------------------------------------------------------------------------

  initialize (void)
    // TODO: open the trace device.

  // --------------------------------------------------------------------------

  write (const void* buf, std::size_t nbyte)
    if (buf == nullptr || nbyte == 0)
        return 0;

    // TODO: write chars from buffer to the trace channel.

    // Return the number of characters successfully sent.
    return (ssize_t) nbyte;

  // --------------------------------------------------------------------------

  flush (void)
    // TODO the trace channel.

// ----------------------------------------------------------------------------

Enabling MICRO_OS_PLUS_TRACE without having these functions defined, results in missing symbols during link.

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 file to be included in user source files is:

#include <micro-os-plus/diag/trace.h>

Source files

The source files to be added to the build are:

  • src/trace.cpp

Preprocessor definitions

There are several preprocessor definitions used to configure the build.

  • MICRO_OS_PLUS_INCLUDE_CONFIG_H - to include <micro-os-plus/config.h>
  • MICRO_OS_PLUS_TRACE - enable support for tracing

If not defined, all functions are defined as empty inlines; thus it is not necessary to brace the calls with #if/#endif.

This definition is usually passed to the compiler on the command line, generally for debug configurations.

Compiler options

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

C++ Namespaces

  • micro_os_plus::trace

micro_os_plus is the top µOS++ namespace, and trace is the library namespace.

C++ Classes

  • none


  • none


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


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

target_link_libraries(your-target PRIVATE



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


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

exe = executable(
  link_with: [
    # Nothing, not static.
  dependencies: [


A simple example showing how to use the trace::printf() functions is presented below and is also available in tests/sample-test.cpp.

#include <micro-os-plus/diag/trace.h>
#include <unistd.h>

using namespace micro_os_plus;

main (int argc, char* argv[])
  trace::initialize ();

  trace::dump_args (argc, argv);

  trace::printf ("Hello %s!\n", "World");
  trace::puts ("one line");
  trace::putchar ('*');

  trace::flush ();

  return 0;

Known problems

  • none


The project is fully tested via GitHub Actions on each push.

The test platforms are GNU/Linux, macOS and Windows; native tests are compiled with GCC and clang; tests for embedded platforms are compiled with arm-none-eabi-gcc and risc-none-elf-gcc, and executed via QEMU.

There are two sets of tests, one that runs on every push, with a limited number of tests, and a set that is triggered manually, usually before releases, and runs all tests on all supported platforms.

The full set can be run manually with the following commands:

xpm run install-all -C ~/Work/micro-os-plus/diag-trace-xpack.git
xpm run test-all -C ~/Work/micro-os-plus/diag-trace-xpack.git

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:

  • v3.x: the weak attribute was removed from initialize() and flush(), so there are no more defaults and both functions must be implemented by the application;
  • v2.x: the C++ namespace was renamed from os to micro_os_plus;
  • v1.x: the code was extracted from the mono-repo µOS++ project.


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

Package Sidebar


npm i @micro-os-plus/diag-trace

Weekly Downloads






Unpacked Size

69.1 kB

Total Files


Last publish


  • ilg