Mynewt log package supports logging of information within a Mynewt application. It allows packages to define their own log streams with separate names. It also allows an application to control the output destination of logs.


In the Mynewt OS, the log package comes in two versions:

  • The sys/log/full package implements the complete log functionality and API.

  • The sys/log/stub package implements stubs for the API.

Both packages export the log API, and any package that uses the log API must list log as a requirement in its pkg.yml file as follows:

    - log

The application's pkg.yml file specifies the version of the log package to use. A project that requires the full logging capability must list the sys/log/full package as a dependency in its pkg.yml file:

    - sys/log/full

You can use the sys/log/stub package if you want to build your application without logging to reduce code size.

Syscfg Settings

The LOG_LEVEL syscfg setting allows you to specify the level of logs to enable in your application. Only logs for levels higher or equal to the value of LOG_LEVEL are enabled. The amount of logs you include affects your application code size. LOG_LEVEL: 0 specifies LOG_LEVEL_DEBUG and includes all logs. You set LOG_LEVEL: 255 to disable all logging. The #defines for the log levels are specified in the sys/log/full/include/log/log.h file. For example the following setting corresponds to LOG_LEVEL_ERROR:

    LOG_LEVEL: 3   

The LOG_LEVEL setting applies to all modules registered with the log package.


Each log stream requires a log structure to define its logging properties.

Log Handler

To use logs, a log handler that handles the I/O from the log is required. The log package comes with three pre-built log handlers:

  • console -- streams log events directly to the console port. Does not support walking and reading.
  • cbmem -- writes/reads log events to a circular buffer. Supports walking and reading for access by newtmgr and shell commands.
  • fcb -- writes/reads log events to a flash circular buffer. Supports walking and reading for access by newtmgr and shell commands.

In addition, it is possible to create custom log handlers for other methods. Examples may include

  • Flash file system
  • Flat flash buffer
  • Streamed over some other interface

To use logging, you typically do not need to create your own log handler. You can use one of the pre-built ones.

A package or an application must define a variable of type struct log and register a log handler for it with the log package. It must call the log_register() function to specify the log handler to use:

log_register(char *name, struct log *log, const struct log_handler *lh, void *arg, uint8_t level)

The parameters are:

  • name- Name of the log stream.
  • log - Log instance to register,
  • lh - Pointer to the log handler. You can specify one of the pre-built ones:
    • &log_console_handler for console
    • &log_cbm_handler for circular buffer
    • &log_fcb_handler for flash circular buffer
  • arg - Opaque argument that the specified log handler uses. The value of this argument depends on the log handler you specify:
    • NULL for the log_console_handler.
    • Pointer to an initialized cbmem structure (see util/cbmem package) for the log_cbm_handler.
    • Pointer to an initialized fcb_log structure (see fs/fcb package) for the log_fcb_handler.

Typically, a package that uses logging defines a global variable, such as my_package_log, of type struct log. The package can call the log_register() function with default values, but usually an application will override the logging properties and where to log to. There are two ways a package can allow an application to override the values:

  • Define system configuration settings that an application can set and the package can then call the log_register() function with the configuration values.
  • Make the my_package_log variable external and let the application call the log_register() function to specify a log handler for its specific purpose.

Configuring Logging for Packages that an Application Uses

Here is an example of how an application can set the log handlers for the logs of the packages that the application includes.

In this example, the package1 package defines the variable package1_log of type struct log and externs the variable. Similarly, the package2 package defines the variable package2_log and externs the variable. The application sets logs for package1 to use console and sets logs for package2 to use a circular buffer.

#include <package1/package1.h>
#include <package2/package2.h>
#include <util/cbmem.h>

#include <log/log.h>

static uint32_t cbmem_buf[MAX_CBMEM_BUF];
static struct cbmem cbmem;

void app_log_init(void)

    log_register("package1_log", &package1_log, &log_console_handler, NULL, LOG_SYSLEVEL);

    cbmem_init(&cbmem, cbmem_buf, MAX_CBMEM_BUF);
    log_register("package2_log", &package2_log, &log_cbmem_handler, &cbmem, LOG_SYSLEVEL);


Implementing a Package that Uses Logging

This example shows how a package logs to console. The package registers default logging properties to use the console, but allows an application to override the values. It defines the my_package_log variable and makes it external so an application can override log handler.

Make the my_package_log variable external:

/* my_package.h*/

/* pick a unique name here */
extern struct log my_package_log;

Define the my_package_log variable and register the console log handler:

/* my_package.c */

struct log my_package_log;


    /* register my log with a name to the system */
    log_register("log", &my_package_log, &log_console_handler, NULL, LOG_LEVEL_DEBUG);

    LOG_DEBUG(&my_package_log, LOG_MODULE_DEFAULT, "bla");
    LOG_DEBUG(&my_package_log, LOG_MODULE_DEFAULT, "bab");

Log API and Log Levels

For more information on the log API and log levels, see the sys/log/full/include/log/log.h header file.