Compile-Time Configuration

This guide describes how Mynewt manages system configuration. It shows you how to tell Mynewt to use default or customized values to configure packages that you develop or use to build a target. This guide:

  • Assumes you have read the Concepts section that describes the Mynewt package hierarchy and its use of the pkg.yml and syscfg.yml files.

  • Assumes you have read the Mynewt Theory of Operations and are familiar with how newt determines package dependencies for your target build.

Mynewt defines several configuration parameters in the pkg.yml and syscfg.yml files. The newt tool uses this information to:

  • Generate a system configuration header file that contains all the package configuration settings and values.

  • Display the system configuration settings and values in the newt target config command.

The benefits with this approach include:

  • Allows Mynewt developers to reuse other packages and easily change their configuration settings without updating source or header files when implementing new packages.

  • Allows application developers to easily view the system configuration settings and values and determine the values to override for a target build.

System Configuration Setting Definitions and Values

A package can optionally:

  • Define and expose the system configuration settings to allow other packages to override the default setting values.

  • Override the system configuration setting values defined by the packages that it depends on.

You use the defs parameter in a syscfg.yml file to define the system configuration settings for a package. defs is a mapping (or associative array) of system configuration setting definitions. It has the following syntax:


Each setting definition consists of the following key-value mapping:

  • A setting name for the key, such as PKGA_SYSCFG_NAME1 in the syntax example above. Note: A system configuration setting name must be unique. The newt tool aborts the build when multiple packages define the same setting.

  • A mapping of fields for the value. Each field itself is a key-value pair of attributes. The field keys are description, value, type, and restrictions. They are described in following table:




    Describes the usage for the setting. This field is optional.


    Specifies the default value for the setting. This field is required. The value depends on the type that you specify and can be an empty string.


    Specifies the data type for the value field. This field is optional. You can specify one of three types:


    The value data is uninterpreted. This is the default type.


    Specifies a Mynewt task priority number. The task priority number assigned to each setting must be unique and between 0 and 239. value can be one of the following:

    A number between 0 and 239 - The task priority number to use for the setting.

    any - Specify any to have newt automatically assign a priority for the setting. newt alphabetically orders all system configuration settings of this type and assigns the next highest available task priority number to each setting.


    Specifies a flash area. The value should be the name of a flash area defined in the BSP flash map for your target board.


    Specifies a list of restrictions on the setting value. This field is optional. You can specify two formats:


    Specifies that the setting cannot have the empty string for a value. It essentially means that an empty string is not a sensible value and a package must override it with an appropriate value.


    Specifies a boolean expression of the form [!]&ltrequired-setting>[if &ltbase-value>]


    restrictions: !LOG_FCB - When this setting is enabled, LOG_FCB must be disabled.

    restrictions: LOG_FCB if 0 - When this setting is disabled, LOG_FCB must be enabled.

Examples of Configuration Settings

Example 1

The following example is an excerpt from the sys/log/full package syscfg.yml file. It defines the LOG_LEVEL configuration setting to specify the log level and the LOG_NEWTMGR configuration setting to specify whether to enable or disable the newtmgr logging feature.

        description: 'Log Level'
        value: 0
        type: raw


        description: 'Enables or disables newtmgr command tool logging'
        value: 0

Example 2

The following example is an excerpt from the net/nimble/controller package syscfg.yml file. It defines the BLE_LL_PRIO configuration setting with a task_priority type and assigns task priority 0 to the BLE link layer task.

        description: 'BLE link layer task priority'
        type: 'task_priority'
        value: 0

Example 3

The following example is an excerpt from the fs/nffs package syscfg.yml file.

        description: 'The flash area to use for the Newtron Flash File System'
        type: flash_owner
            - $notnull

It defines the NFFS_FLASH_AREA configuration setting with a flash_owner type indicating that a flash area needs to be specified for the Newtron Flash File System. The flash areas are typically defined by the BSP in its bsp.yml file. For example, the bsp.yml for nrf52dk board (hw/bsp/nrf52dk/bsp.yml) defines an area named FLASH_AREA_NFFS:

    user_id: 1
    device: 0
    offset: 0x0007d000
    size: 12kB

The syscfg.yml file for the same board (hw/bsp/nrf52dk/syscfg.yml) specifies that the above area be used for NFFS_FLASH_AREA.


Note that the fs/nffs/syscfg.yml file indicates that the NFFS_FLASH_AREA setting cannot be a null string; so a higher priority package must set a non-null value to it. That is exactly what the BSP package does. For more on priority of packages in setting values, see the next section.

Overriding System Configuration Setting Values

A package may use the vals parameter in its syscfg.yml file to override the configuration values defined by other packages. This mechanism allows:

  • Mynewt developers to implement a package and easily override the system configuration setting values that are defined by the packages it depends on.

  • Application developers to easily and cleanly override default configuration settings in a single place and build a customized target. You can use the newt target config show <target-name> command to check all the system configuration setting definitions and values in your target to determine the setting values to override. See newt target.

vals specifies the mappings of system configuration setting name-value pairs as follows:


Note: The newt tool ignores overrides of undefined system configuration settings.

Resolving Override Conflicts

The newt tool uses package priorities to determine whether a package can override a value and resolve conflicts when multiple packages override the same system configuration setting. The following rules apply:

  • A package can only override the default values of system configuration settings that are defined by lower priority packages.

  • When packages with different priorities override the same system configuration setting value, newt uses the value from the highest priority package.

  • Packages of equal priority cannot override the same system configuration setting with different values. newt aborts the build unless a higher priority package also overrides the value.

The following package types are listed from highest to lowest priority:

  • Target

  • App

  • unittest - A target can include either an app or unit test package, but not both.

  • BSP

  • Lib - Includes all other system level packages such as os, lib, sdk, and compiler. (Note that a Lib package cannot override other Lib package settings.)

It is recommended that you override defaults at the target level instead of updating individual package syscfg.yml files.

Examples of Overrides

Example 4

The following example is an excerpt from the apps/slinky package syscfg.yml file. The application package overrides, in addition to other packages, the sys/log/full package system configuration settings defined in Example 1. It changes the LOG_NEWTMGR system configuration setting value from 0 to 1.

    # Enable the shell task.


    # Enable newtmgr commands.

Example 5

The following example are excerpts from the hw/bsp/native package bsp.yml and syscfg.yml files. The package defines the flash areas for the BSP flash map in the bsp.yml file, and sets the NFFS_FLASH_AREA configuration setting value to use the flash area named FLASH_AREA_NFFS in the syscfg.yml file.

        # System areas.
            device: 0
            offset: 0x00000000
            size: 16kB


        # User areas.
            user_id: 0
            device: 0
            offset: 0x00004000
            size: 16kB
            user_id: 1
            device: 0
            offset: 0x00008000
            size: 32kB


Conditional Settings

Setings in most Mynewt YAML files can be made conditional on syscfg settings. For example, a package might depend on a second package only if a syscfg setting has a particular value. The condition can be the value of a single syscfg setting or an arbitrary expression involving many settings.

Examples of Conditional Settings

Example 6

In this example, a package depends on lib/pkg2 only if MY_SETTING has a true value.

    # Only depend on pkg2 if MY_SETTING is true.
    - lib/pkg2

A setting is “true” if it has a value other than 0 or the empty string. Undefined settings are not true.

Example 7

In this example, a package overrides the setting FOO only if BAR is greater than 5 and BAZ is not true.

syscfg.vals.'(BAR > 5 && !BAZ):
    # Only override FOO if BAR is greater than 5 and BAZ is untrue.
    FOO: 35

Generated syscfg.h and Referencing System Configuration Settings

The newt tool processes all the package syscfg.yml files and generates the bin/<target-path>/generated/include/syscfg/syscfg.h include file with #define statements for each system configuration setting defined. Newt creates a #define for a setting name as follows:

  • Adds the prefix MYNEWT_VAL_.

  • Replaces all occurrences of “/”, “-“, and ” ” in the setting name with “_”.

  • Converts all characters to upper case.

For example, the #define for my-config-name setting name is MYNEWT_VAL_MY_CONFIG_NAME.

Newt groups the settings in syscfg.h by the packages that defined them. It also indicates the package that changed a system configuration setting value.

You must use the MYNEWT_VAL() macro to reference a #define of a setting name in your header and source files. For example, to reference the my-config-name setting name, you use MYNEWT_VAL(MY_CONFIG_NAME).

Note: You only need to include syscfg/syscfg.h in your source files to access the syscfg.h file. The newt tool sets the correct include path to build your target.

Example of syscfg.h and How to Reference a Setting Name

Example 6: The following example are excerpts from a sample syscfg.h file generated for an app/slinky target and from the sys/log/full package log.c file that shows how to reference a setting name.

The syscfg.h file shows the sys/log/full package definitions and also indicates that app/slinky changed the value for the LOG_NEWTMGR settings.

 * This file was generated by Apache Newt version: 1.0.0-dev


 * This macro exists to ensure code includes this header when needed.  If code
 * checks the existence of a setting directly via ifdef without including this
 * header, the setting macro will silently evaluate to 0.  In contrast, an
 * attempt to use these macros without including this header will result in a
 * compiler error.
#define MYNEWT_VAL(x)                           MYNEWT_VAL_ ## x

/* ... */

/*** kernel/os */


/* ... */

/*** sys/log/full */


/* ... */

/* Overridden by apps/slinky (defined by sys/log/full) */


The log_init() function in the sys/log/full/src/log.c file initializes the sys/log/full package. It checks the LOG_NEWTMGR setting value, using MYNEWT_VAL(LOG_NEWTMGR), to determine whether the target application has enabled the newtmgr log functionality. It only registers the the callbacks to process the newtmgr log commands when the setting value is non-zero.

    int rc;

    /* Ensure this function only gets called by sysinit. */


    if (log_inited) {
    log_inited = 1;

    /* ... */

    rc = log_nmgr_register_group();