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About the configuration system

The mbed configuration system can be used to customize the compile time configuration of various mbed components (targets, libraries and applications). Each such component can define a number of configuration parameters. The values of these configuration parameters can then be overridden in various ways. Configuration is defined using JSON. Some examples of configuration parameters:

the sampling period for a data acquisition application.
the default stack size for a newly created OS thread.
the receive buffer size of a serial communication library.
the flash and RAM memory size of a mbed target.

The configuration system gathers and interprets all the configuration defined in the source tree. The output of the configuration system is a list of macros that are automatically defined when compiling the code.

Defining configuration parameters

The configuration system understands configuration data defined in targets, libraries and applications. While there are some slight differences in the way the configuration system works in these cases, the configuration parameters are always defined in a JSON object called "config". An example is given below:

{
    "config": {
        "param1": {
            "help": "The first configuration parameter",
            "macro_name": "CUSTOM_MACRO_NAME",
            "value": 0
        },
        "param2": {
            "help": "The second configuration parameter",
            "required": true
        },
        "param3": 10
    }
}

The JSON fragment above defines 3 configuration parameters named param1, param2 and param3. There are two ways to define a configuration parameter:

the short way: by name and value. param3 above is an example of a short definition for a parameter named param3 with value 10.
the long way: by name and description (another JSON object), like param1 and param2 above. The JSON description object can have the following keys:
- help: an optional help message that describes the purpose of the parameter.
- value: an optional field that defines the value of the parameter.
- required: an optional key that specifies if the parameter must be given a value before compiling the code (false by default). It's not possible to compile a source tree with one or more required parameters that don't have a value. Generally, it makes sense to define a required parameter only when it doesn't have a value key.
- macro_name: an optional name for the macro defined at compile time for this configuration parameter. The configuration system will automatically figure out the corresponding macro name for a configuration parameter, but the user can override this automatically computed name by specifying macro_name.

Note that the name of a parameter in config can't contain a dot (.) character.

The configuration system automatically appends an implicit prefix to the name of each parameter, so you don't have to worry about a name clash if you define a parameter with the same name in a library and a target, for example. The implicit prefix is:

target. if the parameter is defined in a target.
app. if the parameter is defined in the application.
the name of the library followed by a dot (.) if the parameter is defined in a library.

Configuration data in libraries

Each mbed library can have an optional mbed_lib.json file located in the root folder of the library that defines its configuration. For a library called mylib, the configuration file could look like this:

{
    "name": "mylib",
    "config": {
        "buffer_size": 1024,
        "timer_period": {
            "help": "The timer period (in us)",
            "macro_name": "INTERNAL_GPTMR_PERIOD",
            "required": true
        },
        "queue_size": {
            "help": "Size of event queue (entries)",
            "value": 10
        }
    },
    "macros": ["MYMOD_MACRO1", "MYMOD_MACRO2=\"TEST\""],
    "target_overrides": {
        "K64F": {
             "timer_period": 100,
             "queue_size": 40
        },
        "NXP": {
             "queue_size": 20,
             "buffer_size": 128
        }
    }
}

In this JSON file:

name is the name of the library. This is a required field.
config defines the configuration parameters of the library, as explained here.
macros is a list of extra macros that will be defined when compiling a project that includes this library. A macro can be defined without a value (like MYMOD_MACRO1 above) or with a value (like MYMOD_MACRO2 above).
target_overrides is a dictionary with target-specific values for the configuration parameters.

target_overrides is used to override the values of the parameters depending on the current compilation target. The keys in target_overrides are matched against toolchain labels (a description of mbed targets can be found here). If a key inside target_overrides matches one of the target labels, the parameter values are changed according to the value of the key. In the example above:

config is always processed first, independent of the target. config might define values for some of the parameters. In this case, buffer_size will be set to 1024, queue_size will be set to 10 and timer_period will not have a value.
if the library is compiled for the K64F target, timer_period will be set to 100 and queue_size will be set to 40, since they are overridden by the K64F key in target_overrides. buffer_size will be set to 1024, as defined in config.
assuming that NXP is a label defined by all NXP based targets, if the library is compiled for any NXP target (like LPC1768 or LPC11U24), buffer_size will be set to 128 and queue_size will be set to 20, while timer_period will not have a value (since it doesn't get one neither in config, nor in the NXP override).
the keys in target_overrides are processed in order: if a hypothetical target defines both K64F and NXP as labels, timer_period will be set to 100, queue_size will be set to 20 and buffer_size will be set to 128.
if the library is compiled for a target that doesn't have K64F or NXP as labels, the values of the parameters will be the ones set in config.

Except name, all the above keys in the JSON file are optional, but if target_overrides is defined, config must also be defined.

As explained here, the parameters have an implicit mylib. prefix. Outside mylib, buffer_size is accessible using the name mylib.buffer_size. An application will be able to override the value of this parameter, as described in this section.

If the source tree has code for more than one library, each library needs its own mbed_lib.json file in its root folder.

Configuration data in targets

Like libraries, targets can define their own configuration data. Additionally, tables can override the configuration of the target(s) they inherit from (for more details about how do define a target and target inheritance, check this link). Target configuration data is defined in targets.json using config, as described here. An example for a hypothetical Base target is given below:

"Base": {
    "core": "Cortex-M0",
    "extra_labels": ["BASE_LABEL"],
    "config": {
        "serial_console_speed": {
            "help": "Baud rate of the serial console",
            "value": 115200,
            "macro_name": "MBED_SERIAL_UART_SPEED"
        },
        "stack_size": {
            "help": "Initial stack size of the application",
            "value": 128
        }
    }
}

Similar to libraries, the target defined parameters have an implicit prefix. For a target, the prefix is always called target (no matter what the actual target name is), so the above configuration parameters will be accessible outside the definition in Base (and any other target) as target.serial_console_speed and target.stack_size.

Targets can inherit from other targets, and their configuration data is also inherited. A target that inherits from one or more other targets can add new parameters in its own config section and can also override the configuration parameters defined by its parent(s) in a overrides section. For example:

"Derived": {
    "inherits": ["Base"],
    "extra_labels_add": ["NXP"],
    "config": {
        "my_own_config": {
            "help": "My very own configuration parameter",
            "value": 0
        }
    },
    "overrides": {
        "stack_size": 256
    }
}

Derived above defines its own configuration parameter called my_own_config and inherits the configuration parameters from Base, so its configuration parameters are serial_console_speed, stack_size and my_own_config. It also overrides the value of the stack_size parameter defined in Base. This means that:

when compiling for Base, the target will define two configuration parameters: serial_console_speed with the value 115200 and stack_size with the value 128.
when compiling for Derived, the target will define three configuration parameters: serial_console_speed with the value 115200, stack_size with the value 256 and my_own_config with the value 0.

It is an error for a derived target to re-define a configuration parameter already defined by its parent(s) in its config section. It is also an error for a derived target to override a configuration parameter that was not defined by its parent(s) in its overrides section.

Configuration data in applications

Like the configuration for targets and libraries, application configuration is optional; if it exists, it must be defined in a mbed_app.json file. Unlike library configuration, there can be a single mbed_app.json file in the source tree.

There are quite a few similarities between configuration data in applications and libraries:

applications define their configuration parameters in the config section of mbed_app.json, as explained here.
applications can specify target-dependent values in their target_overrides section, as described in the [library configuration paragraph][#configuration-data-in-libraries) (but see below for differences).
applications can define macros that will be define at compile time by declaring them in macros.

There are also a few differences:

applications can't have a name key in mbed_app.json. The prefix for the configuration parameters defined in an application is always app..
applications can also override configuration of libraries and targets in addition to its own configuration in its target_overrides section.

The last point above is important. The application can freely override the configuration of any of the libraries it depends on, as well as the configuration data in targets, so it has complete control over the configuration of the whole build. For an application called myapp that depends on mylib above, the configuration can look like this:

{
    "config": {
        "welcome_string": {
            "help": "The string printed on the display on start-up",
            "value": "\"Hello!\""
        }
    },
    "target_overrides": {
        "*": {
            "target.serial_console_speed": 2400,
            "mylib.timer_period": 100
        },
        "Base": {
            "target.serial_console_speed": 9600
        }
    }
}

target_overrides works a lot like it does in libraries, but there are a few differences:

since the application can override any configuration parameter, it must specify them using their prefix (like mylib.timer_period). If an overridden parameter doesn't have a prefix, it is assumed that it is one of the parameters defined by the application in its own config section.
the * key in target_overrides will match any target. It is possible to use the * key in a library's target_overrides too, but it'd make little sense to do so, since it will always override the values defined in the library's config section. In an application it might make sense to use the * key, since it can be used to override the configuration defined by the target or the dependent libraries, no matter which target is used for building.

Other than this, target_overrides works exactly like it does for libraries. Keys in target_overrides are still processed in the order they are defined, so for the example above, the * override is always processed first (since it matches all targets) and then Base is only processed for the Base target.

myapp above defines its own configuration parameter (welcome_string) and overrides the configuration in both the target (target.serial_console_speed) and its mylib dependency (mylib.timer_period):

when compiling for Base, app.welcome_string will be set to "Hello!", target.serial_console_speed will be set to 9600 (from the Base override) and mylib.timer_period will be set to 100 (from the * override).
when compiling for Derived, app.welcome_string will be set to "Hello!", target.serial_console_speed will be set to 2400 (from the * override) and mylib.timer_period will be set to 100 (also from the * override).

It is an error for the application configuration to override configuration parameters that were not defined.

Custom targets

Application configuration can optionally define application-specific targets. These are mbed targets that are needed just to compile this specific application, so it doesn't make sense to add them to the list of official mbed targets; on the contrary, since they're part of mbed_app.json, they're versioned together with the application and only known by the application. Application-specific targets are defined with the key custom_targets in the mbed_app.json file and have the same syntax as a regular target definition, for example:

{
    "custom_targets": {
       "k64f_myapp": {
           "inherits": ["K64F"],
           "extra_labels_add": ["CUSTOM_K64F_LIB"]
       }
    }
}

This will define a new target named k64f_myapp that inherits from the K64F mbed target, but with an extra label defined, which will change the way the build system looks for sources in the tree.

Configuration data precedence

The order in which the various bits of configurations are considered is this:

the configuration defined by an inherited target overrides the configuration defined by its parent(s), as described above.
the configuration of the top level application overrides the configuration defined by the target and any of the libraries on which it depends.

For myapp above:

the value of target.serial_console_speed will be 9600 when compiling for Base because of the Base override in myapp's target_overrides.
the value of target.serial_console_speed will be 2400 when compiling for any other target because of the * override in myapp's target_overrides.
the value of target.stack_size will be 256 when compiling for Derived and 128 when compiling for Base or any other target that derives from Base (assuming of course that Derived is the only target that redefines stack_size).
the value of mylib.timer_period will be 100, since that's overridden by the application and thus takes precedence over the values defined in mylib.
when compiling for Base, the values of mylib.buffer_size and mylib.queue_size will be 1024 and 10 respectively, as defined in the config section of mylib.
when compiling for Derived, the values of mylib.buffer_size and mylib.queue_size will be 128 and 20 respectively, since Derived defines the NXP label and mylib defines a specific configuration for this label. Also, since Derived has its own my_own_config configuration parameter, target.my_own_config will also be defined in this case.

Using configuration data in the code

When compiling, the configuration system will automatically generate macro definitions for the configuration parameters and all the macros defined in libraries and the application in their macros keys. These definitions will be appended to the compiler's command line. When compiling myapp for target Base, the following extra macros will be generated (not necessarily in this order):

-DMBED_CONF_MYAPP_WELCOME_STRING="Hello!"
-DMBED_SERIAL_UART_SPEED=9600
-DMBED_CONF_TARGET_STACK_SIZE=128
-DINTERNAL_GPTMR_PERIOD=100
-DMBED_CONF_MYLIB_BUFFER_SIZE=1024
-DMBED_CONF_MYLIB_QUEUE_SIZE=10
-DMYMOD_MACRO1
-DMYMOD_MACRO2="TEST"

When compiling for Derived, the following extra macros will be generated:

-DMBED_CONF_MYAPP_WELCOME_STRING="Hello!"
-DMBED_SERIAL_UART_SPEED=2400
-DMBED_CONF_TARGET_STACK_SIZE=256
-DMBED_CONF_TARGET_MY_OWN_CONFIG=0
-DINTERNAL_GPTMR_PERIOD=100
-DMBED_CONF_MYLIB_BUFFER_SIZE=128
-DMBED_CONF_MYLIB_QUEUE_SIZE=20
-DMYMOD_MACRO1
-DMYMOD_MACRO2="TEST"

Note that a macro definition will not be generated for a parameter that doesn't have a value.

The names of the macros for the configuration parameter (unless explicitly specified by macro_name) are prefixed by MBED_CONF_, followed by the full (prefixed) name of the parameter, capitalized and converted to a valid C macro name (if needed).

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