Common File Format

All of the definition files have the same format. They're divided into sections. Each section type has a set of permitted content, which could be simple, numerical or text values, named items, or other sections (subsections).


A section always starts with a section name followed by a colon (':').

faultAction: restart
maxFileSystemBytes: 200K

Sections that can have multiple content items always have curly braces around their content.


All sections are optional; they can be empty or omitted.

Sections can be in any order, multiple occurrences are permitted, and some sections have sub-sections.

/dev/ttyS0 /dev/uart
/tmp/foo /tmp/

Named Items

A named item always begins with the item name followed by an equals sign ('=').

myPool = 45

If a named item can have multiple content items, then it must have curly braces around its content.

myExe = { myComponent otherComponent }

Numerical Values

Integers are required in some sections.

maxFileDescriptors: 100

Numbers can be expressed in multiples of 1024 by adding the 'K' suffix.

maxFileSystemBytes: 120K

Text Values

Some sections contain text-based values. The format of these varies depending on the type of section.

Environment Variables

It's possible to use environment variables inside of .sdef, .adef, and Component.cdef files.

This works the same way as in shell scripts, by prefixing the environment variable name with a dollar sign ($):


If necessary, the variable name can also be enclosed in curly braces.


A double dollar sign ('$$') is replaced by a dollar sign ('$').

Furthermore, the mk tools will create some environment variables:

  • LEGATO_ROOT = path to where the Legato framework sources are located
  • LEGATO_TARGET = build target (e.g., wp85, localhost)
  • LEGATO_BUILD = shorthand for $LEGATO_ROOT/build/$TARGET
  • LEGATO_SYSROOT = path of the directory containing the target device's system files
  • CURDIR = path of the directory containing the definition file being parsed
  • CC = path to the C compiler
  • CXX = path to the C++ compiler
  • LD = path to the linker
  • AR = path to the archiver to be used to generate static libraries
  • AS = path to the assembler to be used if assembly code source files are found
  • STRIP = path to the debug symbol stripper utility
  • OBJCOPY = path to the object file copier/translator utility
  • READELF = path to the ELF file information extractor utility

Include Directives

Definition file sections or parts of a section may be common between several related systems, applications or components. For example, all systems built for a certain module may need the some common buildVars or cflags set, or a series of related products may all share a base set of apps. To reuse common parts of the definition file, these parts can be separated into a common file, which is then included in each definition file where it is needed. This is done through include directives.

Include directives consist of the word #include followed by the name of the file to include. For example


will be treated as if the contents of appeared at this point. Multiple include directives can be used to include multiple files.

When looking for a file to include, environment variables in the filename will be expanded first. If the filename is absolute, that file will be included. If it is relative, first the directory containing the current file will be searched, followed by the ${LEGATO_ROOT} directory.

If an environment variable is used in an include filename the value of that environment variable can't be changed in a later buildVars section.

If Directives

Some definition file sections or parts of sections may be needed only on certain targets, or in certain configurations. To handle conditionals like this, definition files support the #if, #elif, #else, and #endif directives. These function much like their C counterparts, allowing you to include or exclude certain sections of a Xdef file.

If directives consist of an initial #if directive, zero or more #elif directives, optionally an #else directive then a final #endif directive marking the end of the conditional section. For example:

// Definitions to be used if INCLUDE_FEATURE is 1
#elif dir_exists(apps/optional_app)
// Definitions to be used if INCLUDE_FEATURE is not 1 but apps/optional_app exists
// Definitions to be used if none of the above are true

Conditions which can be used with #if directives are:

  • left_hand_side = right_hand_side: Check if left_hand_side is equal to right_hand_side after expanding environment variables. Example uses are setting different compiler flags depending on the target, or including certain apps only if a specific feature is enabled.
  • dir_exists(dirpath): Checks if the directory dirpath exists. Relative paths are relative to the directory containing the file being processed. Example uses include building a component from source if the source is available, or using a pre-built binary module if not.
  • file_exists(filepath): Checks if the file filepath exists. Relative paths are relative to the directory containing the file being processed. Uses are similar to dir_exists.
This feature is most commonly used in included files. In a top level file it is usually easier to write a separate definition file for each configuration, or move target-specific portions into a target-specific include file.


Tabs, spaces, new-lines, carriage-returns and comments act as token separators only, and are otherwise all ignored. Consequently, choice of indentation and spacing style is relatively unrestricted.


Comments can be included using

// C++-style one-line comments.
/* or
C-style multi-line comments. */


/* Component.cdef file for a hypothetical web server component that provides a control API
* (defined in httpdCtrl.api) that allows other apps to start and stop the web server via IPC.
#include "targets/${LEGATO_TARGET}.def"
// Include all the web pages from the build host in the application, and have them appear
// to the application under /var/www.
htdocs /var/www
provides : // Note that space is allowed between the section name and the colon.
api: { httpdCtrl.api
sources: {httpd.c}