Legacy Development and Integration Guide

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Linux Application Development

This section targets software developers with a little background on Linux software development, presenting some basic concepts about Linux applications that may be unfamiliar to developers with experience in real time operating systems (RTOS).

Linux application development is more like standard PC application development and less like RTOS focused development. The main differences is the clear separation between the applications and the operating system and also that applications are stored on a file system.

This section doesn't replace a good book on Linux/Unix software development, but introduces the minimal concepts that will be useful when porting applications.

Kernel and User Space

Linux is a multi-tasking operating system with protection between executing processes.Therefore applications run in their own address-space which is provided by the kernel using the hardware memory management unit (MMU).

Since applications are separate from the kernel, they live on some file system, therefore Linux needs a least one file system holding the initial applications: the root file system.

The File System

Linux file system is based on Unix file system concepts, and therefore it is composed of a tree of files that are located on some physical storage. The physical storage should provide the root of the tree (the root file system) and other devices can be “mounted” at directories in the file system tree.

Following the Unix mindset, most of the computer universe is represented like a file: files are data containers for more than binary executable files or raw data stored on memory devices, files are also used to represent device drivers, interprocess pipes, or system status.

Unix file system layout is also important, as there are standard locations for configuration files, shared libraries, binaries, shared data, etc.

Pseudo file systems

Linux has several pseudo file systems that when mounted doesn't reflect actual stored files on some memory device, but file handlers that can provide information about the system, a process, a driver, etc. This pseudo file systems generate the file entries and contents dynamically, based on the system status.

The two more common pseudo-file systems are procfs, and sysfs. More information on those can be found on the next section.

Linux File System Layout

Understanding the Linux file system layout is important for the porting process. The following table explains what's the propose for most common locations and give you an idea or where to store the contents of your application.

Location Propose
/bin Intended for executable binaries required for the basic system.
/sbin Intended for executable binaries required for the basic system that should be available only to the super (root) user.
/lib Shared libraries for the basic system.
/etc Text configuration files of the basic system.
/tmp Temporary files: on most embedded systems this directory contents are lost between power cycles.
/var Intended to store data from system programs: web server pages, log files, mail server data, etc.
/proc This is the typical mount point of Linux's pseudo file system procfs, which creates a set of files that contain different aspects of the running system, processes, drivers, etc.
/sys This is the typical mount point of Linux's pseudo file system sysfs, which provides information on, and control of, devices drivers and kernel objects.
/opt Intended for applications that don't form part of the standard system provided by the vendor. These are usually third-party applications. Most of the applications at this location don't conform to the bin/lib separation of the Unix file systems.
/usr/bin Intended for executable binaries that are not required for the basic system.
/usr/sbin Intended for executable binaries that are not required for the basic system that should be available only to the super user.
/usr/lib Shared libraries not required for the basic system.
Embedded Linux File System

The standard Linux file system is designed for use by more than one person at a time. This is rarely the case on an embedded device, so the difference between /bin, /sbin, /usr/bin, /usr/sbin,/opt may seem less useful than for standard desktop computing proposes.

However one of the important uses of this separation is that it simplifies the final layout of the file systems used by the embedded target. RidgeRun applies the following rules to decide where to locate the applications:

This layout permits different mount points to be used for the /usr and /opt directories which allows:

Embedded target file system

RidgeRun SDK creates the target file system on the Linux host machine, and processes it to create the proper target file system image to be downloaded (if required) to the embedded system. The file system of the target system is found under the base (referred to as $DEVDIR) of your SDK: $(DEVDIR)/fs/fs.

This directory only exist once you have built the SDK for the first time, and you can copy files to this location to include them in the target file system image.

Since the file system of the target should have a minimal footprint, the SDK only puts the strictly necessary parts of the applications into this directory, leaving out some run-time unnecessary parts of many programs like include files, manual and info pages, unused localizations, etc. However some of this files (like the includes) may be required during the compilation process for other programs. For this propose the SDK provides the directory $DEVDIR/fs/fsdev, which is the typical installation point of most applications (including all the unnecessary run-time files), and later the scripts of the application copy the minimal parts from $(DEVDIR)/fs/fsdev towards $(DEVDIR)/fs/fs . More information on this procedure will be explained later on this document.

SDK build system

Building software for embedded systems is a complex task as it typically involves cross-compiling. Cross compiling presents new challenges to already established procedures for building software. The RidgeRun SDK uses a build system that simplifies the complexities of cross compiling.

The SDK build system was designed by embedded Linux programmers for embedded Linux programmers, therefore it not only simplifies the process of building embedded Linux applications, but the process of integrating, using or developing open source tools or packages with the SDK.

Introduction to RidgeRun SDK build system

Embedded Linux software development differs in various ways from the two environments it mixes: “embedded” and “Linux”.

Embedded Linux differs from most embedded environments using an RTOS in that Linux uses a file system outside the kernel, and advanced software tools like dynamic loaded libraries, multiple applications, dynamic modules, powerful networking tools and high modularity are supported.

An often overlook but crucial difference between Linux and other RTOS systems has to do with the dynamics of building and integrating open source software. Open source software is build by different communities around the world, and a typical embedded Linux system will include pieces from hundreds of developers and several different open source projects. This factor impacts various processes: system integration, application building, bug reporting, etc.RidgeRun support services minimizes this complexity by providing a single point to get access to embedded Linux support experts, but also by accounting this factor on the design of the build system.

Embedded Linux also differs from standard Linux distributions in its requirements for custom tailored kernel, drivers, and file system. Embedded limitations on footprint, power consumption and processing power create a big gap between embedded and desktop Linux systems.

All the previously mentioned details need to be taken into account by the development procedures for building and packaging the software for embedded Linux. We found three different approaches from SDK providers to build the system (with RidgeRun using the last option):

Binary software distributions: some embedded Linux providers ship the file system applications pre-compiled for the target hardware. This approach provides the advantage that the overhead and work to recompile the basic system is avoided, however also presents several disadvantages:

Source software distributions by emulation: some embedded Linux providers use a source building based system that is compiled by running a virtual machine emulating the target hardware and using the compiler natively on the emulated target. This approach has valuable advantages:

The disadvantages of this system are:

Source software distributions by cross compilation: consists on building the applications from source for the target hardware from the host target. The advantages are:

The disadvantages of this system are:

RidgeRun SDK build system is a source distribution built using cross compilation, and has minimized the complexities of it by defining a centralized system to simplify tool usage, the setting of compilation flags, dependency handling and minimizing host tools requirements.

SDK build tools

The SDK build system is based on a set of technologies commonly found on standard Linux developer machines:

The set of tools were selected are generally available on most systems, and can be extended to integrate with more sophisticated IDEs like Eclipse, or can be used standalone.

The SDK requires a Linux host machine for development, RidgeRun recommends Ubuntu. Please consult the SDK User Guide or RidgeRun support for the latest information on supported distributions.

Anatomy of a build system

An embedded Linux build system can be analyzed by how well it supports the requirements and the components used to meet those requirements. The RidgeRun SDK build system is composed by:

Centralized configuration system: a single system where the user can control the features and components of the software to be build.

Centralized build system: a single system that is responsible for building the software according to the configuration files generated by the configuration tool.

Application build tools: a set of tools and guidelines that provided support for target applications build process (as described in this document), such:

SDK Build system basics

The SDK build system needs to be understood to easily integrate new applications and port existing applications.

Configuration system

The SDK uses a dynamic configuration utility, called kconfig, which is shared by other important projects of the open source community, for example the Linux Kernel and the busybox set of tools.

The kconfig system used by RidgeRun differs from standard versions:

The build system provides a way to add menus to the configuration system and associate that value to a variable that can later be accessed by the Makefiles to effect decisions or translate it as a compile definition. For example the definition:

   config FS_APPS_MYAPP
         bool “myapp”

will create a definition:

   CONFIG_FS_APPS_MYAPP=y

if selected as true on the configuration system. Note that the suffix “CONFIG_” was added to the variable name. Those definitions can be used by the Makefiles by including the file $DEVDIR/bsp/mach/Make.conf

For more information on the syntax of the configuration files used by the kconfig system and the different options please consult $DEVDIR/bsp/bspc/kconfig-language.txt.

Build stages

The SDK build system builds difference components and this order may be important when building software that relays on other definitions or libraries. The SDK build order is:

The applications from the file system are separated in different directories, and are build in the following order:

The order in which the directories are built inside the specific fs stages defaults to alphabetically sorted directory names. If some application requires a dependent code to be build first, the order can be changed by using the metainfo system explained later on this document.

Makefile integration with the configuration system

The different makefiles of the system can be integrated with the configuration system by including a couple of files providing:

All three are described in detail throughout the rest of this document.

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