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.