Embedded Linux Primer: A Practical, Real-World Approach

}

Listing 2-5 shows the console output that this program produces. Notice that the process called hello thinks it is executing somewhere in high RAM just above the 256MB boundary (0x10000418). Notice also that the stack address is roughly halfway into a 32-bit address space, well beyond our 256MB of RAM (0x7ff8ebb0). How can this be? DRAM is usually contiguous in systems like these. To the casual observer, it appears that we have nearly 2GB of DRAM available for our use. These virtual addresses were assigned by the kernel and are backed by physical RAM somewhere within the 256MB range of available memory on the Yosemite board.

Listing 2-5. Hello Output

root@amcc:~# ./hello
Hello, World! Main is executing at 0x10000418
This address (0x7ff8ebb0) is in our stack frame
This address (0x10010a1c) is in our bss section
This address (0x10010a18) is in our data section
root@amcc:~#

One of the characteristics of a virtual memory system is that when available physical RAM goes below a designated threshold, the kernel can swap memory pages out to a bulk storage medium, usually a hard disk drive. The kernel examines its active memory regions, determines which areas in memory have been least recently used, and swaps these memory regions out to disk, to free them up for the current process. Developers of embedded systems often disable swapping on embedded systems because of performance or resource constraints. For example, it would be ridiculous in most cases to use a relatively slow Flash memory device with limited write life cycles as a swap device. Without a swap device, you must carefully design your applications to exist within the limitations of your available physical memory.

2.3.8. Cross-Development Environment

Before we can develop applications and device drivers for an embedded system, we need a set of tools (compiler, utilities, and so on) that will generate binary executables in the proper format for the target system. Consider a simple application written on your desktop PC, such as the traditional 'Hello World' example. After you have created the source code on your desktop, you invoke the compiler that came with your desktop system (or that you purchased and installed) to generate a binary executable image. That image file is properly formatted to execute on the machine on which it was compiled. This is referred to as native compilation. That is, using compilers on your desktop system, you generate code that will execute on that desktop system.

Note that native does not imply an architecture. Indeed, if you have a toolchain that runs on your target board, you can natively compile applications for your target's architecture. In fact, one great way to test a new kernel and custom board is to repeatedly compile the Linux kernel on it.

Developing software in a cross-development environment requires that the compiler running on your development host output a binary executable that is incompatible with the desktop development workstation on which it was compiled. The primary reason these tools exist is that it is often impractical or impossible to develop and compile software natively on the embedded system because of resource (typically memory and CPU horsepower) constraints.

Numerous hidden traps to this approach often catch the unwary newcomer to embedded development. When a given program is compiled, the compiler often knows how to find include files, and where to find libraries that might be required for the compilation to succeed. To illustrate these concepts, let's look again at the 'Hello World' program. The example reproduced in Listing 2-4 above was compiled with the following command line:

gcc -Wall -o hello hello.c

From Listing 2-4, we see an include the file stdio.h. This file does not reside in the same directory as the hello.c file specified on the gcc command line. So how does the compiler find them? Also, the printf() function is not defined in the file hello.c. Therefore, when hello.c is compiled, it will contain an unresolved reference for this symbol. How does the linker resolve this reference at link time?

Compilers have built-in defaults for locating include files. When the reference to the include file is encountered, the compiler searches its default list of locations to locate the file. A similar process exists for the linker to resolve the reference to the external symbol printf(). The linker knows by default to search the C library (libc-*) for unresolved references. Again, this default behavior is built into the toolchain.

Now consider that you are building an application targeting a PowerPC embedded system. Obviously, you will need a cross-compiler to generate binary executables compatible with the PowerPC processor architecture. If you issue a similar compilation command using your cross-compiler to compile the hello.c example above, it is possible that your binary executable could end up being accidentally linked with an x86 version of the C library on your development system, attempting to resolve the reference to printf(). Of course, the results of running this bogus hybrid executable, containing a mix of PowerPC and x86 binary instructions[14] are predictable: crash!

The solution to this predicament is to instruct the cross-compiler to look in nonstandard locations to pick up the header files and target specific libraries. We cover this topic in much more detail in Chapter 12, 'Embedded Development Environment.' The intent of this example was to illustrate the differences between a native development environment, and a development environment targeted at cross-compilation for embedded systems. This is but one of the complexities of a cross-development environment. The same issue and solutions apply to cross-debugging, as you will see starting in Chapter 14, 'Kernel Debugging Techniques.' A proper cross- development environment is crucial to your success and involves much more than just compilers, as we shall soon see in Chapter 12, 'Embedded Development Environment.'

2.4. Embedded Linux Distributions

What exactly is a distribution anyway? After the Linux kernel boots, it expects to find and mount a root file system. When a suitable root file system has been mounted, startup scripts launch a number of programs and utilities that the system requires. These programs often invoke other programs to do specific tasks, such as spawn a login shell, initialize network interfaces, and launch a user's applications. Each of these programs has specific requirements of the system. Most Linux application programs depend on one or more system libraries. Other programs require configuration and log files, and so on. In summary, even a small embedded Linux system needs many dozens of files populated in an appropriate directory structure on a root file system.

Full-blown desktop systems have many thousands of files on the root file system. These files come from packages that are usually grouped by functionality. The packages are typically installed and managed using a package manager. Red Hat's Package Manager (rpm) is a popular example and is widely used for installing, removing, and updating packages on a Linux system. If your Linux workstation is based on Red Hat, including the Fedora Core series, typing rpm -qa at a command prompt lists all the packages installed on your system.

A package can consist of many files; indeed, some packages contain hundreds of files. A complete Linux distribution can contain hundreds or even thousands of packages. These are some examples of packages that you might find on an embedded Linux distribution, and their purpose:

• initscripts Contains basic system startup and shutdown scripts.