Christopher Hallinan: Embedded Linux Primer: A Practical, Real-World Approach

When busybox is invoked via the sh symbolic link, it spawns a shell. We cover this in detail in Chapter 11.

This inittab is a nice example of a small, purpose-built embedded system.

It just so happens that on this particular board, our physical SDRAM starts at 256MB.

Out of necessity (space), this is a very simplified description of the sequence of events. The actual mechanism is similar in concept, but several significant details are omitted for clarity. You are encouraged to consult the kernel source code for more details. See .../init/main.c and .../init/do_mounts*.c.

do_basic_setup is called from .../init/main.c and calls do_initcalls(). This causes driver module initialization routines to be called. This was described in detail in Chapter 5 and shown in Listing 5-10 .

Some embedded designs protect the bootloader and provide callbacks to bootloader routines, but this is almost never a good design approach. Linux is far more capable than bootloaders, so there is often little point in doing so.

The details differ, depending upon architecture, processor, and details of the hardware design.

This data was taken directly from the 405GP user's manual, referenced at the end of this chapter.

This is mostly for historical reasons. From the early days of PCs, BIOS programs loaded only the first sector of a disk drive and passed control to it.

In an acknowledgment of the number of bootloaders in existence, the YAMON user's guide bills itself as Yet Another MONitor.

The terms module and device driver are used here interchangeably.

This path is used by Red Hat and Fedora distributions, and is also required by the File System Hierarchy Standard referenced at the end of this chapter. Other distributions might use different locations in the file system for kernel modules.

Hosting a target board and NFS root mount are covered in detail in Chapter 12, "Embedded Development Environment".

If you don't see the messages on the console, either disable your syslogd logger or lower the console loglevel. We describe how to do this in Chapter 14, "Kernel Debugging Techniques".

/proc/modules is part of the proc file system, which is introduced in Chapter 9, "File Systems".

Reference to this standard is found in the " Suggestions for Additional Reading," at the end of this chapter.

Actually, the open() call is a C library wrapper function around the Linux sys_open() system call.

This practice is not unique to open source. Copyright and patent infringement is an ongoing concern for all developers.

The term cylinder was borrowed from the unit of storage on a rotational media. It consists of the data under a group of heads on a given sector of a disk device. Here it is used for compatibility purposes with the existing file system utilities.

File systems can be made mountable by nonroot users, as with cdrom.

A file on a file system is represented by an internal ext2 data structure called an inode.

Metadata is data about the file, as opposed to the file's data itself. Examples include a file's date, time, size, blocks used, and so on.

Converting a file system in this manner should be considered a development activity only.

It is certainly possible to mount /proc anywhere you like on your file system, but all the utilities (including mount) that require proc expect to find it mounted on /proc.

The size was fixed in the kernel configuration when we enabled the MTD RAM test device in the Linux kernel configuration.

The kernel can be configured to operate with a wrong-endian MTD file system, at the cost of reduced performance. In some configurations (such as multiprocessor designs), this can be a useful feature.

We cover the details of symbolic links shortly.

We covered the details of System V initialization in Chapter 6.

Webster's defines nonsense as "an idea that is absurd or contrary to good sense." It is my opinion that developing embedded Linux platforms on a non-Linux/UNIX host is nonsensical.

See SIG_KERNEL_COREDUMP_MASK in .../kernel/signal.c for a definition of which signals generate a core dump.

Signals and their associated numbers are defined in .../asm-/signal.h in your Linux kernel source tree.

Actually, support for the underlying engine that cbrowser uses is in the Linux build system.

See man ldconfig for details on creating a linker cache for your target system.

Sometimes an all-zeros address is appropriate in this context. However, we are investigating why the program bailed abnormally, so we should consider this suspect.

See man hosts for details of this system administration file.

The analysis utility is a Perl script supplied with the mTRace package.

A reference for the Dwarf2 Debug Information Specification is provided at the end of this chapter.

S-record files are an ASCII representation of a binary file, used by many device programmers and software binary utilities.

See the GCC manual referenced at the end of this chapter in Section 14.6.1, "Suggestions for Additional Reading" for details on the optimization levels.

Inline functions are like macros, but with the advantage of compile-time type checking.

Notwithstanding the comments made earlier about KGDB over Ethernet.

As pointed out earlier, the gdb remote protocol is detailed in the gdb manual cited at the end of this chapter in Section 14.6.1, "Suggestions for Additional Reading."

Remember to use your cross-version of readelffor example, ppc_44x-readelf for the PowerPC 44x architecture.

A reference for the Dwarf debug specification appears at the end of this chapter in Section 14.6.1, "Suggestions for Additional Reading."

A helpful shortcut for macro development is the gdb source command. This command opens and reads a source file containing macro definitions.

The symbol _end is defined in the linker script file during the final link.

As of this writing, there is a bug in gdb that prevents this technique from working properly. Hopefully, by the time you read this, it will be fixed.

An entry in the host system's /etc/hosts file enables the symbolic IP address reference.

All these filenames are unique, so they can be found without full pathname references.