Comprehensive Real-World Guidance for Every Embedded Developer and Engineer
This book brings together indispensable knowledge for building efficient, high-value, Linux-based embedded products: information that has never been assembled in one place before. Drawing on years of experience as an embedded Linux consultant and field application engineer, Christopher Hallinan offers solutions for the specific technical issues you're most likely to face, demonstrates how to build an effective embedded Linux environment, and shows how to use it as productively as possible.
Hallinan begins by touring a typical Linux-based embedded system, introducing key concepts and components, and calling attention to differences between Linux and traditional embedded environments. Writing from the embedded developer's viewpoint, he thoroughly addresses issues ranging from kernel building and initialization to bootloaders, device drivers to file systems.
Hallinan thoroughly covers the increasingly popular BusyBox utilities; presents a step-by-step walkthrough of porting Linux to custom boards; and introduces real-time configuration via CONFIG_RT--one of today's most exciting developments in embedded Linux. You'll find especially detailed coverage of using development tools to analyze and debug embedded systems--including the art of kernel debugging.
• Compare leading embedded Linux processors
• Understand the details of the Linux kernel initialization process
• Learn about the special role of bootloaders in embedded Linux systems, with specific emphasis on U-Boot
• Use embedded Linux file systems, including JFFS2--with detailed guidelines for building Flash-resident file system images
• Understand the Memory Technology Devices subsystem for flash (and other) memory devices
• Master gdb, KGDB, and hardware JTAG debugging
• Learn many tips and techniques for debugging within the Linux kernel
• Maximize your productivity in cross-development environments
• Prepare your entire development environment, including TFTP, DHCP, and NFS target servers
• Configure, build, and initialize BusyBox to support your unique requirements
13.6.5. nm
13.6.5. nm
The nm utility displays symbols from an object file. This can be useful for a variety of tasks. For example, when cross-compiling a large application, you encounter unresolved symbols. You can use nm to find which object module contains those symbols and then modify your build environment to include it.
The nm utility provides attributes for each symbol. For example, you can discover whether this symbol is local or global, or whether it is defined or referenced only in a particular object module. Listing 13-18 reproduces several lines from the output of nm run on the U-Boot ELF image u-boot.
Listing 13-18. Displaying Symbols Using nm
$ ppc_85xx-nm u-boot
...
fff23140 b base_address
fff24c98 B BootFile
fff06d64 T BootpRequest
fff00118 t boot_warm
fff21010 d border
fff23000 A __bss_start
...
Notice the link addresses of these U-Boot symbols. They were linked for a Flash device that lives in the highest portion of the memory map on this particular board. This listing contains only a few example symbols, for discussion purposes. The middle column is the symbol type. A capitalized letter indicates a global symbol, and lower case indicates a local symbol. B indicates that the symbol is located in the .bss section. T indicates that the symbol is located in the .text section. D indicates that the symbol is located in the .data section. A indicates that this address is absolute and is not subject to modification by an additional link stage. This absolute symbol indicates the start of the .bss section and is used by the code that clears the .bss on startup, as required for a C execution environment.
