Книга: Embedded Linux Primer: A Practical, Real-World Approach

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.

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