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To use gdb, you need to be able to access symbols defined within your program, that is, labels for variables and locations in the code They allow you to jump from one place in the program forward to another without single-stepping through the intervening code Such symbols are usually included in the executable file by default but can be stripped out to make the executable file smaller To determine whether symbols are present in one of your executable files, execute the nm utility on that file: nm hilinux No one's ever explained to me what nm stands for; my hunch is it's a scrunched form of "name" (When it was created, people communicated with Unix on electromechanical Teletype machines, which were ponderously slow and difficult to type on There was a big payback in using short names for things) What you'll see when you run nm will either be a list of symbols, or else this message: hilinux: no symbols The list, when you get one, will be quite long and most of the symbols on the list will be unfamiliar to you That's because they're symbols of elements of the C library that have been linked into your program Your symbols, the ones defined in your assembly language program, will be there somewhere alas, they're not all clustered conveniently together Look for main assuming you defined a label main in your program (and you must or you will have trouble linking it with the C library), it will be there, probably near the end of the list Here's a typical nm listing for a very simple program that I present in the next chapter: 0804943c 08049420 080483fc 08049414 08049410 0804941c 08049418 0804940c 0804940c 080494dc 08049400 R A D U 080483b0 t _DYNAMIC _GLOBAL_OFFSET_TABLE_ _IO_stdin_used __CTOR_END__ __CTOR_LIST__ __DTOR_END__ __DTOR_LIST__ __EH_FRAME_BEGIN__ __FRAME_END__ __bss_start __data_start __deregister_frame_info@@GLIBC_20 __do_global_ctors_aux
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08048320 t U U U 080494dc A 080494f4 A 080483dc A 080483dc U 08048274 080482f0 T 08049408 d 08049400 W 08048368 t 0804940c d 0804940c d 08048370 t 08048314 t 08048320 t 080483b0 t 080483dc t 08048390 t 080483d4 t 080483a0 T 080494dc b 08049404 d
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__do_global_dtors_aux __gmon_start__ __libc_start_main@@GLIBC_20 __register_frame_info@@GLIBC_20 _edata _end _etext _fini _fp_hw _init _start completed3 data_start fini_dummy force_to_data force_to_data frame_dummy gcc2_compiled gcc2_compiled gcc2_compiled gcc2_compiled init_dummy init_dummy main object8 p2
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The program this listing was generated from, boilerasm, has only one symbol in it: main The capital T to the left of the symbol main indicates that the symbol resides in the [text] section of your program, and the fact that it's capitalized indicates that main is global Local symbols (which means all symbols you do not explicitly mark as global) are indicated by small letters here Because nm is virtually undocumented, I don't know what all the various tags mean, but t or T indicates items residing in the [text] section, and d or D indicates items residing in the [data] section What are all the rest of these symbols Mostly, they're code labels and data items from the C library, which gcc links into your program Most of the time you won't have to fool with them, especially when you're just getting started
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When you're done debugging a program, you can strip the symbols out of the program to reduce the size of the program on disk For small programs the reduction can be substantial; the eatlinuxasm program I show you in the next chapter went from 12 to 3K bytes after I stripped the symbols out of it Stripping the symbols out of a program is trivial You use the strip command, followed by the name of the executable: strip eatlinux And that's it! Like most Unix utilities, strip is a taciturn creature and won't say anything unless something goes wrong No comment, no problems
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