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nm

  • nm (Unix) - Wikipedia

    • The nm command ships with a number of later versions of Unix and similar operating systems.
    • nm is used to EXAMINE BINARY FILES (including libraries, compiled object modules, shared-object files, and standalone executables) and to DISPLAY THE CONTENTS of those files, or META INFORMATION stored in them, specifically the SYMBOL TABLE.
    • The output from nm distinguishes between various SYMBOL TYPES. For example, it differentiates between a function that is supplied by an object module and a function that is required by it. nm is used as an AID FOR DEBUGGING, to help resolve problems arising from name conflicts and C++ NAME MANGLING, and to validate other parts of the toolchain.
    • The GNU Project ships an implementation of nm as part of the GNU Binutils package.

新手上路 {: #getting-started }

  • nm output sample - nm (Unix) - Wikipedia

    • 分別用 gcc -c test.c 與 g++ -c test.cpp 產生 test.o，只是 C 跟 C++ compiler 的結果會不太一樣，C++ 版本的 symbol table 名字很亂，因為 C+ compiler 會做 name mangling (為了達到 unique names) 的關係。

    • If the previous code is compiled with the gcc C compiler, the output of the nm command is the following:

      # nm test.o
0000000a T global_function
00000025 T global_function2
00000004 C global_var
00000000 D global_var_init
00000004 b local_static_var.1255
00000008 d local_static_var_init.1256
0000003b T main
00000036 T non_mangled_function
00000000 t static_function
00000000 b static_var
00000004 d static_var_init

      無法顯示變數的初始值 ??

    • When the C++ compiler is used, the output differs:

      # nm test.o
0000000a T _Z15global_functioni
00000025 T _Z16global_function2v
00000004 b _ZL10static_var
00000000 t _ZL15static_functionv
00000004 d _ZL15static_var_init
00000008 b _ZZ15global_functioniE16local_static_var
00000008 d _ZZ15global_functioniE21local_static_var_init
         U __gxx_personality_v0
00000000 B global_var
00000000 D global_var_init
0000003b T main
00000036 T non_mangled_function

  • man page nm section 1 - Mac OS X 10.12.6 #ril

    • 在 macOS High Sierra (10.13.6) 執行 man nm，發現 man page 跟 FreeBSD 也不一樣，執行 nm --reverse 吐 nm: Unknown command line argument '--reverse'. Try: '/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/nm -help' 的錯誤，才發現它跟 Xcode 綁得很緊。
    • As of Xcode 8.0 the default nm(1) tool is llvm-nm(1). They for the most part have the same options except for -f and -s which the differences are noted below. More help on options for llvm-nm(1) is provided when running it with the --help option. 原來是 llvm-nm，但跟 LLVM 官方文件 也差好多。
    • 列出 object file (x.o) 或 archive (libx.a) 裡每個 object file 的 name list (symbol table)；甚至寫成 libx.a(x.o) 則只會看 archive 裡的特定 object file。如果沒有給定 file，則會看 a.out。
    • 每一行都是一個 symbol，依序印出 value、type 跟 name，其中 type 若是 undefined 則 value 會是留白，而 type 若沒有使用 -m 則會用一個字母來表示 -- U (undefined)、A (absolute)、T (text section symbol)、D (data section symbol)、B (bss section symbol)、C (common symbol)、- (debugger symbol)、S (在上述 section 之外) 及 I (indirect symbol)；Symbol type 的字母有大小寫之分，大寫表示 external，小寫表示 local (non-external)。

  • nm (GNU Binary Utilities) #ril

    • GNU nm lists the symbols from object files objfile…. If no object files are listed as arguments, nm assumes the file a.out.

參考資料 {: #reference }

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