很多时候我们并不首要关注库本身的实现，或者根本无法看到其底层逻辑，但又必须确认某些函数或变量的命名（如排查定义冲突）问题，这时候就需要用到一个很基础但是很有用的工具“nm”了。
1 nm 命令介绍
NAME
nm - list symbols from object files
SYNOPSIS
nm [-A|-o|--print-file-name] [-a|--debug-syms]
[-B|--format=bsd] [-C|--demangle[=style]]
[-D|--dynamic] [-fformat|--format=format]
[-g|--extern-only] [-h|--help]
[-l|--line-numbers] [-n|-v|--numeric-sort]
[-P|--portability] [-p|--no-sort]
[-r|--reverse-sort] [-S|--print-size]
[-s|--print-armap] [-t radix|--radix=radix]
[-u|--undefined-only] [-V|--version]
[-X 32_64] [--defined-only] [--no-demangle]
[--plugin name] [--size-sort] [--special-syms]
[--synthetic] [--target=bfdname]
[objfile...]
DESCRIPTION
GNU nm lists the symbols from object files objfile.... If no object
files are listed as arguments, nm assumes the file a.out.
For each symbol, nm shows:
# ... run man nm for detials.
简单说的话，就是可以帮你列举出该目标中定义的符合要求的符号。要求可以很多，主要通过参数实现：外部引入的、内部定义的、动态的… 也可以添加参数使nm同时打印行号、文件名等相关信息。
2 nm 结果含义
nm 将找到的符号值使用十六进制缺省表示，并在函数前添加其类型，类型主要有：
| Value | Descripition | Note |
|---|---|---|
| A | The symbol’s value is absolute, and will not be changed by further linking. | 符号绝对，链接过程不会改变 |
| B/b | The symbol is in the uninitialized data section (known as BSS). | 非初始化符号 |
| C | The symbol is common. | 公有符号，链接时会被同名符号覆盖 |
| D/d | The symbol is in the initialized data section. | 初始化符号 |
| G/g | The symbol is in an initialized data section for small objects. | 初始化符号，面向小数据访问优化 |
| I | The symbol is an indirect reference to another symbol. | 其它符号的间接引用 |
| N | The symbol is a debugging symbol. | 调试符号 |
| P | The symbols is in a stack unwind section. | 栈区符号（清空） |
| R/r | The symbol is in a read only data section. | 符号只读 |
| S/s | The symbol is in an uninitialized data section for small objects. | 非初始化符号，面向小数据访问优化 |
| T/t | The symbol is in the text (code) section. | 代码区符号 |
| U | The symbol is undefined. | 未定义或在外部定义的符号 |
| u | The symbol is a unique global symbol. | 全局唯一，GNU保留符 |
| V/v | The symbol is a weak object. | 弱定义符（详见C++强弱符号定义） |
| W/w | The symbol is a weak symbol that has not been specifically tagged as a weak object symbol. | emm…绕口令符号 |
| – | The symbol is a stabs symbol in an a.out object file. | stabs格式符号 |
| ? | The symbol type is unknown, or object file format specific. | NM也不认识的符号 |
简单的举个栗子：
rew@rew:/usr/lib64$ nm libpthread.a
nptl-init.o:
U __default_pthread_attr
U __default_pthread_attr_lock
U _dl_cpuclock_offset
U _dl_get_tls_static_info
U _dl_init_static_tls
U _dl_pagesize
U _dl_wait_lookup_done
U __fork_generation
U __getrlimit
U __is_smp
U __libc_fatal
0000000000000008 C __libc_multiple_threads_ptr
U __libc_pthread_init
U __libc_setup_tls
U __libc_sigaction
U __libc_stack_end
U __lll_lock_wait_private
U __lll_unlock_wake_private
0000000000000000 b __nptl_initial_report_events
00000000000001b0 T __nptl_set_robust
U __nptl_setxid_error
0000000000000000 r nptl_version
00000000000004b0 T __pthread_get_minstack
00000000000001d0 T __pthread_initialize_minimal
00000000000001d0 T __pthread_initialize_minimal_internal
有了nm的分析结果搭配上符号定义的解释，就可以很愉快的找到想要的符号啦！
