最好是阅读 man nm
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nm - list symbols from object files
If lowercase, the symbol is local; if uppercase, the symbol is global (external)
其输出结果：
"A" The symbol’s value is absolute, and will not be changed by further linking.
"B"/"b" The symbol is in the uninitialized data section (known as BSS).
"C" The symbol is common. Common symbols are uninitialized data. When linking, multiple common symbols may appear with the same name. If the symbol is defined anywhere, the common symbols are treated as undefined references.
"D"/"d" The symbol is in the initialized data section.
"T"/"t" The symbol is in the text (code) section.
"U" The symbol is undefined.
"u" The symbol is a unique global symbol. This is a GNU extension to the standard set of ELF symbol bindings. For such a symbol the dynamic linker will make sure that in the entire process there is just one symbol with this name and type in use.
"S"/"s" The symbol is in an uninitialized data section for small objects.
| A | Global absolute 符号。 |
| a | Local absolute 符号。 |
| B | Global bss 符号。 |
| b | Local bss 符号。 |
| D | Global data 符号。 |
| d | Local data 符号。 |
| f | 源文件名称符号。 |
| T | Global text 符号。 |
| t | Local text 符号。 |
| U | 未定义符号。 |