nm命令报告许多符号类型，例如：
"U" The symbol is undefined.
"D"
"d" The symbol is in the initialized data section.
"B"
"b" The symbol is in the uninitialized data section (known as BSS).
"T"
"t" The symbol is in the text (code) section.
"I" The symbol is an indirect reference to another symbol.
等是否所有不等于“ u”和“ U”的符号类型都是已定义的符号？
更具体地说，假设我们将nm用于共享库utils.so：
nm utils.so
是否所有在utils.so中定义了所有标记有不同于“ u”，“ U”的类型的符号？