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摘要：詳細(xì)介紹了C++中的Name Mangling的原理和gcc中對(duì)應(yīng)的實(shí)現(xiàn)，通過(guò)程序代碼和nm c++filt等工具來(lái)驗(yàn)證這些原理。對(duì)于詳細(xì)了解程序的鏈接過(guò)程有一定的幫助。
Name Mangling概述
C++的語(yǔ)言特性比C豐富的多，C++支持的函數(shù)重載功能是需要Name Mangling技術(shù)的最直接的例子。對(duì)于重載的函數(shù)，不能僅依靠函數(shù)名稱(chēng)來(lái)區(qū)分不同的函數(shù)，因?yàn)镃++中重載函數(shù)的區(qū)分是建立在以下規(guī)則上的：
當(dāng)然，C++還有很多其他的地方需要Name Mangling，如namespace, class, template等等。
- /*
- * simple_test.c
- * a demo to show that different name mangling technology in C++ and C
- * Author: Chaos Lee
- */
- #include
- int rect_area(int x1,int x2,int y1,int y2)
- {
- return (x2-x1) * (y2-y1);
- }
- int elipse_area(int a,int b)
- {
- return 3.14 * a * b;
- }
- int main(int argc,char *argv[])
- {
- int x1 = 10, x2 = 20, y1 = 30, y2 = 40;
- int a = 3,b=4;
- int result1 = rect_area(x1,x2,y1,y2);
- int result2 = elipse_area(a,b);
- return 0;
- }
- [lichao@sg01 name_mangling]$ gcc -c simple_test.c
- [lichao@sg01 name_mangling]$ nm simple_test.o
- 0000000000000027 T elipse_area
- 0000000000000051 T main
- 0000000000000000 T rect_area
- [lichao@sg01 name_mangling]$ nm simple_test.o
- 0000000000000028 T _Z11elipse_areaii
- 0000000000000000 T _Z9rect_areaiiii
- U __gxx_personality_v0
- 0000000000000052 T main
l C++語(yǔ)言中規(guī)定 ：以下劃線(xiàn)并緊挨著大寫(xiě)字母開(kāi)頭或者以?xún)蓚€(gè)下劃線(xiàn)開(kāi)頭的標(biāo)識(shí)符都是C++語(yǔ)言中保留的標(biāo)示符。所以_Z9rect_areaiiii是保留的標(biāo)識(shí)符，g++編譯的目標(biāo)文件中的符號(hào)使用_Z開(kāi)頭（C99標(biāo)準(zhǔn)）。
- /*
- * simple_test.c
- * a demo to show that different name mangling technology in C++ and C
- * Author: Chaos Lee
- */
- #include
- #ifdef __cplusplus
- extern "C" {
- #endif
- int rect_area(int x1,int x2,int y1,int y2)
- {
- return (x2-x1) * (y2-y1);
- }
- int elipse_area(int a,int b)
- {
- return (int)(3.14 * a * b);
- }
- #ifdef __cplusplus
- }
- #endif
- int main(int argc,char *argv[])
- {
- int x1 = 10, x2 = 20, y1 = 30, y2 = 40;
- int a = 3,b=4;
- int result1 = rect_area(x1,x2,y1,y2);
- int result2 = elipse_area(a,b);
- return 0;
- }
- [lichao@sg01 name_mangling]$ gcc -c simple_test.c
- [lichao@sg01 name_mangling]$ nm simple_test.o
- 0000000000000027 T elipse_area
- 0000000000000051 T main
- 0000000000000000 T rect_area
- [lichao@sg01 name_mangling]$ g++ -c simple_test.c
- [lichao@sg01 name_mangling]$ nm simple_test.o
- U __gxx_personality_v0
- 0000000000000028 T elipse_area
- 0000000000000052 T main
- 0000000000000000 T rect_area
事實(shí)上，C標(biāo)準(zhǔn)庫(kù)中使用了大量的extern “C”關(guān)鍵字，因?yàn)镃標(biāo)準(zhǔn)庫(kù)也是可以用C++編譯器編譯的，但是要確保編譯之后仍然保持C的接口而不是C++的接口（因?yàn)槭荂標(biāo)準(zhǔn)庫(kù)），所以需要使用extern “C”關(guān)鍵字。
- /*
- * libc_test.c
- * a demo program to show that how the standard C
- * library are compiled when encountering a C++ compiler
- */
- #include
- int main(int argc,char * argv[])
- {
- puts("hello world.\n");
- return 0;
- }
搜索一下puts，我們并沒(méi)有看到extern “C”.奇怪么？
- [lichao@sg01 name_mangling]$ g++ -E libc_test.c | grep 'puts'
- extern int fputs (__const char *__restrict __s, FILE *__restrict __stream);
- extern int puts (__const char *__s);
- extern int fputs_unlocked (__const char *__restrict __s,
- puts("hello world.\n");
- [lichao@sg01 name_mangling]$ g++ -E libc_test.c | grep 'extern "C"'
- extern "C" {
- extern "C" {
不同編譯器使用不同的方式進(jìn)行name mangling, 你可能會(huì)問(wèn)為什么不將C++的 name mangling標(biāo)準(zhǔn)化，這樣就能實(shí)現(xiàn)各個(gè)編譯器之間的互操作了。事實(shí)上，在C++的FAQ列表上有對(duì)此問(wèn)題的回答：
"Compilers differ as to how objects are laid out, how multiple inheritance is implemented, how virtual function calls are handled, and so on, so if the name mangling were made the same, your programs would link against libraries provided from other compilers but then crash when run. For this reason, the ARM (Annotated C++ Reference Manual) encourages compiler writers to make their name mangling different from that of other compilers for the same platform. Incompatible libraries are then detected at link time, rather than at run time."
GCC采用IA 64的name mangling方案，此方案定義于Intel IA64 standard ABI.在g++的FAQ列表中有以下一段話(huà)：
"GNU C++ does not do name mangling in the same way as other C++ compilers.
This means that object files compiled with one compiler cannot be used with
GNU C++的name mangling方案和其他C++編譯器方案不同，所以一種編譯器生成的目標(biāo)文件并不能被另外一種編譯器生成的目標(biāo)文件使用。
- Builtin types encoding
::= v # void - ::= w # wchar_t
- ::= b # bool
- ::= c # char
- ::= a # signed char
- ::= h # unsigned char
- ::= s # short
- ::= t # unsigned short
- ::= i # int
- ::= j # unsigned int
- ::= l # long
- ::= m # unsigned long
- ::= x # long long, __int64
- ::= y # unsigned long long, __int64
- ::= n # __int128
- ::= o # unsigned __int128
- ::= f # float
- ::= d # double
- ::= e # long double, __float80
- ::= g # __float128
- ::= z # ellipsis
- ::= u
# vendor extended type
Operator encoding
::= nw # new - ::= na # new[]
- ::= dl # delete
- ::= da # delete[]
- ::= ps # + (unary)
- ::= ng # - (unary)
- ::= ad # & (unary)
- ::= de # * (unary)
- ::= co # ~
- ::= pl # +
- ::= mi # -
- ::= ml # *
- ::= dv # /
- ::= rm # %
- ::= an # &
- ::= or # |
- ::= eo # ^
- ::= aS # =
- ::= pL # +=
- ::= mI # -=
- ::= mL # *=
- ::= dV # /=
- ::= rM # %=
- ::= aN # &=
- ::= oR # |=
- ::= eO # ^=
- ::= ls # <<
- ::= rs # >>
- ::= lS # <<=
- ::= rS # >>=
- ::= eq # ==
- ::= ne # !=
- ::= lt # <
- ::= gt # >
- ::= le # <=
- ::= ge # >=
- ::= nt # !
- ::= aa # &&
- ::= oo # ||
- ::= pp # ++
- ::= mm # --
- ::= cm # ,
- ::= pm # ->*
- ::= pt # ->
- ::= cl # ()
- ::= ix # []
- ::= qu # ?
- ::= st # sizeof (a type)
- ::= sz # sizeof (an expression)
- ::= cv
# (cast) - ::= v
# vendor extended operator
::= - ::= P
# pointer-to - ::= R
# reference-to - ::= O
# rvalue reference-to (C++0x) - ::= C
# complex pair (C 2000) - ::= G
# imaginary (C 2000) - ::= U
# vendor extended type qualifier
- /*
- * Author: Chaos Lee
- * Description: A simple demo to show how the rules used to mangle functions' names work
- * Date:2012/05/06
- */
- #include
- #include
- using namespace std;
- int test_func(int & tmpInt,const char * ptr,double dou,string str,float f)
- {
- return 0;
- }
- int main(int argc,char * argv[])
- {
- char * test="test";
- int intNum = 10;
- double dou = 10.012;
- string str="str";
- float f = 1.2;
- test_func(intNum,test,dou,str,f);
- return 0;
- }
- [lichao@sg01 name_mangling]$ g++ -c func.cpp
- [lichao@sg01 name_mangling]$ nm func.cpp
- nm: func.cpp: File format not recognized
- [lichao@sg01 name_mangling]$ nm func.o
- 0000000000000060 t _GLOBAL__I__Z9test_funcRiPKcdSsf
- U _Unwind_Resume
- 0000000000000022 t _Z41__static_initialization_and_destruction_0ii
- 0000000000000000 T _Z9test_funcRiPKcdSsf
- U _ZNSaIcEC1Ev
- U _ZNSaIcED1Ev
- U _ZNSsC1EPKcRKSaIcE
- U _ZNSsC1ERKSs
- U _ZNSsD1Ev
- U _ZNSt8ios_base4InitC1Ev
- U _ZNSt8ios_base4InitD1Ev
- 0000000000000000 b _ZSt8__ioinit
- U __cxa_atexit
- U __dso_handle
- U __gxx_personality_v0
- 0000000000000076 t __tcf_0
- 000000000000008e T main
加粗的那行就是函數(shù)test_func經(jīng)過(guò)name mangling之后的結(jié)果，其中：
C++的name mangling技術(shù)一般使得函數(shù)變得面目全非，而很多情況下我們?cè)诓榭催@些符號(hào)的時(shí)候并不需要看到這些函數(shù)name mangling之后的效果，而是想看看是否定義了某個(gè)函數(shù)，或者是否引用了某個(gè)函數(shù)，這對(duì)于我們調(diào)試程序是非常有幫助的。
所以需要一種方法從name mangling之后的符號(hào)變換為name mangling之前的符號(hào)，這個(gè)過(guò)程稱(chēng)之為name demangling.事實(shí)上有很多工具提供這些功能，最常用的就是c++file命令，c++filt命令接受一個(gè)name mangling之后的符號(hào)作為輸入并輸出demangling之后的符號(hào)。例如：
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