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--allow-multiple-definition
-z muldefs
Normally when a symbol is defined multiple times, the linker will report a fatal error. These options allow multiple definitions and the first definition will be used.

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int subfunc(int a, int b) {
    return a - b;
}

int funcB(int a, int b) {
    return subfunc(a, b);
}

static void foo() {
}

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# g++ -c libB.cpp
# readelf -a libB.o
Symbol table '.symtab' contains 11 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     5: 0000000000000035     6 FUNC    LOCAL  DEFAULT    1 _ZL3foov
     9: 0000000000000000    22 FUNC    GLOBAL DEFAULT    1 _Z7subfuncii
    10: 0000000000000016    31 FUNC    GLOBAL DEFAULT    1 _Z5funcBii

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--localize-symbol=symbolname
Make symbol symbolname local to the file, so that it is not visible externally. This option may be given more than once.

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# readelf -a libB.a | grep fun
     9: 0000000000000000    22 FUNC    LOCAL  DEFAULT    1 _Z7subfuncii
    10: 0000000000000016    31 FUNC    GLOBAL DEFAULT    1 _Z5funcBii

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--localize-symbols=filename
Apply --localize-symbol option to each symbol listed in the file filename. filename is simply a flat file, with one symbol name per line. Line comments may be introduced by the hash character. This option may be given more than once.

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libB.a(libB.o): In function `subfunc(int, int)':
libB.cpp:(.text+0x0): multiple definition of `subfunc(int, int)'
libA.a(libA.o):libA.cpp:(.text+0x0): first defined here

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1. 不允许多个同名的强符号
2. 有同名的一个强符号和多个弱符号，使用强符号的实现
3. 有同名的多个弱符号，选择任意一个弱符号的实现

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--weaken-symbol=symbolname
Make symbol symbolname weak. This option may be given more than once.

--weaken-symbols=filename
Apply --weaken-symbol option to each symbol listed in the file filename. filename is simply a flat file, with one symbol name per line. Line comments may be introduced by the hash character. This option may be given more than once.

--weaken
Change all global symbols in the file to be weak. This can be useful when building an object which will be linked against other objects using the -R option to the linker. This option is only effective when using an object file format which supports weak symbols.

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# readelf -a libB.a | grep subfunc
     8: 0000000000000000    22 FUNC    GLOBAL DEFAULT    1 _Z7subfuncii

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# readelf -a libB.a | grep subfunc
     8: 0000000000000000    22 FUNC    WEAK   DEFAULT    1 _Z7subfuncii

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int subfunc_c(int a, int b) {
    return a + b;
}
int funcAA(int a, int b) {
    return subfunc_c(a, b);
}

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int subfunc_c(int a, int b);
int funcBB(int a, int b) {
    return subfunc_c(a, b);
}

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int subfunc_c(int a, int b) {
    return a - b;
}

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#include <cstdio>
int funcAA(int, int);
int funcBB(int, int);
int main() {
    printf("%d,", funcAA(2, 1));
    printf("%d\n", funcBB(2, 1));
    return 0;
}

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# g++ -c libA.cpp
# g++ -c libB.cpp
# g++ -c libC.cpp
# ar rvs libA.a libA.o
# ar rvs libB.a libB.o libC.o
# g++ main.cpp libA.a libB.a
# ./a.out
3,3

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1. 初始E,U,D都为空。
2. 对于传入的每个文件，判断是.o文件还是.a文件。
如果是.o文件，扫描.o文件中的符号，全部添加到E集合中，根据每个符号是否定义，更新对应的未定义集合U和已定义集合D。扫描过程如果发现某个符号已经在已定义集合D中，报错符号重定义。
如果是.a文件，.a文件也是由一系列.o文件组成。链接器扫描每个.o文件，判断该.o文件中是否有未定义集合U中需要的符号，如果有，把该.o的符号全部添加到E集合中，更新未定义集合U和已定义集合D，如果没有，直接跳过该.o文件，不会把该.o文件合并到可执行文件中。
3. 全部输入文件扫描完成后，如果未定义集合U不是空的，报错有未定义符号。

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--redefine-sym old=new
Change the name of a symbol old, to new. This can be useful when one is trying link two things together for which you have no source, and there are name collisions.
--redefine-syms=filename
Apply --redefine-sym to each symbol pair "old new" listed in the file filename. filename is simply a flat file, with one symbol pair per line. Line comments may be introduced by the hash character. This option may be given more than once.