如何让GCC链接强烈符号静态库覆盖弱的象征？ [英] How to make gcc link strong symbol in static library to overwrite weak symbol?

问题描述

我的问题可以总结如下：

My problem can be summarised in the following:

bar.c

#include <stdio.h>

void bar() {
    printf("bar\n");
}

的main.c

#include <stdio.h>

void __attribute__((weak)) bar() {
    printf("foo\n");
}

int main() {
    bar();
    return 0;
}

的Makefile

all:
    gcc -c bar.c
    ar -rc libbar.a bar.o
    gcc main.c -L. -lbar

输出

$ ./a.out
foo

所以main.c中的薄弱符号栏不因bar.c bar.c的强烈符号覆盖链接到的main.c静态库libbar.a。

So the weak symbol bar in main.c is not overwritten by the strong symbol in bar.c due to bar.c being linked to main.c in a static library libbar.a.

我怎么能告诉GCC来使libbar.a强烈的符号覆盖弱符号main.c中？

How can I tell gcc to make the strong symbol in libbar.a to overwritten the weak symbol in main.c?

推荐答案

一般来说：如果你不把执行不力到主，链接器将解决它终于在运行时。但是，如果你在的main.c 实现它，你只能用强结合（ bar.c ）联系起来时，静态

Generally speaking: if you don't put a weak implementation into your main, the linker will resolve it at last at runtime. But if you implement it in main.c, you will only be able to override it with a strong bound (bar.c) when linking this static.

请阅读 http://www.bottomupcs.com/libraries_and_the_linker.html - 它包含关于这个话题有很多有趣的东西。

Please read http://www.bottomupcs.com/libraries_and_the_linker.html - it contains a lot of interesting stuff on this topic.

我做了一个测试自己：

bar.c

#include <stdio.h>

void bar()
{
        puts("bar.c: i'm the strong bar()");
}

baz.c

#include <stdio.h>

void __attribute__((weak)) bar() 
{
        puts("baz.c: i'm the weak bar()");
}

的main.c

main.c

#include <stdio.h>

#ifdef V2
        void __attribute__((weak)) bar()
        {
                puts("main: i'm the build in weak bar()");
        }
#else
        void __attribute__((weak)) bar();
#endif

int main()
{
    bar();
    return 0;
}

我的Makefile：

My Makefile:

all:
    gcc -c -o bar.o bar.c
    gcc -shared -fPIC -o libbar.so bar.o
    gcc -c -o baz.o baz.c
    gcc -shared -fPIC -o libbaz.so baz.o
    gcc -o main1 main.c -L. -lbar -lbaz
    gcc -o main2 main.c -L. -lbaz -lbar
    LD_LIBRARY_PATH=. ./main1                                   # => bar.c
    LD_LIBRARY_PATH=. ./main2                                   # => baz.c
    LD_LIBRARY_PATH=. LD_PRELOAD=libbaz.so ./main1              # => baz.c (!!)
    LD_LIBRARY_PATH=. LD_PRELOAD=libbaz.so ./main2              # => baz.c
    gcc -o main3 main.c bar.o baz.o
    gcc -o main4 main.c baz.o bar.o
    ./main3                                                     # => bar.c
    ./main4                                                     # => bar.c
    gcc -DV2 -o main5 main.c -L. -lbar -lbaz
    gcc -DV2 -o main6 main.c -L. -lbaz -lbar
    LD_LIBRARY_PATH=. ./main5                                   # => main's implementation
    LD_LIBRARY_PATH=. ./main6                                   # => main's implementation
    gcc -DV2 -o main7 main.c -L. -lbar -lbaz
    gcc -DV2 -o main8 main.c -L. -lbaz -lbar
    LD_LIBRARY_PATH=. LD_PRELOAD=libbaz.so ./main7              # => main's implementation
    LD_LIBRARY_PATH=. LD_PRELOAD=libbaz.so ./main8              # => main's implementation
    gcc -DV2 -o main9  main.c -L. -lbar -lbaz
    gcc -DV2 -o main10 main.c -L. -lbaz -lbar
    LD_LIBRARY_PATH=. LD_PRELOAD=libbar.so ./main9              # => main's implementation
    LD_LIBRARY_PATH=. LD_PRELOAD=libbar.so ./main10             # => main's implementation
    gcc -c bar.c
    gcc -c baz.c
    gcc -o main11 main.c bar.o baz.o
    gcc -o main12 main.c baz.o bar.o
    ./main11                                                    # => bar.c
    ./main12                                                    # => bar.c
    gcc -o main13 -DV2 main.c bar.o baz.o
    gcc -o main14 -DV2 main.c baz.o bar.o
    ./main13                                                    # => bar.c
    ./main14                                                    # => bar.c

看看MAIN1＆安培;＆安培; MAIN2 ......如果你不把任何执行不力成的main.c ，但保持弱者在图书馆和强之一，另一个lib中，你会能够覆盖弱的，如果强lib中定义了一个强有力的执行巴（）。

Take a look at main1 && main2... if you don't put any weak implementation into main.c but keep the weak one in a library and the strong one in another lib., you'll be able to override the weak one if the strong lib defines a strong implementation of bar().

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