手动指定特定链接符号的重新映射 [英] Manually specify remapping of specific linking symbols

问题描述

是否无需修改这两个源文件，是否有办法获取通过编译它们而生成的目标文件，并说服链接程序将main_v1.c中的foo链接到bar.c中的bar?

Without modifying these two source files, is there a way to take the object files produced by compiling them, and convincing a linker to link foo in main_v1.c to bar in bar.c?

main_v1.c

void foo(void);

int main(void)
{
    foo();
}

bar.c

#include <stdio.h>

void bar(void)
{
    puts("bar()!");
}

修改目标文件本身是一个公平的游戏，但是假设我们甚至可能没有可用的源代码.该平台是Linux.

Modifying the object files themselves is fair game but assume that we might not even have the source code available. The platform is Linux.

通过坚持对main_v1.c进行适度更改，并链接到其他映射"目标文件，这是一种仅使用标准C即可几乎获得所需结果的方法.

By insisting on a modest change to main_v1.c, and linking in an additional "mapping" object file, here's a way to almost get the desired result with just standard C.

main_v2.c

extern void (*const foo)(void);

int main(void)
{
    foo();
}

bar.c 不变.

map.c

void bar(void);
void (*const foo)(void) = bar;

如果目标文件是使用lto编译的，则甚至消除了函数指针取消引用(使用最新的gcc).这是一个相当不错的结果，但是如果将main()修改为可以直接调用bar()，则在链接后会直接内嵌bar()本身，因此还有改进的余地.

If the object files are compiled with lto, the function pointer dereference is even elided (using a recent gcc). This is a pretty good result but if main() is modified to have a direct call to bar(), then bar() itself is inlined after linking so there's room for improvement.

推荐答案

这将是

This would be a job for the GNU ld --wrap option

我们假设您已经编译了main_v1.o，并且可能与您的 main_v1.c，同样是根据您的bar.c编译而成的bar.o.

You have, we assume, a main_v1.o compiled and perhaps unchangeable from your main_v1.c and likewise a bar.o compiled from your bar.c.

现在编写另一个源文件:

Now write another source file:

wrap.c

extern void bar(void);

void __wrap_foo(void)
{
    bar();
}

将其编译为wrap.o并将其与先前的目标文件链接，如下所示:

Compile it to wrap.o and link it with the prior object files like so:

$ gcc -c wrap.c
$ gcc -o prog -Wl,--wrap=foo main_v1.o bar.o wrap.o

然后:

$ ./prog
bar()!

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