如何在 C 中为外部定义的函数创建别名? [英] How I can make alias on external defined function in C?

问题描述

在编译这个 C 代码时

During compiling this C code

extern void Default_Handler(void);
void NMI_Handler(void) __attribute__ ((weak, alias ("Default_Handler")));

我已经收到了

error: 'NMI_Handler' aliased to undefined symbol 'Default_Handler'

如何为外部定义的函数创建别名?

How I can make alias on external defined function?

编译器:

gcc version 7.2.1 20170904 (release) [ARM/embedded-7-branch revision 255204] (GNU Tools for Arm Embedded Processors 7-2017-q4-major)

推荐答案

要给外部函数取别名，可以使用 objcopy.举个例子:

To alias an external function, you can use objcopy. Here's an example:

假设我有一个函数的定义并且我给它起了别名，就像下面的程序(叫做 myimp.c):

Say I have the definition of a function and I alias it, like in the following program (called myimp.c):

// myimp.c
int
myadd(int x, int y)
{
    return x+y;
}

int
coolguy (int x, int y) __attribute__((alias("myadd")));

如果我们编译这个(但不链接)，我们会得到一个目标文件，我们可以检查它的符号:

If we compile this (but don't link) we get an object file and we can check out its symbols:

# compile
$ cc -c myimp.c
# look at the symbols
$ nm myimp.o
0000000000000000 T coolguy
0000000000000000 T myadd

所以我们可以看到 __attribute__((alias("myadd")) 只是添加了一个符号 coolguy，其值与 相同，0000000000000000myadd.如果你查看 nm 的 man 页面，它会说 T 表示它是 .text 部分.这是有道理的，因为该函数不是 static 并且包含指令(与数据相反).

So we can see that the __attribute__((alias("myadd")) is simply adding a symbol coolguy with the same value, 0000000000000000, as myadd. If you look in the man page for nm, it will say that T means it is a global symbol in the .text section. This makes sense because the function is not static and comprises instructions (as opposed to data).

所以如果我们有目标文件，但没有源文件，并且我们想添加一个函数别名，我们可以使用 objcopy --add-symbol 来实现.

So if we have the object file, but not the source, and we want to add a function alias we can do so using objcopy --add-symbol.

假设我们有编译这个源代码的目标文件:

Say we have the object file resulting from compiling this source code:

// myimp2.c
int
myadd(int x, int y)
{
    return x+y;
}

如上编译，我们将得到myimp2.o，其符号表工具如下:

Compiling as above, we'll get myimp2.o, whose symbol table tools like this:

# look at the symbols
$ nm myimp2.o
0000000000000000 T myadd

所以我们要添加coolguy符号，我们这样做

So we want to add the coolguy symbol, which we do as follows

# objcopy --add-symbol coolguy=.text:0000000000000000,global myimp2.o myimp2_augmented.o

查看objcopy man 页面中关于--add-symbol 的文档.基本上 .text 指定了部分，在冒号之后是值，然后 global 我通过传递一个错误类型发现并且 objcopy 失败并告诉我有效类型的列表，我从中选择了 global.

Look at the documentation on --add-symbol in the objcopy man page. Basically .text specifies the section, after the colon the value, and then global I found by passing a bad type and objcopy failed and told me a list of the valid types, from which I chose global.

尝试在 myimp2_augmented.o 上运行 nm，您会看到我们添加了符号.

Try running nm on myimp2_augmented.o, you'll see that we've added the symbol.

现在您应该能够使用 myimp2_augmented.o 而不是 myimp.o 进行链接，并且您的程序可以调用 coolguy 而不会出现链接错误.

Now you should be able to link with myimp2_augmented.o instead of myimp.o and your program can call coolguy without linking errors.

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