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C language: how to give an alias to a global variable?

2022-06-13 07:25:00 【qq_ forty-three million four hundred and seventy-nine thousand 】

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do person ： Doug ,10+ Embedded development veteran , Focus on ：C/C++、 The embedded 、Linux.
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Catalog

Catalog * What's an alias ？

The experience of others , Our steps ！

What's an alias ？

stay stackoverflow I saw an interesting topic on ： How to set an alias for a variable ?（How to assign to a variable an alias?）

The so-called variable alias , Different identifiers pass through , To represent the same variable .

We know , Variable names are for programmers .

In the eyes of the compiler , All variables become addresses .

Please note that ： The alias discussed here , Just reference the same variable through different identifiers .
And strong sign 、 Weak symbols don't matter , That's another topic .

In the above post , The author's first thought is to define , Rename the variable .

The way to do it , It will be in the preprocessing phase before compilation , Replace macro identifier with variable identifier .

In the answers replied by netizens , Most of them are implemented through pointers ： Let different identifiers point to the same variable .

Anyway? , This is also an alias .

however , These answers have one limitation ： These codes must be compiled together to , Otherwise, there may be an error message that the symbol cannot be found .

Plug in programming is very popular now , If the developer wants to reference the variables in the main program through a variable alias in the plug-in , How to deal with this ？

This article provides two ways to achieve this , And through two simple example code to demonstrate .

At the end of the article, there is the download address of the sample code .

Method 1： Reverse registration

I've been in touch with some before CodeSys Code for , The code quality inside is really very high , Especially the software architecture design .

Legend has it that ：CodySys It's from the industrial control industry Android.

There is an idea of reverse registration , It can be used for variable aliases .

There are... In the sample code 2 File ：main.c and plugin.c.

main.c A global variable array is defined in , Compile into executable program main.

plugin.c Used by an alias in main.c Global variable in .

plugin.c Compiled into a dynamic link library , By executable program main Dynamic loading (dlopen).

stay plugin.c in , Provide a function func_init, When the dynamic library is maindlopen after , This function is called , And pass the address of the real global variable through the parameter .

In this case , In the plug-in, you can use real variables through an alias （ such as ： Change the value of the variable ）.

Essentially , This is still a reference through a pointer .

Just use the idea of dynamic registration , Isolate the binding relationship between pointer and variable in time and space .

plugin.c Source file

#include 

int *alias_data = NULL;

void func\_init(int *data)
{
	printf("libplugin.so: func\_init is called. \n");
	alias_data = data;
}

void func\_stage1(void)
{
	printf("libplugin.so: func\_stage1 is called. \n");
	if (alias_data)
	{
		alias_data[0] = 100;
		alias_data[1] = 200;
	}
}

main.c Source file

#include 
#include 
#include  

// defined in libplugin.so
typedef void (*pfunc\_init)(int *);
typedef void (*pfunc\_stage1)(void);

int data[100] = { 0 };


void main(void)
{
	data[0] = 10;
	data[1] = 20;

	printf("data[0] = %d \n", data[0]);
	printf("data[1] = %d \n", data[1]);
	
	// open libplugin.so
	void *handle = dlopen("./libplugin.so", RTLD_NOW);
	if (!handle)
	{
		printf("dlopen failed. \n");
		return;
	}

	// get and call init function in libplugin.so
	pfunc_init func_init =  (pfunc_init) dlsym(handle, "func\_init");
	if (!func_init)
	{
		printf("get func\_init failed. \n");
		return;
	}
	func\_init(data);

	// get and call routine function in libplugin.so
	pfunc_stage1 func_stage1 =  (pfunc_stage1) dlsym(handle, "func\_stage1");
	if (!func_stage1)
	{
		printf("get func\_stage1 failed. \n");
		return;
	}
	func\_stage1();

	printf("data[0] = %d \n", data[0]);
	printf("data[1] = %d \n", data[1]);

	return;
}

The compilation instructions are as follows ：

gcc -m32 -fPIC --shared plugin.c -o libplugin.so
gcc -m32 -o main main.c -ldl

Execution results ：

data[0] = 10 
data[1] = 20 
libplugin.so: func_init is called. 
libplugin.so: func_stage1 is called. 
data[0] = 100 
data[1] = 200

You can take a look at the symbol table of the dynamic link library ：

readelf -s libplugin.so | grep data

You can see alias_data identifier , And is the global variable defined in this document .

【 About author 】

Master ： Doug , More than ten years of embedded development veteran , Focus on embedded systems + Linux field , Yes Single chip microcomputer 、 Have done Smart home 、 Studied PLC and Industrial robot , Project development experience is very rich .

His articles mainly include C/C++、Linux operating system 、 The Internet of things 、 MCU and embedded .

whole 、 The perspective-taking , Summarize the article from the perspective of readers .

Every output , It's not just dry goods , It also guides you to think deeply step by step , Improve yourself from the bottom logic .

Method 2： Embedded assembly code

Use variable aliases in dynamically loaded plug-ins , In addition to the dynamic registration method demonstrated above , You can also embed assembly code to : Set a global label to achieve .

Go directly to the sample code ：

`plugin.c` Source file

#include 

asm(".Global alias\_data");
asm("alias\_data = data");

extern int alias_data[];

void func\_stage1(void)
{
	printf("libplugin.so: func\_stage1 is called. \n");
	
	*(alias_data + 0) = 100;
	*(alias_data + 1) = 200;
}

`main.c` Source file

#include 
#include 
#include  

// defined in libplugin.so
typedef void (*pfunc\_init)(int *);
typedef void (*pfunc\_stage1)(void);

int data[100] = { 0 };


void main(void)
{
	data[0] = 10;
	data[1] = 20;

	printf("data[0] = %d \n", data[0]);
	printf("data[1] = %d \n", data[1]);
	
	// open libplugin.so
	void *handle = dlopen("./libplugin.so", RTLD_NOW);
	if (!handle)
	{
		printf("dlopen failed. \n");
		return;
	}

	// get and call routine function in libplugin.so
	pfunc_stage1 func_stage1 =  (pfunc_stage1) dlsym(handle, "func\_stage1");
	if (!func_stage1)
	{
		printf("get func\_stage1 failed. \n");
		return;
	}
	func\_stage1();

	printf("data[0] = %d \n", data[0]);
	printf("data[1] = %d \n", data[1]);

	return;
}

Compile instructions ：

gcc -m32 -fPIC --shared plugin.c -o libplugin.so
gcc -m32 -rdynamic -o main main.c -ldl

Execution results ：

data[0] = 10 
data[1] = 20 
libplugin.so: func_stage1 is called. 
data[0] = 100 
data[1] = 200

Let's also take a look at libplugin.so Symbol information in ：

readelf -s libplugin.so | grep data

Summary

This document uses two sample code , It discusses how to use in plug-ins ( Dynamic link library ), Access to real variables through aliases .

I wonder if you have such a question ： Use it directly extern Just declare the externally defined variables , Why bother ？

The truth is right ！

however , In some special fields or scenes ( For example, in some secondary development ), Such needs do exist , And it's a strong demand .

If you have any questions , Or there is a task error in the text , Feel free to leave a comment 、 correct .

------ End ------

In the public, 【IOT Internet of things town 】 Background reply keyword ：20522, You can get the download address of the sample code .

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If you reprint this article , At the end of the text, it must be noted that ：“ The official account is from WeChat ：IOT Internet of things town ”.