Linux内核驱动开发的EXPORT_SYMBOL

简介

本文主要来讲讲Linux内核驱动中，EXPORT_SYMBOL()宏定义的用法。
在阅读的Linux内核驱动源码的时候，我们会发现很多的函数带有EXPORT_SYMBOL()宏定义。
从这个宏定义的理解为输出符号。那么他究竟有什么作用。

EXPORT_SYMBOL()宏定义作用

EXPORT_SYMBOL宏定义定义的函数或者符号将对内核代码公开，不用修改内核代码就在其他的内核模块中直接调用，即使用EXPORT_SYMBOL可以将一个函数以符号的方式导出给其他模块使用。

使用方法

在模块函数定义之后使用"EXPORT_SYMBOL(函数名)"来导出。
```
static int rice_func(void)
{
return 0;
}
EXPORT_SYMBOL(rice_func);
```
在调用该函数的另外一个模块中使用extern对之声明。
```
extern int rice_func(void);
```
先加载定义该函数的模块，然后再加载调用该函数的模块，先后顺序必须注意。

实验

编写代码

编写两个模块：rice_export.ko 和 rice_import.ko，其中：
rice_export.ko：导出定义的函数
rice_import.ko：调用导出的函数

导出函数模块的代码(rice_export.c)

导出函数为：rice_drv_export，函数含义：外部输入一个字符串，然后打印出来

#include "rice_export.h"

#define CLASS_NAME  "rice_export"
#define DEVICE_NAME "rice_export"

typedef struct {
    int major_number;
    struct device *device;
    struct class *class;
} Rice_Driver;

Rice_Driver rice_drv;

static int rice_drv_export(char *name) {
    
    printk(KERN_ALERT "Rice Export: %s\n", name);
    return 0;
}
EXPORT_SYMBOL(rice_drv_export);

static int __init rice_export_init(void) {
    rice_drv.major_number = register_chrdev(0, DEVICE_NAME, NULL);

    if (rice_drv.major_number < 0) {
        printk(KERN_ALERT "Register fail!!\n");
        return rice_drv.major_number;
    }

    printk(KERN_ALERT "Registe success, major number is %d\n", rice_drv.major_number);

    rice_drv.class = class_create(THIS_MODULE, CLASS_NAME);

    if (IS_ERR(rice_drv.class)) {
        unregister_chrdev(rice_drv.major_number, DEVICE_NAME);
        return PTR_ERR(rice_drv.class);
    }

    rice_drv.device = device_create(rice_drv.class, NULL, MKDEV(rice_drv.major_number, 0), NULL, DEVICE_NAME);

    if (IS_ERR(rice_drv.device)) {
        class_destroy(rice_drv.class);
        unregister_chrdev(rice_drv.major_number, DEVICE_NAME);
        return PTR_ERR(rice_drv.device);
    }

    printk(KERN_ALERT "rice export ko init!!\n");

    return 0;
}

static void __exit rice_export_exit(void) {
    device_destroy(rice_drv.class, MKDEV(rice_drv.major_number, 0));
    class_unregister(rice_drv.class);
    class_destroy(rice_drv.class);
    unregister_chrdev(rice_drv.major_number, DEVICE_NAME);

    printk(KERN_ALERT "rice export ko exit!!\n");
}

module_init(rice_export_init);
module_exit(rice_export_exit);
MODULE_AUTHOR("RieChen");
MODULE_LICENSE("GPL");

电泳函数模块的代码(rice_import.c)

调用函数声明：extern int rice_drv_export(char * name);，含义：声明外部函数

#include "rice_import.h"

#define CLASS_NAME  "rice_import"
#define DEVICE_NAME "rice_import"

typedef struct {
 int major_number;
 struct device *device;
 struct class *class;
} Rice_Driver;

Rice_Driver rice_drv;


extern int rice_drv_export(char *name);


static int __init rice_import_init(void) {
 rice_drv.major_number = register_chrdev(0, DEVICE_NAME, NULL);

 if (rice_drv.major_number < 0) {
     printk(KERN_ALERT "Register fail!!\n");
     return rice_drv.major_number;
 }

 printk(KERN_ALERT "Registe success, major number is %d\n", rice_drv.major_number);

 rice_drv.class = class_create(THIS_MODULE, CLASS_NAME);

 if (IS_ERR(rice_drv.class)) {
     unregister_chrdev(rice_drv.major_number, DEVICE_NAME);
     return PTR_ERR(rice_drv.class);
 }

 rice_drv.device = device_create(rice_drv.class, NULL, MKDEV(rice_drv.major_number, 0), NULL, DEVICE_NAME);

 if (IS_ERR(rice_drv.device)) {
     class_destroy(rice_drv.class);
     unregister_chrdev(rice_drv.major_number, DEVICE_NAME);
     return PTR_ERR(rice_drv.device);
 }

 printk(KERN_ALERT "rice import ko init!!\n");

 rice_drv_export("RiceChen");

 return 0;
}

static void __exit rice_import_exit(void) {
 device_destroy(rice_drv.class, MKDEV(rice_drv.major_number, 0));
 class_unregister(rice_drv.class);
 class_destroy(rice_drv.class);
 unregister_chrdev(rice_drv.major_number, DEVICE_NAME);

 printk(KERN_ALERT "rice import ko exit!!\n");
}

module_init(rice_import_init);
module_exit(rice_import_exit);
MODULE_AUTHOR("RieChen");
MODULE_LICENSE("GPL");

编译运行

将两个模块编译完，push到板子，先加载导出模块--rice_export.ko，然后再加载调用模块--rice_import.ko
运行结果:

首发：Rice 嵌入式开发技术分享
作者：RiceDIY

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简介

EXPORT_SYMBOL()宏定义作用

使用方法

实验

编写代码

编译运行

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