completion是一种轻量级的机制，它允许一个线程告诉另一个线程工作已经完成。可以利用下面的宏静态创建completion：

DECLARE_COMPLETION(my_completion);

        如果运行时创建completion，则必须采用以下方法动态创建和初始化：

struct compltion my_completion;                           init_completion(&my_completion);

        completion的相关定义包含在kernel/include/linux/completion.h中：

struct completion {

                                     unsigned int done;                                      wait_queue_head_t wait;                          };

#define COMPLETION_INITIALIZER(work) \                                                            { 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }

#define DECLARE_COMPLETION(work) \

                                                      struct completion work = COMPLETION_INITIALIZER(work)

static inline void init_completion(struct completion *x)

{           x->done = 0;           init_waitqueue_head(&x->wait); }

       要等待completion，可进行如下调用：

                    void wait_for_completion(struct completion *c);

       触发completion事件，调用：

                   void complete(struct completion *c);    //唤醒一个等待线程                    void complete_all(struct completion *c);//唤醒所有的等待线程

        为说明completion的使用方法，将《Linux设备驱动程序》一书中的complete模块的代码摘抄如下：

#include <linux/module.h>

#include <linux/init.h>

#include <linux/sched.h>  

#include <linux/kernel.h> #include <linux/fs.h>       #include <linux/types.h>   #include <linux/completion.h>

MODULE_LICENSE("Dual BSD/GPL");

static int complete_major = 253;//指定主设备号

DECLARE_COMPLETION(comp);

ssize_t complete_read (struct file *filp, char __user *buf, size_t count, loff_t *pos)

{          printk(KERN_DEBUG "process %i (%s) going to sleep\n",          current->pid, current->comm);          wait_for_completion(&comp);          printk(KERN_DEBUG "awoken %i (%s)\n", current->pid, current->comm);          return 0; }

ssize_t complete_write (struct file *filp, const char __user *buf, size_t count,

    loff_t *pos) {          printk(KERN_DEBUG "process %i (%s) awakening the readers...\n",          current->pid, current->comm);          complete(&comp);          return count; }

struct file_operations complete_fops = {          .owner = THIS_MODULE,          .read =    complete_read,          .write = complete_write, };

int complete_init(void) {          int result;

        result = register_chrdev(complete_major, "complete", &complete_fops);         if (result < 0)                 return result;         if (complete_major == 0)                 complete_major = result;         return 0; }

void complete_cleanup(void)

{          unregister_chrdev(complete_major, "complete"); }

module_init(complete_init);

module_exit(complete_cleanup);

        该模块定义了一个简单的completion设备：任何试图从该设备中读取的进程都将等待，直到其他设备写入该设备为止。编译此模块的Makefile如下： obj-m := complete.o KDIR := /lib/modules/$(shell uname -r)/build PWD := $(shell pwd) default: $(MAKE) -C $(KDIR) M=$(PWD) modules clean: rm -f *.ko *.o *.mod.c

在linux终端中执行以下命令，编译生成模块，并进行动态加载。

#make #mknod completion c 253 0 #insmod complete.ko 再打开三个终端，一个用于读进程： #cat completion 一个用于写进程： #echo >completion 另一个查看系统日志： #tail -f /var/log/messages

         值得注意的是，当我们使用的complete_all接口时，如果要重复使用一个completion结构，则必须执行 INIT_COMPLETION(struct completion c)来重新初始化它。可以在kernel/include/linux/completion.h中找到这个宏的定义：

          #define INIT_COMPLETION(x) ((x).done = 0)

        以下代码对书中原有的代码进行了一番变动，将唤醒接口由原来的complete换成了complete_all，并且为了重复利用completion结构，所有读进程都结束后就重新初始化completion结构，具体代码如下：

#include <linux/module.h> #include <linux/init.h>

#include <linux/sched.h>

#include <linux/kernel.h> #include <linux/fs.h> #include <linux/types.h> #include <linux/completion.h>

MODULE_LICENSE("Dual BSD/GPL");

#undef KERN_DEBUG

#define KERN_DEBUG "<1>"

static int complete_major=253;

static int reader_count = 0;

DECLARE_COMPLETION(comp);

ssize_t complete_read (struct file *filp,char __user *buf,size_t count,loff_t *pos)

{            printk(KERN_DEBUG "process %i (%s) going to sleep,waiting for writer\n",current->pid,current->comm);            reader_count++;            printk(KERN_DEBUG "In read ,before comletion: reader count = %d \n",reader_count);            wait_for_completion(&comp);            reader_count--;            printk(KERN_DEBUG "awoken %s (%i) \n",current->comm,current->pid);            printk(KERN_DEBUG "In read,after completion : reader count = %d \n",reader_count);

           if(reader_count == 0)                        INIT_COMPLETION(comp);            return 0; }

ssize_t complete_write(struct file *filp,const char __user *buf,size_t count,loff_t *pos)

{            printk(KERN_DEBUG "process %i (%s) awoking the readers...\n",current->pid,current->comm);            printk(KERN_DEBUG "In write ,before do complete_all : reader count = %d \n",reader_count);            if(reader_count != 0)                      complete_all(&comp);            printk(KERN_DEBUG "In write ,after do complete_all : reader count = %d \n",reader_count);            return count; }

struct file_operations complete_fops={

           .owner = THIS_MODULE,            .read = complete_read,            .write = complete_write, };

int complete_init(void)

{            int result;

           result=register_chrdev(complete_major,"complete",&complete_fops);

           if(result<0)                     return result;            if(complete_major==0)                    complete_major =result;

           printk(KERN_DEBUG    "complete driver test init! complete_major=%d\n",complete_major);

           printk(KERN_DEBUG "静态初始化completion\n");            return 0; }

void complete_exit(void)

{            unregister_chrdev(complete_major,"complete");            printk(KERN_DEBUG    "complete driver    is removed\n"); }

module_init(complete_init);

module_exit(complete_exit);

这里测试步骤和上述一样，只不过需要多打开几个终端来执行多个进程同时读操作。