如何实现一套linux进程间通信的机制

我们知道linux的进程的间通信的组件有管道，消息队列，socket, 信号量，共享内存等。但是我们如果自己实现一套进程间通信的机制的话，要怎么做？了解android 开发的可能会知道，android里面有个binder机制，简单来说，就是一个进程往binder里面写数据，另一个进程从binder里面读出数据。

所以我们也可以按照binder的思路来设计一个自己的进程间通信组件。

原理

我们的设计思路很简单，我们首先需要注册一个字符设备文件叫**/dev/channel**, 同时需要为这个设备编写驱动，此时某个进程A向设备文件写数据，同时如果该设备可读，我们就通知另一个进程B去读该进程。 我们怎么知道该设备是否可读可写呢？使用poll来管理，因为该设备驱动属于一个IO, 打开一个设备就有fd, 有了fd我们就可以使用poll来管理。

代码实现

首先驱动程序:

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include


#ifndef CHANNEL_MAJOR
#define CHANNEL_MAJOR 96
#endif

#ifndef CHANNEL_NR_DEVS
#define CHANNEL_NR_DEVS 2
#endif

#ifndef CHANNEL_SIZE
#define CHANNEL_SIZE 4096
#endif

#define ENABLE_POLL 1

struct channel {
char *data;
unsigned long size;
#if ENABLE_POLL
wait_queue_head_t inq;
#endif
};

static int channel_major = CHANNEL_MAJOR;
module_param(channel_major, int, S_IRUGO);

struct channel *channel_devp;
struct cdev cdev;

char have_data = 0;

int channel_open (struct inode *inode, struct file *filp) {
struct channel *channel;

int num = MINOR(inode->i_rdev); //设备读了多少次
if (num >= CHANNEL_NR_DEVS)
return -ENODEV;

channel = &channel_devp[num];
filp->private_data = channel;

return 0;
}

int channel_release (struct inode *inode, struct file *filp) {
return 0;
}

#if ENABLE_POLL
unsigned int channel_poll (struct file *filp, struct poll_table_struct *wait) {
struct channel *channel = filp->private_data;
unsigned int mask = 0;

poll_wait(filp, &channel->inq, wait); // poll 阻塞

if (have_data)
mask |= (POLLIN | POLLRDNORM);

return mask;
}
#endif


int channel_mmap (struct file *filp, struct vm_area_struct *vma) {
struct channel *channel = filp->private_data;

vma->vm_flags |= VM_IO;
vma->vm_flags |= (VM_DONTEXPAND | VM_DONTDUMP);

if (remap_pfn_range(vma, vma->vm_start, virt_to_phys(channel->data) >> PAGE_SHIFT,
vma->vm_end-vma->vm_start, vma->vm_page_prot)) {
return -EAGAIN;
}

return 0;
}

ssize_t channel_read (struct file *filp, char __user * buffer, size_t size, loff_t *ppos) {
unsigned long p = *ppos;
unsigned int count = size;

int ret = 0;
struct channel *channel = filp->private_data; // 读私有空间

if (p >= CHANNEL_SIZE) return 0;
if (count > CHANNEL_SIZE- p)
count = CHANNEL_SIZE- p;

#if ENABLE_POLL
while (!have_data) {
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;

wait_event_interruptible(channel->inq, have_data);
}
#endif
if (copy_to_user(buffer, (void*)(channel->data + p), count)) { //拷贝到用户空间
ret = -EFAULT;
} else {
ret = strlen(buffer);
channel->size -= ret;
printk(KERN_INFO "read %d byte(s) from %ldn", ret, p);
}

have_data = 0;
return ret;
}

ssize_t channel_write (struct file *filp , const char __user * buffer, size_t size, loff_t *ppos) {
int ret = 0;
unsigned long p = *ppos;
unsigned int count = size;

struct channel *channel = filp->private_data; // 写道文件的私有空间
if (p >= CHANNEL_SIZE) return 0;
if (count > CHANNEL_SIZE- p)
count = CHANNEL_SIZE- p;

if (copy_from_user(channel->data +p, buffer, count)) { // 从user -> kernel
return -EFAULT;
} else {
*ppos += count;
ret = count;
channel->size += count;
*(channel->data+p + count) = '�';

printk(KERN_INFO "written %d byte(s) from %ldn", count, p);
}

#if ENABLE_POLL
have_data = 1;
wake_up(&channel->inq);
#endif

return ret;
}

loff_t channel_llseek (struct file *filp, loff_t offset, int whence) { //偏移
loff_t newpos;

switch (whence)
{
case 0:
newpos = offset;
break;
case 1:
newpos = filp->f_pos + offset;
break;
case 2:
newpos = CHANNEL_SIZE - 1 + offset;
break;
default:
return -EINVAL;
}

if (newpos < 0 || newpos > CHANNEL_SIZE) return -EINVAL;

filp->f_pos = newpos;

return newpos;
}

static const struct file_operations channel_fops =
{
.owner = THIS_MODULE,
.llseek = channel_llseek,
.read = channel_read,
.write = channel_write,
.open = channel_open,
.release = channel_release,
.poll = channel_poll,
.mmap = channel_mmap,
};


static int channel_init(void) {
int reslut;
int i;

dev_t devno = MKDEV(channel_major, 0); // 创建一个主设备号为96，次设备号为0的设备
if (channel_major) {
reslut = register_chrdev_region(devno, CHANNEL_NR_DEVS, "channel"); // 注册设备
} else {
reslut = alloc_chrdev_region(&devno, 0, CHANNEL_NR_DEVS, "channel");
}

if (reslut < 0) return reslut;

cdev_init(&cdev, &channel_fops); //初始化字符设备
cdev.owner = THIS_MODULE;

cdev_add(&cdev, MKDEV(channel_major, 0), CHANNEL_NR_DEVS); //添加到字符设备中

channel_devp = kmalloc(CHANNEL_NR_DEVS *sizeof(struct channel), GFP_KERNEL); //为 我们的buffer 分配一块空间
if (!channel_devp) {
reslut = -ENOMEM;
goto fail_malloc;
}
memset(channel_devp, 0, sizeof(struct channel));

for (i = 0; i < CHANNEL_NR_DEVS; i++) {
channel_devp[i].size = CHANNEL_SIZE;
channel_devp[i].data = kmalloc(CHANNEL_SIZE, GFP_KERNEL);
memset(channel_devp[i].data, 0, CHANNEL_SIZE);
#if ENABLE_POLL
init_waitqueue_head(&(channel_devp[i].inq));
#endif
}
printk(KERN_INFO "ntychannel_init");

return 0;
fail_malloc:
unregister_chrdev_region(devno, 1);
return reslut;
}

static void channel_exit(void) {
printk(KERN_INFO "channel_exit");
cdev_del(&cdev);
kfree(channel_devp);

unregister_chrdev_region(MKDEV(channel_major, 0), 2);
}


MODULE_AUTHOR("birate");
MODULE_LICENSE("GPL");

module_init(channel_init); // 设备初始化
module_exit(channel_exit); //设备退出

编写Makefile文件:

obj-m += channel.o
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
all:
make -C $(KERNELDIR) M=$(PWD) modules
clean:
make -C $(KERNELDIR) M=$(PWD) clean

1. 使用 make 命令。编译出我们需要的channel.ko文件。
2. 使用 insmod channel.ko， 向kernel中插入 我们的module
3. 使用mknod /dev/channel c 96 0, 创建一个/dev/channel 的字符设备，主设备号为96，次设备号为0；

编写我们的应用程序:

channel_app.c

#include
#include
#include

#include
#include
#include
#include
#include

#include

#define BUFFER_LENGTH 128

int main () {
int fd = open("/dev/channel", O_RDWR);
if (fd < 0) {
printf("open failed: errno : %sn", strerror(errno));
return -1;
}

char *buffer = (char *)malloc(BUFFER_LENGTH);
memset(buffer, 0, BUFFER_LENGTH);

char *start = mmap(NULL, BUFFER_LENGTH, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

fd_set rds;
FD_ZERO(&rds);
FD_SET(fd, &rds);

while(1) {
int ret = select(fd+1, &rds, NULL, NULL, NULL);
if (ret < 0) {
printf("select errorn");
exit(1);
}
if (FD_ISSET(fd, &rds)) {
#if 0
strcpy(buffer, start);
printf("channel: %sn", buffer);
#else
read(fd, buffer, BUFFER_LENGTH);
printf("channel: %sn", buffer);
#endif
}
}

munmap(start, BUFFER_LENGTH);
free(buffer);
close(fd);

return 0;
}

应用程序很简单，我们使用 gcc -o channel_app channel_app.c ， 编译出可执行文件，在一个进程中执行channel_app， 另一个进程使用echo " " > /dev/channel 去向设备文件中写就可以了。