Linux源码阅读笔记13-进程通信组件中

架构图

代码分析

c 复制代码

loff_t lnchannel_llseek(struct file *filp, loff_t offset, int whence) {

    loff_t newpos;

    switch(whence) {

        case 0:
            newpos = offset;
            break;
        case 1:
            newpos = filp->f_pos + offset;
            break;
        case 2:
            return -EINVAL;
    }

    if (newpos < 0 || newpos > LNCHANNEL_SIZE) {

        return -EINVAL;
    }

    filp->f_pos = newpos;

    return newpos;
}

这段代码是一个文件操作函数lnchannel_llseek()，它用于在文件中进行定位。具体来说，根据参数whenc e的值不同，函数将会根据指定的偏移量进行定位，并返回新的位置。

如果whence为0，则将偏移量直接作为新位置。
如果whence为1，则将当前位置(filp->f_pos)与偏移量相加得到新位置。
如果whence为2，则返回无效参数错误(EINVAL)。

在完成计算新位置之后，函数会对新位置进行合法性检查。如果新位置小于0或大于预定义的文件大小(LNCHANNEL_SIZE)，则会返回无效参数错误。

最后，函数更新文件结构体中的当前位置(filp->f_pos)为新位置，并返回新位置值。

c 复制代码

unsigned int lnchannel_poll(struct file *filp, struct poll_table_struct *wait) {

    struct lnchannel* channel = filp->private_data;
    unsigned int mask = 0;

    poll_wait(filp, &channel->inq, wait);
    if (have_data) {

        mask |= (POLLIN | POLLRDNORM);
    }

    return mask;
}

这段代码是一个文件操作函数lnchannel_poll()，用于进行文件的轮询操作。它主要用于检查是否有数据可读，并返回相应的事件掩码。

首先，函数从文件结构体中获取通道指针channel。

接下来，使用poll_wait()函数将当前进程加入到等待队列中，以便在有新数据到达时唤醒进程。

然后，在判断是否有数据可读之前，代码中缺少了一个变量have_data的定义和赋值过程。在此处假设该变量为布尔类型，并且已经被正确初始化。

如果存在可读数据，则将事件掩码的对应位(POLLIN | POLLRDNORM)设置为1，表示可读事件发生。

最后，函数返回事件掩码mask给调用者，表示当前的文件状态。

需要注意的是，这只是代码片段，并没有提供完整的上下文环境。如果需要更详细的解释或有其他问题，请随时提问。

完整代码实现

c 复制代码

#include <linux/module.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/io.h>
#include <linux/uaccess.h>
#include <linux/slab.h>
#include <linux/poll.h>

#ifndef LNCHANNEL_MAJOR
#define LNCHANNEL_MAJOR     96
#endif

#ifndef LNCHANNEL_NR_DEVS
#define LNCHANNEL_NR_DEVS   2
#endif

#ifndef LNCHANNEL_SIZE
#define LNCHANNEL_SIZE      4096
#endif

#define ENABLE_POLL     1

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

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

struct lnchannel* channel_devp;
struct cdev cdev;

char have_data = 0;

loff_t lnchannel_llseek(struct file *filp, loff_t offset, int whence) {

    loff_t newpos;

    switch(whence) {

        case 0:
            newpos = offset;
            break;
        case 1:
            newpos = filp->f_pos + offset;
            break;
        case 2:
            return -EINVAL;
    }

    if (newpos < 0 || newpos > LNCHANNEL_SIZE) {

        return -EINVAL;
    }

    filp->f_pos = newpos;

    return newpos;
}

ssize_t lnchannel_read(struct file *filp, char __user *buffer, size_t size, loff_t *ppos) {

    int ret = 0;
    unsigned long count = size;
    unsigned long long p = *ppos;

    struct lnchannel* channel = filp->private_data;

    if (p >= LNCHANNEL_SIZE) {

        return 0;
    }

    if (count > LNCHANNEL_SIZE - p) {

        count = LNCHANNEL_SIZE - p;
    }

    while(!have_data) {

        if (filp->f_flags & O_NONBLOCK) {

            return -EAGAIN;
        }
            
        wait_event_interruptible(channel->inq, have_data);
    }

    if (copy_to_user(buffer, (void*)(channel->data + p), count)) {

        ret = -EFAULT;
    }
    else {

        ret = strlen(buffer);
        channel->size -= ret;
        printk(KERN_INFO "read %d btye(s) from %ld\n", ret, p);
    }

    have_data = 0;

    return ret;
}

ssize_t lnchannel_write(struct file *filp, const char __user *buffer, size_t size, loff_t * ppos) {

    int ret = 0;
    unsigned long count = size;
    unsigned long long p = *ppos;
    struct lnchannel* channel = filp->private_data;

    if (p >= LNCHANNEL_SIZE) {

        return ret;
    }

    if (count >= LNCHANNEL_SIZE - p) {

        count = LNCHANNEL_SIZE - p;
    }

    if (copy_from_user(channel->data + p, buffer, count)) {

        return -EFAULT;
    }
    else {

        *(channel->data + p + count) = '\0';
        channel->size += count;
        ret = count;
        *ppos += count;

        printk(KERN_INFO, "written %d btyes from %ld\n", count, p);
    }

    have_data = 1;
    wake_up(channel->inq);

    return ret;
}

int lnchannel_mmap(struct file *filp, struct vm_area_struct *vma) {

    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;
}

int lnchannel_open(struct inode *inode, struct file *filp) {

    struct lnchannel* channel;

    int num = MINOR(inode->i_cdev);
    if (num >= LNCHANNEL_NR_DEVS) {

        return -ENODEV;
    }

    channel = &channel_devp[num];

    filp->private_data = channel;

    return 0;

}

int lnchannel_release(struct inode *, struct file *) {

    return 0;
}

unsigned int lnchannel_poll(struct file *filp, struct poll_table_struct *wait) {

    struct lnchannel* channel = filp->private_data;
    unsigned int mask = 0;

    poll_wait(filp, &channel->inq, wait);
    if (have_data) {

        mask |= (POLLIN | POLLRDNORM);
    }

    return mask;
}

static const struct file_operations lnchannel_fops = {
    .owner = THIS_MODULE,
    .llseek = lnchannel_llseek,
    .open = lnchannel_open,
    .write = lnchannel_write,
    .open = lnchannel_open,
    .release = lnchannel_release,
    .mmap = lnchannel_mmap,
    .poll = lnchannel_poll,
};

static int lnchannel_init(void) {

    int result;
    int i;

    dev_t devno = MKDEV(channel_major, 0);
    if (channel_major) {

        result = register_chrdev_region(devno, LNCHANNEL_NR_DEVS, "lnchannel");
    }
    else {

        result = alloc_chrdev_region(&devno, 0, LNCHANNEL_NR_DEVS, "lnchannel");
        channel_major = MAJOR(devno);
    }

    if (result < 0) return result;

    cdev_init(&cdev, &lnchannel_fops);
    cdev.owner = THIS_MODULE;

    cdev_add(&cdev, MKDEV(channel_major, 0), LNCHANNEL_NR_DEVS);

    channel_devp = kmalloc(LNCHANNEL_NR_DEVS * (struct lnchannel), GFP_KERNEL);
    if (!channel_devp) {

        result = -ENOMEM;
        goto fail_malloc;
    }
    memset(channel_devp, 0, sizeof(struct lnchannel));

    for(i = 0; i < LNCHANNEL_NR_DEVS; i++) {

        channel_devp[i].size = LNCHANNEL_SIZE;
        channel_devp[i].data = kmalloc(LNCHANNEL_SIZE, GFP_KERNEL);
        memset(channel_devp[i].data, 0, LNCHANNEL_SIZE);
        init_waitqueue_head(&(channel_devp[i].inq));
    }

    printk(KERN_INFO "linchannel_init");

    return 0;

fail_malloc:
    unregister_chrdev_region(devno, 1);

    return result;
}

static void lnchannel_exit(void) {

    printk(KERN_INFO "lnchannel_exit");
    cdev_del(&cdev);
    kfree(channel_devp);

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

MODULE_AUTHOR("Lenn Louis");
MODULE_LICENSE("GPL");

module_init(lnchannel_init);
module_init(lnchannel_exit);

Makefile 复制代码

#!/bin/bash

ccflags_y += -O2

ifneq ($(KERNELRELEASE),)
obj-m := lnchannel.o
else
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

default:
	$(MAKE) -C $(KERNELDIR) M=$(PWD) modules 
endif

clean:
	rm -rf *.o *.ko *.mod.c

depend .depend dep:
	$(CC)  -M *.c > .depend