学步园

struct seq_operations {
    void * (*start) (struct seq_file *m, loff_t *pos);
    void (*stop) (struct seq_file *m, void *v);
    void * (*next) (struct seq_file *m, void *v, loff_t *pos);
    int (*show) (struct seq_file *m, void *v);
};

start()：
主要实现初始化工作，在遍历一个链接对象开始时，调用。返回一个链接对象的偏移或者SEQ_START_TOKEN（表征这是所有循环的开始）。出错返回ERR_PTR。
stop():
当所有链接对象遍历结束时调用。主要完成一些清理工作。
next():
用来在遍历中寻找下一个链接对象。返回下一个链接对象或者NULL（遍历结束）。
show():
对遍历对象进行操作的函数。主要是调用seq_printf(), seq_puts()之类的函数，打印出这个对象节点的信息。
下图描述了seq_file函数对一个链表的遍历。

2、重要的数据结构
除了struct seq_operations以外，另一个最重要的数据结构是struct seq_file：
struct seq_file {
        char *buf;
        size_t size;
        size_t from;
        size_t count;
        loff_t index;
        u64 version;
        struct mutex lock;
        const struct seq_operations *op;
        void *private;
};
该结构会在seq_open函数调用中分配，然后作为参数传递给每个seq_file的操作函数。Privat变量可以用来在各个操作函数之间传递参数。

seq_hello.c
#include <net/net_namespace.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>

#define PROC_NAME "test_proc"
#define MAX_LINES 10

typedef struct item
{
    unsigned long key;
    unsigned char value;
}user_item;

user_item items[4];

MODULE_AUTHOR("ZHANG JIE：iptabler@mail.com");
MODULE_LICENSE("GPL");

static void *my_seq_start(struct seq_file *s, loff_t *pos)
{
        static unsigned long counter = 0;
        printk(KERN_INFO"Invoke start/n");

        if ( *pos == 0 )
        {
                /* yes => return a non null value to begin the sequence */
                return &counter;
        }
        else
        {
                /* no => it's the end of the sequence, return end to stop reading */
                *pos = 0;
                return NULL;
        }
}

static void *my_seq_next(struct seq_file *s, void *v, loff_t *pos)
{
    unsigned long *tmp_v = (unsigned long *)v;
     if (*pos < MAX_LINES) {
        (*tmp_v)++;
        (*pos)++;
        return tmp_v;
        }
    else
    {
        *pos = 0;
        return NULL;
    }
}

static void my_seq_stop(struct seq_file *s, void *v)
{
    printk("Invoke stop/n");
}

static int my_seq_show(struct seq_file *s, void *v)
{
       int i;
        loff_t *spos = (loff_t *) v;
    for (i = 0; i < 4; i++)
    {
     items[i].key = *spos;
    }
      items[0].value = '0';
      items[1].value = '1';
      items[2].value = '2';
      items[3].value = '3';
      seq_printf(s, "%ld=%c,%ld=%c,%ld=%c,%ld=%c;/n", items[0].key,
         items[0].value, items[1].key, items[1].value, items[2].key,
         items[2].value, items[3].key, items[3].value);
        return 0;
}

static struct seq_operations my_seq_ops = {
        .start = my_seq_start,
        .next = my_seq_next,
        .stop = my_seq_stop,
        .show = my_seq_show
};

static int my_open(struct inode *inode, struct file *file)
{
        return seq_open(file, &my_seq_ops);
};

static struct file_operations my_file_ops = {
        .owner = THIS_MODULE,
        .open = my_open,
        .read = seq_read,
        .llseek = seq_lseek,
        .release = seq_release
};

int init_module(void)
{
        struct proc_dir_entry *entry;
        entry = create_proc_entry(PROC_NAME, 0, init_net.proc_net);
        if (entry) {
                entry->proc_fops = &my_file_ops;
        }
        printk(KERN_INFO"Initialze /proc/net/test_proc success!/n");
        return 0;
}

void cleanup_module(void)
{
        remove_proc_entry(PROC_NAME, init_net.proc_net);
        printk(KERN_INFO"Remove /proc/net/test_proc success!/n");
}

Makefile
obj-m := seq_hello.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

default:
    $(MAKE) -C $(KDIR) M=$(PWD) modules
clean:
    $(RM) *.o *.mod.c *.ko *.symvers *.markers *.order