seq_file学习—— seq_file

Posted 2020-10-24 摩斯电码

作者

彭东林

[e-mail]

 

平台

Linux-4.14.13

Qemu + vexpress

 

概述

前面介绍了single_open，下面结合一个简单的demo驱动，学习一下seq_file的用法。

下面是一张示意图：

 

正文

seq_demo驱动里实现了一个简单的链表，在init的时候会依次创建7个节点并加入链表，然后向用户空间导出一个seq_demo的节点，读取这个节点，就会调用seq_file相关函数对链表进行遍历，输出每个节点的相关信息。

一、seq_demo驱动

#include <linux/init.h> #include <linux/module.h> #include <linux/seq_file.h> #include <linux/debugfs.h> #include <linux/fs.h> #include <linux/list.h> #include <linux/slab.h> static struct dentry *seq_demo_dir; static LIST_HEAD(seq_demo_list); static DEFINE_MUTEX(seq_demo_lock); struct seq_demo_node { char name[10]; struct list_head list; }; static void *seq_demo_start(struct seq_file *s, loff_t *pos) { mutex_lock(&seq_demo_lock); return seq_list_start(&seq_demo_list, *pos); } static void *seq_demo_next(struct seq_file *s, void *v, loff_t *pos) { return seq_list_next(v, &seq_demo_list, pos); } static void seq_demo_stop(struct seq_file *s, void *v) { mutex_unlock(&seq_demo_lock); } static int seq_demo_show(struct seq_file *s, void *v) { struct seq_demo_node *node = list_entry(v, struct seq_demo_node, list); seq_printf(s, "name: %s, addr: 0x%p\\n", node->name, node); return 0; } static const struct seq_operations seq_demo_ops = { .start = seq_demo_start, .next = seq_demo_next, .stop = seq_demo_stop, .show = seq_demo_show, }; static int seq_demo_open(struct inode *inode, struct file *file) { return seq_open(file, &seq_demo_ops); } static const struct file_operations seq_demo_fops = { .owner = THIS_MODULE, .open = seq_demo_open, .read = seq_read, .llseek = seq_lseek, .release = seq_release, }; static int __init seq_demo_init(void) { int i; struct seq_demo_node *node; for (i = 0; i < 7; i++) { node = kzalloc(sizeof(struct seq_demo_node), GFP_KERNEL); sprintf(node->name, "node%d", i); INIT_LIST_HEAD(&node->list); list_add_tail(&node->list, &seq_demo_list); } seq_demo_dir = debugfs_create_file("seq_demo", 0444, NULL, NULL, &seq_demo_fops); return 0; } static void __exit seq_demo_exit(void) { struct seq_demo_node *node_pos, *node_n; if (seq_demo_dir) { debugfs_remove(seq_demo_dir); list_for_each_entry_safe(node_pos, node_n, &seq_demo_list, list) if (node_pos) { printk("%s: release %s\\n", __func__, node_pos->name); kfree(node_pos); } } } module_init(seq_demo_init); module_exit(seq_demo_exit); MODULE_LICENSE("GPL");

 1 #include <linux/init.h>
 2 #include <linux/module.h>
 3 #include <linux/seq_file.h>
 4 #include <linux/debugfs.h>
 5 #include <linux/fs.h>
 6 #include <linux/list.h>
 7 #include <linux/slab.h>
 8 
 9 static struct dentry *seq_demo_dir;
10 static LIST_HEAD(seq_demo_list);
11 static DEFINE_MUTEX(seq_demo_lock);
12 
13 struct seq_demo_node {
14     char name[10];
15     struct list_head list;
16 };
17 
18 static void *seq_demo_start(struct seq_file *s, loff_t *pos)
19 {
20     mutex_lock(&seq_demo_lock);
21 
22     return seq_list_start(&seq_demo_list, *pos);
23 }
24 
25 static void *seq_demo_next(struct seq_file *s, void *v, loff_t *pos)
26 {
27     return seq_list_next(v, &seq_demo_list, pos);
28 }
29 
30 static void seq_demo_stop(struct seq_file *s, void *v)
31 {
32     mutex_unlock(&seq_demo_lock);
33 }
34 
35 static int seq_demo_show(struct seq_file *s, void *v)
36 {
37     struct seq_demo_node *node = list_entry(v, struct seq_demo_node, list);
38 
39     seq_printf(s, "name: %s, addr: 0x%p\\n", node->name, node);
40 
41     return 0;
42 }
43 
44 static const struct seq_operations seq_demo_ops = {
45     .start = seq_demo_start,
46     .next = seq_demo_next,
47     .stop = seq_demo_stop,
48     .show = seq_demo_show,
49 };
50 
51 static int seq_demo_open(struct inode *inode, struct file *file)
52 {
53     return seq_open(file, &seq_demo_ops);
54 }
55 
56 static const struct file_operations seq_demo_fops = {
57     .owner = THIS_MODULE,
58     .open = seq_demo_open,
59     .read = seq_read,
60     .llseek = seq_lseek,
61     .release = seq_release,
62 };
63 
64 static int __init seq_demo_init(void)
65 {
66     int i;
67     struct seq_demo_node *node;
68 
69     for (i = 0; i < 7; i++) {
70         node = kzalloc(sizeof(struct seq_demo_node), GFP_KERNEL);
71         sprintf(node->name, "node%d", i);
72 
73         INIT_LIST_HEAD(&node->list);
74         list_add_tail(&node->list, &seq_demo_list);
75     }
76 
77     seq_demo_dir = debugfs_create_file("seq_demo", 0444, NULL,
78         NULL, &seq_demo_fops);
79     return 0;
80 }
81 
82 static void __exit seq_demo_exit(void)
83 {
84     struct seq_demo_node *node_pos, *node_n;
85 
86     if (seq_demo_dir) {
87         debugfs_remove(seq_demo_dir);
88         list_for_each_entry_safe(node_pos, node_n, &seq_demo_list, list)
89             if (node_pos) {
90                 printk("%s: release %s\\n", __func__, node_pos->name);
91                 kfree(node_pos);
92             }
93     }
94 }
95 
96 module_init(seq_demo_init);
97 module_exit(seq_demo_exit);
98 MODULE_LICENSE("GPL");

下面是运行结果：

[root@vexpress mnt]# cat /d/seq_demo 
name: node0, addr: 0xef252000
name: node1, addr: 0xef252680
name: node2, addr: 0xef252380
name: node3, addr: 0xef252740
name: node4, addr: 0xef252b00
name: node5, addr: 0xee80e480
name: node6, addr: 0xeeb9fd40
name: node7, addr: 0xeeb9fd00

二、分析

在遍历链表的时候使用seq_file提供的通用接口函数，当然也可以自己实现，只需要遵循如下原则：

start：根据索引编号pos找到对应的node，并返回该node的地址，也就是show和next方法里的v

next：根据当前node的地址和索引编号计算下一个node的地址和索引编号pos，返回值就是下一个节点的地址

show：输出传入的node的信息

stop：如果在start里有加锁，那么在这里需要释放锁

 

结合上面的驱动分析一下：

seq_list_start：

 1 struct list_head *seq_list_start(struct list_head *head, loff_t pos)
 2 {
 3     struct list_head *lh;
 4 
 5     list_for_each(lh, head)
 6         if (pos-- == 0)
 7             return lh;
 8 
 9     return NULL;
10 }

遍历链表，寻找索引为pos的项，找到的话，返回地址，否则返回NULL。当返回NULL的时候， stop会被调用。

 

seq_list_next：

1 struct list_head *seq_list_next(void *v, struct list_head *head, loff_t *ppos)
2 {
3     struct list_head *lh;
4 
5     lh = ((struct list_head *)v)->next;
6     ++*ppos;
7     return lh == head ? NULL : lh;
8 }

计算下一项的地址和索引，如果遍历结束，即lh==head，返回NULL，否则返回下一项的地址。当返回NULL的时候，stop会被调用并把缓冲区中的内容吐给用户

 

 

未完待续