linux内核seq操作

头文件linux/seq_file.h

seq相关函数的实现在fs/seq_file.c

struct seq_file {
char *buf;
size_t size;
size_t from;
size_t count;
loff_t index;
loff_t read_pos;
u64 version;
struct mutex lock;
const struct seq_operations *op;
int poll_event;
void *private;
};

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实现初始化工作，在遍历一个链接对象开始时调用

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，返回下一个对象或者NULL

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。

//seq操作包括以下一系列函数
int seq_open(struct file *, const struct seq_operations *);
ssize_t seq_read(struct file *, char __user *, size_t, loff_t *);
loff_t seq_lseek(struct file *, loff_t, int);
int seq_release(struct inode *, struct file *);

实现例子

/*
* Documentation/filesystem/seq_file.txt
*/
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>

//#define DEBUG_SEQ

#ifdef DEBUG_SEQ
#define log_seq(...) printk(__VA_ARGS__)
#else
#define log_seq(...)
#endif

static void *ct_seq_start(struct seq_file *s, loff_t *pos)
{
int *count = s->private;

log_seq("%s\n", __func__);

if ((long long)*pos < *count) {
printk("start pos %lld\n", (long long)*pos);
return pos;
}

return NULL;
}

static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
{
int *count = s->private;

log_seq("%s\n", __func__);

++*pos;
if ((long long)*pos < *count) {
printk("next pos %lld\n", (long long)*pos);
return pos;
}

return NULL;
}

static void ct_seq_stop(struct seq_file *s, void *v)
{
log_seq("%s\n", __func__);
}

static int ct_seq_show(struct seq_file *s, void *v)
{
loff_t *pos = v;

log_seq("%s\n", __func__);

seq_printf(s, "%lld\n", (long long)*pos);
return 0;
}

static const struct seq_operations ct_seq_ops = {
.start = ct_seq_start,
.next = ct_seq_next,
.stop = ct_seq_stop,
.show = ct_seq_show
};

static int ct_open(struct inode *inode, struct file *file)
{
int ret;
struct seq_file *s;

ret = seq_open(file, &ct_seq_ops);

s = file->private_data;
s->private = (void *)kmalloc(sizeof(int), GFP_KERNEL);
*((int *)s->private) = 5;

return ret;
}

static int ct_close(struct inode *inode, struct file *file)
{
struct seq_file *s = file->private_data;

kfree(s->private);
return seq_release(inode, file);
}

static const struct file_operations ct_file_ops = {
.owner = THIS_MODULE,
.open = ct_open,
.read = seq_read,
//      .write = seq_write,
.llseek = seq_lseek,
.release = ct_close
};

static int __init ct_init(void)
{
struct proc_dir_entry *entry;

entry = proc_create("sequence", 0, NULL, &ct_file_ops);
return 0;
}

static void __exit ct_exit(void)
{
remove_proc_entry("sequence", NULL);
}

module_init(ct_init);
module_exit(ct_exit);

整体看来，用户态调用一次读操作，seq_file流程为：该函数调用struct seq_operations结构提顺序为：start->show->next->show…->next->show->next->stop->start->stop来读取文件

参考文章

读取proc文件之seq_file

参考资源

实现内核seq操作的例子