Driver porting: The seq_file interface

Creating those files correctly has always been a bit of a challenge, however. It is not that hard to make a /proc file which returns a string. But life gets trickier if the output is long - anything greater than an application is likely to read in a single operation. Handling multiple reads (and seeks) requires careful attention to the reader's position within the virtual file - that position is, likely as not, in the middle of a line of output. The Linux kernel is full of /proc file implementations that get this wrong.

The 2.6 kernel contains a set of functions (implemented by Alexander Viro) which are designed to make it easy for virtual file creators to get it right. This interface (called "seq_file") is not strictly a 2.6 feature - it was also merged into 2.4.15. But 2.6 is where the feature is starting to see serious use, so it is worth describing here.

The seq_file interface is available via <linux/seq_file.h>. There are three aspects to seq_file:

• An iterator interface which lets a virtual file implementation step through the objects it is presenting.

• Some utility functions for formatting objects for output without needing to worry about things like output buffers.

• A set of canned file_operations which implement most operations on the virtual file.

We'll look at the seq_file interface via an extremely simple example: a loadable module which creates a file called /proc/sequence. The file, when read, simply produces a set of increasing integer values, one per line. The sequence will continue until the user loses patience and finds something better to do. The file is seekable, in that one can do something like the following:

    dd if=/proc/sequence of=out1 count=1
    dd if=/proc/sequence skip=1 out=out2 count=1

Then concatenate the output files out1 and out2 and get the right result. Yes, it is a thoroughly useless module, but the point is to show how the mechanism works without getting lost in other details. (Those wanting to see the full source for this module can find it here).

The iterator interface

The /proc/sequence iterator just uses the count of the next number it will output as its position.

Four functions must be implemented to make the iterator work. The first, called start() takes a position as an argument and returns an iterator which will start reading at that position. For our simple sequence example, the start() function looks like:

static void *ct_seq_start(struct seq_file *s, loff_t *pos)
{
	loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
	if (! spos)
		return NULL;
	*spos = *pos;
	return spos;
}

The entire data structure for this iterator is a single loff_t value holding the current position. There is no upper bound for the sequence iterator, but that will not be the case for most other seq_file implementations; in most cases the start() function should check for a "past end of file" condition and return NULL if need be.

For more complicated applications, the private field of the seq_file structure can be used. There is also a special value whch can be returned by the start() function called SEQ_START_TOKEN; it can be used if you wish to instruct your show() function (described below) to print a header at the top of the output. SEQ_START_TOKEN should only be used if the offset is zero, however.

The next function to implement is called, amazingly, next(); its job is to move the iterator forward to the next position in the sequence. The example module can simply increment the position by one; more useful modules will do what is needed to step through some data structure. The next() function returns a new iterator, or NULL if the sequence is complete. Here's the example version:

static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
{
	loff_t *spos = (loff_t *) v;
	*pos = ++(*spos);
	return spos;
}

The stop() function is called when iteration is complete; its job, of course, is to clean up. If dynamic memory is allocated for the iterator, stop() is the place to return it.

static void ct_seq_stop(struct seq_file *s, void *v)
{
	kfree (v);
}

Finally, the show() function should format the object currently pointed to by the iterator for output. It should return zero, or an error code if something goes wrong. The example module's show() function is:

static int ct_seq_show(struct seq_file *s, void *v)
{
	loff_t *spos = (loff_t *) v;
	seq_printf(s, "%Ld\n", *spos);
	return 0;
}

We will look at seq_printf() in a moment. But first, the definition of the seq_file iterator is finished by creating a seq_operations structure with the four functions we have just defined:

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

This structure will be needed to tie our iterator to the /proc file in a little bit.

It's worth noting that the interator value returned by start() and manipulated by the other functions is considered to be completely opaque by the seq_file code. It can thus be anything that is useful in stepping through the data to be output. Counters can be useful, but it could also be a direct pointer into an array or linked list. Anything goes, as long as the programmer is aware that things can happen between calls to the iterator function. However, the seq_file code (by design) will not sleep between the calls to start() and stop(), so holding a lock during that time is a reasonable thing to do. The seq_file code will also avoid taking any other locks while the iterator is active.

Formatted output

Most code will simply use seq_printf(), which works pretty much like printk(), but which requires the seq_file pointer as an argument. It is common to ignore the return value from seq_printf(), but a function producing complicated output may want to check that value and quit if something non-zero is returned; an error return means that the seq_file buffer has been filled and further output will be discarded.

For straight character output, the following functions may be used:

int seq_putc(struct seq_file *m, char c);
int seq_puts(struct seq_file *m, const char *s);
int seq_escape(struct seq_file *m, const char *s, const char *esc);

The first two output a single character and a string, just like one would expect. seq_escape() is like seq_puts(), except that any character in s which is in the string esc will be represented in octal form in the output.

There is also a function for printing filenames:

int seq_path(struct seq_file *m, struct vfsmount *mnt, 
	     struct dentry *dentry, char *esc);

Here, mnt and dentry indicate the file of interest, and esc is a set of characters which should be escaped in the output. This function is more suited to filesystem code than device drivers, however.

Making it all work

static int ct_open(struct inode *inode, struct file *file)
{
	return seq_open(file, &ct_seq_ops);
};

Here, the call to seq_open() takes the seq_operations structure we created before, and gets set up to iterate through the virtual file.

On a successful open, seq_open() stores the struct seq_file pointer in file->private_data. If you have an application where the same iterator can be used for more than one file, you can store an arbitrary pointer in the private field of the seq_file structure; that value can then be retrieved by the iterator functions.

The other operations of interest - read(), llseek(), and release() - are all implemented by the seq_file code itself. So a virtual file's file_operations structure will look like:

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

The final step is the creation of the /proc file itself. In the example code, that is done in the initialization code in the usual way:

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

	entry = create_proc_entry("sequence", 0, NULL);
	if (entry)
		entry->proc_fops = &ct_file_ops;
	return 0;
}

module_init(ct_init);

And that is pretty much it.

The extra-simple version

int single_open(struct file *file, 
                int (*show)(struct seq_file *m, void *p), 
		void *data);

When output time comes, the show() function will be called once. The data value given to single_open() can be found in the private field of the seq_file structure. When using single_open(), the programmer should use single_release() instead of seq_release() in the file_operations structure to avoid a memory leak.