seq_file.txt

The seq_file interface

	Copyright 2003 Jonathan Corbet <corbet@lwn.net>
	This file is originally from the LWN.net Driver Porting series at
	http://lwn.net/Articles/driver-porting/


There are numerous ways for a device driver (or other kernel component) to
provide information to the user or system administrator.  One useful
technique is the creation of virtual files, in debugfs, /proc or elsewhere.
Virtual files can provide human-readable output that is easy to get at
without any special utility programs; they can also make life easier for
script writers. It is not surprising that the use of virtual files has
grown over the years.

Creating those files correctly has always been a bit of a challenge,
however. It is not that hard to make a virtual 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 kernel has traditionally had a number of
implementations that got 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.

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 of=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 at
http://lwn.net/Articles/22359/).


The iterator interface

Modules implementing a virtual file with seq_file must implement a simple
iterator object that allows stepping through the data of interest.
Iterators must be able to move to a specific position - like the file they
implement - but the interpretation of that position is up to the iterator
itself. A seq_file implementation that is formatting firewall rules, for
example, could interpret position N as the Nth rule in the chain.
Positioning can thus be done in whatever way makes the most sense for the
generator of the data, which need not be aware of how a position translates
to an offset in the virtual file. The one obvious exception is that a
position of zero should indicate the beginning of the file.

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 which 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 = 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 free 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.  The example module's show() function is:

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

If all is well, the show() function should return zero.  A negative error
code in the usual manner indicates that something went wrong; it will be
passed back to user space.  This function can also return SEQ_SKIP, which
causes the current item to be skipped; if the show() function has already
generated output before returning SEQ_SKIP, that output will be dropped.

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 const 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 iterator 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

The seq_file code manages positioning within the output created by the
iterator and getting it into the user's buffer. But, for that to work, that
output must be passed to the seq_file code. Some utility functions have
been defined which make this task easy.

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 pair of functions for printing filenames:

	int seq_path(struct seq_file *m, struct path *path, char *esc);
	int seq_path_root(struct seq_file *m, struct path *path,
			  struct path *root, char *esc)

Here, path indicates the file of interest, and esc is a set of characters
which should be escaped in the output.  A call to seq_path() will output
the path relative to the current process's filesystem root.  If a different
root is desired, it can be used with seq_path_root().  Note that, if it
turns out that path cannot be reached from root, the value of root will be
changed in seq_file_root() to a root which *does* work.


Making it all work

So far, we have a nice set of functions which can produce output within the
seq_file system, but we have not yet turned them into a file that a user
can see. Creating a file within the kernel requires, of course, the
creation of a set of file_operations which implement the operations on that
file. The seq_file interface provides a set of canned operations which do
most of the work. The virtual file author still must implement the open()
method, however, to hook everything up. The open function is often a single
line, as in the example module:

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

There is also a seq_release_private() which passes the contents of the
seq_file private field to kfree() before releasing the structure.

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.


seq_list

If your file will be iterating through a linked list, you may find these
routines useful:

	struct list_head *seq_list_start(struct list_head *head,
	       		 		 loff_t pos);
	struct list_head *seq_list_start_head(struct list_head *head,
			 		      loff_t pos);
	struct list_head *seq_list_next(void *v, struct list_head *head,
					loff_t *ppos);

These helpers will interpret pos as a position within the list and iterate
accordingly.  Your start() and next() functions need only invoke the
seq_list_* helpers with a pointer to the appropriate list_head structure.


The extra-simple version

For extremely simple virtual files, there is an even easier interface.  A
module can define only the show() function, which should create all the
output that the virtual file will contain. The file's open() method then
calls:

	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.