Add the seq_file documentation

	- info on relay, for efficient streaming from kernel to user space.

romfs.txt

	- description of the ROMFS filesystem.

seq_file.txt

	- how to use the seq_file API

sharedsubtree.txt

	- a description of shared subtrees for namespaces.

smbfs.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 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 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 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 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. 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

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 function for printing filenames:

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

Here, path indicates the file of interest, and esc is a set of characters

which should be escaped in the output.

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 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.