Re: [PATCH] Add seq_file howto to Documentation/

[Randy Dunlap]

On Mon, 23 Jul 2007 14:09:08 -0600 Jonathan Corbet wrote:

> > +[Another seq_file reference is "Driver porting: The seq_file interface"
> > +at <http://lwn.net/Articles/22355/>, which is part of the
> > +LWN.net series "Porting Drivers to 2.5" that is located at
> > +<http://lwn.net/Articles/driver-porting/>.]
> 
> Funny thing, that...I had sent in a version of that document for
> consideration back in 2003.  Guess that was still the Good Old Days when
> the patch management system was rather lossier, so I never heard back.
> Here it is again just in case anybody's interested.
> 
> In general, I'm more than happy to have anything I put on LWN go into
> Documentation/ if there's interest - just say the word.
> 
> jon

Jon,
Can we get this merged??  or does it need to be updated first?


> To: linux-kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH] seq_file documentation
> cc: torvalds@xxxxxxxx
> From: Jonathan Corbet <corbet@xxxxxxx>
> Date: Thu, 13 Nov 2003 10:16:31 -0700
> Sender: corbet
> 
> I went ahead and packaged up my seq_file document as a basic text file, in
> the interests of getting something out there.  
> 
> jon
> 
> Jonathan Corbet
> Executive editor, LWN.net
> corbet@xxxxxxx
> 
> 
> diff -urN test9-vanilla/Documentation/filesystems/seq_file.txt test9/Documentation/filesystems/seq_file.txt
> --- test9-vanilla/Documentation/filesystems/seq_file.txt	Wed Dec 31 17:00:00 1969
> +++ test9/Documentation/filesystems/seq_file.txt	Fri Nov 14 01:03:58 2003
> @@ -0,0 +1,268 @@
> +The seq_file interface
> +
> +	Copyright 2003 Jonathan Corbet <corbet@xxxxxxx>
> +	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 very useful
> +technique is the creation of virtual files, in /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 /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 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.
> +
> +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 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
> +
> +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 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. 
> +
> +
> +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.
> +
> +
> +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.

---
~Randy
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