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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.

1	The seq_file interface
2	
3		Copyright 2003 Jonathan Corbet <corbet@lwn.net>
4		This file is originally from the LWN.net Driver Porting series at
5		http://lwn.net/Articles/driver-porting/
6	
7	
8	There are numerous ways for a device driver (or other kernel component) to
9	provide information to the user or system administrator.  One useful
10	technique is the creation of virtual files, in debugfs, /proc or elsewhere.
11	Virtual files can provide human-readable output that is easy to get at
12	without any special utility programs; they can also make life easier for
13	script writers. It is not surprising that the use of virtual files has
14	grown over the years.
15	
16	Creating those files correctly has always been a bit of a challenge,
17	however. It is not that hard to make a virtual file which returns a
18	string. But life gets trickier if the output is long - anything greater
19	than an application is likely to read in a single operation.  Handling
20	multiple reads (and seeks) requires careful attention to the reader's
21	position within the virtual file - that position is, likely as not, in the
22	middle of a line of output. The kernel has traditionally had a number of
23	implementations that got this wrong.
24	
25	The 2.6 kernel contains a set of functions (implemented by Alexander Viro)
26	which are designed to make it easy for virtual file creators to get it
27	right.
28	
29	The seq_file interface is available via <linux/seq_file.h>. There are
30	three aspects to seq_file:
31	
32	     * An iterator interface which lets a virtual file implementation
33	       step through the objects it is presenting.
34	
35	     * Some utility functions for formatting objects for output without
36	       needing to worry about things like output buffers.
37	
38	     * A set of canned file_operations which implement most operations on
39	       the virtual file.
40	
41	We'll look at the seq_file interface via an extremely simple example: a
42	loadable module which creates a file called /proc/sequence. The file, when
43	read, simply produces a set of increasing integer values, one per line. The
44	sequence will continue until the user loses patience and finds something
45	better to do. The file is seekable, in that one can do something like the
46	following:
47	
48	    dd if=/proc/sequence of=out1 count=1
49	    dd if=/proc/sequence skip=1 of=out2 count=1
50	
51	Then concatenate the output files out1 and out2 and get the right
52	result. Yes, it is a thoroughly useless module, but the point is to show
53	how the mechanism works without getting lost in other details.  (Those
54	wanting to see the full source for this module can find it at
55	http://lwn.net/Articles/22359/).
56	
57	Deprecated create_proc_entry
58	
59	Note that the above article uses create_proc_entry which was removed in
60	kernel 3.10. Current versions require the following update
61	
62	-	entry = create_proc_entry("sequence", 0, NULL);
63	-	if (entry)
64	-		entry->proc_fops = &ct_file_ops;
65	+	entry = proc_create("sequence", 0, NULL, &ct_file_ops);
66	
67	The iterator interface
68	
69	Modules implementing a virtual file with seq_file must implement a simple
70	iterator object that allows stepping through the data of interest.
71	Iterators must be able to move to a specific position - like the file they
72	implement - but the interpretation of that position is up to the iterator
73	itself. A seq_file implementation that is formatting firewall rules, for
74	example, could interpret position N as the Nth rule in the chain.
75	Positioning can thus be done in whatever way makes the most sense for the
76	generator of the data, which need not be aware of how a position translates
77	to an offset in the virtual file. The one obvious exception is that a
78	position of zero should indicate the beginning of the file.
79	
80	The /proc/sequence iterator just uses the count of the next number it
81	will output as its position.
82	
83	Four functions must be implemented to make the iterator work. The first,
84	called start() takes a position as an argument and returns an iterator
85	which will start reading at that position. For our simple sequence example,
86	the start() function looks like:
87	
88		static void *ct_seq_start(struct seq_file *s, loff_t *pos)
89		{
90		        loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
91		        if (! spos)
92		                return NULL;
93		        *spos = *pos;
94		        return spos;
95		}
96	
97	The entire data structure for this iterator is a single loff_t value
98	holding the current position. There is no upper bound for the sequence
99	iterator, but that will not be the case for most other seq_file
100	implementations; in most cases the start() function should check for a
101	"past end of file" condition and return NULL if need be.
102	
103	For more complicated applications, the private field of the seq_file
104	structure can be used. There is also a special value which can be returned
105	by the start() function called SEQ_START_TOKEN; it can be used if you wish
106	to instruct your show() function (described below) to print a header at the
107	top of the output. SEQ_START_TOKEN should only be used if the offset is
108	zero, however.
109	
110	The next function to implement is called, amazingly, next(); its job is to
111	move the iterator forward to the next position in the sequence.  The
112	example module can simply increment the position by one; more useful
113	modules will do what is needed to step through some data structure. The
114	next() function returns a new iterator, or NULL if the sequence is
115	complete. Here's the example version:
116	
117		static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
118		{
119		        loff_t *spos = v;
120		        *pos = ++*spos;
121		        return spos;
122		}
123	
124	The stop() function is called when iteration is complete; its job, of
125	course, is to clean up. If dynamic memory is allocated for the iterator,
126	stop() is the place to free it.
127	
128		static void ct_seq_stop(struct seq_file *s, void *v)
129		{
130		        kfree(v);
131		}
132	
133	Finally, the show() function should format the object currently pointed to
134	by the iterator for output.  The example module's show() function is:
135	
136		static int ct_seq_show(struct seq_file *s, void *v)
137		{
138		        loff_t *spos = v;
139		        seq_printf(s, "%lld\n", (long long)*spos);
140		        return 0;
141		}
142	
143	If all is well, the show() function should return zero.  A negative error
144	code in the usual manner indicates that something went wrong; it will be
145	passed back to user space.  This function can also return SEQ_SKIP, which
146	causes the current item to be skipped; if the show() function has already
147	generated output before returning SEQ_SKIP, that output will be dropped.
148	
149	We will look at seq_printf() in a moment. But first, the definition of the
150	seq_file iterator is finished by creating a seq_operations structure with
151	the four functions we have just defined:
152	
153		static const struct seq_operations ct_seq_ops = {
154		        .start = ct_seq_start,
155		        .next  = ct_seq_next,
156		        .stop  = ct_seq_stop,
157		        .show  = ct_seq_show
158		};
159	
160	This structure will be needed to tie our iterator to the /proc file in
161	a little bit.
162	
163	It's worth noting that the iterator value returned by start() and
164	manipulated by the other functions is considered to be completely opaque by
165	the seq_file code. It can thus be anything that is useful in stepping
166	through the data to be output. Counters can be useful, but it could also be
167	a direct pointer into an array or linked list. Anything goes, as long as
168	the programmer is aware that things can happen between calls to the
169	iterator function. However, the seq_file code (by design) will not sleep
170	between the calls to start() and stop(), so holding a lock during that time
171	is a reasonable thing to do. The seq_file code will also avoid taking any
172	other locks while the iterator is active.
173	
174	
175	Formatted output
176	
177	The seq_file code manages positioning within the output created by the
178	iterator and getting it into the user's buffer. But, for that to work, that
179	output must be passed to the seq_file code. Some utility functions have
180	been defined which make this task easy.
181	
182	Most code will simply use seq_printf(), which works pretty much like
183	printk(), but which requires the seq_file pointer as an argument. It is
184	common to ignore the return value from seq_printf(), but a function
185	producing complicated output may want to check that value and quit if
186	something non-zero is returned; an error return means that the seq_file
187	buffer has been filled and further output will be discarded.
188	
189	For straight character output, the following functions may be used:
190	
191		int seq_putc(struct seq_file *m, char c);
192		int seq_puts(struct seq_file *m, const char *s);
193		int seq_escape(struct seq_file *m, const char *s, const char *esc);
194	
195	The first two output a single character and a string, just like one would
196	expect. seq_escape() is like seq_puts(), except that any character in s
197	which is in the string esc will be represented in octal form in the output.
198	
199	There is also a pair of functions for printing filenames:
200	
201		int seq_path(struct seq_file *m, struct path *path, char *esc);
202		int seq_path_root(struct seq_file *m, struct path *path,
203				  struct path *root, char *esc)
204	
205	Here, path indicates the file of interest, and esc is a set of characters
206	which should be escaped in the output.  A call to seq_path() will output
207	the path relative to the current process's filesystem root.  If a different
208	root is desired, it can be used with seq_path_root().  Note that, if it
209	turns out that path cannot be reached from root, the value of root will be
210	changed in seq_file_root() to a root which *does* work.
211	
212	
213	Making it all work
214	
215	So far, we have a nice set of functions which can produce output within the
216	seq_file system, but we have not yet turned them into a file that a user
217	can see. Creating a file within the kernel requires, of course, the
218	creation of a set of file_operations which implement the operations on that
219	file. The seq_file interface provides a set of canned operations which do
220	most of the work. The virtual file author still must implement the open()
221	method, however, to hook everything up. The open function is often a single
222	line, as in the example module:
223	
224		static int ct_open(struct inode *inode, struct file *file)
225		{
226			return seq_open(file, &ct_seq_ops);
227		}
228	
229	Here, the call to seq_open() takes the seq_operations structure we created
230	before, and gets set up to iterate through the virtual file.
231	
232	On a successful open, seq_open() stores the struct seq_file pointer in
233	file->private_data. If you have an application where the same iterator can
234	be used for more than one file, you can store an arbitrary pointer in the
235	private field of the seq_file structure; that value can then be retrieved
236	by the iterator functions.
237	
238	The other operations of interest - read(), llseek(), and release() - are
239	all implemented by the seq_file code itself. So a virtual file's
240	file_operations structure will look like:
241	
242		static const struct file_operations ct_file_ops = {
243		        .owner   = THIS_MODULE,
244		        .open    = ct_open,
245		        .read    = seq_read,
246		        .llseek  = seq_lseek,
247		        .release = seq_release
248		};
249	
250	There is also a seq_release_private() which passes the contents of the
251	seq_file private field to kfree() before releasing the structure.
252	
253	The final step is the creation of the /proc file itself. In the example
254	code, that is done in the initialization code in the usual way:
255	
256		static int ct_init(void)
257		{
258		        struct proc_dir_entry *entry;
259	
260		        proc_create("sequence", 0, NULL, &ct_file_ops);
261		        return 0;
262		}
263	
264		module_init(ct_init);
265	
266	And that is pretty much it.
267	
268	
269	seq_list
270	
271	If your file will be iterating through a linked list, you may find these
272	routines useful:
273	
274		struct list_head *seq_list_start(struct list_head *head,
275		       		 		 loff_t pos);
276		struct list_head *seq_list_start_head(struct list_head *head,
277				 		      loff_t pos);
278		struct list_head *seq_list_next(void *v, struct list_head *head,
279						loff_t *ppos);
280	
281	These helpers will interpret pos as a position within the list and iterate
282	accordingly.  Your start() and next() functions need only invoke the
283	seq_list_* helpers with a pointer to the appropriate list_head structure.
284	
285	
286	The extra-simple version
287	
288	For extremely simple virtual files, there is an even easier interface.  A
289	module can define only the show() function, which should create all the
290	output that the virtual file will contain. The file's open() method then
291	calls:
292	
293		int single_open(struct file *file,
294		                int (*show)(struct seq_file *m, void *p),
295		                void *data);
296	
297	When output time comes, the show() function will be called once. The data
298	value given to single_open() can be found in the private field of the
299	seq_file structure. When using single_open(), the programmer should use
300	single_release() instead of seq_release() in the file_operations structure
301	to avoid a memory leak.
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