About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / filesystems / seq_file.txt




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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	
58	The iterator interface
59	
60	Modules implementing a virtual file with seq_file must implement a simple
61	iterator object that allows stepping through the data of interest.
62	Iterators must be able to move to a specific position - like the file they
63	implement - but the interpretation of that position is up to the iterator
64	itself. A seq_file implementation that is formatting firewall rules, for
65	example, could interpret position N as the Nth rule in the chain.
66	Positioning can thus be done in whatever way makes the most sense for the
67	generator of the data, which need not be aware of how a position translates
68	to an offset in the virtual file. The one obvious exception is that a
69	position of zero should indicate the beginning of the file.
70	
71	The /proc/sequence iterator just uses the count of the next number it
72	will output as its position.
73	
74	Four functions must be implemented to make the iterator work. The first,
75	called start() takes a position as an argument and returns an iterator
76	which will start reading at that position. For our simple sequence example,
77	the start() function looks like:
78	
79		static void *ct_seq_start(struct seq_file *s, loff_t *pos)
80		{
81		        loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
82		        if (! spos)
83		                return NULL;
84		        *spos = *pos;
85		        return spos;
86		}
87	
88	The entire data structure for this iterator is a single loff_t value
89	holding the current position. There is no upper bound for the sequence
90	iterator, but that will not be the case for most other seq_file
91	implementations; in most cases the start() function should check for a
92	"past end of file" condition and return NULL if need be.
93	
94	For more complicated applications, the private field of the seq_file
95	structure can be used. There is also a special value which can be returned
96	by the start() function called SEQ_START_TOKEN; it can be used if you wish
97	to instruct your show() function (described below) to print a header at the
98	top of the output. SEQ_START_TOKEN should only be used if the offset is
99	zero, however.
100	
101	The next function to implement is called, amazingly, next(); its job is to
102	move the iterator forward to the next position in the sequence.  The
103	example module can simply increment the position by one; more useful
104	modules will do what is needed to step through some data structure. The
105	next() function returns a new iterator, or NULL if the sequence is
106	complete. Here's the example version:
107	
108		static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
109		{
110		        loff_t *spos = v;
111		        *pos = ++*spos;
112		        return spos;
113		}
114	
115	The stop() function is called when iteration is complete; its job, of
116	course, is to clean up. If dynamic memory is allocated for the iterator,
117	stop() is the place to free it.
118	
119		static void ct_seq_stop(struct seq_file *s, void *v)
120		{
121		        kfree(v);
122		}
123	
124	Finally, the show() function should format the object currently pointed to
125	by the iterator for output.  The example module's show() function is:
126	
127		static int ct_seq_show(struct seq_file *s, void *v)
128		{
129		        loff_t *spos = v;
130		        seq_printf(s, "%lld\n", (long long)*spos);
131		        return 0;
132		}
133	
134	If all is well, the show() function should return zero.  A negative error
135	code in the usual manner indicates that something went wrong; it will be
136	passed back to user space.  This function can also return SEQ_SKIP, which
137	causes the current item to be skipped; if the show() function has already
138	generated output before returning SEQ_SKIP, that output will be dropped.
139	
140	We will look at seq_printf() in a moment. But first, the definition of the
141	seq_file iterator is finished by creating a seq_operations structure with
142	the four functions we have just defined:
143	
144		static const struct seq_operations ct_seq_ops = {
145		        .start = ct_seq_start,
146		        .next  = ct_seq_next,
147		        .stop  = ct_seq_stop,
148		        .show  = ct_seq_show
149		};
150	
151	This structure will be needed to tie our iterator to the /proc file in
152	a little bit.
153	
154	It's worth noting that the iterator value returned by start() and
155	manipulated by the other functions is considered to be completely opaque by
156	the seq_file code. It can thus be anything that is useful in stepping
157	through the data to be output. Counters can be useful, but it could also be
158	a direct pointer into an array or linked list. Anything goes, as long as
159	the programmer is aware that things can happen between calls to the
160	iterator function. However, the seq_file code (by design) will not sleep
161	between the calls to start() and stop(), so holding a lock during that time
162	is a reasonable thing to do. The seq_file code will also avoid taking any
163	other locks while the iterator is active.
164	
165	
166	Formatted output
167	
168	The seq_file code manages positioning within the output created by the
169	iterator and getting it into the user's buffer. But, for that to work, that
170	output must be passed to the seq_file code. Some utility functions have
171	been defined which make this task easy.
172	
173	Most code will simply use seq_printf(), which works pretty much like
174	printk(), but which requires the seq_file pointer as an argument. It is
175	common to ignore the return value from seq_printf(), but a function
176	producing complicated output may want to check that value and quit if
177	something non-zero is returned; an error return means that the seq_file
178	buffer has been filled and further output will be discarded.
179	
180	For straight character output, the following functions may be used:
181	
182		int seq_putc(struct seq_file *m, char c);
183		int seq_puts(struct seq_file *m, const char *s);
184		int seq_escape(struct seq_file *m, const char *s, const char *esc);
185	
186	The first two output a single character and a string, just like one would
187	expect. seq_escape() is like seq_puts(), except that any character in s
188	which is in the string esc will be represented in octal form in the output.
189	
190	There is also a pair of functions for printing filenames:
191	
192		int seq_path(struct seq_file *m, struct path *path, char *esc);
193		int seq_path_root(struct seq_file *m, struct path *path,
194				  struct path *root, char *esc)
195	
196	Here, path indicates the file of interest, and esc is a set of characters
197	which should be escaped in the output.  A call to seq_path() will output
198	the path relative to the current process's filesystem root.  If a different
199	root is desired, it can be used with seq_path_root().  Note that, if it
200	turns out that path cannot be reached from root, the value of root will be
201	changed in seq_file_root() to a root which *does* work.
202	
203	
204	Making it all work
205	
206	So far, we have a nice set of functions which can produce output within the
207	seq_file system, but we have not yet turned them into a file that a user
208	can see. Creating a file within the kernel requires, of course, the
209	creation of a set of file_operations which implement the operations on that
210	file. The seq_file interface provides a set of canned operations which do
211	most of the work. The virtual file author still must implement the open()
212	method, however, to hook everything up. The open function is often a single
213	line, as in the example module:
214	
215		static int ct_open(struct inode *inode, struct file *file)
216		{
217			return seq_open(file, &ct_seq_ops);
218		}
219	
220	Here, the call to seq_open() takes the seq_operations structure we created
221	before, and gets set up to iterate through the virtual file.
222	
223	On a successful open, seq_open() stores the struct seq_file pointer in
224	file->private_data. If you have an application where the same iterator can
225	be used for more than one file, you can store an arbitrary pointer in the
226	private field of the seq_file structure; that value can then be retrieved
227	by the iterator functions.
228	
229	The other operations of interest - read(), llseek(), and release() - are
230	all implemented by the seq_file code itself. So a virtual file's
231	file_operations structure will look like:
232	
233		static const struct file_operations ct_file_ops = {
234		        .owner   = THIS_MODULE,
235		        .open    = ct_open,
236		        .read    = seq_read,
237		        .llseek  = seq_lseek,
238		        .release = seq_release
239		};
240	
241	There is also a seq_release_private() which passes the contents of the
242	seq_file private field to kfree() before releasing the structure.
243	
244	The final step is the creation of the /proc file itself. In the example
245	code, that is done in the initialization code in the usual way:
246	
247		static int ct_init(void)
248		{
249		        struct proc_dir_entry *entry;
250	
251		        proc_create("sequence", 0, NULL, &ct_file_ops);
252		        return 0;
253		}
254	
255		module_init(ct_init);
256	
257	And that is pretty much it.
258	
259	
260	seq_list
261	
262	If your file will be iterating through a linked list, you may find these
263	routines useful:
264	
265		struct list_head *seq_list_start(struct list_head *head,
266		       		 		 loff_t pos);
267		struct list_head *seq_list_start_head(struct list_head *head,
268				 		      loff_t pos);
269		struct list_head *seq_list_next(void *v, struct list_head *head,
270						loff_t *ppos);
271	
272	These helpers will interpret pos as a position within the list and iterate
273	accordingly.  Your start() and next() functions need only invoke the
274	seq_list_* helpers with a pointer to the appropriate list_head structure.
275	
276	
277	The extra-simple version
278	
279	For extremely simple virtual files, there is an even easier interface.  A
280	module can define only the show() function, which should create all the
281	output that the virtual file will contain. The file's open() method then
282	calls:
283	
284		int single_open(struct file *file,
285		                int (*show)(struct seq_file *m, void *p),
286		                void *data);
287	
288	When output time comes, the show() function will be called once. The data
289	value given to single_open() can be found in the private field of the
290	seq_file structure. When using single_open(), the programmer should use
291	single_release() instead of seq_release() in the file_operations structure
292	to avoid a memory leak.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.