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Jonathan Corbetded49262008-03-28 11:19:56 -06001The 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
8There are numerous ways for a device driver (or other kernel component) to
9provide information to the user or system administrator. One useful
10technique is the creation of virtual files, in debugfs, /proc or elsewhere.
11Virtual files can provide human-readable output that is easy to get at
12without any special utility programs; they can also make life easier for
13script writers. It is not surprising that the use of virtual files has
14grown over the years.
15
16Creating those files correctly has always been a bit of a challenge,
17however. It is not that hard to make a virtual file which returns a
18string. But life gets trickier if the output is long - anything greater
19than an application is likely to read in a single operation. Handling
20multiple reads (and seeks) requires careful attention to the reader's
21position within the virtual file - that position is, likely as not, in the
22middle of a line of output. The kernel has traditionally had a number of
23implementations that got this wrong.
24
25The 2.6 kernel contains a set of functions (implemented by Alexander Viro)
26which are designed to make it easy for virtual file creators to get it
27right.
28
29The seq_file interface is available via <linux/seq_file.h>. There are
30three 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
41We'll look at the seq_file interface via an extremely simple example: a
42loadable module which creates a file called /proc/sequence. The file, when
43read, simply produces a set of increasing integer values, one per line. The
44sequence will continue until the user loses patience and finds something
45better to do. The file is seekable, in that one can do something like the
46following:
47
48 dd if=/proc/sequence of=out1 count=1
Jesper Dangaard Brouere8188802009-05-26 15:18:52 +020049 dd if=/proc/sequence skip=1 of=out2 count=1
Jonathan Corbetded49262008-03-28 11:19:56 -060050
51Then concatenate the output files out1 and out2 and get the right
52result. Yes, it is a thoroughly useless module, but the point is to show
53how the mechanism works without getting lost in other details. (Those
54wanting to see the full source for this module can find it at
55http://lwn.net/Articles/22359/).
56
57
58The iterator interface
59
60Modules implementing a virtual file with seq_file must implement a simple
61iterator object that allows stepping through the data of interest.
62Iterators must be able to move to a specific position - like the file they
63implement - but the interpretation of that position is up to the iterator
64itself. A seq_file implementation that is formatting firewall rules, for
65example, could interpret position N as the Nth rule in the chain.
66Positioning can thus be done in whatever way makes the most sense for the
67generator of the data, which need not be aware of how a position translates
68to an offset in the virtual file. The one obvious exception is that a
69position of zero should indicate the beginning of the file.
70
71The /proc/sequence iterator just uses the count of the next number it
72will output as its position.
73
74Four functions must be implemented to make the iterator work. The first,
75called start() takes a position as an argument and returns an iterator
76which will start reading at that position. For our simple sequence example,
77the 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
88The entire data structure for this iterator is a single loff_t value
89holding the current position. There is no upper bound for the sequence
90iterator, but that will not be the case for most other seq_file
91implementations; in most cases the start() function should check for a
92"past end of file" condition and return NULL if need be.
93
94For more complicated applications, the private field of the seq_file
Dmitri Vorobievb82d4042008-04-15 14:34:40 -070095structure can be used. There is also a special value which can be returned
Jonathan Corbetded49262008-03-28 11:19:56 -060096by the start() function called SEQ_START_TOKEN; it can be used if you wish
97to instruct your show() function (described below) to print a header at the
98top of the output. SEQ_START_TOKEN should only be used if the offset is
99zero, however.
100
101The next function to implement is called, amazingly, next(); its job is to
102move the iterator forward to the next position in the sequence. The
103example module can simply increment the position by one; more useful
104modules will do what is needed to step through some data structure. The
105next() function returns a new iterator, or NULL if the sequence is
106complete. Here's the example version:
107
108 static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
109 {
Jan Engelhardtf3271f62008-03-28 20:09:39 +0100110 loff_t *spos = v;
111 *pos = ++*spos;
Jonathan Corbetded49262008-03-28 11:19:56 -0600112 return spos;
113 }
114
115The stop() function is called when iteration is complete; its job, of
116course, is to clean up. If dynamic memory is allocated for the iterator,
117stop() 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
124Finally, the show() function should format the object currently pointed to
Jonathan Corbet22c36d12008-04-23 10:34:52 -0600125by the iterator for output. The example module's show() function is:
Jonathan Corbetded49262008-03-28 11:19:56 -0600126
127 static int ct_seq_show(struct seq_file *s, void *v)
128 {
Jan Engelhardtf3271f62008-03-28 20:09:39 +0100129 loff_t *spos = v;
130 seq_printf(s, "%lld\n", (long long)*spos);
Jonathan Corbetded49262008-03-28 11:19:56 -0600131 return 0;
132 }
133
Jonathan Corbet22c36d12008-04-23 10:34:52 -0600134If all is well, the show() function should return zero. A negative error
135code in the usual manner indicates that something went wrong; it will be
136passed back to user space. This function can also return SEQ_SKIP, which
137causes the current item to be skipped; if the show() function has already
138generated output before returning SEQ_SKIP, that output will be dropped.
139
Jonathan Corbetded49262008-03-28 11:19:56 -0600140We will look at seq_printf() in a moment. But first, the definition of the
141seq_file iterator is finished by creating a seq_operations structure with
142the four functions we have just defined:
143
Jan Engelhardtf3271f62008-03-28 20:09:39 +0100144 static const struct seq_operations ct_seq_ops = {
Jonathan Corbetded49262008-03-28 11:19:56 -0600145 .start = ct_seq_start,
146 .next = ct_seq_next,
147 .stop = ct_seq_stop,
148 .show = ct_seq_show
149 };
150
151This structure will be needed to tie our iterator to the /proc file in
152a little bit.
153
Dmitri Vorobievb82d4042008-04-15 14:34:40 -0700154It's worth noting that the iterator value returned by start() and
Jonathan Corbetded49262008-03-28 11:19:56 -0600155manipulated by the other functions is considered to be completely opaque by
156the seq_file code. It can thus be anything that is useful in stepping
157through the data to be output. Counters can be useful, but it could also be
158a direct pointer into an array or linked list. Anything goes, as long as
159the programmer is aware that things can happen between calls to the
160iterator function. However, the seq_file code (by design) will not sleep
161between the calls to start() and stop(), so holding a lock during that time
162is a reasonable thing to do. The seq_file code will also avoid taking any
163other locks while the iterator is active.
164
165
166Formatted output
167
168The seq_file code manages positioning within the output created by the
169iterator and getting it into the user's buffer. But, for that to work, that
170output must be passed to the seq_file code. Some utility functions have
171been defined which make this task easy.
172
173Most code will simply use seq_printf(), which works pretty much like
174printk(), but which requires the seq_file pointer as an argument. It is
175common to ignore the return value from seq_printf(), but a function
176producing complicated output may want to check that value and quit if
177something non-zero is returned; an error return means that the seq_file
178buffer has been filled and further output will be discarded.
179
180For 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
186The first two output a single character and a string, just like one would
187expect. seq_escape() is like seq_puts(), except that any character in s
188which is in the string esc will be represented in octal form in the output.
189
Jonathan Corbet9f4def92008-04-25 11:56:37 -0600190There is also a pair of functions for printing filenames:
Jonathan Corbetded49262008-03-28 11:19:56 -0600191
192 int seq_path(struct seq_file *m, struct path *path, char *esc);
Jonathan Corbet9f4def92008-04-25 11:56:37 -0600193 int seq_path_root(struct seq_file *m, struct path *path,
194 struct path *root, char *esc)
Jonathan Corbetded49262008-03-28 11:19:56 -0600195
196Here, path indicates the file of interest, and esc is a set of characters
Jonathan Corbet9f4def92008-04-25 11:56:37 -0600197which should be escaped in the output. A call to seq_path() will output
198the path relative to the current process's filesystem root. If a different
199root is desired, it can be used with seq_path_root(). Note that, if it
200turns out that path cannot be reached from root, the value of root will be
201changed in seq_file_root() to a root which *does* work.
Jonathan Corbetded49262008-03-28 11:19:56 -0600202
203
204Making it all work
205
206So far, we have a nice set of functions which can produce output within the
207seq_file system, but we have not yet turned them into a file that a user
208can see. Creating a file within the kernel requires, of course, the
209creation of a set of file_operations which implement the operations on that
210file. The seq_file interface provides a set of canned operations which do
211most of the work. The virtual file author still must implement the open()
212method, however, to hook everything up. The open function is often a single
213line, 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);
Jan Engelhardtf3271f62008-03-28 20:09:39 +0100218 }
Jonathan Corbetded49262008-03-28 11:19:56 -0600219
220Here, the call to seq_open() takes the seq_operations structure we created
221before, and gets set up to iterate through the virtual file.
222
223On a successful open, seq_open() stores the struct seq_file pointer in
224file->private_data. If you have an application where the same iterator can
225be used for more than one file, you can store an arbitrary pointer in the
226private field of the seq_file structure; that value can then be retrieved
227by the iterator functions.
228
229The other operations of interest - read(), llseek(), and release() - are
230all implemented by the seq_file code itself. So a virtual file's
231file_operations structure will look like:
232
Jan Engelhardtf3271f62008-03-28 20:09:39 +0100233 static const struct file_operations ct_file_ops = {
Jonathan Corbetded49262008-03-28 11:19:56 -0600234 .owner = THIS_MODULE,
235 .open = ct_open,
236 .read = seq_read,
237 .llseek = seq_lseek,
238 .release = seq_release
239 };
240
241There is also a seq_release_private() which passes the contents of the
242seq_file private field to kfree() before releasing the structure.
243
244The final step is the creation of the /proc file itself. In the example
245code, 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
Alexey Dobriyan6be4b782009-12-15 16:47:00 -0800251 proc_create("sequence", 0, NULL, &ct_file_ops);
Jonathan Corbetded49262008-03-28 11:19:56 -0600252 return 0;
253 }
254
255 module_init(ct_init);
256
257And that is pretty much it.
258
259
260seq_list
261
262If your file will be iterating through a linked list, you may find these
263routines 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
272These helpers will interpret pos as a position within the list and iterate
273accordingly. Your start() and next() functions need only invoke the
Dmitri Vorobievb82d4042008-04-15 14:34:40 -0700274seq_list_* helpers with a pointer to the appropriate list_head structure.
Jonathan Corbetded49262008-03-28 11:19:56 -0600275
276
277The extra-simple version
278
279For extremely simple virtual files, there is an even easier interface. A
280module can define only the show() function, which should create all the
281output that the virtual file will contain. The file's open() method then
282calls:
283
284 int single_open(struct file *file,
285 int (*show)(struct seq_file *m, void *p),
286 void *data);
287
288When output time comes, the show() function will be called once. The data
289value given to single_open() can be found in the private field of the
290seq_file structure. When using single_open(), the programmer should use
291single_release() instead of seq_release() in the file_operations structure
292to avoid a memory leak.