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authorJonathan Corbet <corbet@lwn.net>2008-03-28 13:19:56 -0400
committerJonathan Corbet <corbet@lwn.net>2008-03-30 13:05:04 -0400
commitded4926aa28992efcb67dd27a642ddf139ac572b (patch)
treee4a9e66bea9888a8174b8212c043db66b6807a40 /Documentation/filesystems
parentaf8be4e4b316df36a00c1e52a9970c253783b57e (diff)
Add the seq_file documentation
This is an updated version of the document describing the seq_file interface. Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/00-INDEX2
-rw-r--r--Documentation/filesystems/seq_file.txt283
2 files changed, 285 insertions, 0 deletions
diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX
index e68021c08fbd..e731196410b3 100644
--- a/Documentation/filesystems/00-INDEX
+++ b/Documentation/filesystems/00-INDEX
@@ -82,6 +82,8 @@ relay.txt
82 - info on relay, for efficient streaming from kernel to user space. 82 - info on relay, for efficient streaming from kernel to user space.
83romfs.txt 83romfs.txt
84 - description of the ROMFS filesystem. 84 - description of the ROMFS filesystem.
85seq_file.txt
86 - how to use the seq_file API
85sharedsubtree.txt 87sharedsubtree.txt
86 - a description of shared subtrees for namespaces. 88 - a description of shared subtrees for namespaces.
87smbfs.txt 89smbfs.txt
diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt
new file mode 100644
index 000000000000..92975ee7942c
--- /dev/null
+++ b/Documentation/filesystems/seq_file.txt
@@ -0,0 +1,283 @@
1The 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
49 dd if=/proc/sequence skip=1 out=out2 count=1
50
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
95structure can be used. There is also a special value whch can be returned
96by 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 {
110 loff_t *spos = (loff_t *) v;
111 *pos = ++(*spos);
112 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
125by the iterator for output. It should return zero, or an error code if
126something goes wrong. The example module's show() function is:
127
128 static int ct_seq_show(struct seq_file *s, void *v)
129 {
130 loff_t *spos = (loff_t *) v;
131 seq_printf(s, "%Ld\n", *spos);
132 return 0;
133 }
134
135We will look at seq_printf() in a moment. But first, the definition of the
136seq_file iterator is finished by creating a seq_operations structure with
137the four functions we have just defined:
138
139 static struct seq_operations ct_seq_ops = {
140 .start = ct_seq_start,
141 .next = ct_seq_next,
142 .stop = ct_seq_stop,
143 .show = ct_seq_show
144 };
145
146This structure will be needed to tie our iterator to the /proc file in
147a little bit.
148
149It's worth noting that the interator value returned by start() and
150manipulated by the other functions is considered to be completely opaque by
151the seq_file code. It can thus be anything that is useful in stepping
152through the data to be output. Counters can be useful, but it could also be
153a direct pointer into an array or linked list. Anything goes, as long as
154the programmer is aware that things can happen between calls to the
155iterator function. However, the seq_file code (by design) will not sleep
156between the calls to start() and stop(), so holding a lock during that time
157is a reasonable thing to do. The seq_file code will also avoid taking any
158other locks while the iterator is active.
159
160
161Formatted output
162
163The seq_file code manages positioning within the output created by the
164iterator and getting it into the user's buffer. But, for that to work, that
165output must be passed to the seq_file code. Some utility functions have
166been defined which make this task easy.
167
168Most code will simply use seq_printf(), which works pretty much like
169printk(), but which requires the seq_file pointer as an argument. It is
170common to ignore the return value from seq_printf(), but a function
171producing complicated output may want to check that value and quit if
172something non-zero is returned; an error return means that the seq_file
173buffer has been filled and further output will be discarded.
174
175For straight character output, the following functions may be used:
176
177 int seq_putc(struct seq_file *m, char c);
178 int seq_puts(struct seq_file *m, const char *s);
179 int seq_escape(struct seq_file *m, const char *s, const char *esc);
180
181The first two output a single character and a string, just like one would
182expect. seq_escape() is like seq_puts(), except that any character in s
183which is in the string esc will be represented in octal form in the output.
184
185There is also a function for printing filenames:
186
187 int seq_path(struct seq_file *m, struct path *path, char *esc);
188
189Here, path indicates the file of interest, and esc is a set of characters
190which should be escaped in the output.
191
192
193Making it all work
194
195So far, we have a nice set of functions which can produce output within the
196seq_file system, but we have not yet turned them into a file that a user
197can see. Creating a file within the kernel requires, of course, the
198creation of a set of file_operations which implement the operations on that
199file. The seq_file interface provides a set of canned operations which do
200most of the work. The virtual file author still must implement the open()
201method, however, to hook everything up. The open function is often a single
202line, as in the example module:
203
204 static int ct_open(struct inode *inode, struct file *file)
205 {
206 return seq_open(file, &ct_seq_ops);
207 };
208
209Here, the call to seq_open() takes the seq_operations structure we created
210before, and gets set up to iterate through the virtual file.
211
212On a successful open, seq_open() stores the struct seq_file pointer in
213file->private_data. If you have an application where the same iterator can
214be used for more than one file, you can store an arbitrary pointer in the
215private field of the seq_file structure; that value can then be retrieved
216by the iterator functions.
217
218The other operations of interest - read(), llseek(), and release() - are
219all implemented by the seq_file code itself. So a virtual file's
220file_operations structure will look like:
221
222 static struct file_operations ct_file_ops = {
223 .owner = THIS_MODULE,
224 .open = ct_open,
225 .read = seq_read,
226 .llseek = seq_lseek,
227 .release = seq_release
228 };
229
230There is also a seq_release_private() which passes the contents of the
231seq_file private field to kfree() before releasing the structure.
232
233The final step is the creation of the /proc file itself. In the example
234code, that is done in the initialization code in the usual way:
235
236 static int ct_init(void)
237 {
238 struct proc_dir_entry *entry;
239
240 entry = create_proc_entry("sequence", 0, NULL);
241 if (entry)
242 entry->proc_fops = &ct_file_ops;
243 return 0;
244 }
245
246 module_init(ct_init);
247
248And that is pretty much it.
249
250
251seq_list
252
253If your file will be iterating through a linked list, you may find these
254routines useful:
255
256 struct list_head *seq_list_start(struct list_head *head,
257 loff_t pos);
258 struct list_head *seq_list_start_head(struct list_head *head,
259 loff_t pos);
260 struct list_head *seq_list_next(void *v, struct list_head *head,
261 loff_t *ppos);
262
263These helpers will interpret pos as a position within the list and iterate
264accordingly. Your start() and next() functions need only invoke the
265seq_list_* helpers with a pointer to the appropriate list_head structure.
266
267
268The extra-simple version
269
270For extremely simple virtual files, there is an even easier interface. A
271module can define only the show() function, which should create all the
272output that the virtual file will contain. The file's open() method then
273calls:
274
275 int single_open(struct file *file,
276 int (*show)(struct seq_file *m, void *p),
277 void *data);
278
279When output time comes, the show() function will be called once. The data
280value given to single_open() can be found in the private field of the
281seq_file structure. When using single_open(), the programmer should use
282single_release() instead of seq_release() in the file_operations structure
283to avoid a memory leak.