1 .. SPDX-License-Identifier: GPL-2.0
7 Copyright 2003 Jonathan Corbet <corbet@lwn.net>
9 This file is originally from the LWN.net Driver Porting series at
10 https://lwn.net/Articles/driver-porting/
13 There are numerous ways for a device driver (or other kernel component) to
14 provide information to the user or system administrator. One useful
15 technique is the creation of virtual files, in debugfs, /proc or elsewhere.
16 Virtual files can provide human-readable output that is easy to get at
17 without any special utility programs; they can also make life easier for
18 script writers. It is not surprising that the use of virtual files has
21 Creating those files correctly has always been a bit of a challenge,
22 however. It is not that hard to make a virtual file which returns a
23 string. But life gets trickier if the output is long - anything greater
24 than an application is likely to read in a single operation. Handling
25 multiple reads (and seeks) requires careful attention to the reader's
26 position within the virtual file - that position is, likely as not, in the
27 middle of a line of output. The kernel has traditionally had a number of
28 implementations that got this wrong.
30 The 2.6 kernel contains a set of functions (implemented by Alexander Viro)
31 which are designed to make it easy for virtual file creators to get it
34 The seq_file interface is available via <linux/seq_file.h>. There are
35 three aspects to seq_file:
37 * An iterator interface which lets a virtual file implementation
38 step through the objects it is presenting.
40 * Some utility functions for formatting objects for output without
41 needing to worry about things like output buffers.
43 * A set of canned file_operations which implement most operations on
46 We'll look at the seq_file interface via an extremely simple example: a
47 loadable module which creates a file called /proc/sequence. The file, when
48 read, simply produces a set of increasing integer values, one per line. The
49 sequence will continue until the user loses patience and finds something
50 better to do. The file is seekable, in that one can do something like the
53 dd if=/proc/sequence of=out1 count=1
54 dd if=/proc/sequence skip=1 of=out2 count=1
56 Then concatenate the output files out1 and out2 and get the right
57 result. Yes, it is a thoroughly useless module, but the point is to show
58 how the mechanism works without getting lost in other details. (Those
59 wanting to see the full source for this module can find it at
60 https://lwn.net/Articles/22359/).
62 Deprecated create_proc_entry
63 ============================
65 Note that the above article uses create_proc_entry which was removed in
66 kernel 3.10. Current versions require the following update::
68 - entry = create_proc_entry("sequence", 0, NULL);
70 - entry->proc_fops = &ct_file_ops;
71 + entry = proc_create("sequence", 0, NULL, &ct_file_ops);
73 The iterator interface
74 ======================
76 Modules implementing a virtual file with seq_file must implement an
77 iterator object that allows stepping through the data of interest
78 during a "session" (roughly one read() system call). If the iterator
79 is able to move to a specific position - like the file they implement,
80 though with freedom to map the position number to a sequence location
81 in whatever way is convenient - the iterator need only exist
82 transiently during a session. If the iterator cannot easily find a
83 numerical position but works well with a first/next interface, the
84 iterator can be stored in the private data area and continue from one
87 A seq_file implementation that is formatting firewall rules from a
88 table, for example, could provide a simple iterator that interprets
89 position N as the Nth rule in the chain. A seq_file implementation
90 that presents the content of a, potentially volatile, linked list
91 might record a pointer into that list, providing that can be done
92 without risk of the current location being removed.
94 Positioning can thus be done in whatever way makes the most sense for
95 the generator of the data, which need not be aware of how a position
96 translates to an offset in the virtual file. The one obvious exception
97 is that a position of zero should indicate the beginning of the file.
99 The /proc/sequence iterator just uses the count of the next number it
100 will output as its position.
102 Four functions must be implemented to make the iterator work. The
103 first, called start(), starts a session and takes a position as an
104 argument, returning an iterator which will start reading at that
105 position. The pos passed to start() will always be either zero, or
106 the most recent pos used in the previous session.
108 For our simple sequence example,
109 the start() function looks like::
111 static void *ct_seq_start(struct seq_file *s, loff_t *pos)
113 loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
120 The entire data structure for this iterator is a single loff_t value
121 holding the current position. There is no upper bound for the sequence
122 iterator, but that will not be the case for most other seq_file
123 implementations; in most cases the start() function should check for a
124 "past end of file" condition and return NULL if need be.
126 For more complicated applications, the private field of the seq_file
127 structure can be used to hold state from session to session. There is
128 also a special value which can be returned by the start() function
129 called SEQ_START_TOKEN; it can be used if you wish to instruct your
130 show() function (described below) to print a header at the top of the
131 output. SEQ_START_TOKEN should only be used if the offset is zero,
134 The next function to implement is called, amazingly, next(); its job is to
135 move the iterator forward to the next position in the sequence. The
136 example module can simply increment the position by one; more useful
137 modules will do what is needed to step through some data structure. The
138 next() function returns a new iterator, or NULL if the sequence is
139 complete. Here's the example version::
141 static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
148 The stop() function closes a session; its job, of course, is to clean
149 up. If dynamic memory is allocated for the iterator, stop() is the
150 place to free it; if a lock was taken by start(), stop() must release
151 that lock. The value that ``*pos`` was set to by the last next() call
152 before stop() is remembered, and used for the first start() call of
153 the next session unless lseek() has been called on the file; in that
154 case next start() will be asked to start at position zero::
156 static void ct_seq_stop(struct seq_file *s, void *v)
161 Finally, the show() function should format the object currently pointed to
162 by the iterator for output. The example module's show() function is::
164 static int ct_seq_show(struct seq_file *s, void *v)
167 seq_printf(s, "%lld\n", (long long)*spos);
171 If all is well, the show() function should return zero. A negative error
172 code in the usual manner indicates that something went wrong; it will be
173 passed back to user space. This function can also return SEQ_SKIP, which
174 causes the current item to be skipped; if the show() function has already
175 generated output before returning SEQ_SKIP, that output will be dropped.
177 We will look at seq_printf() in a moment. But first, the definition of the
178 seq_file iterator is finished by creating a seq_operations structure with
179 the four functions we have just defined::
181 static const struct seq_operations ct_seq_ops = {
182 .start = ct_seq_start,
188 This structure will be needed to tie our iterator to the /proc file in
191 It's worth noting that the iterator value returned by start() and
192 manipulated by the other functions is considered to be completely opaque by
193 the seq_file code. It can thus be anything that is useful in stepping
194 through the data to be output. Counters can be useful, but it could also be
195 a direct pointer into an array or linked list. Anything goes, as long as
196 the programmer is aware that things can happen between calls to the
197 iterator function. However, the seq_file code (by design) will not sleep
198 between the calls to start() and stop(), so holding a lock during that time
199 is a reasonable thing to do. The seq_file code will also avoid taking any
200 other locks while the iterator is active.
206 The seq_file code manages positioning within the output created by the
207 iterator and getting it into the user's buffer. But, for that to work, that
208 output must be passed to the seq_file code. Some utility functions have
209 been defined which make this task easy.
211 Most code will simply use seq_printf(), which works pretty much like
212 printk(), but which requires the seq_file pointer as an argument.
214 For straight character output, the following functions may be used::
216 seq_putc(struct seq_file *m, char c);
217 seq_puts(struct seq_file *m, const char *s);
218 seq_escape(struct seq_file *m, const char *s, const char *esc);
220 The first two output a single character and a string, just like one would
221 expect. seq_escape() is like seq_puts(), except that any character in s
222 which is in the string esc will be represented in octal form in the output.
224 There are also a pair of functions for printing filenames::
226 int seq_path(struct seq_file *m, const struct path *path,
228 int seq_path_root(struct seq_file *m, const struct path *path,
229 const struct path *root, const char *esc)
231 Here, path indicates the file of interest, and esc is a set of characters
232 which should be escaped in the output. A call to seq_path() will output
233 the path relative to the current process's filesystem root. If a different
234 root is desired, it can be used with seq_path_root(). If it turns out that
235 path cannot be reached from root, seq_path_root() returns SEQ_SKIP.
237 A function producing complicated output may want to check::
239 bool seq_has_overflowed(struct seq_file *m);
241 and avoid further seq_<output> calls if true is returned.
243 A true return from seq_has_overflowed means that the seq_file buffer will
244 be discarded and the seq_show function will attempt to allocate a larger
245 buffer and retry printing.
251 So far, we have a nice set of functions which can produce output within the
252 seq_file system, but we have not yet turned them into a file that a user
253 can see. Creating a file within the kernel requires, of course, the
254 creation of a set of file_operations which implement the operations on that
255 file. The seq_file interface provides a set of canned operations which do
256 most of the work. The virtual file author still must implement the open()
257 method, however, to hook everything up. The open function is often a single
258 line, as in the example module::
260 static int ct_open(struct inode *inode, struct file *file)
262 return seq_open(file, &ct_seq_ops);
265 Here, the call to seq_open() takes the seq_operations structure we created
266 before, and gets set up to iterate through the virtual file.
268 On a successful open, seq_open() stores the struct seq_file pointer in
269 file->private_data. If you have an application where the same iterator can
270 be used for more than one file, you can store an arbitrary pointer in the
271 private field of the seq_file structure; that value can then be retrieved
272 by the iterator functions.
274 There is also a wrapper function to seq_open() called seq_open_private(). It
275 kmallocs a zero filled block of memory and stores a pointer to it in the
276 private field of the seq_file structure, returning 0 on success. The
277 block size is specified in a third parameter to the function, e.g.::
279 static int ct_open(struct inode *inode, struct file *file)
281 return seq_open_private(file, &ct_seq_ops,
282 sizeof(struct mystruct));
285 There is also a variant function, __seq_open_private(), which is functionally
286 identical except that, if successful, it returns the pointer to the allocated
287 memory block, allowing further initialisation e.g.::
289 static int ct_open(struct inode *inode, struct file *file)
292 __seq_open_private(file, &ct_seq_ops, sizeof(*p));
297 p->foo = bar; /* initialize my stuff */
304 A corresponding close function, seq_release_private() is available which
305 frees the memory allocated in the corresponding open.
307 The other operations of interest - read(), llseek(), and release() - are
308 all implemented by the seq_file code itself. So a virtual file's
309 file_operations structure will look like::
311 static const struct file_operations ct_file_ops = {
312 .owner = THIS_MODULE,
316 .release = seq_release
319 There is also a seq_release_private() which passes the contents of the
320 seq_file private field to kfree() before releasing the structure.
322 The final step is the creation of the /proc file itself. In the example
323 code, that is done in the initialization code in the usual way::
325 static int ct_init(void)
327 struct proc_dir_entry *entry;
329 proc_create("sequence", 0, NULL, &ct_file_ops);
333 module_init(ct_init);
335 And that is pretty much it.
341 If your file will be iterating through a linked list, you may find these
344 struct list_head *seq_list_start(struct list_head *head,
346 struct list_head *seq_list_start_head(struct list_head *head,
348 struct list_head *seq_list_next(void *v, struct list_head *head,
351 These helpers will interpret pos as a position within the list and iterate
352 accordingly. Your start() and next() functions need only invoke the
353 ``seq_list_*`` helpers with a pointer to the appropriate list_head structure.
356 The extra-simple version
357 ========================
359 For extremely simple virtual files, there is an even easier interface. A
360 module can define only the show() function, which should create all the
361 output that the virtual file will contain. The file's open() method then
364 int single_open(struct file *file,
365 int (*show)(struct seq_file *m, void *p),
368 When output time comes, the show() function will be called once. The data
369 value given to single_open() can be found in the private field of the
370 seq_file structure. When using single_open(), the programmer should use
371 single_release() instead of seq_release() in the file_operations structure
372 to avoid a memory leak.