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ded4926a JC |
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 | |
e8188807 | 49 | dd if=/proc/sequence skip=1 of=out2 count=1 |
ded4926a JC |
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 | ||
0b07cb82 FF |
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); | |
ded4926a JC |
66 | |
67 | The iterator interface | |
68 | ||
1f4aace6 N |
69 | Modules implementing a virtual file with seq_file must implement an |
70 | iterator object that allows stepping through the data of interest | |
71 | during a "session" (roughly one read() system call). If the iterator | |
72 | is able to move to a specific position - like the file they implement, | |
73 | though with freedom to map the position number to a sequence location | |
74 | in whatever way is convenient - the iterator need only exist | |
75 | transiently during a session. If the iterator cannot easily find a | |
76 | numerical position but works well with a first/next interface, the | |
77 | iterator can be stored in the private data area and continue from one | |
78 | session to the next. | |
79 | ||
80 | A seq_file implementation that is formatting firewall rules from a | |
81 | table, for example, could provide a simple iterator that interprets | |
82 | position N as the Nth rule in the chain. A seq_file implementation | |
83 | that presents the content of a, potentially volatile, linked list | |
84 | might record a pointer into that list, providing that can be done | |
85 | without risk of the current location being removed. | |
86 | ||
87 | Positioning can thus be done in whatever way makes the most sense for | |
88 | the generator of the data, which need not be aware of how a position | |
89 | translates to an offset in the virtual file. The one obvious exception | |
90 | is that a position of zero should indicate the beginning of the file. | |
ded4926a JC |
91 | |
92 | The /proc/sequence iterator just uses the count of the next number it | |
93 | will output as its position. | |
94 | ||
1f4aace6 N |
95 | Four functions must be implemented to make the iterator work. The |
96 | first, called start(), starts a session and takes a position as an | |
97 | argument, returning an iterator which will start reading at that | |
98 | position. The pos passed to start() will always be either zero, or | |
99 | the most recent pos used in the previous session. | |
100 | ||
101 | For our simple sequence example, | |
ded4926a JC |
102 | the start() function looks like: |
103 | ||
104 | static void *ct_seq_start(struct seq_file *s, loff_t *pos) | |
105 | { | |
106 | loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL); | |
107 | if (! spos) | |
108 | return NULL; | |
109 | *spos = *pos; | |
110 | return spos; | |
111 | } | |
112 | ||
113 | The entire data structure for this iterator is a single loff_t value | |
114 | holding the current position. There is no upper bound for the sequence | |
115 | iterator, but that will not be the case for most other seq_file | |
116 | implementations; in most cases the start() function should check for a | |
117 | "past end of file" condition and return NULL if need be. | |
118 | ||
119 | For more complicated applications, the private field of the seq_file | |
1f4aace6 N |
120 | structure can be used to hold state from session to session. There is |
121 | also a special value which can be returned by the start() function | |
122 | called SEQ_START_TOKEN; it can be used if you wish to instruct your | |
123 | show() function (described below) to print a header at the top of the | |
124 | output. SEQ_START_TOKEN should only be used if the offset is zero, | |
125 | however. | |
ded4926a JC |
126 | |
127 | The next function to implement is called, amazingly, next(); its job is to | |
128 | move the iterator forward to the next position in the sequence. The | |
129 | example module can simply increment the position by one; more useful | |
130 | modules will do what is needed to step through some data structure. The | |
131 | next() function returns a new iterator, or NULL if the sequence is | |
132 | complete. Here's the example version: | |
133 | ||
134 | static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos) | |
135 | { | |
f3271f65 JE |
136 | loff_t *spos = v; |
137 | *pos = ++*spos; | |
ded4926a JC |
138 | return spos; |
139 | } | |
140 | ||
1f4aace6 N |
141 | The stop() function closes a session; its job, of course, is to clean |
142 | up. If dynamic memory is allocated for the iterator, stop() is the | |
143 | place to free it; if a lock was taken by start(), stop() must release | |
144 | that lock. The value that *pos was set to by the last next() call | |
145 | before stop() is remembered, and used for the first start() call of | |
146 | the next session unless lseek() has been called on the file; in that | |
147 | case next start() will be asked to start at position zero. | |
ded4926a JC |
148 | |
149 | static void ct_seq_stop(struct seq_file *s, void *v) | |
150 | { | |
151 | kfree(v); | |
152 | } | |
153 | ||
154 | Finally, the show() function should format the object currently pointed to | |
22c36d18 | 155 | by the iterator for output. The example module's show() function is: |
ded4926a JC |
156 | |
157 | static int ct_seq_show(struct seq_file *s, void *v) | |
158 | { | |
f3271f65 JE |
159 | loff_t *spos = v; |
160 | seq_printf(s, "%lld\n", (long long)*spos); | |
ded4926a JC |
161 | return 0; |
162 | } | |
163 | ||
22c36d18 JC |
164 | If all is well, the show() function should return zero. A negative error |
165 | code in the usual manner indicates that something went wrong; it will be | |
166 | passed back to user space. This function can also return SEQ_SKIP, which | |
167 | causes the current item to be skipped; if the show() function has already | |
168 | generated output before returning SEQ_SKIP, that output will be dropped. | |
169 | ||
ded4926a JC |
170 | We will look at seq_printf() in a moment. But first, the definition of the |
171 | seq_file iterator is finished by creating a seq_operations structure with | |
172 | the four functions we have just defined: | |
173 | ||
f3271f65 | 174 | static const struct seq_operations ct_seq_ops = { |
ded4926a JC |
175 | .start = ct_seq_start, |
176 | .next = ct_seq_next, | |
177 | .stop = ct_seq_stop, | |
178 | .show = ct_seq_show | |
179 | }; | |
180 | ||
181 | This structure will be needed to tie our iterator to the /proc file in | |
182 | a little bit. | |
183 | ||
b82d4043 | 184 | It's worth noting that the iterator value returned by start() and |
ded4926a JC |
185 | manipulated by the other functions is considered to be completely opaque by |
186 | the seq_file code. It can thus be anything that is useful in stepping | |
187 | through the data to be output. Counters can be useful, but it could also be | |
188 | a direct pointer into an array or linked list. Anything goes, as long as | |
189 | the programmer is aware that things can happen between calls to the | |
190 | iterator function. However, the seq_file code (by design) will not sleep | |
191 | between the calls to start() and stop(), so holding a lock during that time | |
192 | is a reasonable thing to do. The seq_file code will also avoid taking any | |
193 | other locks while the iterator is active. | |
194 | ||
195 | ||
196 | Formatted output | |
197 | ||
198 | The seq_file code manages positioning within the output created by the | |
199 | iterator and getting it into the user's buffer. But, for that to work, that | |
200 | output must be passed to the seq_file code. Some utility functions have | |
201 | been defined which make this task easy. | |
202 | ||
203 | Most code will simply use seq_printf(), which works pretty much like | |
1f33c41c | 204 | printk(), but which requires the seq_file pointer as an argument. |
ded4926a JC |
205 | |
206 | For straight character output, the following functions may be used: | |
207 | ||
1f33c41c JP |
208 | seq_putc(struct seq_file *m, char c); |
209 | seq_puts(struct seq_file *m, const char *s); | |
210 | seq_escape(struct seq_file *m, const char *s, const char *esc); | |
ded4926a JC |
211 | |
212 | The first two output a single character and a string, just like one would | |
213 | expect. seq_escape() is like seq_puts(), except that any character in s | |
214 | which is in the string esc will be represented in octal form in the output. | |
215 | ||
1f33c41c | 216 | There are also a pair of functions for printing filenames: |
ded4926a | 217 | |
38094536 DL |
218 | int seq_path(struct seq_file *m, const struct path *path, |
219 | const char *esc); | |
220 | int seq_path_root(struct seq_file *m, const struct path *path, | |
221 | const struct path *root, const char *esc) | |
ded4926a JC |
222 | |
223 | Here, path indicates the file of interest, and esc is a set of characters | |
9f4def9a JC |
224 | which should be escaped in the output. A call to seq_path() will output |
225 | the path relative to the current process's filesystem root. If a different | |
38094536 DL |
226 | root is desired, it can be used with seq_path_root(). If it turns out that |
227 | path cannot be reached from root, seq_path_root() returns SEQ_SKIP. | |
ded4926a | 228 | |
1f33c41c JP |
229 | A function producing complicated output may want to check |
230 | bool seq_has_overflowed(struct seq_file *m); | |
231 | and avoid further seq_<output> calls if true is returned. | |
232 | ||
233 | A true return from seq_has_overflowed means that the seq_file buffer will | |
234 | be discarded and the seq_show function will attempt to allocate a larger | |
235 | buffer and retry printing. | |
236 | ||
ded4926a JC |
237 | |
238 | Making it all work | |
239 | ||
240 | So far, we have a nice set of functions which can produce output within the | |
241 | seq_file system, but we have not yet turned them into a file that a user | |
242 | can see. Creating a file within the kernel requires, of course, the | |
243 | creation of a set of file_operations which implement the operations on that | |
244 | file. The seq_file interface provides a set of canned operations which do | |
245 | most of the work. The virtual file author still must implement the open() | |
246 | method, however, to hook everything up. The open function is often a single | |
247 | line, as in the example module: | |
248 | ||
249 | static int ct_open(struct inode *inode, struct file *file) | |
250 | { | |
251 | return seq_open(file, &ct_seq_ops); | |
f3271f65 | 252 | } |
ded4926a JC |
253 | |
254 | Here, the call to seq_open() takes the seq_operations structure we created | |
255 | before, and gets set up to iterate through the virtual file. | |
256 | ||
257 | On a successful open, seq_open() stores the struct seq_file pointer in | |
258 | file->private_data. If you have an application where the same iterator can | |
259 | be used for more than one file, you can store an arbitrary pointer in the | |
260 | private field of the seq_file structure; that value can then be retrieved | |
261 | by the iterator functions. | |
262 | ||
77be4daf RJ |
263 | There is also a wrapper function to seq_open() called seq_open_private(). It |
264 | kmallocs a zero filled block of memory and stores a pointer to it in the | |
265 | private field of the seq_file structure, returning 0 on success. The | |
266 | block size is specified in a third parameter to the function, e.g.: | |
267 | ||
268 | static int ct_open(struct inode *inode, struct file *file) | |
269 | { | |
270 | return seq_open_private(file, &ct_seq_ops, | |
271 | sizeof(struct mystruct)); | |
272 | } | |
273 | ||
274 | There is also a variant function, __seq_open_private(), which is functionally | |
275 | identical except that, if successful, it returns the pointer to the allocated | |
276 | memory block, allowing further initialisation e.g.: | |
277 | ||
278 | static int ct_open(struct inode *inode, struct file *file) | |
279 | { | |
280 | struct mystruct *p = | |
281 | __seq_open_private(file, &ct_seq_ops, sizeof(*p)); | |
282 | ||
283 | if (!p) | |
284 | return -ENOMEM; | |
285 | ||
286 | p->foo = bar; /* initialize my stuff */ | |
287 | ... | |
288 | p->baz = true; | |
289 | ||
290 | return 0; | |
291 | } | |
292 | ||
293 | A corresponding close function, seq_release_private() is available which | |
294 | frees the memory allocated in the corresponding open. | |
295 | ||
ded4926a JC |
296 | The other operations of interest - read(), llseek(), and release() - are |
297 | all implemented by the seq_file code itself. So a virtual file's | |
298 | file_operations structure will look like: | |
299 | ||
f3271f65 | 300 | static const struct file_operations ct_file_ops = { |
ded4926a JC |
301 | .owner = THIS_MODULE, |
302 | .open = ct_open, | |
303 | .read = seq_read, | |
304 | .llseek = seq_lseek, | |
305 | .release = seq_release | |
306 | }; | |
307 | ||
308 | There is also a seq_release_private() which passes the contents of the | |
309 | seq_file private field to kfree() before releasing the structure. | |
310 | ||
311 | The final step is the creation of the /proc file itself. In the example | |
312 | code, that is done in the initialization code in the usual way: | |
313 | ||
314 | static int ct_init(void) | |
315 | { | |
316 | struct proc_dir_entry *entry; | |
317 | ||
6be4b789 | 318 | proc_create("sequence", 0, NULL, &ct_file_ops); |
ded4926a JC |
319 | return 0; |
320 | } | |
321 | ||
322 | module_init(ct_init); | |
323 | ||
324 | And that is pretty much it. | |
325 | ||
326 | ||
327 | seq_list | |
328 | ||
329 | If your file will be iterating through a linked list, you may find these | |
330 | routines useful: | |
331 | ||
332 | struct list_head *seq_list_start(struct list_head *head, | |
333 | loff_t pos); | |
334 | struct list_head *seq_list_start_head(struct list_head *head, | |
335 | loff_t pos); | |
336 | struct list_head *seq_list_next(void *v, struct list_head *head, | |
337 | loff_t *ppos); | |
338 | ||
339 | These helpers will interpret pos as a position within the list and iterate | |
340 | accordingly. Your start() and next() functions need only invoke the | |
b82d4043 | 341 | seq_list_* helpers with a pointer to the appropriate list_head structure. |
ded4926a JC |
342 | |
343 | ||
344 | The extra-simple version | |
345 | ||
346 | For extremely simple virtual files, there is an even easier interface. A | |
347 | module can define only the show() function, which should create all the | |
348 | output that the virtual file will contain. The file's open() method then | |
349 | calls: | |
350 | ||
351 | int single_open(struct file *file, | |
352 | int (*show)(struct seq_file *m, void *p), | |
353 | void *data); | |
354 | ||
355 | When output time comes, the show() function will be called once. The data | |
356 | value given to single_open() can be found in the private field of the | |
357 | seq_file structure. When using single_open(), the programmer should use | |
358 | single_release() instead of seq_release() in the file_operations structure | |
359 | to avoid a memory leak. |