1.. SPDX-License-Identifier: GPL-2.0 2 3====================== 4The seq_file Interface 5====================== 6 7 Copyright 2003 Jonathan Corbet <corbet@lwn.net> 8 9 This file is originally from the LWN.net Driver Porting series at 10 https://lwn.net/Articles/driver-porting/ 11 12 13There are numerous ways for a device driver (or other kernel component) to 14provide information to the user or system administrator. One useful 15technique is the creation of virtual files, in debugfs, /proc or elsewhere. 16Virtual files can provide human-readable output that is easy to get at 17without any special utility programs; they can also make life easier for 18script writers. It is not surprising that the use of virtual files has 19grown over the years. 20 21Creating those files correctly has always been a bit of a challenge, 22however. It is not that hard to make a virtual file which returns a 23string. But life gets trickier if the output is long - anything greater 24than an application is likely to read in a single operation. Handling 25multiple reads (and seeks) requires careful attention to the reader's 26position within the virtual file - that position is, likely as not, in the 27middle of a line of output. The kernel has traditionally had a number of 28implementations that got this wrong. 29 30The 2.6 kernel contains a set of functions (implemented by Alexander Viro) 31which are designed to make it easy for virtual file creators to get it 32right. 33 34The seq_file interface is available via <linux/seq_file.h>. There are 35three aspects to seq_file: 36 37 * An iterator interface which lets a virtual file implementation 38 step through the objects it is presenting. 39 40 * Some utility functions for formatting objects for output without 41 needing to worry about things like output buffers. 42 43 * A set of canned file_operations which implement most operations on 44 the virtual file. 45 46We'll look at the seq_file interface via an extremely simple example: a 47loadable module which creates a file called /proc/sequence. The file, when 48read, simply produces a set of increasing integer values, one per line. The 49sequence will continue until the user loses patience and finds something 50better to do. The file is seekable, in that one can do something like the 51following:: 52 53 dd if=/proc/sequence of=out1 count=1 54 dd if=/proc/sequence skip=1 of=out2 count=1 55 56Then concatenate the output files out1 and out2 and get the right 57result. Yes, it is a thoroughly useless module, but the point is to show 58how the mechanism works without getting lost in other details. (Those 59wanting to see the full source for this module can find it at 60https://lwn.net/Articles/22359/). 61 62Deprecated create_proc_entry 63============================ 64 65Note that the above article uses create_proc_entry which was removed in 66kernel 3.10. Current versions require the following update:: 67 68 - entry = create_proc_entry("sequence", 0, NULL); 69 - if (entry) 70 - entry->proc_fops = &ct_file_ops; 71 + entry = proc_create("sequence", 0, NULL, &ct_file_ops); 72 73The iterator interface 74====================== 75 76Modules implementing a virtual file with seq_file must implement an 77iterator object that allows stepping through the data of interest 78during a "session" (roughly one read() system call). If the iterator 79is able to move to a specific position - like the file they implement, 80though with freedom to map the position number to a sequence location 81in whatever way is convenient - the iterator need only exist 82transiently during a session. If the iterator cannot easily find a 83numerical position but works well with a first/next interface, the 84iterator can be stored in the private data area and continue from one 85session to the next. 86 87A seq_file implementation that is formatting firewall rules from a 88table, for example, could provide a simple iterator that interprets 89position N as the Nth rule in the chain. A seq_file implementation 90that presents the content of a, potentially volatile, linked list 91might record a pointer into that list, providing that can be done 92without risk of the current location being removed. 93 94Positioning can thus be done in whatever way makes the most sense for 95the generator of the data, which need not be aware of how a position 96translates to an offset in the virtual file. The one obvious exception 97is that a position of zero should indicate the beginning of the file. 98 99The /proc/sequence iterator just uses the count of the next number it 100will output as its position. 101 102Four functions must be implemented to make the iterator work. The 103first, called start(), starts a session and takes a position as an 104argument, returning an iterator which will start reading at that 105position. The pos passed to start() will always be either zero, or 106the most recent pos used in the previous session. 107 108For our simple sequence example, 109the start() function looks like:: 110 111 static void *ct_seq_start(struct seq_file *s, loff_t *pos) 112 { 113 loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL); 114 if (! spos) 115 return NULL; 116 *spos = *pos; 117 return spos; 118 } 119 120The entire data structure for this iterator is a single loff_t value 121holding the current position. There is no upper bound for the sequence 122iterator, but that will not be the case for most other seq_file 123implementations; in most cases the start() function should check for a 124"past end of file" condition and return NULL if need be. 125 126For more complicated applications, the private field of the seq_file 127structure can be used to hold state from session to session. There is 128also a special value which can be returned by the start() function 129called SEQ_START_TOKEN; it can be used if you wish to instruct your 130show() function (described below) to print a header at the top of the 131output. SEQ_START_TOKEN should only be used if the offset is zero, 132however. SEQ_START_TOKEN has no special meaning to the core seq_file 133code. It is provided as a convenience for a start() funciton to 134communicate with the next() and show() functions. 135 136The next function to implement is called, amazingly, next(); its job is to 137move the iterator forward to the next position in the sequence. The 138example module can simply increment the position by one; more useful 139modules will do what is needed to step through some data structure. The 140next() function returns a new iterator, or NULL if the sequence is 141complete. Here's the example version:: 142 143 static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos) 144 { 145 loff_t *spos = v; 146 *pos = ++*spos; 147 return spos; 148 } 149 150The next() function should set ``*pos`` to a value that start() can use 151to find the new location in the sequence. When the iterator is being 152stored in the private data area, rather than being reinitialized on each 153start(), it might seem sufficient to simply set ``*pos`` to any non-zero 154value (zero always tells start() to restart the sequence). This is not 155sufficient due to historical problems. 156 157Historically, many next() functions have *not* updated ``*pos`` at 158end-of-file. If the value is then used by start() to initialise the 159iterator, this can result in corner cases where the last entry in the 160sequence is reported twice in the file. In order to discourage this bug 161from being resurrected, the core seq_file code now produces a warning if 162a next() function does not change the value of ``*pos``. Consequently a 163next() function *must* change the value of ``*pos``, and of course must 164set it to a non-zero value. 165 166The stop() function closes a session; its job, of course, is to clean 167up. If dynamic memory is allocated for the iterator, stop() is the 168place to free it; if a lock was taken by start(), stop() must release 169that lock. The value that ``*pos`` was set to by the last next() call 170before stop() is remembered, and used for the first start() call of 171the next session unless lseek() has been called on the file; in that 172case next start() will be asked to start at position zero:: 173 174 static void ct_seq_stop(struct seq_file *s, void *v) 175 { 176 kfree(v); 177 } 178 179Finally, the show() function should format the object currently pointed to 180by the iterator for output. The example module's show() function is:: 181 182 static int ct_seq_show(struct seq_file *s, void *v) 183 { 184 loff_t *spos = v; 185 seq_printf(s, "%lld\n", (long long)*spos); 186 return 0; 187 } 188 189If all is well, the show() function should return zero. A negative error 190code in the usual manner indicates that something went wrong; it will be 191passed back to user space. This function can also return SEQ_SKIP, which 192causes the current item to be skipped; if the show() function has already 193generated output before returning SEQ_SKIP, that output will be dropped. 194 195We will look at seq_printf() in a moment. But first, the definition of the 196seq_file iterator is finished by creating a seq_operations structure with 197the four functions we have just defined:: 198 199 static const struct seq_operations ct_seq_ops = { 200 .start = ct_seq_start, 201 .next = ct_seq_next, 202 .stop = ct_seq_stop, 203 .show = ct_seq_show 204 }; 205 206This structure will be needed to tie our iterator to the /proc file in 207a little bit. 208 209It's worth noting that the iterator value returned by start() and 210manipulated by the other functions is considered to be completely opaque by 211the seq_file code. It can thus be anything that is useful in stepping 212through the data to be output. Counters can be useful, but it could also be 213a direct pointer into an array or linked list. Anything goes, as long as 214the programmer is aware that things can happen between calls to the 215iterator function. However, the seq_file code (by design) will not sleep 216between the calls to start() and stop(), so holding a lock during that time 217is a reasonable thing to do. The seq_file code will also avoid taking any 218other locks while the iterator is active. 219 220The iterater value returned by start() or next() is guaranteed to be 221passed to a subsequent next() or stop() call. This allows resources 222such as locks that were taken to be reliably released. There is *no* 223guarantee that the iterator will be passed to show(), though in practice 224it often will be. 225 226 227Formatted output 228================ 229 230The seq_file code manages positioning within the output created by the 231iterator and getting it into the user's buffer. But, for that to work, that 232output must be passed to the seq_file code. Some utility functions have 233been defined which make this task easy. 234 235Most code will simply use seq_printf(), which works pretty much like 236printk(), but which requires the seq_file pointer as an argument. 237 238For straight character output, the following functions may be used:: 239 240 seq_putc(struct seq_file *m, char c); 241 seq_puts(struct seq_file *m, const char *s); 242 seq_escape(struct seq_file *m, const char *s, const char *esc); 243 244The first two output a single character and a string, just like one would 245expect. seq_escape() is like seq_puts(), except that any character in s 246which is in the string esc will be represented in octal form in the output. 247 248There are also a pair of functions for printing filenames:: 249 250 int seq_path(struct seq_file *m, const struct path *path, 251 const char *esc); 252 int seq_path_root(struct seq_file *m, const struct path *path, 253 const struct path *root, const char *esc) 254 255Here, path indicates the file of interest, and esc is a set of characters 256which should be escaped in the output. A call to seq_path() will output 257the path relative to the current process's filesystem root. If a different 258root is desired, it can be used with seq_path_root(). If it turns out that 259path cannot be reached from root, seq_path_root() returns SEQ_SKIP. 260 261A function producing complicated output may want to check:: 262 263 bool seq_has_overflowed(struct seq_file *m); 264 265and avoid further seq_<output> calls if true is returned. 266 267A true return from seq_has_overflowed means that the seq_file buffer will 268be discarded and the seq_show function will attempt to allocate a larger 269buffer and retry printing. 270 271 272Making it all work 273================== 274 275So far, we have a nice set of functions which can produce output within the 276seq_file system, but we have not yet turned them into a file that a user 277can see. Creating a file within the kernel requires, of course, the 278creation of a set of file_operations which implement the operations on that 279file. The seq_file interface provides a set of canned operations which do 280most of the work. The virtual file author still must implement the open() 281method, however, to hook everything up. The open function is often a single 282line, as in the example module:: 283 284 static int ct_open(struct inode *inode, struct file *file) 285 { 286 return seq_open(file, &ct_seq_ops); 287 } 288 289Here, the call to seq_open() takes the seq_operations structure we created 290before, and gets set up to iterate through the virtual file. 291 292On a successful open, seq_open() stores the struct seq_file pointer in 293file->private_data. If you have an application where the same iterator can 294be used for more than one file, you can store an arbitrary pointer in the 295private field of the seq_file structure; that value can then be retrieved 296by the iterator functions. 297 298There is also a wrapper function to seq_open() called seq_open_private(). It 299kmallocs a zero filled block of memory and stores a pointer to it in the 300private field of the seq_file structure, returning 0 on success. The 301block size is specified in a third parameter to the function, e.g.:: 302 303 static int ct_open(struct inode *inode, struct file *file) 304 { 305 return seq_open_private(file, &ct_seq_ops, 306 sizeof(struct mystruct)); 307 } 308 309There is also a variant function, __seq_open_private(), which is functionally 310identical except that, if successful, it returns the pointer to the allocated 311memory block, allowing further initialisation e.g.:: 312 313 static int ct_open(struct inode *inode, struct file *file) 314 { 315 struct mystruct *p = 316 __seq_open_private(file, &ct_seq_ops, sizeof(*p)); 317 318 if (!p) 319 return -ENOMEM; 320 321 p->foo = bar; /* initialize my stuff */ 322 ... 323 p->baz = true; 324 325 return 0; 326 } 327 328A corresponding close function, seq_release_private() is available which 329frees the memory allocated in the corresponding open. 330 331The other operations of interest - read(), llseek(), and release() - are 332all implemented by the seq_file code itself. So a virtual file's 333file_operations structure will look like:: 334 335 static const struct file_operations ct_file_ops = { 336 .owner = THIS_MODULE, 337 .open = ct_open, 338 .read = seq_read, 339 .llseek = seq_lseek, 340 .release = seq_release 341 }; 342 343There is also a seq_release_private() which passes the contents of the 344seq_file private field to kfree() before releasing the structure. 345 346The final step is the creation of the /proc file itself. In the example 347code, that is done in the initialization code in the usual way:: 348 349 static int ct_init(void) 350 { 351 struct proc_dir_entry *entry; 352 353 proc_create("sequence", 0, NULL, &ct_file_ops); 354 return 0; 355 } 356 357 module_init(ct_init); 358 359And that is pretty much it. 360 361 362seq_list 363======== 364 365If your file will be iterating through a linked list, you may find these 366routines useful:: 367 368 struct list_head *seq_list_start(struct list_head *head, 369 loff_t pos); 370 struct list_head *seq_list_start_head(struct list_head *head, 371 loff_t pos); 372 struct list_head *seq_list_next(void *v, struct list_head *head, 373 loff_t *ppos); 374 375These helpers will interpret pos as a position within the list and iterate 376accordingly. Your start() and next() functions need only invoke the 377``seq_list_*`` helpers with a pointer to the appropriate list_head structure. 378 379 380The extra-simple version 381======================== 382 383For extremely simple virtual files, there is an even easier interface. A 384module can define only the show() function, which should create all the 385output that the virtual file will contain. The file's open() method then 386calls:: 387 388 int single_open(struct file *file, 389 int (*show)(struct seq_file *m, void *p), 390 void *data); 391 392When output time comes, the show() function will be called once. The data 393value given to single_open() can be found in the private field of the 394seq_file structure. When using single_open(), the programmer should use 395single_release() instead of seq_release() in the file_operations structure 396to avoid a memory leak. 397