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 of=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 57Deprecated create_proc_entry 58 59Note that the above article uses create_proc_entry which was removed in 60kernel 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); 66 67The iterator interface 68 69Modules implementing a virtual file with seq_file must implement a simple 70iterator object that allows stepping through the data of interest. 71Iterators must be able to move to a specific position - like the file they 72implement - but the interpretation of that position is up to the iterator 73itself. A seq_file implementation that is formatting firewall rules, for 74example, could interpret position N as the Nth rule in the chain. 75Positioning can thus be done in whatever way makes the most sense for the 76generator of the data, which need not be aware of how a position translates 77to an offset in the virtual file. The one obvious exception is that a 78position of zero should indicate the beginning of the file. 79 80The /proc/sequence iterator just uses the count of the next number it 81will output as its position. 82 83Four functions must be implemented to make the iterator work. The first, 84called start() takes a position as an argument and returns an iterator 85which will start reading at that position. For our simple sequence example, 86the start() function looks like: 87 88 static void *ct_seq_start(struct seq_file *s, loff_t *pos) 89 { 90 loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL); 91 if (! spos) 92 return NULL; 93 *spos = *pos; 94 return spos; 95 } 96 97The entire data structure for this iterator is a single loff_t value 98holding the current position. There is no upper bound for the sequence 99iterator, but that will not be the case for most other seq_file 100implementations; in most cases the start() function should check for a 101"past end of file" condition and return NULL if need be. 102 103For more complicated applications, the private field of the seq_file 104structure can be used. There is also a special value which can be returned 105by the start() function called SEQ_START_TOKEN; it can be used if you wish 106to instruct your show() function (described below) to print a header at the 107top of the output. SEQ_START_TOKEN should only be used if the offset is 108zero, however. 109 110The next function to implement is called, amazingly, next(); its job is to 111move the iterator forward to the next position in the sequence. The 112example module can simply increment the position by one; more useful 113modules will do what is needed to step through some data structure. The 114next() function returns a new iterator, or NULL if the sequence is 115complete. Here's the example version: 116 117 static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos) 118 { 119 loff_t *spos = v; 120 *pos = ++*spos; 121 return spos; 122 } 123 124The stop() function is called when iteration is complete; its job, of 125course, is to clean up. If dynamic memory is allocated for the iterator, 126stop() is the place to free it. 127 128 static void ct_seq_stop(struct seq_file *s, void *v) 129 { 130 kfree(v); 131 } 132 133Finally, the show() function should format the object currently pointed to 134by the iterator for output. The example module's show() function is: 135 136 static int ct_seq_show(struct seq_file *s, void *v) 137 { 138 loff_t *spos = v; 139 seq_printf(s, "%lld\n", (long long)*spos); 140 return 0; 141 } 142 143If all is well, the show() function should return zero. A negative error 144code in the usual manner indicates that something went wrong; it will be 145passed back to user space. This function can also return SEQ_SKIP, which 146causes the current item to be skipped; if the show() function has already 147generated output before returning SEQ_SKIP, that output will be dropped. 148 149We will look at seq_printf() in a moment. But first, the definition of the 150seq_file iterator is finished by creating a seq_operations structure with 151the four functions we have just defined: 152 153 static const struct seq_operations ct_seq_ops = { 154 .start = ct_seq_start, 155 .next = ct_seq_next, 156 .stop = ct_seq_stop, 157 .show = ct_seq_show 158 }; 159 160This structure will be needed to tie our iterator to the /proc file in 161a little bit. 162 163It's worth noting that the iterator value returned by start() and 164manipulated by the other functions is considered to be completely opaque by 165the seq_file code. It can thus be anything that is useful in stepping 166through the data to be output. Counters can be useful, but it could also be 167a direct pointer into an array or linked list. Anything goes, as long as 168the programmer is aware that things can happen between calls to the 169iterator function. However, the seq_file code (by design) will not sleep 170between the calls to start() and stop(), so holding a lock during that time 171is a reasonable thing to do. The seq_file code will also avoid taking any 172other locks while the iterator is active. 173 174 175Formatted output 176 177The seq_file code manages positioning within the output created by the 178iterator and getting it into the user's buffer. But, for that to work, that 179output must be passed to the seq_file code. Some utility functions have 180been defined which make this task easy. 181 182Most code will simply use seq_printf(), which works pretty much like 183printk(), but which requires the seq_file pointer as an argument. 184 185For straight character output, the following functions may be used: 186 187 seq_putc(struct seq_file *m, char c); 188 seq_puts(struct seq_file *m, const char *s); 189 seq_escape(struct seq_file *m, const char *s, const char *esc); 190 191The first two output a single character and a string, just like one would 192expect. seq_escape() is like seq_puts(), except that any character in s 193which is in the string esc will be represented in octal form in the output. 194 195There are also a pair of functions for printing filenames: 196 197 int seq_path(struct seq_file *m, const struct path *path, 198 const char *esc); 199 int seq_path_root(struct seq_file *m, const struct path *path, 200 const struct path *root, const char *esc) 201 202Here, path indicates the file of interest, and esc is a set of characters 203which should be escaped in the output. A call to seq_path() will output 204the path relative to the current process's filesystem root. If a different 205root is desired, it can be used with seq_path_root(). If it turns out that 206path cannot be reached from root, seq_path_root() returns SEQ_SKIP. 207 208A function producing complicated output may want to check 209 bool seq_has_overflowed(struct seq_file *m); 210and avoid further seq_<output> calls if true is returned. 211 212A true return from seq_has_overflowed means that the seq_file buffer will 213be discarded and the seq_show function will attempt to allocate a larger 214buffer and retry printing. 215 216 217Making it all work 218 219So far, we have a nice set of functions which can produce output within the 220seq_file system, but we have not yet turned them into a file that a user 221can see. Creating a file within the kernel requires, of course, the 222creation of a set of file_operations which implement the operations on that 223file. The seq_file interface provides a set of canned operations which do 224most of the work. The virtual file author still must implement the open() 225method, however, to hook everything up. The open function is often a single 226line, as in the example module: 227 228 static int ct_open(struct inode *inode, struct file *file) 229 { 230 return seq_open(file, &ct_seq_ops); 231 } 232 233Here, the call to seq_open() takes the seq_operations structure we created 234before, and gets set up to iterate through the virtual file. 235 236On a successful open, seq_open() stores the struct seq_file pointer in 237file->private_data. If you have an application where the same iterator can 238be used for more than one file, you can store an arbitrary pointer in the 239private field of the seq_file structure; that value can then be retrieved 240by the iterator functions. 241 242There is also a wrapper function to seq_open() called seq_open_private(). It 243kmallocs a zero filled block of memory and stores a pointer to it in the 244private field of the seq_file structure, returning 0 on success. The 245block size is specified in a third parameter to the function, e.g.: 246 247 static int ct_open(struct inode *inode, struct file *file) 248 { 249 return seq_open_private(file, &ct_seq_ops, 250 sizeof(struct mystruct)); 251 } 252 253There is also a variant function, __seq_open_private(), which is functionally 254identical except that, if successful, it returns the pointer to the allocated 255memory block, allowing further initialisation e.g.: 256 257 static int ct_open(struct inode *inode, struct file *file) 258 { 259 struct mystruct *p = 260 __seq_open_private(file, &ct_seq_ops, sizeof(*p)); 261 262 if (!p) 263 return -ENOMEM; 264 265 p->foo = bar; /* initialize my stuff */ 266 ... 267 p->baz = true; 268 269 return 0; 270 } 271 272A corresponding close function, seq_release_private() is available which 273frees the memory allocated in the corresponding open. 274 275The other operations of interest - read(), llseek(), and release() - are 276all implemented by the seq_file code itself. So a virtual file's 277file_operations structure will look like: 278 279 static const struct file_operations ct_file_ops = { 280 .owner = THIS_MODULE, 281 .open = ct_open, 282 .read = seq_read, 283 .llseek = seq_lseek, 284 .release = seq_release 285 }; 286 287There is also a seq_release_private() which passes the contents of the 288seq_file private field to kfree() before releasing the structure. 289 290The final step is the creation of the /proc file itself. In the example 291code, that is done in the initialization code in the usual way: 292 293 static int ct_init(void) 294 { 295 struct proc_dir_entry *entry; 296 297 proc_create("sequence", 0, NULL, &ct_file_ops); 298 return 0; 299 } 300 301 module_init(ct_init); 302 303And that is pretty much it. 304 305 306seq_list 307 308If your file will be iterating through a linked list, you may find these 309routines useful: 310 311 struct list_head *seq_list_start(struct list_head *head, 312 loff_t pos); 313 struct list_head *seq_list_start_head(struct list_head *head, 314 loff_t pos); 315 struct list_head *seq_list_next(void *v, struct list_head *head, 316 loff_t *ppos); 317 318These helpers will interpret pos as a position within the list and iterate 319accordingly. Your start() and next() functions need only invoke the 320seq_list_* helpers with a pointer to the appropriate list_head structure. 321 322 323The extra-simple version 324 325For extremely simple virtual files, there is an even easier interface. A 326module can define only the show() function, which should create all the 327output that the virtual file will contain. The file's open() method then 328calls: 329 330 int single_open(struct file *file, 331 int (*show)(struct seq_file *m, void *p), 332 void *data); 333 334When output time comes, the show() function will be called once. The data 335value given to single_open() can be found in the private field of the 336seq_file structure. When using single_open(), the programmer should use 337single_release() instead of seq_release() in the file_operations structure 338to avoid a memory leak. 339