Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 The seq_file interface 2 3 Copyright 2003 Jonathan Corbet <corbet[AT]lwn[DOT]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 49 dd if=/proc/sequence skip=1 out=out2 count=1 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 57 58 The iterator interface 59 60 Modules implementing a virtual file with seq_file must implement a simple 61 iterator object that allows stepping through the data of interest. 62 Iterators must be able to move to a specific position - like the file they 63 implement - but the interpretation of that position is up to the iterator 64 itself. A seq_file implementation that is formatting firewall rules, for 65 example, could interpret position N as the Nth rule in the chain. 66 Positioning can thus be done in whatever way makes the most sense for the 67 generator of the data, which need not be aware of how a position translates 68 to an offset in the virtual file. The one obvious exception is that a 69 position of zero should indicate the beginning of the file. 70 71 The /proc/sequence iterator just uses the count of the next number it 72 will output as its position. 73 74 Four functions must be implemented to make the iterator work. The first, 75 called start() takes a position as an argument and returns an iterator 76 which will start reading at that position. For our simple sequence example, 77 the 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 88 The entire data structure for this iterator is a single loff_t value 89 holding the current position. There is no upper bound for the sequence 90 iterator, but that will not be the case for most other seq_file 91 implementations; in most cases the start() function should check for a 92 "past end of file" condition and return NULL if need be. 93 94 For more complicated applications, the private field of the seq_file 95 structure can be used. There is also a special value which can be returned 96 by the start() function called SEQ_START_TOKEN; it can be used if you wish 97 to instruct your show() function (described below) to print a header at the 98 top of the output. SEQ_START_TOKEN should only be used if the offset is 99 zero, however. 100 101 The next function to implement is called, amazingly, next(); its job is to 102 move the iterator forward to the next position in the sequence. The 103 example module can simply increment the position by one; more useful 104 modules will do what is needed to step through some data structure. The 105 next() function returns a new iterator, or NULL if the sequence is 106 complete. 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 = v; 111 *pos = ++*spos; 112 return spos; 113 } 114 115 The stop() function is called when iteration is complete; its job, of 116 course, is to clean up. If dynamic memory is allocated for the iterator, 117 stop() is the place to free it. 118 119 static void ct_seq_stop(struct seq_file *s, void *v) 120 { 121 kfree(v); 122 } 123 124 Finally, the show() function should format the object currently pointed to 125 by the iterator for output. It should return zero, or an error code if 126 something 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 = v; 131 seq_printf(s, "%lld\n", (long long)*spos); 132 return 0; 133 } 134 135 We will look at seq_printf() in a moment. But first, the definition of the 136 seq_file iterator is finished by creating a seq_operations structure with 137 the four functions we have just defined: 138 139 static const 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 }; 145 146 This structure will be needed to tie our iterator to the /proc file in 147 a little bit. 148 149 It's worth noting that the iterator value returned by start() and 150 manipulated by the other functions is considered to be completely opaque by 151 the seq_file code. It can thus be anything that is useful in stepping 152 through the data to be output. Counters can be useful, but it could also be 153 a direct pointer into an array or linked list. Anything goes, as long as 154 the programmer is aware that things can happen between calls to the 155 iterator function. However, the seq_file code (by design) will not sleep 156 between the calls to start() and stop(), so holding a lock during that time 157 is a reasonable thing to do. The seq_file code will also avoid taking any 158 other locks while the iterator is active. 159 160 161 Formatted output 162 163 The seq_file code manages positioning within the output created by the 164 iterator and getting it into the user's buffer. But, for that to work, that 165 output must be passed to the seq_file code. Some utility functions have 166 been defined which make this task easy. 167 168 Most code will simply use seq_printf(), which works pretty much like 169 printk(), but which requires the seq_file pointer as an argument. It is 170 common to ignore the return value from seq_printf(), but a function 171 producing complicated output may want to check that value and quit if 172 something non-zero is returned; an error return means that the seq_file 173 buffer has been filled and further output will be discarded. 174 175 For 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); 180 181 The first two output a single character and a string, just like one would 182 expect. seq_escape() is like seq_puts(), except that any character in s 183 which is in the string esc will be represented in octal form in the output. 184 185 There is also a function for printing filenames: 186 187 int seq_path(struct seq_file *m, struct path *path, char *esc); 188 189 Here, path indicates the file of interest, and esc is a set of characters 190 which should be escaped in the output. 191 192 193 Making it all work 194 195 So far, we have a nice set of functions which can produce output within the 196 seq_file system, but we have not yet turned them into a file that a user 197 can see. Creating a file within the kernel requires, of course, the 198 creation of a set of file_operations which implement the operations on that 199 file. The seq_file interface provides a set of canned operations which do 200 most of the work. The virtual file author still must implement the open() 201 method, however, to hook everything up. The open function is often a single 202 line, 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 } 208 209 Here, the call to seq_open() takes the seq_operations structure we created 210 before, and gets set up to iterate through the virtual file. 211 212 On a successful open, seq_open() stores the struct seq_file pointer in 213 file->private_data. If you have an application where the same iterator can 214 be used for more than one file, you can store an arbitrary pointer in the 215 private field of the seq_file structure; that value can then be retrieved 216 by the iterator functions. 217 218 The other operations of interest - read(), llseek(), and release() - are 219 all implemented by the seq_file code itself. So a virtual file's 220 file_operations structure will look like: 221 222 static const 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 }; 229 230 There is also a seq_release_private() which passes the contents of the 231 seq_file private field to kfree() before releasing the structure. 232 233 The final step is the creation of the /proc file itself. In the example 234 code, 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); 247 248 And that is pretty much it. 249 250 251 seq_list 252 253 If your file will be iterating through a linked list, you may find these 254 routines 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); 262 263 These helpers will interpret pos as a position within the list and iterate 264 accordingly. Your start() and next() functions need only invoke the 265 seq_list_* helpers with a pointer to the appropriate list_head structure. 266 267 268 The extra-simple version 269 270 For extremely simple virtual files, there is an even easier interface. A 271 module can define only the show() function, which should create all the 272 output that the virtual file will contain. The file's open() method then 273 calls: 274 275 int single_open(struct file *file, 276 int (*show)(struct seq_file *m, void *p), 277 void *data); 278 279 When output time comes, the show() function will be called once. The data 280 value given to single_open() can be found in the private field of the 281 seq_file structure. When using single_open(), the programmer should use 282 single_release() instead of seq_release() in the file_operations structure 283 to avoid a memory leak.