linux/Documentation/filesystems/seq_file.txt
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   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
  57
  58The iterator interface
  59
  60Modules implementing a virtual file with seq_file must implement a simple
  61iterator object that allows stepping through the data of interest.
  62Iterators must be able to move to a specific position - like the file they
  63implement - but the interpretation of that position is up to the iterator
  64itself. A seq_file implementation that is formatting firewall rules, for
  65example, could interpret position N as the Nth rule in the chain.
  66Positioning can thus be done in whatever way makes the most sense for the
  67generator of the data, which need not be aware of how a position translates
  68to an offset in the virtual file. The one obvious exception is that a
  69position of zero should indicate the beginning of the file.
  70
  71The /proc/sequence iterator just uses the count of the next number it
  72will output as its position.
  73
  74Four functions must be implemented to make the iterator work. The first,
  75called start() takes a position as an argument and returns an iterator
  76which will start reading at that position. For our simple sequence example,
  77the 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
  88The entire data structure for this iterator is a single loff_t value
  89holding the current position. There is no upper bound for the sequence
  90iterator, but that will not be the case for most other seq_file
  91implementations; in most cases the start() function should check for a
  92"past end of file" condition and return NULL if need be.
  93
  94For more complicated applications, the private field of the seq_file
  95structure can be used. There is also a special value which can be returned
  96by the start() function called SEQ_START_TOKEN; it can be used if you wish
  97to instruct your show() function (described below) to print a header at the
  98top of the output. SEQ_START_TOKEN should only be used if the offset is
  99zero, however.
 100
 101The next function to implement is called, amazingly, next(); its job is to
 102move the iterator forward to the next position in the sequence.  The
 103example module can simply increment the position by one; more useful
 104modules will do what is needed to step through some data structure. The
 105next() function returns a new iterator, or NULL if the sequence is
 106complete. 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
 115The stop() function is called when iteration is complete; its job, of
 116course, is to clean up. If dynamic memory is allocated for the iterator,
 117stop() 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
 124Finally, the show() function should format the object currently pointed to
 125by the iterator for output.  The example module's show() function is:
 126
 127        static int ct_seq_show(struct seq_file *s, void *v)
 128        {
 129                loff_t *spos = v;
 130                seq_printf(s, "%lld\n", (long long)*spos);
 131                return 0;
 132        }
 133
 134If all is well, the show() function should return zero.  A negative error
 135code in the usual manner indicates that something went wrong; it will be
 136passed back to user space.  This function can also return SEQ_SKIP, which
 137causes the current item to be skipped; if the show() function has already
 138generated output before returning SEQ_SKIP, that output will be dropped.
 139
 140We will look at seq_printf() in a moment. But first, the definition of the
 141seq_file iterator is finished by creating a seq_operations structure with
 142the four functions we have just defined:
 143
 144        static const struct seq_operations ct_seq_ops = {
 145                .start = ct_seq_start,
 146                .next  = ct_seq_next,
 147                .stop  = ct_seq_stop,
 148                .show  = ct_seq_show
 149        };
 150
 151This structure will be needed to tie our iterator to the /proc file in
 152a little bit.
 153
 154It's worth noting that the iterator value returned by start() and
 155manipulated by the other functions is considered to be completely opaque by
 156the seq_file code. It can thus be anything that is useful in stepping
 157through the data to be output. Counters can be useful, but it could also be
 158a direct pointer into an array or linked list. Anything goes, as long as
 159the programmer is aware that things can happen between calls to the
 160iterator function. However, the seq_file code (by design) will not sleep
 161between the calls to start() and stop(), so holding a lock during that time
 162is a reasonable thing to do. The seq_file code will also avoid taking any
 163other locks while the iterator is active.
 164
 165
 166Formatted output
 167
 168The seq_file code manages positioning within the output created by the
 169iterator and getting it into the user's buffer. But, for that to work, that
 170output must be passed to the seq_file code. Some utility functions have
 171been defined which make this task easy.
 172
 173Most code will simply use seq_printf(), which works pretty much like
 174printk(), but which requires the seq_file pointer as an argument. It is
 175common to ignore the return value from seq_printf(), but a function
 176producing complicated output may want to check that value and quit if
 177something non-zero is returned; an error return means that the seq_file
 178buffer has been filled and further output will be discarded.
 179
 180For straight character output, the following functions may be used:
 181
 182        int seq_putc(struct seq_file *m, char c);
 183        int seq_puts(struct seq_file *m, const char *s);
 184        int seq_escape(struct seq_file *m, const char *s, const char *esc);
 185
 186The first two output a single character and a string, just like one would
 187expect. seq_escape() is like seq_puts(), except that any character in s
 188which is in the string esc will be represented in octal form in the output.
 189
 190There is also a pair of functions for printing filenames:
 191
 192        int seq_path(struct seq_file *m, struct path *path, char *esc);
 193        int seq_path_root(struct seq_file *m, struct path *path,
 194                          struct path *root, char *esc)
 195
 196Here, path indicates the file of interest, and esc is a set of characters
 197which should be escaped in the output.  A call to seq_path() will output
 198the path relative to the current process's filesystem root.  If a different
 199root is desired, it can be used with seq_path_root().  Note that, if it
 200turns out that path cannot be reached from root, the value of root will be
 201changed in seq_file_root() to a root which *does* work.
 202
 203
 204Making it all work
 205
 206So far, we have a nice set of functions which can produce output within the
 207seq_file system, but we have not yet turned them into a file that a user
 208can see. Creating a file within the kernel requires, of course, the
 209creation of a set of file_operations which implement the operations on that
 210file. The seq_file interface provides a set of canned operations which do
 211most of the work. The virtual file author still must implement the open()
 212method, however, to hook everything up. The open function is often a single
 213line, as in the example module:
 214
 215        static int ct_open(struct inode *inode, struct file *file)
 216        {
 217                return seq_open(file, &ct_seq_ops);
 218        }
 219
 220Here, the call to seq_open() takes the seq_operations structure we created
 221before, and gets set up to iterate through the virtual file.
 222
 223On a successful open, seq_open() stores the struct seq_file pointer in
 224file->private_data. If you have an application where the same iterator can
 225be used for more than one file, you can store an arbitrary pointer in the
 226private field of the seq_file structure; that value can then be retrieved
 227by the iterator functions.
 228
 229The other operations of interest - read(), llseek(), and release() - are
 230all implemented by the seq_file code itself. So a virtual file's
 231file_operations structure will look like:
 232
 233        static const struct file_operations ct_file_ops = {
 234                .owner   = THIS_MODULE,
 235                .open    = ct_open,
 236                .read    = seq_read,
 237                .llseek  = seq_lseek,
 238                .release = seq_release
 239        };
 240
 241There is also a seq_release_private() which passes the contents of the
 242seq_file private field to kfree() before releasing the structure.
 243
 244The final step is the creation of the /proc file itself. In the example
 245code, that is done in the initialization code in the usual way:
 246
 247        static int ct_init(void)
 248        {
 249                struct proc_dir_entry *entry;
 250
 251                proc_create("sequence", 0, NULL, &ct_file_ops);
 252                return 0;
 253        }
 254
 255        module_init(ct_init);
 256
 257And that is pretty much it.
 258
 259
 260seq_list
 261
 262If your file will be iterating through a linked list, you may find these
 263routines useful:
 264
 265        struct list_head *seq_list_start(struct list_head *head,
 266                                         loff_t pos);
 267        struct list_head *seq_list_start_head(struct list_head *head,
 268                                              loff_t pos);
 269        struct list_head *seq_list_next(void *v, struct list_head *head,
 270                                        loff_t *ppos);
 271
 272These helpers will interpret pos as a position within the list and iterate
 273accordingly.  Your start() and next() functions need only invoke the
 274seq_list_* helpers with a pointer to the appropriate list_head structure.
 275
 276
 277The extra-simple version
 278
 279For extremely simple virtual files, there is an even easier interface.  A
 280module can define only the show() function, which should create all the
 281output that the virtual file will contain. The file's open() method then
 282calls:
 283
 284        int single_open(struct file *file,
 285                        int (*show)(struct seq_file *m, void *p),
 286                        void *data);
 287
 288When output time comes, the show() function will be called once. The data
 289value given to single_open() can be found in the private field of the
 290seq_file structure. When using single_open(), the programmer should use
 291single_release() instead of seq_release() in the file_operations structure
 292to avoid a memory leak.
 293
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