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 ======================
 The seq_file Interface
 ======================
 
         Copyright 2003 Jonathan Corbet <corbet@lwn.net>
 
         This file is originally from the LWN.net Driver Porting series at
         https://lwn.net/Articles/driver-porting/
 
 
 There are numerous ways for a device driver (or other kernel component) to
 provide information to the user or system administrator.  One useful
 technique is the creation of virtual files, in debugfs, /proc or elsewhere.
 Virtual files can provide human-readable output that is easy to get at
 without any special utility programs; they can also make life easier for
 script writers. It is not surprising that the use of virtual files has
 grown over the years.
 
 Creating those files correctly has always been a bit of a challenge,
 however. It is not that hard to make a virtual file which returns a
 string. But life gets trickier if the output is long - anything greater
 than an application is likely to read in a single operation.  Handling
 multiple reads (and seeks) requires careful attention to the reader's
 position within the virtual file - that position is, likely as not, in the
 middle of a line of output. The kernel has traditionally had a number of
 implementations that got this wrong.
 
 The 2.6 kernel contains a set of functions (implemented by Alexander Viro)
 which are designed to make it easy for virtual file creators to get it
 right.
 
 The seq_file interface is available via <linux/seq_file.h>. There are
 three aspects to seq_file:
 
      * An iterator interface which lets a virtual file implementation
        step through the objects it is presenting.
 
      * Some utility functions for formatting objects for output without
        needing to worry about things like output buffers.
 
      * A set of canned file_operations which implement most operations on
        the virtual file.
 
 We'll look at the seq_file interface via an extremely simple example: a
 loadable module which creates a file called /proc/sequence. The file, when
 read, simply produces a set of increasing integer values, one per line. The
 sequence will continue until the user loses patience and finds something
 better to do. The file is seekable, in that one can do something like the
 following::
 
     dd if=/proc/sequence of=out1 count=1
     dd if=/proc/sequence skip=1 of=out2 count=1
 
 Then concatenate the output files out1 and out2 and get the right
 result. Yes, it is a thoroughly useless module, but the point is to show
 how the mechanism works without getting lost in other details.  (Those
 wanting to see the full source for this module can find it at
 https://lwn.net/Articles/22359/).
 
 Deprecated create_proc_entry
 ============================
 
 Note that the above article uses create_proc_entry which was removed in
 kernel 3.10. Current versions require the following update::
 
     -   entry = create_proc_entry("sequence", 0, NULL);
     -   if (entry)
     -           entry->proc_fops = &ct_file_ops;
     +   entry = proc_create("sequence", 0, NULL, &ct_file_ops);
 
 The iterator interface
 ======================
 
 Modules implementing a virtual file with seq_file must implement an
 iterator object that allows stepping through the data of interest
 during a "session" (roughly one read() system call).  If the iterator
 is able to move to a specific position - like the file they implement,
 though with freedom to map the position number to a sequence location
 in whatever way is convenient - the iterator need only exist
 transiently during a session.  If the iterator cannot easily find a
 numerical position but works well with a first/next interface, the
 iterator can be stored in the private data area and continue from one
 session to the next.
 
 A seq_file implementation that is formatting firewall rules from a
 table, for example, could provide a simple iterator that interprets
 position N as the Nth rule in the chain.  A seq_file implementation
 that presents the content of a, potentially volatile, linked list
 might record a pointer into that list, providing that can be done
 without risk of the current location being removed.
 
 Positioning can thus be done in whatever way makes the most sense for
 the generator of the data, which need not be aware of how a position
 translates to an offset in the virtual file. The one obvious exception
 is that a position of zero should indicate the beginning of the file.
 
 The /proc/sequence iterator just uses the count of the next number it
 will output as its position.
 
 Four functions must be implemented to make the iterator work. The
 first, called start(), starts a session and takes a position as an
 argument, returning an iterator which will start reading at that
 position.  The pos passed to start() will always be either zero, or
 the most recent pos used in the previous session.
 
 For our simple sequence example,
 the start() function looks like::
 
         static void *ct_seq_start(struct seq_file *s, loff_t *pos)
         {
                 loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
                 if (! spos)
                         return NULL;
                 *spos = *pos;
                 return spos;
         }
 
 The entire data structure for this iterator is a single loff_t value
 holding the current position. There is no upper bound for the sequence
 iterator, but that will not be the case for most other seq_file
 implementations; in most cases the start() function should check for a
 "past end of file" condition and return NULL if need be.
 
 For more complicated applications, the private field of the seq_file
 structure can be used to hold state from session to session.  There is
 also a special value which can be returned by the start() function
 called SEQ_START_TOKEN; it can be used if you wish to instruct your
 show() function (described below) to print a header at the top of the
 output. SEQ_START_TOKEN should only be used if the offset is zero,
 however.  SEQ_START_TOKEN has no special meaning to the core seq_file
 code.  It is provided as a convenience for a start() funciton to
 communicate with the next() and show() functions.
 
 The next function to implement is called, amazingly, next(); its job is to
 move the iterator forward to the next position in the sequence.  The
 example module can simply increment the position by one; more useful
 modules will do what is needed to step through some data structure. The
 next() function returns a new iterator, or NULL if the sequence is
 complete. Here's the example version::
 
         static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
         {
                 loff_t *spos = v;
                 *pos = ++*spos;
                 return spos;
         }
 
 The next() function should set ``*pos`` to a value that start() can use
 to find the new location in the sequence.  When the iterator is being
 stored in the private data area, rather than being reinitialized on each
 start(), it might seem sufficient to simply set ``*pos`` to any non-zero
 value (zero always tells start() to restart the sequence).  This is not
 sufficient due to historical problems.
 
 Historically, many next() functions have *not* updated ``*pos`` at
 end-of-file.  If the value is then used by start() to initialise the
 iterator, this can result in corner cases where the last entry in the
 sequence is reported twice in the file.  In order to discourage this bug
 from being resurrected, the core seq_file code now produces a warning if
 a next() function does not change the value of ``*pos``.  Consequently a
 next() function *must* change the value of ``*pos``, and of course must
 set it to a non-zero value.
 
 The stop() function closes a session; its job, of course, is to clean
 up. If dynamic memory is allocated for the iterator, stop() is the
 place to free it; if a lock was taken by start(), stop() must release
 that lock.  The value that ``*pos`` was set to by the last next() call
 before stop() is remembered, and used for the first start() call of
 the next session unless lseek() has been called on the file; in that
 case next start() will be asked to start at position zero::
 
         static void ct_seq_stop(struct seq_file *s, void *v)
         {
                 kfree(v);
         }
 
 Finally, the show() function should format the object currently pointed to
 by the iterator for output.  The example module's show() function is::
 
         static int ct_seq_show(struct seq_file *s, void *v)
         {
                 loff_t *spos = v;
                 seq_printf(s, "%lld\n", (long long)*spos);
                 return 0;
         }
 
 If all is well, the show() function should return zero.  A negative error
 code in the usual manner indicates that something went wrong; it will be
 passed back to user space.  This function can also return SEQ_SKIP, which
 causes the current item to be skipped; if the show() function has already
 generated output before returning SEQ_SKIP, that output will be dropped.
 
 We will look at seq_printf() in a moment. But first, the definition of the
 seq_file iterator is finished by creating a seq_operations structure with
 the four functions we have just defined::
 
         static const struct seq_operations ct_seq_ops = {
                 .start = ct_seq_start,
                 .next  = ct_seq_next,
                 .stop  = ct_seq_stop,
                 .show  = ct_seq_show
         };
 
 This structure will be needed to tie our iterator to the /proc file in
 a little bit.
 
 It's worth noting that the iterator value returned by start() and
 manipulated by the other functions is considered to be completely opaque by
 the seq_file code. It can thus be anything that is useful in stepping
 through the data to be output. Counters can be useful, but it could also be
 a direct pointer into an array or linked list. Anything goes, as long as
 the programmer is aware that things can happen between calls to the
 iterator function. However, the seq_file code (by design) will not sleep
 between the calls to start() and stop(), so holding a lock during that time
 is a reasonable thing to do. The seq_file code will also avoid taking any
 other locks while the iterator is active.
 
 The iterater value returned by start() or next() is guaranteed to be
 passed to a subsequent next() or stop() call.  This allows resources
 such as locks that were taken to be reliably released.  There is *no*
 guarantee that the iterator will be passed to show(), though in practice
 it often will be.
 
 
 Formatted output
 ================
 
 The seq_file code manages positioning within the output created by the
 iterator and getting it into the user's buffer. But, for that to work, that
 output must be passed to the seq_file code. Some utility functions have
 been defined which make this task easy.
 
 Most code will simply use seq_printf(), which works pretty much like
 printk(), but which requires the seq_file pointer as an argument.
 
 For straight character output, the following functions may be used::
 
         seq_putc(struct seq_file *m, char c);
         seq_puts(struct seq_file *m, const char *s);
         seq_escape(struct seq_file *m, const char *s, const char *esc);
 
 The first two output a single character and a string, just like one would
 expect. seq_escape() is like seq_puts(), except that any character in s
 which is in the string esc will be represented in octal form in the output.
 
 There are also a pair of functions for printing filenames::
 
         int seq_path(struct seq_file *m, const struct path *path,
                      const char *esc);
         int seq_path_root(struct seq_file *m, const struct path *path,
                           const struct path *root, const char *esc)
 
 Here, path indicates the file of interest, and esc is a set of characters
 which should be escaped in the output.  A call to seq_path() will output
 the path relative to the current process's filesystem root.  If a different
 root is desired, it can be used with seq_path_root().  If it turns out that
 path cannot be reached from root, seq_path_root() returns SEQ_SKIP.
 
 A function producing complicated output may want to check::
 
         bool seq_has_overflowed(struct seq_file *m);
 
 and avoid further seq_<output> calls if true is returned.
 
 A true return from seq_has_overflowed means that the seq_file buffer will
 be discarded and the seq_show function will attempt to allocate a larger
 buffer and retry printing.
 
 
 Making it all work
 ==================
 
 So far, we have a nice set of functions which can produce output within the
 seq_file system, but we have not yet turned them into a file that a user
 can see. Creating a file within the kernel requires, of course, the
 creation of a set of file_operations which implement the operations on that
 file. The seq_file interface provides a set of canned operations which do
 most of the work. The virtual file author still must implement the open()
 method, however, to hook everything up. The open function is often a single
 line, as in the example module::
 
         static int ct_open(struct inode *inode, struct file *file)
         {
                 return seq_open(file, &ct_seq_ops);
         }
 
 Here, the call to seq_open() takes the seq_operations structure we created
 before, and gets set up to iterate through the virtual file.
 
 On a successful open, seq_open() stores the struct seq_file pointer in
 file->private_data. If you have an application where the same iterator can
 be used for more than one file, you can store an arbitrary pointer in the
 private field of the seq_file structure; that value can then be retrieved
 by the iterator functions.
 
 There is also a wrapper function to seq_open() called seq_open_private(). It
 kmallocs a zero filled block of memory and stores a pointer to it in the
 private field of the seq_file structure, returning 0 on success. The
 block size is specified in a third parameter to the function, e.g.::
 
         static int ct_open(struct inode *inode, struct file *file)
         {
                 return seq_open_private(file, &ct_seq_ops,
                                         sizeof(struct mystruct));
         }
 
 There is also a variant function, __seq_open_private(), which is functionally
 identical except that, if successful, it returns the pointer to the allocated
 memory block, allowing further initialisation e.g.::
 
         static int ct_open(struct inode *inode, struct file *file)
         {
                 struct mystruct *p =
                         __seq_open_private(file, &ct_seq_ops, sizeof(*p));
 
                 if (!p)
                         return -ENOMEM;
 
                 p->foo = bar; /* initialize my stuff */
                         ...
                 p->baz = true;
 
                 return 0;
         }
 
 A corresponding close function, seq_release_private() is available which
 frees the memory allocated in the corresponding open.
 
 The other operations of interest - read(), llseek(), and release() - are
 all implemented by the seq_file code itself. So a virtual file's
 file_operations structure will look like::
 
         static const struct file_operations ct_file_ops = {
                 .owner   = THIS_MODULE,
                 .open    = ct_open,
                 .read    = seq_read,
                 .llseek  = seq_lseek,
                 .release = seq_release
         };
 
 There is also a seq_release_private() which passes the contents of the
 seq_file private field to kfree() before releasing the structure.
 
 The final step is the creation of the /proc file itself. In the example
 code, that is done in the initialization code in the usual way::
 
         static int ct_init(void)
         {
                 struct proc_dir_entry *entry;
 
                 proc_create("sequence", 0, NULL, &ct_file_ops);
                 return 0;
         }
 
         module_init(ct_init);
 
 And that is pretty much it.
 
 
 seq_list
 ========
 
 If your file will be iterating through a linked list, you may find these
 routines useful::
 
         struct list_head *seq_list_start(struct list_head *head,
                                          loff_t pos);
         struct list_head *seq_list_start_head(struct list_head *head,
                                               loff_t pos);
         struct list_head *seq_list_next(void *v, struct list_head *head,
                                         loff_t *ppos);
 
 These helpers will interpret pos as a position within the list and iterate
 accordingly.  Your start() and next() functions need only invoke the
 ``seq_list_*`` helpers with a pointer to the appropriate list_head structure.
 
 
 The extra-simple version
 ========================
 
 For extremely simple virtual files, there is an even easier interface.  A
 module can define only the show() function, which should create all the
 output that the virtual file will contain. The file's open() method then
 calls::
 
         int single_open(struct file *file,
                         int (*show)(struct seq_file *m, void *p),
                         void *data);
 
 When output time comes, the show() function will be called once. The data
 value given to single_open() can be found in the private field of the
 seq_file structure. When using single_open(), the programmer should use
 single_release() instead of seq_release() in the file_operations structure
 to avoid a memory leak.

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