e8188807b7
Small error in the "dd" command example, "out=" should be "of=". Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
295 lines
12 KiB
Plaintext
295 lines
12 KiB
Plaintext
The seq_file interface
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Copyright 2003 Jonathan Corbet <corbet@lwn.net>
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This file is originally from the LWN.net Driver Porting series at
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http://lwn.net/Articles/driver-porting/
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There are numerous ways for a device driver (or other kernel component) to
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provide information to the user or system administrator. One useful
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technique is the creation of virtual files, in debugfs, /proc or elsewhere.
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Virtual files can provide human-readable output that is easy to get at
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without any special utility programs; they can also make life easier for
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script writers. It is not surprising that the use of virtual files has
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grown over the years.
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Creating those files correctly has always been a bit of a challenge,
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however. It is not that hard to make a virtual file which returns a
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string. But life gets trickier if the output is long - anything greater
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than an application is likely to read in a single operation. Handling
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multiple reads (and seeks) requires careful attention to the reader's
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position within the virtual file - that position is, likely as not, in the
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middle of a line of output. The kernel has traditionally had a number of
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implementations that got this wrong.
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The 2.6 kernel contains a set of functions (implemented by Alexander Viro)
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which are designed to make it easy for virtual file creators to get it
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right.
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The seq_file interface is available via <linux/seq_file.h>. There are
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three aspects to seq_file:
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* An iterator interface which lets a virtual file implementation
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step through the objects it is presenting.
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* Some utility functions for formatting objects for output without
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needing to worry about things like output buffers.
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* A set of canned file_operations which implement most operations on
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the virtual file.
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We'll look at the seq_file interface via an extremely simple example: a
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loadable module which creates a file called /proc/sequence. The file, when
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read, simply produces a set of increasing integer values, one per line. The
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sequence will continue until the user loses patience and finds something
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better to do. The file is seekable, in that one can do something like the
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following:
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dd if=/proc/sequence of=out1 count=1
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dd if=/proc/sequence skip=1 of=out2 count=1
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Then concatenate the output files out1 and out2 and get the right
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result. Yes, it is a thoroughly useless module, but the point is to show
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how the mechanism works without getting lost in other details. (Those
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wanting to see the full source for this module can find it at
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http://lwn.net/Articles/22359/).
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The iterator interface
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Modules implementing a virtual file with seq_file must implement a simple
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iterator object that allows stepping through the data of interest.
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Iterators must be able to move to a specific position - like the file they
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implement - but the interpretation of that position is up to the iterator
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itself. A seq_file implementation that is formatting firewall rules, for
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example, could interpret position N as the Nth rule in the chain.
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Positioning can thus be done in whatever way makes the most sense for the
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generator of the data, which need not be aware of how a position translates
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to an offset in the virtual file. The one obvious exception is that a
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position of zero should indicate the beginning of the file.
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The /proc/sequence iterator just uses the count of the next number it
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will output as its position.
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Four functions must be implemented to make the iterator work. The first,
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called start() takes a position as an argument and returns an iterator
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which will start reading at that position. For our simple sequence example,
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the start() function looks like:
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static void *ct_seq_start(struct seq_file *s, loff_t *pos)
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{
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loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
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if (! spos)
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return NULL;
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*spos = *pos;
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return spos;
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}
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The entire data structure for this iterator is a single loff_t value
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holding the current position. There is no upper bound for the sequence
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iterator, but that will not be the case for most other seq_file
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implementations; in most cases the start() function should check for a
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"past end of file" condition and return NULL if need be.
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For more complicated applications, the private field of the seq_file
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structure can be used. There is also a special value which can be returned
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by the start() function called SEQ_START_TOKEN; it can be used if you wish
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to instruct your show() function (described below) to print a header at the
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top of the output. SEQ_START_TOKEN should only be used if the offset is
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zero, however.
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The next function to implement is called, amazingly, next(); its job is to
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move the iterator forward to the next position in the sequence. The
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example module can simply increment the position by one; more useful
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modules will do what is needed to step through some data structure. The
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next() function returns a new iterator, or NULL if the sequence is
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complete. Here's the example version:
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static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos)
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{
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loff_t *spos = v;
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*pos = ++*spos;
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return spos;
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}
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The stop() function is called when iteration is complete; its job, of
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course, is to clean up. If dynamic memory is allocated for the iterator,
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stop() is the place to free it.
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static void ct_seq_stop(struct seq_file *s, void *v)
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{
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kfree(v);
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}
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Finally, the show() function should format the object currently pointed to
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by the iterator for output. The example module's show() function is:
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static int ct_seq_show(struct seq_file *s, void *v)
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{
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loff_t *spos = v;
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seq_printf(s, "%lld\n", (long long)*spos);
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return 0;
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}
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If all is well, the show() function should return zero. A negative error
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code in the usual manner indicates that something went wrong; it will be
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passed back to user space. This function can also return SEQ_SKIP, which
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causes the current item to be skipped; if the show() function has already
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generated output before returning SEQ_SKIP, that output will be dropped.
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We will look at seq_printf() in a moment. But first, the definition of the
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seq_file iterator is finished by creating a seq_operations structure with
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the four functions we have just defined:
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static const struct seq_operations ct_seq_ops = {
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.start = ct_seq_start,
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.next = ct_seq_next,
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.stop = ct_seq_stop,
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.show = ct_seq_show
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};
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This structure will be needed to tie our iterator to the /proc file in
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a little bit.
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It's worth noting that the iterator value returned by start() and
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manipulated by the other functions is considered to be completely opaque by
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the seq_file code. It can thus be anything that is useful in stepping
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through the data to be output. Counters can be useful, but it could also be
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a direct pointer into an array or linked list. Anything goes, as long as
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the programmer is aware that things can happen between calls to the
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iterator function. However, the seq_file code (by design) will not sleep
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between the calls to start() and stop(), so holding a lock during that time
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is a reasonable thing to do. The seq_file code will also avoid taking any
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other locks while the iterator is active.
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Formatted output
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The seq_file code manages positioning within the output created by the
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iterator and getting it into the user's buffer. But, for that to work, that
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output must be passed to the seq_file code. Some utility functions have
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been defined which make this task easy.
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Most code will simply use seq_printf(), which works pretty much like
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printk(), but which requires the seq_file pointer as an argument. It is
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common to ignore the return value from seq_printf(), but a function
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producing complicated output may want to check that value and quit if
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something non-zero is returned; an error return means that the seq_file
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buffer has been filled and further output will be discarded.
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For straight character output, the following functions may be used:
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int seq_putc(struct seq_file *m, char c);
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int seq_puts(struct seq_file *m, const char *s);
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int seq_escape(struct seq_file *m, const char *s, const char *esc);
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The first two output a single character and a string, just like one would
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expect. seq_escape() is like seq_puts(), except that any character in s
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which is in the string esc will be represented in octal form in the output.
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There is also a pair of functions for printing filenames:
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int seq_path(struct seq_file *m, struct path *path, char *esc);
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int seq_path_root(struct seq_file *m, struct path *path,
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struct path *root, char *esc)
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Here, path indicates the file of interest, and esc is a set of characters
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which should be escaped in the output. A call to seq_path() will output
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the path relative to the current process's filesystem root. If a different
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root is desired, it can be used with seq_path_root(). Note that, if it
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turns out that path cannot be reached from root, the value of root will be
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changed in seq_file_root() to a root which *does* work.
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Making it all work
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So far, we have a nice set of functions which can produce output within the
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seq_file system, but we have not yet turned them into a file that a user
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can see. Creating a file within the kernel requires, of course, the
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creation of a set of file_operations which implement the operations on that
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file. The seq_file interface provides a set of canned operations which do
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most of the work. The virtual file author still must implement the open()
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method, however, to hook everything up. The open function is often a single
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line, as in the example module:
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static int ct_open(struct inode *inode, struct file *file)
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{
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return seq_open(file, &ct_seq_ops);
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}
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Here, the call to seq_open() takes the seq_operations structure we created
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before, and gets set up to iterate through the virtual file.
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On a successful open, seq_open() stores the struct seq_file pointer in
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file->private_data. If you have an application where the same iterator can
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be used for more than one file, you can store an arbitrary pointer in the
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private field of the seq_file structure; that value can then be retrieved
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by the iterator functions.
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The other operations of interest - read(), llseek(), and release() - are
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all implemented by the seq_file code itself. So a virtual file's
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file_operations structure will look like:
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static const struct file_operations ct_file_ops = {
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.owner = THIS_MODULE,
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.open = ct_open,
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.read = seq_read,
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.llseek = seq_lseek,
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.release = seq_release
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};
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There is also a seq_release_private() which passes the contents of the
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seq_file private field to kfree() before releasing the structure.
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The final step is the creation of the /proc file itself. In the example
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code, that is done in the initialization code in the usual way:
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static int ct_init(void)
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{
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struct proc_dir_entry *entry;
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entry = create_proc_entry("sequence", 0, NULL);
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if (entry)
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entry->proc_fops = &ct_file_ops;
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return 0;
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}
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module_init(ct_init);
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And that is pretty much it.
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seq_list
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If your file will be iterating through a linked list, you may find these
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routines useful:
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struct list_head *seq_list_start(struct list_head *head,
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loff_t pos);
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struct list_head *seq_list_start_head(struct list_head *head,
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loff_t pos);
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struct list_head *seq_list_next(void *v, struct list_head *head,
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loff_t *ppos);
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These helpers will interpret pos as a position within the list and iterate
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accordingly. Your start() and next() functions need only invoke the
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seq_list_* helpers with a pointer to the appropriate list_head structure.
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The extra-simple version
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For extremely simple virtual files, there is an even easier interface. A
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module can define only the show() function, which should create all the
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output that the virtual file will contain. The file's open() method then
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calls:
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int single_open(struct file *file,
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int (*show)(struct seq_file *m, void *p),
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void *data);
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When output time comes, the show() function will be called once. The data
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value given to single_open() can be found in the private field of the
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seq_file structure. When using single_open(), the programmer should use
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single_release() instead of seq_release() in the file_operations structure
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to avoid a memory leak.
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