339 lines
13 KiB
Plaintext
339 lines
13 KiB
Plaintext
|
The seq_file interface
|
||
|
|
|
||
|
|
Copyright 2003 Jonathan Corbet <corbet@lwn.net>
|
||
|
|
This file is originally from the LWN.net Driver Porting series at
|
||
|
|
http://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
|
||
|
|
http://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 a simple
|
||
|
|
iterator object that allows stepping through the data of interest.
|
||
|
|
Iterators must be able to move to a specific position - like the file they
|
||
|
|
implement - but the interpretation of that position is up to the iterator
|
||
|
|
itself. A seq_file implementation that is formatting firewall rules, for
|
||
|
|
example, could interpret position N as the Nth rule in the chain.
|
||
|
|
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() takes a position as an argument and returns an iterator
|
||
|
|
which will start reading at that position. 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. 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.
|
||
|
|
|
||
|
|
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 stop() function is called when iteration is complete; its job, of
|
||
|
|
course, is to clean up. If dynamic memory is allocated for the iterator,
|
||
|
|
stop() is the place to free it.
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
|
||
|
|
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.
|