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diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt new file mode 100644 index 00000000000..92975ee7942 --- /dev/null +++ b/Documentation/filesystems/seq_file.txt @@ -0,0 +1,283 @@ +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 out=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/). + + +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 whch 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 = (loff_t *) 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. It should return zero, or an error code if +something goes wrong. The example module's show() function is: + + static int ct_seq_show(struct seq_file *s, void *v) + { + loff_t *spos = (loff_t *) v; + seq_printf(s, "%Ld\n", *spos); + return 0; + } + +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 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 interator 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. It is +common to ignore the return value from seq_printf(), but a function +producing complicated output may want to check that value and quit if +something non-zero is returned; an error return means that the seq_file +buffer has been filled and further output will be discarded. + +For straight character output, the following functions may be used: + + int seq_putc(struct seq_file *m, char c); + int seq_puts(struct seq_file *m, const char *s); + int 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 is also a function for printing filenames: + + int seq_path(struct seq_file *m, struct path *path, char *esc); + +Here, path indicates the file of interest, and esc is a set of characters +which should be escaped in the output. + + +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. + +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 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; + + entry = create_proc_entry("sequence", 0, NULL); + if (entry) + entry->proc_fops = &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. |