| 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 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. |
| |
| 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 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. |
| |
| |
| 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. |