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Diffstat (limited to 'Documentation/filesystems/seq_file.txt')
| -rw-r--r-- | Documentation/filesystems/seq_file.txt | 63 |
1 files changed, 42 insertions, 21 deletions
diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt index 9de4303201e1..d412b236a9d6 100644 --- a/Documentation/filesystems/seq_file.txt +++ b/Documentation/filesystems/seq_file.txt @@ -66,23 +66,39 @@ kernel 3.10. Current versions require the following update 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. +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() takes a position as an argument and returns an iterator -which will start reading at that position. For our simple sequence example, +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) @@ -101,11 +117,12 @@ 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. +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 @@ -121,9 +138,13 @@ complete. Here's the example version: 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. +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) { |