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-rw-r--r--Documentation/filesystems/seq_file.rst20
1 files changed, 19 insertions, 1 deletions
diff --git a/Documentation/filesystems/seq_file.rst b/Documentation/filesystems/seq_file.rst
index 7f7ee06b2693..56856481dc8d 100644
--- a/Documentation/filesystems/seq_file.rst
+++ b/Documentation/filesystems/seq_file.rst
@@ -129,7 +129,9 @@ 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.
+however. SEQ_START_TOKEN has no special meaning to the core seq_file
+code. It is provided as a convenience for a start() funciton to
+communicate with the next() and show() functions.
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
@@ -145,6 +147,22 @@ complete. Here's the example version::
return spos;
}
+The next() function should set ``*pos`` to a value that start() can use
+to find the new location in the sequence. When the iterator is being
+stored in the private data area, rather than being reinitialized on each
+start(), it might seem sufficient to simply set ``*pos`` to any non-zero
+value (zero always tells start() to restart the sequence). This is not
+sufficient due to historical problems.
+
+Historically, many next() functions have *not* updated ``*pos`` at
+end-of-file. If the value is then used by start() to initialise the
+iterator, this can result in corner cases where the last entry in the
+sequence is reported twice in the file. In order to discourage this bug
+from being resurrected, the core seq_file code now produces a warning if
+a next() function does not change the value of ``*pos``. Consequently a
+next() function *must* change the value of ``*pos``, and of course must
+set it to a non-zero value.
+
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