[PATCH] Documentation for proc_file_read

From: Thomas Hood (jdthood@mail.com)
Date: Sat Feb 02 2002 - 09:47:42 EST


Here is a patch that improves (I hope) the documentation
of how user proc-file-read functions should interface with
proc_file_read(). At this stage I just ask that any
interested parties review what I have written. // Thomas

--- linux-2.4.18-pre7_ORIG/fs/proc/generic.c Fri Sep 7 13:53:59 2001
+++ linux-2.4.18-pre7/fs/proc/generic.c Sat Feb 2 09:45:14 2002
@@ -67,23 +67,60 @@
 
                 start = NULL;
                 if (dp->get_info) {
- /*
- * Handle backwards compatibility with the old net
- * routines.
- */
+ /* Handle old net routines */
                         n = dp->get_info(page, &start, *ppos, count);
                         if (n < count)
                                 eof = 1;
                 } else if (dp->read_proc) {
+ /*
+ * How to be a proc read function
+ * ------------------------------
+ *
+ * Prototype:
+ * int f(char *buffer, char **start, off_t offset,
+ * int count, int *peof, void *dat)
+ *
+ * You may assume that the buffer provided is at least
+ * 3 KiB in size.
+ *
+ * If you know you have supplied all the data there is,
+ * set the *peof bit.
+ *
+ * Three ways to return data:
+ * A) Set *start = NULL
+ * Put the data of the requested offset at that
+ * offset within the buffer. Return the number (n)
+ * of bytes there are from the beginning of the
+ * buffer up to the last byte of data. If the
+ * number of supplied bytes (= n - offset) is less
+ * than the requested count, you will be called
+ * again with the requested offset advanced by n.
+ * This interface is useful for files no larger
+ * than the buffer.
+ * B) Set *start = a small value (less than buffer!)
+ * Put the data of the requested offset at the
+ * beginning of the buffer. Return the number of
+ * bytes of data placed there. If this number is
+ * less than the requested count, you will be
+ * called again with the requested offset advanced
+ * by *start. This interface is useful when you
+ * have a large file consisting of a series of
+ * data blocks which you want to count and return
+ * as wholes.
+ * (hack by Paul.Russell@rustcorp.com.au)
+ * C) Set *start = a location within the buffer
+ * Put the data of the requested offset at *start.
+ * Return the number (n) of bytes of data placed
+ * there. If this number is less than the
+ * requested count, you will be called again with
+ * the requested offset advanced by n.
+ */
                         n = dp->read_proc(page, &start, *ppos,
                                           count, &eof, dp->data);
                 } else
                         break;
 
- if (!start) {
- /*
- * For proc files that are less than 4k
- */
+ if (start == NULL) {
                         start = page + *ppos;
                         n -= *ppos;
                         if (n <= 0)
@@ -91,19 +128,14 @@
                         if (n > count)
                                 n = count;
                 }
- if (n == 0)
- break; /* End of file */
- if (n < 0) {
+ if (n == 0) /* end of file */
+ break;
+ if (n < 0) { /* error */
                         if (retval == 0)
                                 retval = n;
                         break;
                 }
                 
- /* This is a hack to allow mangling of file pos independent
- * of actual bytes read. Simply place the data at page,
- * return the bytes, and set `start' to the desired offset
- * as an unsigned int. - Paul.Russell@rustcorp.com.au
- */
                  n -= copy_to_user(buf, start < page ? page : start, n);
                 if (n == 0) {
                         if (retval == 0)
@@ -111,7 +143,7 @@
                         break;
                 }
 
- *ppos += start < page ? (long)start : n; /* Move down the file */
+ *ppos += start < page ? (long)start : n;
                 nbytes -= n;
                 buf += n;
                 retval += n;

-
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