rewrite iov_* functions

This changes implementations of all iov_*
functions, completing the previous step.

All iov_* functions now ensure that this offset
argument is within the iovec (using assertion),
but lets to specify `bytes' value larger than
actual length of the iovec - in this case they
stops at the actual end of iovec.  It is also
suggested to use convinient `-1' value as `bytes'
to mean just this -- "up to the end".

There's one very minor semantic change here: new
requiriment is that `offset' points to inside of
iovec.  This is checked just at the end of functions
(assert()), it does not actually need to be enforced,
but using any of these functions with offset pointing
past the end of iovec is wrong anyway.

Note: the new code in iov.c uses arithmetic with
void pointers.  I thought this is not supported
everywhere and is a GCC extension (indeed, the C
standard does not define void arithmetic).  However,
the original code already use void arith in
iov_from_buf() function:
  (memcpy(..., buf + buf_off,...)
which apparently works well so far (it is this
way in qemu 1.0).  So I left it this way and used
it in other places.

While at it, add a unit-test file test-iov.c,
to check various corner cases with iov_from_buf(),
iov_to_buf() and iov_memset().

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

1 files changed, 10 insertions, 2 deletions

diff --git a/iov.h b/iov.h
index 0b4acf4a51..19ee3b3acd 100644
--- a/iov.h
+++ b/iov.h
@@ -1,10 +1,11 @@
 /*
- * Helpers for getting linearized buffers from iov / filling buffers into iovs
+ * Helpers for using (partial) iovecs.
  *
  * Copyright (C) 2010 Red Hat, Inc.
  *
  * Author(s):
  *  Amit Shah <amit.shah@redhat.com>
+ *  Michael Tokarev <mjt@tls.msk.ru>
  *
  * This work is licensed under the terms of the GNU GPL, version 2.  See
  * the COPYING file in the top-level directory.
@@ -28,6 +29,12 @@ size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt);
  * only part of data will be copied, up to the end of the iovec.
  * Number of bytes actually copied will be returned, which is
  *  min(bytes, iov_size(iov)-offset)
+ * `Offset' must point to the inside of iovec.
+ * It is okay to use very large value for `bytes' since we're
+ * limited by the size of the iovec anyway, provided that the
+ * buffer pointed to by buf has enough space.  One possible
+ * such "large" value is -1 (sinice size_t is unsigned),
+ * so specifying `-1' as `bytes' means 'up to the end of iovec'.
  */
 size_t iov_from_buf(struct iovec *iov, unsigned int iov_cnt,
                     size_t offset, const void *buf, size_t bytes);
@@ -37,11 +44,12 @@ size_t iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
 /**
  * Set data bytes pointed out by iovec `iov' of size `iov_cnt' elements,
  * starting at byte offset `start', to value `fillc', repeating it
- * `bytes' number of times.
+ * `bytes' number of times.  `Offset' must point to the inside of iovec.
  * If `bytes' is large enough, only last bytes portion of iovec,
  * up to the end of it, will be filled with the specified value.
  * Function return actual number of bytes processed, which is
  * min(size, iov_size(iov) - offset).
+ * Again, it is okay to use large value for `bytes' to mean "up to the end".
  */
 size_t iov_memset(const struct iovec *iov, const unsigned int iov_cnt,
                   size_t offset, int fillc, size_t bytes);