Re: [PATCH] alloc_buffer: Return unqualified pointer type in alloc_buffer_next

[Florian Weimer <fweimer at redhat dot com>]

* Carlos O'Donell:

> On 3/8/19 3:46 PM, Florian Weimer wrote:
>> alloc_buffer_next is useful for peeking to the remaining part of the
>> buffer and update it, with subsequent allocation (once the length
>> is known) using alloc_buffer_alloc_bytes.  This is not as robust
>> as the other interfaces, but it allows using alloc_buffer with
>> string-writing interfaces such as snprintf and ns_name_ntop.
>
> Until now alloc_buffer_next was only used for testing alloc_buffer itself.
>
> Please add a detailed example in the comments for how this API should be
> used. The use case is interesting enough that it needs comments.
>
> OK if you add detailed comments in the header for the use case intended.

Okay.  I think this interface should only be used very sparingly, but
I've written a longer comment.

Of course, the patch is now substantially larger than before, so it's a
bit odd that you pre-approved it. 8-/

Thanks,
Florian

alloc_buffer: Return unqualified pointer type in alloc_buffer_next

alloc_buffer_next is useful for peeking to the remaining part of the
buffer and update it, with subsequent allocation (once the length
is known) using alloc_buffer_alloc_bytes.  This is not as robust
as the other interfaces, but it allows using alloc_buffer with
string-writing interfaces such as snprintf and ns_name_ntop.

2019-04-08  Florian Weimer  <fweimer@redhat.com>

	* include/alloc_buffer.h (alloc_buffer_alloc_bytes): Update
	comment.
	(alloc_buffer_next): Change return type to non-const.  Update
	comment.

diff --git a/include/alloc_buffer.h b/include/alloc_buffer.h
index f27cbb65ca..9c469b9e8b 100644
--- a/include/alloc_buffer.h
+++ b/include/alloc_buffer.h
@@ -183,7 +183,7 @@ alloc_buffer_add_byte (struct alloc_buffer *buf, unsigned char b)
    NULL is returned if there is not enough room, and the buffer is
    marked as failed, or if the buffer has already failed.
    (Zero-length allocations from an empty buffer which has not yet
-   failed succeed.)  */
+   failed succeed.)  The buffer contents is not modified.  */
 static inline __attribute__ ((nonnull (1))) void *
 alloc_buffer_alloc_bytes (struct alloc_buffer *buf, size_t length)
 {
@@ -300,11 +300,32 @@ __alloc_buffer_next (struct alloc_buffer *buf, size_t align)
 /* Like alloc_buffer_alloc, but do not advance the pointer beyond the
    object (so a subseqent call to alloc_buffer_next or
    alloc_buffer_alloc returns the same pointer).  Note that the buffer
-   is still aligned according to the requirements of TYPE.  The effect
-   of this function is similar to allocating a zero-length array from
-   the buffer.  */
+   is still aligned according to the requirements of TYPE, potentially
+   consuming buffer space.  The effect of this function is similar to
+   allocating a zero-length array from the buffer.
+
+   It is possible to use the return pointer to write to the buffer and
+   consume the written bytes using alloc_buffer_alloc_bytes (which
+   does not change the buffer contents), but the calling code needs to
+   perform manual length checks using alloc_buffer_size.  For example,
+   to read as many int32_t values that are available in the input file
+   and can fit into the remaining buffer space, you can use this:
+
+     int32_t array = alloc_buffer_next (buf, int32_t);
+     size_t ret = fread (array, sizeof (int32_t),
+                         alloc_buffer_size (buf) / sizeof (int32_t), fp);
+     if (ferror (fp))
+       handle_error ();
+     alloc_buffer_alloc_array (buf, int32_t, ret);
+
+   The alloc_buffer_alloc_array call makes the actually-used part of
+   the buffer permanent.  The remaining part of the buffer (not filled
+   with data from the file) can be used for something else.
+
+   This manual length checking can easily introduce errors, so this
+   coding style is not recommended.  */
 #define alloc_buffer_next(buf, type)				\
-  ((const type *) __alloc_buffer_next				\
+  ((type *) __alloc_buffer_next					\
    (buf, __alloc_buffer_assert_align (__alignof__ (type))))
 
 /* Internal function.  Allocate an array.  */