[PATCH] alloc_buffer: Return unqualified pointer type in alloc_buffer_next

[Florian Weimer <fweimer at redhat dot com>]

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.

2019-03-08  Florian Weimer  <fweimer@redhat.com>

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

diff --git a/include/alloc_buffer.h b/include/alloc_buffer.h
index f27cbb65ca..7f68f46eac 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)
 {
@@ -302,9 +302,13 @@ __alloc_buffer_next (struct alloc_buffer *buf, size_t align)
    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.  */
+   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.  */
 #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.  */