This is the mail archive of the
mailing list for the glibc project.
Re: [PATCH] alloc_buffer: Return unqualified pointer type in alloc_buffer_next
- From: Florian Weimer <fweimer at redhat dot com>
- To: Carlos O'Donell <codonell at redhat dot com>
- Cc: libc-alpha at sourceware dot org
- Date: Mon, 08 Apr 2019 17:29:47 +0200
- Subject: Re: [PATCH] alloc_buffer: Return unqualified pointer type in alloc_buffer_next
- References: <20190308204633.49DCE80DD6B5@oldenburg2.str.redhat.com> <firstname.lastname@example.org>
* 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-/
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 <email@example.com>
* include/alloc_buffer.h (alloc_buffer_alloc_bytes): Update
(alloc_buffer_next): Change return type to non-const. Update
diff --git a/include/alloc_buffer.h b/include/alloc_buffer.h
index f27cbb65ca..9c469b9e8b 100644
@@ -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. */