Hi Lizardo,
On 11-03-04 11:17 AM, Anderson Lizardo wrote:
Hi Brian,
On Fri, Mar 4, 2011 at 2:57 PM, Brian Gix<bgix@xxxxxxxxxxxxxx> wrote:
On 11-03-04 10:32 AM, Elvis Pfützenreuter wrote:
I don't believe this line to be always portable between platforms. This
line makes the assumption that a uint16_t is two native units (not always
true) and can therefore be directly memcopy'd into into an array of uint8_t
array members. This is not always true. memcpy should never be used in
portable code when packing data for network datagram usage models (anything
that treats data as an octet stream).
If we had used "unsigned char" and "unsigned short int", I'd agree, but
uint8_t and uint16_t are *explicitly* 8-bit and 16-bit types, and thus there
is a guaranteed 2:1 size relationship.
Unfortunately, this is only part true. There are already CPU architectures
in the marketplace which define the smallest addressable unit as being 16
bits, which means that even if you have defined a uint8_t that has a valid
range of only 0-255, the sizeof(uint8_t) will == 1, and the sizeof(uint16_t)
will also == 1, and so a uint16_t of 0x1234 memcpy'd into a uint8_t array
will not appear as {0x12, 0x34} but rather as {0x34, 0x00} regardless of
your endian-ness (the Most Sig 8 bits hidden by the allowed range). Memcpy
is only portability-safe when used to copy between two identical data types.
It is also the reason you see things like sizeof(uint16_t) being passed as
arguments to the mem* functions.
That axiom is also true when copying data between unions and/or stucts with
different underlying definitions. The layout of raw bits can never be
assumed.
While your explanation looks really interesting (I always thought that
sizeof() returned the number of bytes), keep in mind that there are
tons of memcpy() calls beween different datatypes, at least on BlueZ
and on Linux kernel. I'm not arguing that we should keep it as is, but
I suppose there will be a lot of changes to be done even before BlueZ
(or even the kernel) can run on these CPUs.
If I understand your explanation, even memcpy(..., ...,
sizeof(uint16_t)) would not work on these CPUs, as it copies the
specified number of *bytes*.
Simply grep for "memcpy" on BlueZ sources to get an idea.
My two cents,
I suppose you are correct that for Linux and BlueZ, the damage is
already done as far as mem* function usage, and that therefore all of my
other objections are moot, and withdraw my fundamental memcpy objection.
However:
You are also correct that sizeof() returns the number of bytes, but
bytes are not in fact the same thing as octets. An octet is used in
networking, and other inter-architecture communication, and is defined
as being exactly eight bits of data.
A byte on the other hand is architecturally defined and is neither
limited to, nor gaurenteed to be at least, 8 bits. It is rather "the
smallest addressable unit" of the target architecture. That is why if
you read networking documents, you are more likely to hear the
underlying data referred to as "octets" instead of bytes. Octets are
precise, and bytes are imprecise.
byte != octet
So memcpy(a1, a2, sizeof(uint16_t)) would work as expected, assuming a1
and a2 were both pointers to objects with underlying 16 bit definitions.
--
Brian Gix
bgix@xxxxxxxxxxxxxx
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum
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