[C-safe-secure-studygroup] MISRA and packed structures

[Wilson, Charles]

I’m not sure I’d agree that this is an extension.

I believe that it falls under the same category as pragmas, that is, guidance to the compiler.

MSVC https://docs.microsoft.com/en-us/cpp/preprocessor/pack?view=vs-2017
gcc https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Variable-Attributes.html#Variable%20Attributes
keil http://www.keil.com/support/man/docs/armclang_ref/armclang_ref_chr1384946440202.htm

The clang sources provide an interesting view:
https://github.com/llvm-mirror/clang/blob/master/test/Sema/struct-packed-align.c

__packed is used in the keil ARM compiler
http://www.keil.com/support/man/docs/armcc/armcc_chr1359124968737.htm

as well as the gcc ARM compiler
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491e/CJAFJHJD.html

An interesting side note is that all underscore prefixed things belong to the compiler.

From: C-safe-secure-studygroup [mailto:c-safe-secure-studygroup-bounces at lists.trustable.io] On Behalf Of Clive Pygott
Sent: Wednesday, November 28, 2018 4:58 AM
To: C Safety and Security Study Group Discussion <c-safe-secure-studygroup at lists.trustable.io>
Subject: Re: [C-safe-secure-studygroup] MISRA and packed structures

Hi Robert

What compiler are you using, as  __packed  isn't defined in either C11 or C17.    If its a compiler extension then you've already broken rule 1.2 "language extensions should not be used".  Its only advisory, so easy to deviate - but its a warning that you are now taking responsibility for the behaviour of the program.

As far as memcpy is concerned, MISRA C:2012 has a number of examples that use memcpy, so its not totally banned. The examples all show array copying (where the implied pointer arithmetic is explicitly allowed), but my belief would be that copying a tMyStruct to another tMyStruct would be allowed using memcpy. What wouldn't be allowed would be copying to/from something of a different type, say an array of char.

    Clive




On Tue, Nov 27, 2018 at 3:34 PM Robert Seacord <rcseacord at gmail.com<mailto:rcseacord at gmail.com>> wrote:
Clive,

In the following example:

typedef __packed struct
{
              uint16 a;
              uint8 b[ANNOYING_INCONVENIENT_LENGTH];
              uint16 c;
} tMyStruct;

tMyStruct buf;

Are you saying that 18.1 and 18.2 prohibit memcpy to &buf, ?

Thanks,
rCs

On Tue, Nov 27, 2018 at 8:16 AM Clive Pygott <clivepygott at gmail.com<mailto:clivepygott at gmail.com>> wrote:
What do you mean by a packed structure?  The term doesn't appear in the C17 standard or MISRA C:2012

If you mean using a structure with no or known padding as the target of a block move or I/O, then that has to be done by a deviation as it breaks the rules on pointer arithmetic (18.1/18.2) that only allow pointer arithmetic within an array

     Clive

On Tue, Nov 27, 2018 at 12:05 PM Robert Seacord <rcseacord at gmail.com<mailto:rcseacord at gmail.com>> wrote:
Is there any MISRA rules which disallow the use of packed structures?

rCs
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