Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #77199

Re: Aliasing through a union pointer

Show all headers | View raw

supercat@casperkitty.com wrote:
> On Wednesday, November 25, 2015 at 5:58:21 PM UTC-6, Ian Collins wrote:
>> I see where you are coming from, but I don't think your suggestion goes
>> far enough.  Consider passing the address of second to f() in this snippet:
>>
>> __packed struct S
>> {
>>     uint8_t  first;
>>     uint32_t second;
>> };
>
>> void f( uint32_t* );
>
> With regard to the above code, I'd say that the type of &thing.second should
> be "uint32_t __packed *", which is not compatible with "uint32_t*", but could
> be safely passed to "f" if the function had been declared
>
>    void f( uint32_t __packed* p);
>
> or
>
>    void f( uint32_t __packed* restrict p);

But that would preclude calling any (library) function that didn't have 
the qualification.  That's why I said supporting misaligned access 
impacts the entire program.

<snip>

>> There is - compiler extensions!  Supporting misaligned access impacts
>> the entire program, not just specific accesses.
>
> If the small system and big system alike both used new types whose storage
> format and behavior were specified in precise terms, why would the existence
> of such types need to affect any code using normal types on the PC?

Because misaligned pointers can leak.

>> I have done a lot of work similar to what you describe above as well as
>> running parts of the embedded code on a workstation host.  My solution
>> on SPARC (which can't do packed structs) was to compile the embedded
>> code as C++ and use customised types for the stricture members to handle
>> the packing an unpacking.  A more general solution would be to use
>> opaque types and accessor functions.
>
> I've done something similar so I could run the same code on both the embedded
> target using C, and on a PC using C++; unfortunately, the two languages are
> diverging from each other rather than converging.

That depends on how much of the "new" stuff in C99 you use :)

>> At the time, enabling misaligned access on the SPARC build cased the
>> code to run slower than it did on the embedded target....
>
> Was it slower because the code was trying to do unaligned reads which then
> generated traps, or because the code was using sequences of byte reads and
> putting them together?

The former.

-- 
Ian Collins

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Aliasing through a union pointer Ian Pilcher <arequipeno@gmail.com> - 2015-11-23 10:14 -0600
  Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-23 11:43 -0500
    Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-23 11:49 -0500
    Re: Aliasing through a union pointer Ian Pilcher <arequipeno@gmail.com> - 2015-11-23 13:22 -0600
      Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-23 14:38 -0500
        Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-25 10:38 -0800
          Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-25 14:17 -0500
            Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 13:16 -0800
      Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-23 21:46 +0100
        Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-23 22:20 +0000
          Re: Aliasing through a union pointer Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 16:52 -0600
            Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-23 23:10 +0000
              Re: Aliasing through a union pointer Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 19:05 -0600
          Re: Aliasing through a union pointer Nick Bowler <nbowler@draconx.ca> - 2015-11-23 22:51 +0000
          Re: Aliasing through a union pointer Keith Thompson <kst-u@mib.org> - 2015-11-23 15:49 -0800
            Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-23 16:01 -0800
          Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-24 09:55 +0100
            Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-24 14:29 +0000
              Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-24 22:58 +0100
                Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-24 17:30 -0500
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-24 15:27 -0800
                Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-25 12:57 +1300
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-24 16:31 -0800
                Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-25 18:28 +1300
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 06:54 -0800
                Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 08:59 +1300
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 13:01 -0800
                Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 12:58 +1300
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 16:44 -0800
                Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 14:12 +1300
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 21:34 -0800
                Re: Aliasing through a union pointer Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-25 21:55 -0800
                Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 20:51 +1300
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-26 11:39 -0800
                Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-26 00:06 +0000
            Re: Aliasing through a union pointer Keith Thompson <kst-u@mib.org> - 2015-11-24 11:51 -0800
              Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-24 12:20 -0800
                Re: Aliasing through a union pointer Keith Thompson <kst-u@mib.org> - 2015-11-24 21:52 -0800
                Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 08:30 -0800
              Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-24 23:04 +0100
  Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-25 10:10 -0800
    Re: Aliasing through a union pointer Richard Damon <Richard@Damon-Family.org> - 2015-11-25 11:02 -0800
      Re: Aliasing through a union pointer Philip Lantz <prl@canterey.us> - 2015-11-26 13:06 -0800
        Re: Aliasing through a union pointer Richard Damon <Richard@Damon-Family.org> - 2015-11-26 13:09 -0800
      Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 13:08 -0800

csiph-web