Re: newlib, FreeRTOS, reentrancy, heap and related questions

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.arch.embedded
Subject	Re: newlib, FreeRTOS, reentrancy, heap and related questions
Date	2020-02-08 16:19 +0100
Organization	A noiseless patient Spider
Message-ID	<r1mjig$879$1@dont-email.me> (permalink)
References	(7 earlier) <OW5%F.340316$hD4.318614@fx47.iad> <r1j8e3$kj5$1@dont-email.me> <87imkizs3i.fsf@nightsong.com> <r1jbp7$lug$2@dont-email.me> <87eev6z8u3.fsf@nightsong.com>

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On 07/02/2020 17:00, Paul Rubin wrote:
> David Brown <david.brown@hesbynett.no> writes:
>> The point is that because the compiler knows what malloc and free do -
>> they are specified in the standards - it can use that knowledge for
>> optimisation.
> 
> In this case it has accessed unallocated memory.  I wonder if there is
> an exploit.  Hmm.
> 

No, it hasn't accessed unallocated memory at all.  It did not access 
anything.

The compiler could see that either the malloc worked fine and the result 
would be the value 6, or the malloc would fail (and return 0) in which 
case the program would have undefined behaviour (accessing a null 
pointer).  The compiler can assume that the programmer doesn't care what 
happens when executing undefined behaviour, and thus giving a result of 
6 is perfectly acceptable there too.  So the best code is simply to 
return 6 without any work at run time.

Ironically, if I had checked the result of malloc() for a null pointer, 
it could not have made this optimisation!

Thread

newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-06 13:26 +0100
  Re: newlib, FreeRTOS, reentrancy, heap and related questions Richard Damon <Richard@Damon-Family.org> - 2020-02-06 07:37 -0500
    Re: newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-06 13:57 +0100
      Re: newlib, FreeRTOS, reentrancy, heap and related questions David Brown <david.brown@hesbynett.no> - 2020-02-06 14:58 +0100
        Re: newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-06 16:06 +0100
          Re: newlib, FreeRTOS, reentrancy, heap and related questions David Brown <david.brown@hesbynett.no> - 2020-02-06 21:41 +0100
            Re: newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-06 23:23 +0100
              Re: newlib, FreeRTOS, reentrancy, heap and related questions Richard Damon <Richard@Damon-Family.org> - 2020-02-06 23:44 -0500
                Re: newlib, FreeRTOS, reentrancy, heap and related questions David Brown <david.brown@hesbynett.no> - 2020-02-07 09:51 +0100
                Re: newlib, FreeRTOS, reentrancy, heap and related questions Paul Rubin <no.email@nospam.invalid> - 2020-02-07 01:04 -0800
                Re: newlib, FreeRTOS, reentrancy, heap and related questions David Brown <david.brown@hesbynett.no> - 2020-02-07 10:48 +0100
                Re: newlib, FreeRTOS, reentrancy, heap and related questions Paul Rubin <no.email@nospam.invalid> - 2020-02-07 08:00 -0800
                Re: newlib, FreeRTOS, reentrancy, heap and related questions David Brown <david.brown@hesbynett.no> - 2020-02-08 16:19 +0100
                Re: newlib, FreeRTOS, reentrancy, heap and related questions Paul Rubin <no.email@nospam.invalid> - 2020-02-08 19:42 -0800
                Re: newlib, FreeRTOS, reentrancy, heap and related questions David Brown <david.brown@hesbynett.no> - 2020-02-09 12:11 +0100
              Re: newlib, FreeRTOS, reentrancy, heap and related questions David Brown <david.brown@hesbynett.no> - 2020-02-07 10:35 +0100
      Re: newlib, FreeRTOS, reentrancy, heap and related questions Dave Nadler <drn@nadler.com> - 2020-02-06 07:21 -0800
        Re: newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-06 17:14 +0100
        Re: newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-06 23:26 +0100
          Re: newlib, FreeRTOS, reentrancy, heap and related questions Richard Damon <Richard@Damon-Family.org> - 2020-02-06 23:29 -0500
      Re: newlib, FreeRTOS, reentrancy, heap and related questions Richard Damon <Richard@Damon-Family.org> - 2020-02-06 23:23 -0500
        Re: newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-07 09:19 +0100
  Re: newlib, FreeRTOS, reentrancy, heap and related questions upsidedown@downunder.com - 2020-02-07 11:36 +0200
    Re: newlib, FreeRTOS, reentrancy, heap and related questions pozz <pozzugno@gmail.com> - 2020-02-07 14:31 +0100
      Re: newlib, FreeRTOS, reentrancy, heap and related questions Dave Nadler <drn@nadler.com> - 2020-02-07 05:55 -0800
  Re: newlib, FreeRTOS, reentrancy, heap and related questions Dave Nadler <drn@nadler.com> - 2020-04-05 15:05 -0700

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