[plt-scheme] off (plt) topic (short koan in .c language and a question)

Yes, thank you. I understood your program the first time. I'm saying
that I believe it is not legal C code, due to an obscure part of the
specification of the C programming language. Unfortunately, I cannot
point you to a relevant version of the specification. Apologies and
I'll just be quiet now.

Robby

On Fri, May 7, 2010 at 2:11 PM, Vladimir Konrad <vladimir at ok2home.net> wrote:
>> On Fri, May 7, 2010 at 10:01 AM, vladimir<vladimir at ok2home.net>  wrote:
>>>>
>>>> even without pointers, allocation or dubious-looking type casts:
>>>
>>> but none of that type casts should give any warning in decent .c
>>> compiler (on arch where size of integer equals the size of float in
>>> bits).
>>
>> If I understand the spec properly (which is not clear....) then your
>> program was illegal, but the revision was okay, since the spec (I
>> think) says that you cannot point to the same address with two
>> different types unless one of the types is void*.
>
> I think the program is valid, one can _read_ void* any way
> one likes. The program is kinda like this:
>
>  print 4 bytes below as float
>   |
>   |
> [XXXX]  (initialised 4 bytes - the chunk* - the actual bits,
>   |     not the variable)
>   |
>  print the 4 bytes above as integer
>
>
>
> I have commented the code and at the very end there is simplification...
>
> (a comment above a line is about this line)
>
> #-------------------------
>
> #include <stdlib.h>
> #include <stdio.h>
>
> int main(int argc, char **argv) {
>  /* one wants a pointer to a raw ram - blank "piece of paper" */
>  void* chunk;
>
>  / * make a particular bit pattern - our (old fashioned) "stamp"
>
>    http://cache.gawker.com/assets/images/gizmodo/2009/03/01.stamp.jpg
>
>  this "stamp" has a name "number"
>
>  */
>  const int number = 1234567890;
>
>  /* integer "paper" (used later to hold the bit pattern above,
>   as float
>   */
>  int i;
>
>  /* float "paper" (used later to hold the same bit pattern,
>     but as integer
>  */
>  float f;
>
>  /* allocate untyped chunk of memory -
>     create a "blank piece of paper" */
>  chunk = calloc(sizeof(int), 1);
>
>  if( chunk == NULL ) {
>    printf("cannot allocate\n");
>    return 1;
>  }
>  else {
>    /* initialise the blank memory (to particular bit pattern) using
>    our "stamp" */
>    /* means: initialise (i.e. set bits) in "chunk" as if it
>       was integer, using "number" (which is an integer) */
>    *(int *) chunk = number;
>
>    /* transfer the bit pattern (from "paper") as if it was float,
>       to "paper" f (which is a float */
>    f = *(float *) chunk;
>
>    /* transfer the same bit pattern (from "paper") as if it was
>       integer, to "paper" i (which is an integer) */
>    i = *(int *) chunk;
>
>    /* display 2 different interpretations of the same bit pattern */
>    printf("%i\t%f\n", i, f);
>  }
>
>  return 0;
>
> }
>
> /* eof */
>
> by the way, in the above, one could just write:
>
>    printf("%i\t%f\n", *(int *) chunk, *(float *) chunk);
>
> (kinda denser, but maybe clearer)
>
> It basicaly means: deallocate chunk as int pointer, deallocate chunk as
> float pointer, print each as proper "type" (i.e. using particular
> interpretation of bits in memory)...
>
>
> Kind regards,
>
> Vladimir
>
> --
>
> Just because you have intelligence like a hammer,
> you do not have to treat everyone else like a nail...
>
>    -- Principia Discordia (one of the versions)
>