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Re: packing a "variable length struct"

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On 6/27/2011 14:12, Shao Miller wrote:
> On 6/27/2011 07:19, James Kuyper wrote:
>>
>> You should note, however, that while this technique works with most
>> (?all) C compilers, any program that uses it has undefined behavior.
>
> Bounds checking as undefined behaviour? Say it ain't so!
>...
> The meaning of "bounds" is my least favourite ambiguity in C.

Here's another "fun" bound-checking implementation check:

   /**** Identify insane bounds-checking */
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>

   /* An array type with 'char' elements */
   typedef char a_10char[10];

   char * test_ptr_merge(char * p, a_10char * qa) {
       typedef unsigned char * bp;
       char * q, * result;
       bp pi, qi, ri, re;

       q = *qa;
       /* 'q' now points to first element */

       pi = (bp)&p; qi = (bp)&q; ri = (bp)&result;
       re = ri + sizeof result;
       while (ri < re)
         /* Defect Report #260: "Provenance" */
         *ri++ = *pi++ & *qi++;
       pi = (bp)&p; qi = (bp)&q; ri = (bp)&result;
       while (ri < re)
         if (*ri != *pi++ || *ri++ != *qi++)
           return (void *) 0;
       return result;
     }

   int main(void) {
       void * vp;
       char * cp, * test;
       a_10char * ap;

       /* 15 bytes */
       vp = malloc(sizeof *ap + 5);
       if (!vp) {
           puts("Out of memory!");
           return EXIT_FAILURE;
         }

       cp = vp;
       ap = vp;
       /*
        * 'cp' and 'ap' now both point to the
        * same memory, which still has no
        * effective type.
        */

       test = test_ptr_merge(cp, ap);
       if (!test) {
           puts("Test showed insanity.");
           return EXIT_FAILURE;
         }

       /* What bounds apply to use of 'test'? */
       strcpy(test, "0123456789ABCD");
       puts(test);

       free(test);
       return EXIT_SUCCESS;
     }

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Thread

packing a "variable length struct" fishboy <j.j.fishbat@gmail.com> - 2011-06-27 02:46 -0700
  Re: packing a "variable length struct" Ian Collins <ian-news@hotmail.com> - 2011-06-27 22:48 +1200
  Re: packing a "variable length struct" Alexander Bartolich <alexander.bartolich@gmx.at> - 2011-06-27 11:01 +0000
    Re: packing a "variable length struct" James Kuyper <jameskuyper@verizon.net> - 2011-06-27 07:19 -0400
      Re: packing a "variable length struct" Shao Miller <sha0.miller@gmail.com> - 2011-06-27 14:12 -0400
        Re: packing a "variable length struct" Shao Miller <sha0.miller@gmail.com> - 2011-06-27 15:08 -0400
        Re: packing a "variable length struct" Alexander Bartolich <alexander.bartolich@gmx.at> - 2011-06-27 20:09 +0000
          Re: packing a "variable length struct" Shao Miller <sha0.miller@gmail.com> - 2011-06-27 17:45 -0400
  Re: packing a "variable length struct" Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-27 07:24 -0400
    Re: packing a "variable length struct" Keith Thompson <kst-u@mib.org> - 2011-06-27 09:06 -0700
      Re: packing a "variable length struct" BGB <cr88192@hotmail.com> - 2011-06-27 15:35 -0700
    Re: packing a "variable length struct" fishboy <j.j.fishbat@gmail.com> - 2011-06-30 23:54 -0700

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