title: What belongs in an educational tool to demonstrate the unwarranted assumptions people make in C/C++?
body: <p>I'd like to prepare a little educational tool for SO which should help beginners (and intermediate) programmers to recognize and challenge their unwarranted assumptions in C, C++ and their platforms.</p>
  
  <p>Examples: </p>
  
  <ul>
  <li>"integers wrap around"</li>
  <li>"everyone has ASCII"</li>
  <li>"I can store a function pointer in a void*"</li>
  </ul>
  
  <p>I figured that a small test program could be run on various platforms, which runs the "plausible" assumptions which are, from our experience in SO, usually made by many inexperienced/semiexperienced mainstream developers and record the ways they break on diverse machines.</p>
  
  <p>The goal of this is not to prove that it is "safe" to do something (which would be impossible to do, the tests prove only anything if they break), but instead to demonstrate to even the most uncomprehending individual <strong>how the most inconspicuous expression break on a different machine, if it has a undefined or implementation defined behavior.</strong>. </p>
  
  <p>To achieve this I would like to ask you:</p>
  
  <ul>
  <li>How can this idea be improved?</li>
  <li>Which tests would be good and how should they look like?</li>
  <li>Would you run the tests on the platforms you can get your hands on and post the results, so that we end up with a database of platforms, how they differ and why this difference is allowed?</li>
  </ul>
  
  <hr>
  
  <p>Here's the current version for the test toy:</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;limits.h&gt;
  #include &lt;stdlib.h&gt;
  #include &lt;stddef.h&gt;
  int count=0;
  int total=0;
  void expect(const char *info, const char *expr)
  {
      printf("..%s\n   but '%s' is false.\n",info,expr);
      fflush(stdout);
      count++;
  }
  #define EXPECT(INFO,EXPR) if (total++,!(EXPR)) expect(INFO,#EXPR)
  
  /* stack check..How can I do this better? */
  ptrdiff_t check_grow(int k, int *p)
  {
      if (p==0) p=&amp;k;
      if (k==0) return &amp;k-p;
      else return check_grow(k-1,p);
  }
  #define BITS_PER_INT (sizeof(int)*CHAR_BIT)
  
  int bits_per_int=BITS_PER_INT;
  int int_max=INT_MAX;
  int int_min=INT_MIN;
  
  /* for 21 - left to right */
  int ltr_result=0;
  unsigned ltr_fun(int k)
  {
      ltr_result=ltr_result*10+k;
      return 1;
  }
  
  int main()
  {
      printf("We like to think that:\n");
      /* characters */
      EXPECT("00 we have ASCII",('A'==65));
      EXPECT("01 A-Z is in a block",('Z'-'A')+1==26);
      EXPECT("02 big letters come before small letters",('A'&lt;'a'));
      EXPECT("03 a char is 8 bits",CHAR_BIT==8);
      EXPECT("04 a char is signed",CHAR_MIN==SCHAR_MIN);
  
      /* integers */
      EXPECT("05 int has the size of pointers",sizeof(int)==sizeof(void*));
      /* not true for Windows-64 */
      EXPECT("05a long has at least the size of pointers",sizeof(long)&gt;=sizeof(void*));
  
      EXPECT("06 integers are 2-complement and wrap around",(int_max+1)==(int_min));
      EXPECT("07 integers are 2-complement and *always* wrap around",(INT_MAX+1)==(INT_MIN));
      EXPECT("08 overshifting is okay",(1&lt;&lt;bits_per_int)==0);
      EXPECT("09 overshifting is *always* okay",(1&lt;&lt;BITS_PER_INT)==0);
      {
          int t;
          EXPECT("09a minus shifts backwards",(t=-1,(15&lt;&lt;t)==7));
      }
      /* pointers */
      /* Suggested by jalf */
      EXPECT("10 void* can store function pointers",sizeof(void*)&gt;=sizeof(void(*)()));
      /* execution */
      EXPECT("11 Detecting how the stack grows is easy",check_grow(5,0)!=0);
      EXPECT("12 the stack grows downwards",check_grow(5,0)&lt;0);
  
      {
          int t;
          /* suggested by jk */
          EXPECT("13 The smallest bits always come first",(t=0x1234,0x34==*(char*)&amp;t));
      }
      {
          /* Suggested by S.Lott */
          int a[2]={0,0};
          int i=0;
          EXPECT("14 i++ is strictly left to right",(i=0,a[i++]=i,a[0]==1));
      }
      {
          struct {
              char c;
              int i;
          } char_int;
          EXPECT("15 structs are packed",sizeof(char_int)==(sizeof(char)+sizeof(int)));
      }
      {
          EXPECT("16 malloc()=NULL means out of memory",(malloc(0)!=NULL));
      }
  
      /* suggested by David Thornley */
      EXPECT("17 size_t is unsigned int",sizeof(size_t)==sizeof(unsigned int));
      /* this is true for C99, but not for C90. */
      EXPECT("18 a%b has the same sign as a",((-10%3)==-1) &amp;&amp; ((10%-3)==1));
  
      /* suggested by nos */
      EXPECT("19-1 char&lt;short",sizeof(char)&lt;sizeof(short));
      EXPECT("19-2 short&lt;int",sizeof(short)&lt;sizeof(int));
      EXPECT("19-3 int&lt;long",sizeof(int)&lt;sizeof(long));
      EXPECT("20 ptrdiff_t and size_t have the same size",(sizeof(ptrdiff_t)==sizeof(size_t)));
  #if 0
      {
          /* suggested by R. */
          /* this crashed on TC 3.0++, compact. */
          char buf[10];
          EXPECT("21 You can use snprintf to append a string",
                 (snprintf(buf,10,"OK"),snprintf(buf,10,"%s!!",buf),strcmp(buf,"OK!!")==0));
      }
  #endif
  
      EXPECT("21 Evaluation is left to right",
             (ltr_fun(1)*ltr_fun(2)*ltr_fun(3)*ltr_fun(4),ltr_result==1234));
  
      {
      #ifdef __STDC_IEC_559__
      int STDC_IEC_559_is_defined=1;
      #else 
      /* This either means, there is no FP support
       *or* the compiler is not C99 enough to define  __STDC_IEC_559__
       *or* the FP support is not IEEE compliant. */
      int STDC_IEC_559_is_defined=0;
      #endif
      EXPECT("22 floating point is always IEEE",STDC_IEC_559_is_defined);
      }
  
      printf("From what I can say with my puny test cases, you are %d%% mainstream\n",100-(100*count)/total);
      return 0;
  }
  </code></pre>
  
  <hr>
  
  <p>Oh, and I made this community wiki right from the start because I figured that people want to edit my blabber when they read this.</p>
  
  <p><strong>UPDATE</strong> Thanks for your input. I've added a few cases from your answers and will see if I can set up a github for this like Greg suggested.</p>
  
  <p><strong>UPDATE</strong>:
  I've created a github repo for this, the file is "gotcha.c": </p>
  
  <ul>
  <li><a href="http://github.com/lutherblissett/disenchanter" rel="noreferrer">http://github.com/lutherblissett/disenchanter</a></li>
  </ul>
  
  <p>Please answer here with patches or new ideas, so they can be discussed or clarified here. I will merge them into gotcha.c then.</p>