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  1. #include <stdio.h>
  2.  
  3. void dumpMem(void *start, int numBytes)
  4. {
  5.     printf("memory at %p:", start);
  6.     char *p = (char *)start;
  7.     while((unsigned long)p%8){ p--;  numBytes++;}   // align to 8 byte boundary
  8.     for(int i=0; i<numBytes; i++) 
  9.     {
  10.         if( i%8 == 0 ) printf("\nAddr %p:", p+i);
  11.         printf(" %02x", (unsigned int) (p[i] & 0xff));
  12.     }
  13.     putchar('\n');
  14. }
  15.  
  16. int len;        // static allocation, protect them from stack overwrites
  17. char *from, *to;
  18.  
  19. int main(void)
  20. {
  21.     unsigned int    declStart = 0xaaaaaaaa; // marker
  22.     int             *x = (int *)   0xbbbbbbbbbbbbbbbb;
  23.     double          *y = (double *)0xcccccccccccccccc;
  24.     unsigned int    declEnd = 0xdddddddd;   // marker
  25.  
  26.     printf("Addr. of x: %p,\n      of y: %p\n", &x, &y);
  27.  
  28.     // This is all UB because the pointers are not
  29.     // belonging to the same object. But it should
  30.     // work on standard architectures.
  31.     // All calls to dumpMem() therefore are UB, too.
  32.  
  33.     // Thinking of it, I'd be hard-pressed to find 
  34.     // any defined behavior in this program.
  35.     if( &declStart < &declEnd ) 
  36.     {
  37.         from = (char *)&declStart;
  38.         to = (char *)&declEnd + sizeof(declEnd);
  39.     }
  40.     else
  41.     {
  42.         from = (char *)&declEnd;
  43.         to = (char *)&declStart + sizeof(declStart);
  44.     }
  45.     len = to - from;
  46.  
  47.     printf("len is %d\n", len);
  48.     printf("Memory after initializations:\n");
  49.     dumpMem(from, len);
  50.  
  51.     x = (int *)&x;
  52.     printf("\nMemory after assigning own address %p to x/*x: \n", &x);
  53.     dumpMem(from, len);
  54.  
  55.     *x = 3;
  56.     printf("\nMemory after assigning 3 to x/*x: \n");
  57.     dumpMem(from, len);
  58.  
  59.     //print val of pointer
  60.     printf("x as long: %d\n", (unsigned long)x);
  61.  
  62.     y = (double *)&y;
  63.     *y = 4.0;
  64.     printf("\nMemory after assigning 4.0 to y/*y: \n");
  65.     dumpMem(from, len);
  66.  
  67.     printf("y as float: %f\n", y);
  68.     printf("y as double: %lf\n", y); // prints 0.0
  69.     printf("y, cast as double: %lf\n", *(double *)&y); // prints 4.0
  70.     printf("y as unsigned int: 0x%x\n", y);
  71.     printf("y as unsigned long: 0x%lx\n", y);
  72.  
  73.     return 0;
  74. }
Success #stdin #stdout 0s 2112KB
comments (?)
stdin 
Standard input is empty
stdout 
Addr. of x: 0xbfba00c4,
      of y: 0xbfba00c8
len is 16
Memory after initializations:
memory at 0xbfba00c0:
Addr 0xbfba00c0: aa aa aa aa bb bb bb bb
Addr 0xbfba00c8: cc cc cc cc dd dd dd dd

Memory after assigning own address 0xbfba00c4 to x/*x: 
memory at 0xbfba00c0:
Addr 0xbfba00c0: aa aa aa aa c4 00 ba bf
Addr 0xbfba00c8: cc cc cc cc dd dd dd dd

Memory after assigning 3 to x/*x: 
memory at 0xbfba00c0:
Addr 0xbfba00c0: aa aa aa aa 03 00 00 00
Addr 0xbfba00c8: cc cc cc cc dd dd dd dd
x as long: 3

Memory after assigning 4.0 to y/*y: 
memory at 0xbfba00c0:
Addr 0xbfba00c0: aa aa aa aa 03 00 00 00
Addr 0xbfba00c8: 00 00 00 00 00 00 10 40
y as float: -0.101574
y as double: -0.101574
y, cast as double: 4.000000
y as unsigned int: 0x0
y as unsigned long: 0x0
https://ideone.com/TJAXli
language:
C (gcc 8.3)
created:
8 years ago
visibility:
public

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