Lesson of 17 February 2011

Union and big/little endian

File biglittle.cc

 1// to compile on unix/OSX use
 2// g++ -I. biglittle.cc
 3//
 4
 5// include header for C++ I/O
 6// search stdio.h in standard C/C++ include directory e.g. /usr/include
 7#include <iostream>  // for the definition of cin, cout, ...
 8#include <iomanip>   // for manipulator setw, setprecision ...
 9
10// include header for STL vector and algorithm
11#include <vector>
12#include <algorithm>
13
14#include "pstdint.h" // for uint64_t 
15
16#include <cstdlib> // near equivalent to <stdlib.h>  for exit, rand, ...
17#include <cmath>   // near equivalent to <math.h>    for sin, cos, log, ..
18
19using namespace std ;
20
21int
22main() {
23
24  // declare a struct called pluto
25  struct pluto {
26    float x, y ;
27  }  ;
28
29  // declare a struct called pluto
30  typedef struct { float x, y ; } typeStructWithXYField ;
31
32  // declare a struct called pippo
33  struct pippo {
34    double a ;
35    double b ;
36    double c ;
37    int    d ;
38    struct { // no name struct
39      float x, y ;
40    } ABC ;
41  } ;
42  
43  union checkBigEndian {
44    uint64_t a_number ;
45    uint8_t  a_list[8] ;
46  } ;
47  
48  // define X of type struct pippo
49  struct pippo X ;
50  typeStructWithXYField Z ;
51  
52  X . ABC . x = 1 ;
53
54  union checkBigEndian check ;
55
56  check . a_number = 0x0102030405060708 ;
57  cout << hex 
58       << "a[0] = " << (int)check . a_list[0] << '\n'
59       << "a[1] = " << (int)check . a_list[1] << '\n'
60       << "a[2] = " << (int)check . a_list[2] << '\n'
61       << "a[3] = " << (int)check . a_list[3] << '\n'
62       << "a[4] = " << (int)check . a_list[4] << '\n'
63       << "a[5] = " << (int)check . a_list[5] << '\n'
64       << "a[6] = " << (int)check . a_list[6] << '\n'
65       << "a[7] = " << (int)check . a_list[7] << '\n' ;
66  
67  uint64_t a_value = 0x0102030405060708 ;
68  //uint8_t * a_pointer = (uint8_t*)(&a_value) ;
69  uint8_t * a_pointer = reinterpret_cast<uint8_t*>(&a_value) ;
70
71  cout << hex 
72       << "a[0] = " << (int)a_pointer[0] << '\n'
73       << "a[1] = " << (int)a_pointer[1] << '\n'
74       << "a[2] = " << (int)a_pointer[2] << '\n'
75       << "a[3] = " << (int)a_pointer[3] << '\n'
76       << "a[4] = " << (int)a_pointer[4] << '\n'
77       << "a[5] = " << (int)a_pointer[5] << '\n'
78       << "a[6] = " << (int)a_pointer[6] << '\n'
79       << "a[7] = " << (int)a_pointer[7] << '\n' ;
80  
81
82  return 0 ;
83}

Dynamic allocation on the stack

File Dallocation.cc

 1// to compile on unix/OSX use
 2// g++ -I. Dallocation.cc
 3//
 4
 5// include header for C++ I/O
 6// search stdio.h in standard C/C++ include directory e.g. /usr/include
 7#include <iostream>  // for the definition of cin, cout, ...
 8#include <iomanip>   // for manipulator setw, setprecision ...
 9
10#include <cstdlib> // near equivalent to <stdlib.h>  for exit, rand, ...
11#include <cmath>   // near equivalent to <math.h>    for sin, cos, log, ..
12
13using namespace std ;
14
15#ifdef USE_ALLOCA
16  // alloca use stack for allocation
17  #include <alloca.h>
18#endif
19
20int
21main( int argc, char const * argv []) {
22
23  int dim ;
24  sscanf( argv[1], "%d", &dim ) ;
25
26  {
27    #ifdef USE_ALLOCA
28    unsigned * A = (unsigned*)alloca(dim*sizeof(unsigned)) ;
29    #else
30    unsigned A[dim] ;
31    #endif
32    cout << "A size = " << sizeof( A ) << endl ;
33    for ( unsigned i = 0 ; i < sizeof(A)/sizeof(A[0]) ; ++i )
34      A[i] = i ;
35  }
36  
37  { 
38    #ifdef USE_ALLOCA
39    unsigned * B = (unsigned*)alloca(dim*sizeof(unsigned)) ;
40    #else
41    unsigned B[dim] ;
42    #endif
43    cout << "B size = " << sizeof( B ) << endl ;
44    for ( unsigned i = 0 ; i < sizeof(B)/sizeof(B[0]) ; ++i )
45      B[i] = i ;
46    {
47      #ifdef USE_ALLOCA
48      unsigned * C = (unsigned*)alloca(dim*sizeof(unsigned)) ;
49      #else
50      unsigned C[dim] ;
51      #endif
52      cout << "C size = " << sizeof( C ) << endl ;
53      for ( unsigned i = 0 ; i < sizeof(C)/sizeof(C[0]) ; ++i )
54        C[i] = i ;
55    }
56  }
57  
58  return 0 ;
59}

The C way of dynamic allocation

File Dallocation2.cc

 1// to compile on unix/OSX use
 2// g++ -I. Dallocation.cc
 3//
 4
 5// include header for C++ I/O
 6// search stdio.h in standard C/C++ include directory e.g. /usr/include
 7#include <iostream>  // for the definition of cin, cout, ...
 8#include <iomanip>   // for manipulator setw, setprecision ...
 9
10#include <cstdlib> // near equivalent to <stdlib.h>  for exit, rand, ...
11#include <cmath>   // near equivalent to <math.h>    for sin, cos, log, ..
12
13
14using namespace std ;
15
16// use dynamic allocation of C
17
18int
19main( int argc, char const * argv []) {
20
21  int dim ;
22  sscanf( argv[1], "%d", &dim ) ;
23
24  {
25    unsigned * A = (unsigned*)malloc( dim*sizeof(unsigned) ) ;
26    if ( A == NULL ) {
27      cerr << "Memory allocation failed\n" ;
28      exit(0) ;
29    }
30    
31    cout << "A size = " << dim*sizeof(unsigned) << endl ;
32    for ( unsigned i = 0 ; i < dim ; ++i )
33      A[i] = i ;
34
35    free( A ) ; A = NULL ; // free used memory
36    free( A ) ; // free used memory
37  }
38  
39  { 
40    unsigned * B = (unsigned*)malloc( dim*sizeof(unsigned) ) ;
41    if ( B == NULL ) {
42      cerr << "Memory allocation failed\n" ;
43      exit(0) ;
44    }
45    cout << "B size = " << dim*sizeof(unsigned) << endl ;
46    for ( unsigned i = 0 ; i < dim ; ++i )
47      B[i] = i ;
48    free( B ) ; // free used memory
49    {
50      unsigned * C = (unsigned*)malloc( dim*sizeof(unsigned) ) ;
51      if ( C == NULL ) {
52        cerr << "Memory allocation failed\n" ;
53        exit(0) ;
54      }
55      cout << "C size = " << dim*sizeof(unsigned) << endl ;
56      for ( unsigned i = 0 ; i < dim ; ++i )
57        C[i] = i ;
58    }
59  }
60  
61  return 0 ;
62}

The C++ way of dynamic allocation

File Dallocation3.cc

 1// to compile on unix/OSX use
 2// g++ -I. Dallocation.cc
 3//
 4
 5// include header for C++ I/O
 6// search stdio.h in standard C/C++ include directory e.g. /usr/include
 7#include <iostream>  // for the definition of cin, cout, ...
 8#include <iomanip>   // for manipulator setw, setprecision ...
 9
10#include <cstdlib> // near equivalent to <stdlib.h>  for exit, rand, ...
11#include <cmath>   // near equivalent to <math.h>    for sin, cos, log, ..
12
13
14using namespace std ;
15
16// use dynamic allocation of C++
17
18int
19main( int argc, char const * argv []) {
20
21  if ( argc != 2 ) {
22    cerr << " expect an argument!\n" ;
23    exit(1) ;
24  }
25
26  int dim ;
27  sscanf( argv[1], "%d", &dim ) ;
28
29  {
30    unsigned * A = new unsigned[ dim ] ;
31
32    cout << "A size = " << dim*sizeof(unsigned) << endl ;
33    for ( unsigned i = 0 ; i < dim ; ++i )
34      A[i] = i ;
35      
36    delete [] A ;
37  }
38  
39  { 
40    unsigned * B = new unsigned[ dim ] ;
41
42    cout << "B size = " << dim*sizeof(unsigned) << endl ;
43    for ( unsigned i = 0 ; i < dim ; ++i )
44      B[i] = i ;
45    delete [] B ; // free used memory
46    {
47      unsigned * C = new unsigned[ dim ] ;
48      cout << "C size = " << dim*sizeof(unsigned) << endl ;
49      for ( unsigned i = 0 ; i < dim ; ++i )
50        C[i] = i ;
51      delete [] C ; // free used memory
52    }
53  }
54  return 0 ;
55}