strukt.c

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#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>//offsetof()

struct osoba_s {
char jmeno[20];//v inicializatoru se pak pokusime inicializovat delsi retezcem
int  cislo;
};

//offset clena struktury pocita makro offsetof() stddef.h

//ruzny padding mezi cleny struct-ury, GCC, 64bit: LP64 platoforma
struct datum  { char den; char mesic; short  rok; } ;//sizeof == 4
struct datum1 { char den; short  rok; char mesic; } ;//sizeof == 6 misto 4 !!!
struct datum6 { char den; char mesic; int   rok ; } ;//sizeof  8 misto  6 (1+1+4) !!!
struct ilil   { int i1; long l1; int i2; long l2; } ;//sizeof 32 misto 24 (4+8+4+8) !!!
struct iill   { int i1; int  i2;long l1; long l2; } ;//sizeof 24

//union: cleny struktury se v pameti prekryvaji:
union ruzne_u {
  short s;
  int   i;
  long  l;
  struct datum datum;
  char  ch4[4];
  char  ch8[8];
} ruzne;



int main() {

//da endian se zjistit jen pomoci maker? bez promenne?
static union endian_s {
  short s;
  char ch2[2];
} endian;

struct osoba_s o = {"01234567890123456789ABC"};//inicializace prepise .cislo? => warning: too long ...

struct datum d1 = {1,2,2048};//zkusime pretypovat na int

endian.s = 'L'*256 + 'E';

printf("endian: %c%c .s=%hx\n", endian.ch2[0], endian.ch2[1], endian.s);

printf("o.jmeno=\"%s\"\n", o.jmeno);
printf("o.cislo=%i\n", o.cislo);

printf("sizeof(int)  = %zu\n", sizeof(int));
printf("sizeof(long) = %zu\n", sizeof(long));
printf("sizeof(struct datum)   = %zu\n", sizeof(struct datum));
printf("sizeof(struct datum1)  = %zu\n", sizeof(struct datum1));
printf("datum1.rok   offset: %zu\n", offsetof(struct datum1, rok));
printf("datum1.mesic offset: %zu\n", offsetof(struct datum1, mesic));//padding je za poslednim clenem struct
printf("sizeof(struct datum6) = %zu\n", sizeof(struct datum6));
printf("sizeof(struct ilil)   = %zu\n", sizeof(struct ilil));
printf("sizeof(int+long+int+long) = %zu\n", 2*(sizeof(int)+sizeof(long)));
printf("sizeof(struct iill)   = %zu\n", sizeof(struct iill));

//pretypovat struct datum na int nelze!!! pomoci (int)d1
//musime pouzit union:
ruzne.datum = d1;
printf("datum jako   int=%u\n", ruzne.i);
printf("datum jako   int=%x\n", ruzne.i);//1,2,2048 pozpatku, na 99% little endian platforma! (intel/amd)
printf("datum jako short=%04x\n", ruzne.s);
return 0;

}