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  1. #include <stdio.h>
  2. #include <stdlib.h>
  3.  
  4. typedef struct {
  5.     union {
  6.         struct custom {
  7.             unsigned char Lower8;    // al
  8.             unsigned char Higher8;   // ah
  9.             unsigned short Middle16; // ax
  10.             unsigned int Higher32;   // eax
  11.         } sizes;
  12.         unsigned long long T64bit; // rax
  13.     };
  14. } reg_t;
  15.  
  16. int main(void) {
  17.     reg_t AX_REG;
  18.     printf("sizeof(reg_t) == %u\n", sizeof(reg_t));
  19.     AX_REG.T64bit = 0x1122334455667788;
  20.     printf("Actual: 0x%08lX\n", AX_REG.T64bit);
  21.     AX_REG.sizes.Higher32 = 0xDEADBEEF;
  22.     printf("Actual: 0x%08lX\n", AX_REG.T64bit);
  23.     AX_REG.sizes.Lower8 = 0x00;
  24.     printf("Actual: 0x%08lX\n", AX_REG.T64bit);
  25.     AX_REG.sizes.Higher8 = 0xFF;
  26.     printf("Actual: 0x%08lX\n", AX_REG.T64bit);
  27.     AX_REG.sizes.Middle16 = 0x1234;
  28.     printf("Actual: 0x%08lX\n", AX_REG.T64bit);
  29.     return 0;
  30. }
Success #stdin #stdout 0s 9432KB
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stdin
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Standard input is empty
stdout
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sizeof(reg_t) == 8
Actual: 0x1122334455667788
Actual: 0xDEADBEEF55667788
Actual: 0xDEADBEEF55667700
Actual: 0xDEADBEEF5566FF00
Actual: 0xDEADBEEF1234FF00
https://ideone.com/CfDP2u
language:
C (gcc 8.3)
created:
8 years ago
visibility:
public

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