How is the size of a struct with Bit Fields determined/measured?

by

It is the order that matters. The following code will give Output: 8

#include<stdio.h>

typedef struct size
{
        unsigned int b:32;
        unsigned int a:1;
        unsigned int c:1;
}mystruct;

int main(int argc, char const *argv[])
{
        mystruct a;
        printf("\n %lu \n",sizeof(a));
        return 0;
}

Unsigned int is a 32 bit integer, occupying 4 bytes. Memory is allocated contiguously in memory.

Option 1:

unsigned int a:1;       // First 4 bytes are allocated
unsigned int b:31;      // Will get accomodated in the First 4 bytes
unsigned int c:1;       // Second 4 bytes are allocated

Output: 8

Option 2:

unsigned int a:1;       // First 4 bytes are allocated
unsigned int b:32;      // Will NOT get accomodated in the First 4 bytes, Second 4 bytes are allocated
unsigned int c:1;       // Will NOT get accomodated in the Second 4 bytes, Third 4 bytes are allocated

Output: 12

Option 3:

unsigned int a:1;       // First 4 bytes are allocated
unsigned int b:1;       // Will get accomodated in the First 4 bytes
unsigned int c:1;       // Will get accomodated in the First 4 bytes

Output: 4

Option 4:

unsigned int b:32;      // First 4 bytes are allocated
unsigned int a:1;       // Second 4 bytes are allocated
unsigned int c:1;       // Will get accomodated in the Second 4 bytes

Output: 8

More Related Contents:

  1. Why bit endianness is an issue in bitfields?
  2. How does a C-style struct with a bitfield get represented in a Rust #[repr(C)] struct?
  3. Questions about C bitfields
  4. Signed bit field represetation
  5. Order of fields when using a bit field in C
  6. Storing value of unsigned int variable in char array in C [closed]
  7. Simple C scanf does not work? [duplicate]
  8. Pointers in C: when to use the ampersand and the asterisk?
  9. Creating a daemon in Linux
  10. printf format specifiers for uint32_t and size_t
  11. strtok function thread safety
  12. What happens when a variable goes out of scope?
  13. Wrap around explanation for signed and unsigned variables in C?
  14. Where to find stdio.h functions implementations?
  15. Getting Segmentation Fault
  16. Delete a Line from a file in C Language
  17. Why are global variables bad, in a single threaded, non-os, embedded application
  18. Convert double/float to string
  19. Find malloc() array length in C? [duplicate]
  20. Is there a meaningful distinction between freestanding and hosted implementations?
  21. Deep Analysis of Const Qualifier in C
  22. default value for struct member in C
  23. Store data in array from input [duplicate]
  24. Finding out if a message over tcp was delivered
  25. calling assembly function from c
  26. What is the use of typedef?
  27. Dereference void pointer
  28. cross-platform printing of 64-bit integers with printf
  29. C11 _Generic: how to deal with string literals?
  30. Redefining NULL

Leave a Comment