Why is this struct size 3 instead of 2?

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Because you are using char as the underlying type for your fields, the compiler tries to group bits by bytes, and since it cannot put more than eight bits in each byte, it can only store two fields per byte.

The total sum of bits your struct uses is 15, so the ideal size to fit that much data would be a short.

#include <stdio.h>

typedef struct
{
  char A:3;
  char B:3;
  char C:3;
  char D:3;
  char E:3;
} col; 


typedef struct {
  short A:3;
  short B:3;
  short C:3;
  short D:3;
  short E:3;
} col2; 


int main(){

  printf("size of col: %lu\n", sizeof(col));
  printf("size of col2: %lu\n", sizeof(col2));

}

The above code (for a 64-bit platform like mine) will indeed yield 2 for the second struct. For anything larger than a short, the struct will fill no more than one element of the used type, so – for that same platform – the struct will end up with size four for int , eight for long, etc.

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