C enum as a type in a structure when using bit fields -

it understanding type bit field declarator should of int type. in fact, here line c99 standard

"a bit-field shall have type qualified or unqualified version of _bool, signed >int, unsigned int, or other implementation-defined type."

however, came across code today shows enum type, this.

typedef enum {     = 0,     b = 1 }enum;  typedef struct {     enum id : 8; }struct; 

without comments or documentation, it's hard tell intent. provide insight?

a , b both of int type, signed int. has length of 32 bit, meaning 4 byte.

but enum enum not need much.

0000000000000000000000000000000 equals 0000000000000000000000000000001 equals b 

so creator thought making enum shorter int bitfield of length of 8 bit, minimum length 1 byte.

00000000 or 00000001 

he have taken char type beginning length of 1 byte though.

on compilers can activate feature ensure enum can smaller int. using option --short-enums of gcc, makes use smallest type still fitting values.

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here example how save memory bitfield. see somebits struct smaller twoints struct.

#include "stdio.h"  struct oneint {   int x; };  struct twoints {   int x;   int y; };  struct somebits {   int x:2; // 2 bits   int y:6; // 6 bits };   int main (int argc, char** argv) {   printf("type int = %lu bytes\n", sizeof(int));   printf("oneint = %lu bytes\n", sizeof(struct oneint));   printf("twoints = %lu bytes\n", sizeof(struct twoints));   printf("somebits = %lu bytes\n", sizeof(struct somebits));   return 0; } 

output:

type int = 4 bytes oneint = 4 bytes twoints = 8 bytes somebits = 4 bytes