Question

    static struct{
            unsigned a:5;
            unsigned b:5;
            unsigned c:5;
            unsigned d:5;
    }v={1,2,3,4};

    printf("%d %d %d %d %d",v.a,v.b,v.c,v.d,sizeof(v));

Output is : 1 2 3 4 4

Can someone explain why is the size of this struct 4 ? Shouldn’t it be 16 ?

Answer 1

You’re using 20 bits which fit nicely in 4 bytes. No need for 16 bytes.

EDIT

I think it is actually required that they be packed like that:

6.7.2.1

An implementation may allocate any addressable storage unit large
enough to hold a bit- field. If enough space remains, a bit-field that
immediately follows another bit-field in a structure shall be packed
into adjacent bits of the same unit