Error trying to define a 1,024-bit (128 Byte) Bit Field

Question

I would like to define a large bitfield for the purpose of quickly monitoring the status a very large structure of elements. Here is what I have so far:

#define T         


        

Answer 1

As other have said, the C standard doesn't allow bit-fields to exceed the size of their attached integer type.

I'd suggest using plain arrays with some macro magic:

#include 
#include 
#include 

// SIZE should be a constant expression
// this avoids VLAs and problems resulting from being evaluated twice
#define BITFIELD(SIZE, NAME) \
    unsigned char NAME[(SIZE) / CHAR_BIT + ((SIZE) % CHAR_BIT != 0)]

static inline void setbit(unsigned char field[], size_t idx)
{ field[idx / CHAR_BIT] |= 1u << (idx % CHAR_BIT); }

static inline void unsetbit(unsigned char field[], size_t idx)
{ field[idx / CHAR_BIT] &= ~(1u << (idx % CHAR_BIT)); }

static inline void togglebit(unsigned char field[], size_t idx)
{ field[idx / CHAR_BIT] ^= 1u << (idx % CHAR_BIT); }

static inline _Bool isbitset(unsigned char field[], size_t idx)
{ return field[idx / CHAR_BIT] & (1u << (idx % CHAR_BIT)); }

int main(void)
{
    BITFIELD(1025, foo);
    printf("sizeof foo = %u\n", sizeof foo);

    memset(foo, 0, sizeof foo);
    printf("%i", isbitset(foo, 1011));

    setbit(foo, 1011);
    printf("%i", isbitset(foo, 1011));

    unsetbit(foo, 1011);
    printf("%i", isbitset(foo, 1011));
}

Hopefully, I didn't mess up the bit ops...