Is it possible to use array of bit fields?

Question

I am curious to know, Is it possible to use array of bit fields? Like:

struct st {   unsigned int i[5]: 4; };  

Answer 1

No, you can't. Bit field can only be used with integral type variables.

C11-§6.7.2.1/5

A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type.

Alternatively you can do this

struct st {     unsigned int i: 4;   } arr_st[5];  

but its size will be 5 times the size of a struct (as mentioned in comment by @Jonathan Leffler) having 5 members each with bit field 4. So, it doesn't make much sense here.

More closely you can do this

struct st {     uint8_t i: 4;   // Will take only a byte } arr_st[5];  

Answer 2

C does not support arrays of bit-fields, so the short answer is no.

For very large arrays, it might be worthwhile to pack values, 2 per byte, this way:

#define ARRAY_SIZE  1000000  unsigned char arr[(ARRAY_SIZE + 1) / 2];  int get_4bits(const unsigned char *arr, size_t index) {     return arr[index >> 1] >> ((index & 1) << 2); }  int set_4bits(unsigned char *arr, size_t index, int value) {     arr[index >> 1] &= ~ 0x0F << ((index & 1) << 2);     arr[index >> 1] |= (value & 0x0F) << ((index & 1) << 2); }