Simple gist to illustrate how C bitfields work

bitfields.c

/*
 *      Simple gist to illustrate how C bitfields work.
 */
#include <stdio.h>

struct {
        unsigned char   bit0 : 1;
        unsigned char   bit1 : 1;
        unsigned char   bit2 : 1;
        unsigned char   bit3 : 1;
        unsigned char   bit4 : 1;
        unsigned char   bit5 : 1;
        unsigned char   bit6 : 1;       // 8 1-bit bitfields fit in
        unsigned char   bit7 : 1;       // a single unsigned char.

#ifdef  MORE
        unsigned char   bit8 : 1;       // Adding a 9th 1-bit bitfield
                                        // grows the struct to 2 chars.
#endif  // MORE
}
        bitspace;

int main()
{
        printf("Sizeof bitspace = %ld byte(s)\n", sizeof bitspace);
        printf("    Bit 3 = %d\n", bitspace.bit3);
        printf("    Asserting bit 3\n");
        bitspace.bit3 = 1;
        printf("    Bit 3 = %d\n", bitspace.bit3);

        return  0;
}

makefile

CC      = gcc

all:            bitfields_8 bitfields_9

bitfields_8:	bitfields.c
	$(CC)           bitfields.c -o bitfields_8

bitfields_9:	bitfields.c
	$(CC) -DMORE=1  bitfields.c -o bitfields_9

run:		bitfields_8 bitfields_9
	./bitfields_8
	./bitfields_9

clean:
	rm -f *.o bitfields_[89]

Example build and run:

$ make run

gcc          bitfields.c -o bitfields_8
gcc -DMORE=1 bitfields.c -o bitfields_9

./bitfields_8
Sizeof bitspace = 1 byte(s)
    Bit 3 = 0
    Asserting bit 3
    Bit 3 = 1

./bitfields_9
Sizeof bitspace = 2 byte(s)
    Bit 3 = 0
    Asserting bit 3
    Bit 3 = 1