Bitfields in C

Struct members measured in bits, for packing flags and small counters.

#include <stdio.h>

typedef struct {
    unsigned int visible : 1;
    unsigned int locked : 1;
    unsigned int stars : 2;
    unsigned int tour : 4;
} StepFlags;

void flags(void) {
    StepFlags f = {.visible = 1, .stars = 3, .tour = 11};

    printf("size %zu\n", sizeof f);
    printf("visible %u locked %u stars %u tour %u\n", f.visible, f.locked,
           f.stars, f.tour);

    f.stars = 4;
    printf("wrapped stars to %u\n", f.stars);
}

How it works

1. The width follows the member name after a colon.
2. Packing order and padding are implementation defined.
3. An unsigned type avoids surprises with the sign bit.

Keywords and builtins used here

• flags
• int
• sizeof
• struct
• typedef
• unsigned
• void

The run, in numbers

Lines

19

Characters to type

401

Tokens

113

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 68 seconds.

Type this snippet

Step 3 of 5 in Unions & layout, step 3 of 13 in Unions, bitfields & undefined behavior.

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