If you want to optimize things in C you can use the : notation to specify how
many bits you want to use, read more on
geeksforgeeks
Redis uses this a lot like for this struct:
/* quicklistNode is a 32 byte struct describing a ziplist for a quicklist.
* We use bit fields keep the quicklistNode at 32 bytes.
* count: 16 bits, max 65536 (max zl bytes is 65k, so max count actually < 32k).
* encoding: 2 bits, RAW=1, LZF=2.
* container: 2 bits, NONE=1, ZIPLIST=2.
* recompress: 1 bit, bool, true if node is temporarry decompressed for usage.
* attempted_compress: 1 bit, boolean, used for verifying during testing.
* extra: 10 bits, free for future use; pads out the remainder of 32 bits */
typedef struct quicklistNode {
struct quicklistNode *prev;
struct quicklistNode *next;
unsigned char *zl;
unsigned int sz; /* ziplist size in bytes */
unsigned int count : 16; /* count of items in ziplist */
unsigned int encoding : 2; /* RAW==1 or LZF==2 */
unsigned int container : 2; /* NONE==1 or ZIPLIST==2 */
unsigned int recompress : 1; /* was this node previous compressed? */
unsigned int attempted_compress : 1; /* node can't compress; too small */
unsigned int extra : 10; /* more bits to steal for future usage */
} quicklistNode;https://github.com/redis/redis/blob/6.0.0/src/quicklist.h#L46-L57
Note: it took me a while to figure out why it was 32 bytes, so here you go:
8 for each of the first three pointers, a pointer is always a 64-bit integer on
a 64-bit machine apparently, so that's 24 (8*3) bytes right there.
The first int, sz adds 4, we're at 28.
The next bit field optimized fields all fit in a single int (4 bytes):
16 + 2 + 2 + 1 + 1 + 10 = 32 (8*4)
28 + 4 = 32, 32 bytes, done!