Hacking Strings
The implementation of Redis strings is contained in sds.c
(sds
stands for Simple Dynamic Strings).
The C structure sdshdr
declared in sds.h
represents a Redis string:
struct sdshdr {
long len;
long free;
char buf[];
};
The buf
character array stores the actual string.
The len
field stores the length of buf
. This makes obtaining the length
of a Redis string an O(1) operation.
The free
field stores the number of additional bytes available for use.
Together the len
and free
field can be thought of as holding the metadata of the buf
character array.
Creating Redis Strings
A new data type named sds
is defined in sds.h
to be a synonym for a character pointer:
typedef char *sds;
sdsnewlen
function defined in sds.c
creates a new Redis String:
sds sdsnewlen(const void *init, size_t initlen) {
struct sdshdr *sh;
sh = zmalloc(sizeof(struct sdshdr)+initlen+1);
#ifdef SDS_ABORT_ON_OOM
if (sh == NULL) sdsOomAbort();
#else
if (sh == NULL) return NULL;
#endif
sh->len = initlen;
sh->free = 0;
if (initlen) {
if (init) memcpy(sh->buf, init, initlen);
else memset(sh->buf,0,initlen);
}
sh->buf[initlen] = '\0';
return (char*)sh->buf;
}
Remember a Redis string is a variable of type struct sdshdr
. But sdsnewlen
returns a character pointer!!
That’s a trick and needs some explanation.
Suppose I create a Redis string using sdsnewlen
like below:
sdsnewlen("redis", 5);
This creates a new variable of type struct sdshdr
allocating memory for len
and free
fields as well as for the buf
character array.
sh = zmalloc(sizeof(struct sdshdr)+initlen+1); // initlen is length of init argument.
After sdsnewlen
successfully creates a Redis string the result is something like:
-----------
|5|0|redis|
-----------
^ ^
sh sh->buf
sdsnewlen
returns sh->buf
to the caller.
What do you do if you need to free the Redis string pointed by sh
?
You want the pointer sh
but you only have the pointer sh->buf
.
Can you get the pointer sh
from sh->buf
?
Yes. Pointer arithmetic. Notice from the above ASCII art that if you subtract
the size of two longs from sh->buf
you get the pointer sh
.
The sizeof
two longs happens to be the size of struct sdshdr
.
Look at sdslen
function and see this trick at work:
size_t sdslen(const sds s) {
struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
return sh->len;
}
Knowing this trick you could easily go through the rest of the functions in sds.c
.
The Redis string implementation is hidden behind an interface that accepts only character pointers. The users of Redis strings need not care about how its implemented and treat Redis strings as a character pointer.