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> setbit mykey 1 1
> setbit mykey 7 1

When I store string value 1 and 7 into "mykey", what was exactly stored in redis? And how the getbit works inside redis?

Does anyone try to loop the bit inside this value? I know bitcount will give me number 2, but I also want to get the exact string value 1 and 7 from it, is it possible?

--

I doing some experiment by using erlang redis client to read the output.

> setbit mykey 1 1

erlang output:

<<"@">>

Then I delete this entry:

> del mykey

I do the same thing to offset 2 4 8, here you can see the mapping:

When offset is 1, the output is <<"@">>;
When offset is 2, the output is <<" ">>;
When offset is 4, the output is <<"\b">>;
When offset is 8, the output is <<0,128>>;

Honestly, I am more confused now.

Or someone can explain this "bitops.c"

-- updates ---

Maybe I should mention the reason why I want to do this to make the question more clear. We all know it will be cool to use bitmap to store online users. What I am trying to do is get the exactly user id who is online from what redis stored.

Just finished a quick version to extract offsets from redis. Please feel free to improve it.

Joshua
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2 Answers2

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As its name implies, SETBIT allows you to perform bit operations - namely set a given bit to 0 or 1, at a given bit offset, for a given key.

What is important to understand is that the result not always includes only printable characters. This is why Redis uses a custom function sdscatrepr to format the CLI output:

Append to the sds string "s" an escaped string representation where all the non-printable characters (tested with isprint()) are turned into escapes in the form "\n\r\a...." or "\x".

That being said let's start with a simple example. If you consider the hex number 0x7F (= 127) its binary representation on 8-bit is:

pos: 0 1 2 3 4 5 6 7
bit: 0 1 1 1 1 1 1 1
     ^             ^
     |             |
     MSB           LSB

You can typically use SETBIT to store this value, keeping in mind that offset 0 is MSB and offset 7 is LSB:

redis> SETBIT myval 0 0
(integer) 0
redis> SETBIT myval 1 1
(integer) 0
redis> SETBIT myval 2 1
(integer) 0
redis> SETBIT myval 3 1
(integer) 0
redis> SETBIT myval 4 1
(integer) 0
redis> SETBIT myval 5 1
(integer) 0
redis> SETBIT myval 6 1
(integer) 0
redis> SETBIT myval 7 1
(integer) 0

The get your value to inspect if:

redis> GET myval
"\x7f"

Now what happens with multi bytes? Let's say you want to store 0x52 (= 82) which corresponds to character R in ASCII. The 8-bit representation is 01010010 with bit positions (8, 9, ..., 15) since we want it to be stored right after the first value:

redis> SETBIT myval 8 0
(integer) 0
redis> SETBIT myval 9 1
(integer) 0
redis> SETBIT myval 10 0
(integer) 0
redis> SETBIT myval 11 1
(integer) 0
redis> SETBIT myval 12 0
(integer) 0
redis> SETBIT myval 13 0
(integer) 0
redis> SETBIT myval 14 1
(integer) 0
redis> SETBIT myval 15 0
(integer) 0

And you get:

redis> GET myval
"\x7fR"

Here Redis CLI is able to represent the printable character R.

When I store string value 1 and 7 into "mykey"

It corresponds to 01000001 which is equal to 65 and 0x41 in hex. It corresponds to ASCII character A. So doing:

redis> SETBIT mykey 1 1
(integer) 0
redis> SETBIT mykey 7 1
(integer) 0

Gives:

redis> GET mykey
"A"

how the getbit works inside redis?

It simply returns the value of the bit at the given position. Here:

redis> GETBIT mykey 1
(integer) 1

But bit 0 has not been set (it is 0 by default) thus:

redis> GETBIT mykey 0
(integer) 0
deltheil
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  • Thanks for those information, I am wondering is it possible to get the string values(e.g. 1 and 7) by parsing ASCII characters(e.g. "A") which get from redis? I know redis doesn't provide such kind of functions. It will be cool if someone can give some hints. – Joshua Dec 19 '13 at 12:53
  • What do you mean by string values? (1 and 7 are bit offsets here). That said: you are free to manipulate Redis bulk replies the way you want (this is no more than a byte array) - see http://redis.io/topics/protocol. – deltheil Dec 19 '13 at 15:32
  • Sorry, according to [redis document](http://redis.io/commands/setbit), _"Sets or clears the bit at offset in the string value stored at key."_ What I mean is that is it possible to get the offset from the string value. I am still trying to parse it in Erlang. e.g. If we do `setbit mykey 1 1` and `setbit mykey 2 1`, we get the output `<<96>>`; If we do `setbit mykey 1 1`, `setbit mykey 8 1` and `setbit mykey 16 1`, we get the output `<<64, 128, 128>>`. So what I am trying to do is parse the output and get the offsets out of it. (Sorry, the formats looked weird here.) – Joshua Dec 20 '13 at 02:46
  • Since this is an Erlang topic this time about [bitstrings](http://www.erlang.org/doc/programming_examples/bit_syntax.html) you should open a new dedicated question. – deltheil Dec 22 '13 at 20:42
  • No need now, thanks. I have attached the github repository link in ticket description. So anyone who is interested can take a look of my solution. – Joshua Dec 23 '13 at 08:49
  • @deltheil If the default value of any offset is bit 0, why we have to `SETBIT key offset 0`? – northtree May 23 '17 at 11:38
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u can see the exactly details in http://redis.io

GETBIT key offset

Returns the bit value at offset in the string value stored at key. When offset is beyond the string length, the string is assumed to be a contiguous space with 0 bits. When key does not exist it is assumed to be an empty string, so offset is always out of range and the value is also assumed to be a contiguous space with 0 bits.

examples:

redis> SETBIT mykey 7 1
(integer) 0
redis> GETBIT mykey 0
(integer) 0
redis> GETBIT mykey 7
(integer) 1
redis> GETBIT mykey 100
(integer) 0
redis>
freedoo
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  • thanks, I know how to use it. I want to know what exactly stored in redis. e.g. if I do "get mykey", I get "A\X80", what's this? – Joshua Dec 19 '13 at 05:02
  • @jaylzhang can you please give your own examples and not use what is already easily available in redis docs? Please explain the logic setbit and getbit with your understanding. – Nik Dec 19 '13 at 05:23
  • @Nik Sorry. I think the example is enough, so didn't use my own example yesterday. Now, i think i am no need to show my own example, because another reply is more authoritative. – freedoo Dec 19 '13 at 10:46