CODEWITHSUNDEEP
sql-servert-sql
Nicolas
Nicolas

Reputation: 1116

Unpacking a binary string with TSQL

Good Day,

I have a number of binary strings that were created by a C app with a struct. Imagine, if you will, the struct looks like this:

 struct {
     int foo; 
     double bar; //Assume 8 bytes
     char[20] baz;
 }

Each string is 4 + 8 + 20 = 32 bytes long. The structure of the string looks something like this:

IIIIDDDDDDDDSSSSSSSSSSSSSSSSSSSS

I need to unpack this string in a TSQL stored proc. The string is easy:

baz = SUBSTRING(binarystring, 12, 20)

The int also. And then convert to an integer with bit shifting (well, multiplying by 2^4, 2^8, etc)

foo_string = SUBSTRING(binarystring, 0, 4)
foo = unpack_int(foo_string)

But, the double is a lot more challenging. I am able to do it by following the IEEE754 spec, I am not happy with doing this myself.

Is there a function or something that can unpack the int and double out of a binary string?

Thank you,

P.S. I've never used TSQL myself, so the above fragments may be illegal, but you get the notion. I'm assisting a colleague.

Upvotes: 1

Views: 1690

Answers (1)

stevehem
stevehem

Reputation: 671

There is no built-in function to convert from binary to float. However, you can find user-defined functions in T-SQL to do this conversion, or you can write a clr function to do this conversion using the BitConverter.ToDouble(byte[]) method.

An example of a t-sql function for converting binary to float can be found from a thread at sqlteam:

CREATE FUNCTION [dbo].[fnBinaryFloat2Float]
(
    @BinaryFloat BINARY(8)
)
RETURNS FLOAT
AS
BEGIN
    RETURN  SIGN(CAST(@BinaryFloat AS BIGINT))
        * (1.0 + (CAST(@BinaryFloat AS BIGINT) & 0x000FFFFFFFFFFFFF) * POWER(CAST(2 AS FLOAT), -52))
        * POWER(CAST(2 AS FLOAT), (CAST(@BinaryFloat AS BIGINT) & 0x7ff0000000000000) / 0x0010000000000000 - 1023)
END

Upvotes: 2

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