Friday, August 30, 2013

Endianity conversion for a 24bit Integer

Recently while working on SSL protocol I came across 24-bit (3 byte) integers. I never thought any one would ever use a 24-bit integer. There are no default types or any type in stdint.h to create such a variable. So the best one could do is use 3 byte arrays and keep typecasting them to int - ignoring most significant byte on read and not writing the most significant byte on write.
The problem I faced was that I had to change the endianity of these 24-bit integers as the data is written to and read from network. I did not want to use a function call so I came up with following macros. They dont seem right to me i.e they are more weird looking and complicated than I wanted to so if someone has a simple solution, please share.

The following macro takes a pointer to 3 bytes containing 24 bit unsigned integer, converts the endianity of the integer value and returns it as int with most significant byte always 0
#define GET_NO_UINT24(src)  ((((*(uint32_t*)src)<<16)|((*(uint32_t*)src) & 0xFF00)|(((*(uint32_t*)src)>>16) & 0xFF)) & 0xFFFFFF)
The next macro takes 2 arguments. The first one is a pointer to 3 byte array for storing 24-bit integer and the second argument is the value to be stored.
#define PUT_NO_UINT24(dst,src) ((((unsigned char*)dst)[2]=(unsigned char)((src))), (((unsigned char*)dst)[1]=(unsigned char)((src)>>8)), (((unsigned char*)dst)[0]=(unsigned char)((src)>>16)))

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