This page is about a technique for reducing the memory footprint of C programs – manually repacking C structure declarations for reduced size. To read it, you will require basic knowledge of the C programming language.
You need to know this technique if you intend to write code for memory-constrained embedded systems, or operating-system kernels. It is useful if you are working with application data sets so large that your programs routinely hit memory limits. It is good to know in any application where you really, really care about minimizing cache-line misses.
Finally, knowing this technique is a gateway to other esoteric C topics. You are not an advanced C programmer until you have grasped it. You are not a master of C until you could have written this document yourself and can criticize it intelligently.
http://www.catb.org/esr/structure-packing/
