undefined reference 'shm_open', already add -lrt flag here

Question

I just have a system crash and reinstall Ubuntu 11.10, and my code produces this strange error.

I wrote a simple code sample to test where the problem is:

Answer 1

In Expert C programming Page 108: Where to Put Library Options:Always put the -l library options at the rightmost end of your compilation command line. But it doesn't tell why, so i guess this is somewhat a rule?:)