title: -lpthread option of gcc
body: <p>As I know, if I want to use pthread library in linux environment I must include pthread.h and compile the source code with -lpthread option.
  But I don't understand why I should compile with -lpthread option. I think the option is redundant... because I already declared to include pthread.h header file so that gcc links pthread library. Why does gcc not link pthread library file automatically by reading #include?</p>
  
  <p>Thanks in advance.</p>