create shared C library

How to create and use your own shared C library using gcc? This can be done in 4 steps.

1. write your library code, say sum.c with header file sum.h, and save them in your library folder, say ~/local/lib for the source, and ~/local/include for the header file.
   
   
2. compile it using gcc
   
   gcc -I ~/local/include -c sum.c
   
   This will produce a object file sum.o
   
   
3. create a static library named "hash", do
   
   ar rcs libhash.a sum.o
   
   More than one *.o files can be included in single library
   
   
4. you are ready to use it by :
   
   • include the "sum.h"
     
   • compile it using gcc -I ~/local/include -L ~/local/lib -lhash ...
   To save typing -I xxx and -L xxx every time to use the library, save the path for auto lookup by adding the following lines to ~/.bashrc or ~/.bash_profile:
   
   # for header files
   export C_INCLUDE_PATH=~/local/include:$C_INCLUDE_PATH
   # for linking library
   exprot LD_LIBRARY_PATH=~/local/lib:$LD_LIBRARY_PATH
   export LIBRARY_PATH=~/local/lib:$LIBRARY_PATH

For more readings on differences of shared and static library, see here.