libraries on Linux

Library

1. A library is basically just an archive of object files
2. Libraries is made up of source files that contain just functions.
   It can be called statically or dynamically by programs.

Creating Static Libraries

1. Create c/c++ source files.
2. Compile the source files into object files.
   gcc -c 
3. create a library.
   ar crv libmylib.a objfile1.o objfile2.o objfile3.o
4. creates an index inside the library
   ranlib libmylib.a
5. When linking, specify where the library can be found
   gcc -o foo -L. -lmylib foo.o

Note: When copying the library, use -p to preserve permissions. 

Creating Dynamical Libraries

1. Create a  shared or dynamic library
   gcc -fPIC -c objfile1.c
   gcc -fPIC -c objfile2.c
   gcc -fPIC -c objfile3.c
   gcc -shared -o libmylib.so objfile1.o objfile2.o objfile3.o
2. Shared libraries dynamically access libraries at run-time. 
3. dynamic libraries is through the LD_LIBRARY_PATH.
   export LD_LIBRARY_PATH=/usr/local/lib:${LD_LIBRARY_PATH}
4. ldd - print shared library dependencies

some utilities

ldd

 see which shared libraries are required by a program

 Ex:

 ldd program
   libc.so.6 => /lib/libc.so.6 (0x4002a000)
   /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
 

nm

 list the symbols in object file

 Ex:
   nm client_down
     00403010 b .bss
     ...
     00404000 i .idata$2
     ...
     00402000 r .rdata
     004013e0 t .text
     ...
     00401720 T _GetModuleHandleA@4
     ...
     004040ac i hname
     00401326 t probe