GCC stuff

I've often had to build some programs from source have been a little confused as to what shared libraries, static libraries, .a files, and .so files are, and how they relate to the -I -L -l options for GCC. A wonderful website that cleared all this up for me is http://www.network-theory.co.uk/docs/gccintro/index.html, here are some notes I've just scrapped down for my own reference

What is a Library
A library is

• Useful code that has been compiled already 
• Hence saves having to recompile that same bit of code
• Can be used (linked to) in other programs or by other people
• DRY (don't repeat yourself) principles - don't need to reinvent the wheel for universal functions. Eg you don't want to have to program how to draw a window with the maximize, minimize, close buttons for every program you make, so instead you can link to the windows libraries.

Static vs. Shared (Dynamic)

Static Library

• Stored in archive .a files
• Machine code for external functions is copied directly to your final executable
• Makes your executable larger
• (makes it easier to give to friends, they don't need library installed?)

Shared Library

• Stored as shared object .so files 
• Only a table of functions used is stored in your exe
• when your exe starts, it loads up the linked libraries into memory (dynamic linking)
• Final exe is smaller
• OS will load the linked library into virtual memory, this can be used by multiple programs at once - saving memory usage
• Can update library without recompiling code (unless the APIs change)

Linking with GCC

The long way of linking libraries is with

gcc -Wall my_prog.c /usr/lib/libm.a -o my_prog

Shorter way is with

gcc -Wall my_prog.c -lm -o my_prog

-lNAME  = /usr/lib/libNAME.a

GCC will automatically look in the directories

1. /usr/local/lib
2. /usr/lib

for libraries, and

1. /usr/local/include
2. /usr/include

for header files. 

If you library or header files aren't installed in those directories, you can include search directories with the -I  and -L options

gcc -Wall my_prog.c -L/a/new/directory/library/ -I/a/new/directory/include/ -lmy_library 

By default GCC will dynamically link to available .so over static .a files.

Environment variables

export C_INCLUDE_PATH=/a/new/directory/include
export CPLUS_INCLUDE_PATH=/a/new/directory/for/c/plus/plus/include
export LIBRARY_PATH=/a/new/directory/library

these variables allow us to now simply use

gcc -Wall my_prog -lmy_library -o my_prog

assuming my_library.a or my_library.so exists in the director /a/new/directory/library

Your program also needs to load linked libraries, if they're not located in the default directories the variable LD_LIBRARY_PATH tells your OS where to find them

export LD_LIBRARY_PATH=/my/linked/library/

Multiple Directories

If you have libraries or headers stored in multiple directories you can save the variables as

.:/dir/1:/dir/2:/dir/3

Where . = current directory