Review: Prototypes

• Put prototypes for all your functions before their first call.
• Example of prototypes in use
• Using them defines an interface and helps to prevent mistakes.

Externs and sharing symbols between files

• Putting the extern keyword makes a symbol act as a "placeholder" and doesn't allocate space for the variable/function.
• Only in one file should the variable/function not be externalized.
• Example: (examples/compilation/ex4)

  	cc -c main.c
  	cc -c blah.c
  	cc main.o -o example         [doesn't work]
  	cc main.o blah.o -o example  [works]
  	

Static keyword

• For global functions and variables, makes them inaccessable from other files.
• Example:

  	static int foo;
  	

Header (.h) files

• Used as interface specification
• Included using #include "foo.h"
• The -Idir option to the compiler gives the search path for header files.
• Headers should contain:
  • Prototypes for shared functions
  • extern declaration for shared variables
  • typedefs
  • macros
  • structs, enums, other data types
• Use of preprocessor defines to avoid recursion:

  	#ifndef FOO_H_INC
  	#define FOO_H_INC
  	#include "foo.h"
  	...
  	#endif /* FOO_H_INC */
  	

• Example: (examples/compilation/ex5)

  	cc -c main.c
  	cc -c blah.c
  	cc main.o blah.o -o example 
  	

Libraries

• For putting multiple object files together
• Usually named libfoo.a so that -lfoo with work
• ar - creates file archives

  ar -r archive files...	inserts files into archive with replacement
  ar -t archive	prints table of archive contents

• Generating symbol tables for archive. Depending on platform, use either:

  ranlib archive

  or

  ar -s archive

• The -Ldir option to the compiler gives the search path for libraries.
• Example: (examples/compilation/ex6):

  	
  	
  Creating the library
  	cc -c bar.c
  	cc -c blah.c
  	ar -r libblah.a bar.o blah.o
  	ar -t libblah.a	  	[shows contents]
  	ar -s libblah.a   	[or ranlib]
  	
  Using the library
  	cc -c main.c
  	cc main.o -L. -lblah -o example
  	

• Make sure libraries are self contained so all you need to distribute is them and the header file.
• Shared libs are OS dependent but useful. They often end with or contain .sa or .so. Functions are not linked into the executable but are kept in the shared library.