Overview

• Preliminaries.
  • Admin.
  • Questions.
• Topics for today
  • A C program ecosystem.
  • Steps in compilation.
  • Including files.
  • Detour: Why we have included files.
  • Macros.
  • CPP cnditionals.
  • Detour: Standard header file format.
  • Command-line flags.
  • Hack: Generic data types.

Preliminaries

Admin

• Apologies for not being able to be in class last week.
• Sorry for the confusion about last week and this. I think things are now resolved.
• This week's assignment: Find an interesting use of macros and/or includes in C code in the wild and document it.
• Happy Thunderhail!
• Cool CS activities:
  • Thursday extra: Technical interviews.
  • CS Table Friday: Skip lists (if I ever find the paper).

A C program ecosystem

• Header files
  • provide prototypes for functions
  • some written by the person who is working on this project
  • others are part of the standard installation
  • others come from library code that you've installed
• Source code
  • Your utility/library code
  • Proto-executables
• External things
  • The utility code (traditionally put in library files, .a or .so)
  • The tools that let you build your program (gcc, make, valgrind, gdb, lint, gprof)
• Unit tests!
• Documentation (at least a README) to explain the parts to someone else
  • Automatically generated from the code
• A Makefile that ties everything together
• An appropriate license so that people know what they might be able to do with your code. Also an appropriate disclaimer of fitness.
• Remember that there's a lot of stuff as you think about building programs.

Steps in compilation

• Start with source (.c)

• Preprocess - simple textual manipulation (cc -E)

  • Replace #include with the included file (plus notes)

  • Replace constant definitions with the defined value

    define MAXARRAYSIZE 24

    int values[MAXARRAYSIZE];

    int values[24];

  • Macro processing

    define MAX(X,Y) (X > Y) ? X : Y

    y = MAX(a,b);

    y = (a > b) ? a : b;

    The following is of questionable legality

    (MAX(a,b)) = y; ((a > b) ? a : b) = y;

    The following is dangerous

    y = MAX(a++,b++)

  • Conditional code #ifdef NAME fprintf(stderr, "Hi mom!"); #endif
  • Output: C code, but longer
• Translation into a simpler language
  • Assembly (.s)
• Assemble to relocatable machine code
  • Machine code (.o)
• Link together into an executable
  • Executable

Suppose I have a call to foo() in main.c

• The compiler doesn't know where in the executable "foo" will reside.
• So it generates a "jump to _foo" instruction

The linker figures out where all the instructions go (and what instructions you need) and replaces the _foo with the address.

Including files

#include <stdio.h>
#include "foo.h"

The angle brackets say "It's an installed header file" The lack of angle brackets say "It's a relative path"

Where does it find the stdio.h?

• By default: /usr/include (or whatever your compiler is set to use)
• Probably some environment variable
• Problably some Make environment variable
• -I on the command-line

Detour: Why we have included files

• See the example.

Macros

CPP conditionals

Detour: Standard header file format

Command-line flags

Hack: Generic data types

See examples/generic