Held: Thursday, 20 February 2014

Back to Outline 04 - Decoding Scripts. On to Outline 06 - More Bash.

Summary

We consider the steps in compilation, focusing primarily on the preprocessor.

Related Pages

• EBoard (Source) (HTML) (PDF)

Overview

• 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.

Administrivia

• 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.
• Cool CS activities:
  • Thursday extra: Technical interviews.
  • CS Table Friday: Skip lists (if I ever find the paper).

A C Project "Ecosystem"

If we're going to talk about C, we should think about the structure of a typical project. What do we expect to have? We'll look at Sam's traditional model ofo the project

• External resources
  • Standard tools: GCC, Make, Lint, Valgrind, ...
  • Standard libraries. (The ones included with every C program.)
  • Additional libraries. (Optional; Either already on every system or installed.)
  • Information on those libraries. (typically in .h files).
• Project-specific resources
  • A bunch of project-specific library code (typically in .c files).
  • Information on that library code (typically in .h files)
  • Tests of that library code (typically in .c files).
  • One or more applications that use the code.
  • Potentially custom tools to help with things.
• A Makefile that gives instructions for tying everything together.

Steps in Compilation

• In 161, you learn to compile C code directly to executable code.
• However, there are multiple phases the C compiler goes through in that compilation. These steps are particularly relevant when you are dealing with real ecosystems.
• First, the compiler runs the C Preprocessor. The preprocessor deals with some simple textual substitution.
  • Replace #include "*file*" with the contents of file (relative path).
  • Replace #include <*file*>. with the contents of *file* (found in the include path - starts with /usr/include, but also anything else that you specify with -I*dir* also some environment variable).
  • Replace defined constants with their values.
  • Replace macros with their bodies.
  • Process preprocessor conditionals
• Second, the compiler does the main work of translating to assembly code.
  • (Okay, in most cases, the compiler translates to an internal type of assmebly and then has to spend a bit of extra work generating "real" assembly code.)
• Third, assemble the assembly into relocatable object code.
  • This assembly code allows the variables and procedures to be put at different locations in memory.
  • That is, there are effectively "placeholders" for many things that still need to be filled in (where in static memory a global variable is, where in memory the start of a procedure is)
• Finally, linke the relocatable object code with appropriate libraries to build the executable.
  • For all but the basic libraries, you need to specify the library name with -lname.
  • You may also need to specify a path to the library.
• You can get the compiler to build any of these intermediate forms using flags
  • -E - just the preprocessor
  • -S - up through assembly
  • -c - up through relocatable object code

Including Files

• Useful for headers. (Why?)
• Can include regular code, too. (Why?)
  • E.g., an alternative to separate compilation.

Playing with Macros

• Simplest: Constants. Mostly useful for readability.
• Standard: Alternative to functions.
  • Usually more efficient than function calls.
• But still dangerous if used less carfully.

CPP Conditionals

• Typical use: Different code on different systems.
• Another typical use: Insert code for debugging by printing or whatever.
• Sam sometimes puts unit tests in that way. Your main gets put in an ifdef section.

Detour: Standard Header File Format

• So, why do we write header files in the "standard" format?
• Consider two include files, zebra.h and stripes.h. zebra.h includes stripes.h. What happens?

The Preprocessor and the Command Line

• Can put -Dwhatever
  • Makes it easier to change how you compile your program on the fly.

Some Great Preprocessor Hacks

• Include one file multiple times (with different definitions) to get simlar definitions. (E.g., different kinds of lists.)
• Process the same list multiple times.