The C Pre-Processor

Review: Compilation Steps

• The C Pre-processor takes the .c file and processes some directives, creating a new C source file.
• The compiler translates the C source file to assembly code.
• The assembler translates the assembly code to relocatable object code.
• The linker links the relocatable object code with other object code, creating an executable.

• Note: If you just want to see what the C preprocessor does, you use cc -E.

What Does the C Pre-Processor Do?

• #include directives.
• #if and #ifdef directives.
• Macros and constants (with #define and sometimes with #undef)
• More?

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.

CPP Conditionals

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

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 kins of lists.)
• Process the same list multiple times.