19 - The Preprocessor

How do you control what source code gets compiled?

#if (conditional) code(); #endif

How can you use macros to help with debugging?

#if defined(DEBUG) // debug code #else // debug macros expand to nothing #endif

What is the preprocessor directive for asking if macro X is defined?

#ifdef X

How do you stop the same header from being included more than once?

#ifndef C_H #define C_H // All of the stuff in c.h #endif

How do you put a macro variable in quotes after expansion?

#variableName

How do you choose which macros to enable when compiling?

-DMACRO Sets the value of MACRO to 1 Alternatively, to give it a value: -DMACRO=5 Or put the whole thing in quotes to include spaces in expansion

What is this called? #define SIZE 256 What is it called when it is replaced in the code?

A preprocessor macro A macro expansion

Macros use call-by-_________ What is the benefit of this?

Name We can make macros change variables directly without needing a pointer

What are all commands at the beginning of a program starting with # called?

Preprocessor directives

How do you safely write a macro definition?

Put parentheses around each instance of a variable and around the entire definition

If MAX is a macro, why is calling MAX(x++, y++) a bad idea?

The variables may get incremented multiple times

Can you nest conditional compilation?

Yes

If you have this code: #if defined(LINUX) #define HDR "linux.h" #elif defined(WIN32) #define HDR "windows.h" #else #define HDR "default.h" #endif #include HDR How would you compile the program to include "windows.h"?

gcc -DWIN32 myprog.c

How would you compile myProgram.c to send preprocessed output to standard out?

gcc -E myProgram.c

How do you concatenate two macro variables after expansion?

var1##var2