C Programming Decoded 47 Conditional Compilation and Macro Operators
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CHAPTERS
0:00 One File, Two Programs
0:52 Something Killed the Lines First
1:45 The Preprocessor Runs First
2:35 The Directives on the Clipboard
3:25 One File, Many Lives
4:16 Existence Versus Truth
5:20 The Trap: Defined But Zero
6:14 Quick Check Before You Scroll
6:33 Pick a Platform, Or Refuse
7:21 Code Knows Its Own Address
8:17 Move One of the Macro
9:08 Strings Glue Themselves Together
10:08 Two More Edits at the Door
11:13 Stringify Quotes the Name, Not the Value
11:45 It Judges Only Text
12:26 Make Him Look Twice
13:06 Gate Plus Toolkit: One File, Two Programs
13:51 Two Builds From the Same Switch
14:25 Never Stripped Out. Never Let In.
15:03 The One Sentence to Carry Out
15:56 Next: The Header Guard
KEY IDEAS
The C preprocessor is a pure text pass that runs before the compiler ever sees your code, and it can keep or delete whole blocks of source.
Conditional directives (#ifdef, #ifndef, #if, #else, #elif, #endif) decide which lines survive compilation, all starting with a hash so they address the preprocessor, not the compiler.
A line the preprocessor rejects is never compiled and never even properly read; the compiler literally never sees it.
#ifdef NAME asks only whether a name exists, while #if NAME evaluates an expression and does real arithmetic on preprocessor-visible constants.
The classic exam trap: given #define VERSION 0, #ifdef VERSION is true (it exists) but #if VERSION is false (it equals zero) -- existence is not truth.
Every #if needs a matching #endif, and platform code commonly branches on #ifdef _WIN32 / #elif defined(__linux__) / #else #error.
Predefined macros _FILE_ and _LINE_ expand to the current filename string and line number, letting code report its own location for free.
Adjacent string literals in C are glued into one string, which is why a LOG macro can stitch a format string, a message, and FILE/LINE together.
The # operator stringifies the literal token handed to it (not its value), and ## pastes two tokens into one identifier.
To stringify a macro's value you need a two-layer macro so the name is expanded before it is quoted; wrapping a real LOG behind #ifdef DEBUG turns one file into a chatty debug build or a silent release build.
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