C Preprocessor and Macros
The preprocessor runs before actual compilation, handling directives that start with #, like #include and #define. It performs text substitution and conditional inclusion of code.
Macros defined with #define can be simple constants or function-like macros that take arguments, though they should be used carefully since they're just textual substitutions.
#define NAME value
#define MACRO(x) ((x) * (x))Object-like and function-like macros
A simple `#define PI 3.14` substitutes PI with 3.14 everywhere. A function-like macro like `#define SQUARE(x) ((x) * (x))` takes parameters, but expands as plain text, so parentheses around parameters are important.
Conditional compilation
#ifdef, #ifndef, #else and #endif let you include or exclude code based on whether a macro is defined, which is commonly used for header guards and platform-specific code.
#include <stdio.h>
#define SQUARE(x) ((x) * (x))
int main() {
printf("%d\n", SQUARE(5));
return 0;
}25SQUARE(5) expands to ((5) * (5)) before compilation, giving 25.
#include <stdio.h>
#define DEBUG
int main() {
#ifdef DEBUG
printf("Debug mode\n");
#endif
return 0;
}Debug modeThe block only compiles into the program because DEBUG was defined above.
Key points
- Preprocessor directives start with # and run before compilation.
- #define creates simple or function-like macros via text substitution.
- Always wrap macro parameters in parentheses to avoid precedence bugs.
- #ifdef/#ifndef/#endif enable conditional compilation.