The document discusses the C preprocessor, detailing its role in code compilation, macro definitions, and file inclusions using directives like #include and #define. It explains how macros function, including parameterized macros, and contrasts their behavior with functions, particularly in terms of argument evaluation. Additionally, it covers header files, typedefs, and conditional compilation to manage multiple inclusions and debugging output.

1. See through C
Module 5
Macros and preprocessors
Tushar B Kute
http://tusharkute.com

2. The C preprocessor and its role
2
cpp
(C preprocessor)
cc1
(C compiler)
source
program
compiled
code
C compiler (e.g., gcc)
expanded
code
• expand some kinds of characters
• discard whitespace and comments
– each comment is replaced with a single space
• process directives:
– file inclusion (#include)
– macro expansion (#define)
– conditional compilation (#if, #ifdef, …)

3. #include
• Specifies that the preprocessor should read in the contents of the specified file
– usually used to read in type definitions, prototypes, etc.
– proceeds recursively
• #includes in the included file are read in as well
• Two forms:
– #include <filename>
• searches for filename from a predefined list of directories
• the list can be extended via “gcc –I dir”
– #include “filename”
• looks for filename specified as a relative or absolute path
3

4. #include : Example
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a predefined include file that:
• comes with the system
• gives type declarations,
prototypes for library routines
(printf)
where does it come from?
– man 3 printf :

5. #include: cont’d
• We can also define our own header files:
– a header file has file-extension ‘.h’
– these header files typically contain “public” information
• type declarations
• macros and other definitions
• function prototypes
– often, the public information associated with a code file
foo.c will be placed in a header file foo.h
– these header files are included by files that need that
public information
#include “myheaderfile.h”
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6. Macros
• A macro is a symbol that is recognized by the preprocessor and
replaced by the macro body
– Structure of simple macros:
#define identifier replacement_list
– Examples:
#define BUFFERSZ 1024
#define WORDLEN 64
6

7. Using simple macros
• We just use the macro name in place of the value, e.g.:
#define BUFLEN 1024
#define Pi 3.1416
…
char buffer[BUFLEN];
…
area = Pi * r * r;
7
NOT:
#define BUFLEN = 1024
#define Pi 3.1416;


8. Example 1
8

9. Example 2
9
we can “macroize”
symbols selectively

10. Parameterized macros
• Macros can have parameters
– these resemble functions in some ways:
• macro definition ~ formal parameters
• macro use ~ actual arguments
– Form:
#define macroName(arg1, …, argn) replacement_list
– Example:
#define deref(ptr) *ptr
#define MAX(x,y) x > y ? x : y
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no space here!
(else preprocessor will
assume we’re defining
a simple macro

11. Example
11

12. Macros vs. functions
• Macros may be (slightly) faster
– don’t incur the overhead of function call/return
– however, the resulting code size is usually larger
• this can lead to loss of speed
• Macros are “generic”
– parameters don’t have any associated type
– arguments are not type-checked
• Macros may evaluate their arguments more than once
– a function argument is only evaluated once per call
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13. Macros vs. Functions: Argument Evaluation
• Macros and functions may behave differently if an argument is referenced multiple times:
– a function argument is evaluated once, before the call
– a macro argument is evaluated each time it is encountered
in the macro body.
• Example:
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int dbl(x) { return x + x;}
…
u = 10; v = dbl(u++);
printf(“u = %d, v = %d”, u, v);
prints: u = 11, v = 20
#define Dbl(x) x + x
…
u = 10; v = Dbl(u++);
printf(“u = %d, v = %d”, u, v);
prints: u = 12, v = 21
Dbl(u++)
expands to:
u++ + u++

14. Properties of macros
• Macros may be nested
– in definitions, e.g.:
#define Pi 3.1416
#define Twice_Pi 2*Pi
– in uses, e.g.:
#define double(x) x+x
#define Pi 3.1416
…
if ( x > double(Pi) ) …
• Nested macros are expanded recursively
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15. Header Files
• Have a file extension “.h”
• Contain shared definitions
– typedefs
– macros
– function prototypes
• referenced via “#include” directives
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16. Header files: example
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17. typedefs
• Allow us to define aliases for types
• Syntax:
typedef old_type_name new_type_name;
• new_type_name becomes an alias for old_type_name
• Example:
– typedef int BasePay;
– typedef struct node {
int value;
struct node *next;
} node;
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18. Example
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defines “wcnode” as an
alias for “struct wc”
we can use “wcnode” in
place of“struct wc”
but not here, since
“wcnode” has not yet
been defined

19. What if a file is #included multiple times?
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foo.h
bar1.h bar2.h
bar.c

20. Conditional Compilation: #ifdef
#ifdef identifier
line1
…
linen
#endif
• macros can be defined by the compiler:
– gcc –D macroName
– gcc –D macroName=definition
• macros can be defined without giving them a specific
value, e.g.:
– #define macroName
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line1 … linen will be included if
identifier has been defined as a
macro; otherwise nothing will
happen.

21. Conditional Compilation: #ifndef
#ifndef identifier
line1
…
linen
#endif
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line1 … linen will be
included if identifier
is NOT defined as a
macro; otherwise
nothing will happen.

22. Solution to multiple inclusion problem
The header file is written as follows:
#ifndef file_specific_flag
#define file_specific_flag
…contents of file…
#endif
• file_specific_flag usually constructed from the name of the header file:
E.g.: file = foo.h ⇒ flag = _FOO_H_
– try to avoid macro names starting with ‘_’
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indicates whether or
not this file has been
included already

23. Another use of #ifdefs
• They can be useful for controlling debugging output
– Example 1: guard debugging code with #ifdefs:
#ifdef DEBUG
…debug message…
#endif
– Example 2: use the debug macro to control what
debugging code appears in the program:
#ifdef DEBUG
#define DMSG(msg) printf(msg) // debugging output
#else
#define DMSG(msg) {} // empty statement
#endif
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straightforward, but needs
discipline to use consistently

24. Generalizing #ifdef
#if constant-expression
line1
…
linen
#endif
⇒ line1 … linen included if constant-expression evaluates to a non-zero value
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Common uses:
• #if 1
or
• #if 0

25. __LINE__ current line number of the source file
__FILE__ name of the current source file
__TIME__ time of translation
__STDC__ 1 if the compiler conforms to ANSI C
printf("working on %sn", __FILE__);
Predefined Macros

26. Adapted originally from:
CSc 352
An Introduction to the C Preprocessor
Saumya Debray
Dept. of Computer Science
The University of Arizona, Tucson
debray@cs.arizona.edu
Thank you
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