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
4
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;
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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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