Training of the following the following

1. Sri Balaji College of Engineering & Technology
MACRO in C
SUPERVISOR:-
MR. SANJAY SHARMA
Assistant Professor
SBCET , JAIPUR
SUBMITTED BY:-
NAME:- VIVEK SINGH RAJAWAT
ROLL NO. 19ESBCS020
BRANCH :- CSE
DEPARTMENT OF COMPUTER SCIENCE & ENGINEERING
Sri Balaji College of Engineering & Technology
Rajasthan Technical University , Kota
January 2021

2. A PRESENTATION ON MACRO
MACRO in C

3. CONTENTS
• MACROS:INTRO
• Using simple macros
• Parameterized macros
• Macros vs. functions
• Macros vs. Functions: Argument Evaluation
• Properties of macros
• Typedefs
• Conditional Compilation: #ifdef
• Solution to multiple inclusion problem
• Predefined Macros

4. Macros:INTRO
4
• A macro is a symbol that is recognized by the
preprocessor and replaced by the macro body
– Structure of simple macros:
#defineidentifierreplacement_list
– Examples:
#defin
e
BUFFER
SZ
102
4
#defin
e
WORDLE
N
64

5. Using simple macros
5
• 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;
NOT
: #define BUFLEN =
1024 #define Pi
3.1416;

6. Example 1
6

7. Parameterized macros
7
• 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(pt
r) #define
MAX(x,y
)
*ptr
x > y ? x
: y
no space
here!
(else preprocessor
will assume we’re
defining a simple
macro

8. Example
8

9. 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
9
•
•

10. Macros vs. Functions: Argument Evaluation
10
• 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:
•
int dbl(x) { return x + x;}
…
u = 10; v = dbl(u++);
printf(“u = %d, v = %d”,
u, v);
prints: u = 11, v =
20
u = 10; v = Dbl(u++);
printf(“u = %d, v = %d”, u, v);
prints: u = 12, v =
21
Dbl(u++)
expands
to:
#define Dbl(x) x + x
…u++ + u++

11. Properties of macros
11
• Macros may be nested
– in definitions,
e.g.:
#define Pi 3.1416
2*Pi
#define Twice_Pi
– in uses, e.g.:
#define double(x) x+x
#define Pi 3.1416
…
if ( x > double(Pi)
) …
• Nested macros are expanded
recursively

12. typedefs
12
•
•
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;

13. Conditional Compilation: #ifdef
13
#ifdef identifier
line 1
…
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
line1 … linen will be included
if identifier has been defined
as a macro; otherwise
nothing will happen.

14. Conditional Compilation: #ifndef
14
#ifndefidentifier
line1
…
linen
#endi
f
line1 … linen will be
included if identifier
is NOT defined as a
macro; otherwise
nothing will happen.

15. Solution to multiple inclusion problem
15
The header file is written as
follows:
#ifndeffile_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
‘_’
indicates whether
or not this file has
been included
already

16. Another use of #ifdefs
16
• 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
straightforward, but
needs discipline to use
consistently

17. Generalizing #ifdef
17
#if constant-expression
line1
…
linen
#endif
 line1 … linen included if constant-expression evaluates to a non-zero value
Common
uses:
• #if 1
or
• #if 0

18. current line number of the source
file name of the current source
file
time of translation
1 if the compiler conforms to
ANSI C
LINE
FILE
TIME
STDC
printf("working on %sn", FILE );
Predefined Macros