Working of #define in C

Question

I was going through my C learning by writing small pieces of code and one new question came up

I have written a small piece of code

#include<stdio.h>
#include<conio.h>
#define SUM(a) (a+a)
main()
{
  int w,b=5;
  w=SUM(b++);
  printf("%d\n",w);
  printf("%d\n",b);
  getch();     
}

What i was thinking that it will display the output as

10

6

but it is showing

10

7

Can someone explain why ,i am using Visual Studio 2008

@2501 Correct would be "Can someone explain why?" "I am using Visual Studio 2008.", I don't see any `pause` there, so i assume that's what he wants. — bitcell, Nov 26 '14 at 10:14

No Idea For Name · Accepted Answer · 2014-11-26T10:14:55.927

2

because when you do

w=SUM(b++);

the macro will be replaced by:

w= b++ + b++;

now, if b=5 then you do twice b++ and get b=7

Edit

after reading MSalters comment i did some searching and found out that as he said this code couse UB.

as says here:

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. If there are multiple allowable orderings of the subexpressions of an expression, the behavior is undefined if such an unsequenced side effect occurs in any of the orderings.

edited Nov 26 '14 at 10:14

answered Nov 26 '14 at 10:06

No Idea For Name

11,411
10
42
70

7

Actually, you have Undefined Behavior and b = -42 is equally valid. Yo modify `b` twice without an intervening sequence point. – MSalters Nov 26 '14 at 10:08
When you do `b++` twice you have an undefined behaviour. – Sergej Christoforov Nov 26 '14 at 10:08
@willys i've edit the answer accordingly – No Idea For Name Nov 26 '14 at 10:16
@MSalters if you execute using c compiler in windows the above code results in 10 and 7 which is correct. – Manjunath N Nov 26 '14 at 10:22
1

@Madan: **All** answers are correct. That's what Undefined Behavior means. Wiping your harddisk is also a correct outcome. (And that's not just theoretical, happened at my previous job) – MSalters Nov 26 '14 at 10:24
@MSalters ok. i taught in different sense. fine .. – Manjunath N Nov 26 '14 at 10:25

Working of #define in C

1 Answers1

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