Macro and Pre/Post Increment in C

Question

I have noticed this strange behavior of macro functions while using pre increment operator. I know it is not preferable to use pre increment operator with macros but still I would like to know the reason behind the execution of the following 2 codes:

#include <stdio.h>

#define SQR(x) {x*x }

int main()
{
    int a=4;
    int b=SQR(a++);
    int c=SQR(++a);
    printf("%d.....%d....%d\n",b,c,a*b*c); 
    return 0;
}

The output of this is:

20.....64....10240

The first value b is 20, which is okay 4*5. But why the value of C is 64 i.e. 8*8 instead of 7*8?

And I just replaced the sequence of execution like this:

#include <stdio.h>

#define SQR(x) {x*x }

int main()
{
    int a=4;
    int c=SQR(++a);
    int b=SQR(a++);
    printf("%d.....%d....%d\n",b,c,a*b*c); 
    return 0;
}

The output of this is:

42.....36....12096

Strange isn't it? The pre increment again had some issue. It gave 42 i.e. 6*7 instead of 5*6 and after that the post increment also gave wrong answer 36 i.e. 6*6 instead of 6*7.

It would be a great help if anyone can explain why the outputs are like this?

Answer 1

MACROs are textual replacement, so your code

 SQR(a++)

expands to something like

 a++ * a++

and, int c=SQR(++a); to int c = ++a*++a;

which invokes undefined behavior. To elaborate the why part for this, please refer to this well-written answer.