Answer difference in C

Answer 1

In both cases you are hitting undefined behavior. Anything can happen. In each post-fix operator the compiler must retrieve the value of the variable before performing the operation, but the order of execution of that operation with respect to the other post-fix operations in the code is undefined.

For the simpler expression int c = a++ + a--; all these are valid execution paths according to the standard:

// assume a=10
// option 1
int c = a + a;   // 20
a++; a--;
// option 2
register int tmp = a; // the register keyword is just to show that no real memory is created
a++;
int c = tmp + a; // 21
a--;
// option 3
register int tmp = a;
a--;
int c = tmp + a; // 19 
a++;

Answer 2

The b++ + b-- attempts to modify b twice without an intervening sequence point. This leads to undefined behavior.

Answer 3

To explain the second case, I assume that the following values are used:

c = 2 + 3 + 3 + 4 = 12

In the first case I can only assume that a and b are decremented by the -- before being added giving:

c = 2 + 2 + 3 + 3 = 10

Answer 4

Not on my box.

#include <stdio.h>
void func1(){
int a=2,b=3,c;
c = a++ + a-- + b++ + b--;
printf("func1: %d\n", c);
}

void func2(){
int a=2,b=3;
int c = a++ + a-- + b++ + b--;
printf("func2: %d\n", c);
}


  int main(){
func1();
func2();
return 0;
}

When I compiled and ran it I got "func1: 10" and "func2: 10". I'd try a debugger to see what is going on with your system. In case it matters, I compiled with gcc on a mac.