Undefined Behaviour

For simplicity, let's assume the type of both x and y is int.

int x, y;

We can inspect the final state of x and y with a simple printf call.

printf("x = %d, y = %d\n", x, y);

Now, let's see how different compilers handle our program.

#include <stdio.h>


int main(int argc, char **argv)
{
    int x, y;
    x = 10;
    y = ++x + --x;

    printf("x = %d, y = %d\n", x, y);
}

At first glance, I would expect the right-hand side of the initialization of y to evaluate to 11 + 10, i.e., 21.

gcc (GCC) 13.2.0

incdec.c: In function ‘main’:
incdec.c:8:9: warning: operation on ‘x’ may be undefined [-Wsequence-point]
    8 |     y = ++x + --x;
      |         ^~~

x = 10, y = 20

Unsurprisingly, GCC computes the final value of x to be 10, the same as its initial value, after being incremented and decremented. It's markedly less intuitive to me that the final value of y is 20.

Clang, on the other hand, agrees with me, i.e., computes the final value of y to be 21.

clang version 17.0.6

incdec.c:8:9: warning: multiple unsequenced modifications to 'x' [-Wunsequenced]
    8 |     y = ++x + --x;
      |         ^     ~~
1 warning generated.

x = 10, y = 21

Both compilers warn about unsequenced modifications and the resultant undefined behaviour. Digging around a bit leads to some fascinating discussions. The GCC team maintains a list of further reading, including a few formal models and analyses of sequence points in C. Further exploration is left as an exercise for the reader.