Is this example non-deterministic or just undefined? Sure you should not rely on this code to give you consistent results between compilers and even optimisation levels, but what ever happens should be consistent between runs.
$ gcc -Wall test.c
test.c: In function ‘main’:
test.c:12:13: warning: operation on ‘a’ may be undefined [-Wsequence-point]
test(a++, a++);
~^~
$ clang test.c
test.c:12:8: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
test(a++, a++);
^ ~~
1 warning generated. #include <stdio.h>
int postincrement(int *x) { int y = *x; *x = y + 1; return y; }
int main(int argc, char **argv) {
int i = 0;
printf("%d, %d\n", postincrement(&i), postincrement(&i));
return 0;
}