Post increment operator behavior [duplicate]

Answer 1

This is not just unspecified behaviour, it is actually undefined behaviour .

Yes, the order of argument evaluation is unspecified, but it is undefined to both read and modify a single variable without an intervening sequence point unless the read is solely for the purpose of computing the new value. There is no sequence point between the evaluations of function arguments, so f(test,test++) is undefined behaviour: test is being read for one argument and modified for the other. If you move the modification into a function then you're fine:

int preincrement(int* p)
{
    return ++(*p);
}

int test;
printf("%d %d\n",preincrement(&test),test);

This is because there is a sequence point on entry and exit to preincrement, so the call must be evaluated either before or after the simple read. Now the order is just unspecified.

Note also that the comma operator provides a sequence point, so

int dummy;
dummy=test++,test;

is fine --- the increment happens before the read, so dummy is set to the new value.