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cpointersgccoperator-precedence

What happens when you dereference a postincrement C

I am receiving a lot of conflicting answers about this. But as I always understood it.

When we have a pointer in C and use it in a post increment statement, the post increment will always happen after the line of code resolves.

int array[6] = {0,1,2,3,4,5};
int* p = array;
printf("%d", *p++); // This will output 0 then increment pointer to 1

output :

0

Very simple stuff. Now here's where I am receiving a bit of dissonance in the information people are telling me and my own experience.

// Same code as Before
int array[0] = {0,1,2,3,4,5};
int* p = array;
printf("%d", *(p++)); // Issue with this line

output :

0

Now when I run that second version of the code The result is that it will output 0 THEN increments the pointer. The order of operations implied by the parentheses seems to be violated. However some other answers on this site tell me that the proper thing that should happen is that the increment should happen before the dereference. So I guess my question is this: Is my understanding correct? Do post increment statements always execute at the end of the line?

Additional Info:

I am compiling with gcc on linux mint with gcc version ubuntu 4.8.4

I have also tested this on gcc on debian with version debian 4.7.2

Solution

  • OP's "The result is that it will output 0 THEN increments the pointer." is not correct.

    The postfix increment returns the value of the pointer. Consider this value as a copy of the original pointer's value. The pointer is incremented which does not affect the copy.

    The result of the postfix ++ operator is the value of the operand. As a side effect, the value of the operand object is incremented. ... C11dr 6.5.2.4 2

    Then the copy of the pointer is de-referenced and returns the 0. That is the functional sequence of events.

    Since the side-effect of incrementing the pointer and de-referencing that copy of the pointer do not effect each other, which one occurs first is irrelevant. The compiler may optimized as it likes.

    "the end of the line" is not involved in code. It is the end of the expression that is important.