Which argument in a function is called first?

Question

I cannot figure out the reason why I am getting this result. I am not the order in which the function calls it's arguments. I would assume that it would start at the first argument and go until the last. Here's the code...

#include <stdio.h>

int addmeup(int a, int b, int c){

    printf("a:%d\n", a);
    printf("b:%d\n", b);
    printf("c:%d\n", c);

    return a + 2 * b + 3 * c;

}
int main(){

    int x = 1;
    int y = addmeup(x++,x++,x++);

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

    return 0;
}

I don't know why the result I am getting is...

a:3
b:2
c:1
x:4
y:10

For the function addmeup() why wouldn't the a be 1 instead of 3? Then b would be 2 and c would be 3. But it is the opposite. Can't seem to wrap my head around this.

Function arguments are not *called*. They are *evaluated*. And they are evaluated as part of execution of the calling function, not be the called function. — Eric Postpischil, Dec 05 '19 at 09:05

score 1 · Answer 1 · edited Dec 05 '19 at 09:03

1

In C and C++ the evaluation order of function arguments is undefined. It could be left to right, right to left, or randomized by running on multiple threads.

Also your example is even worse than that because function(x++, x++, x++) modifies x multiple times without a sequence point. Function argument commas are not sequence points. Which means x might be sent into the function call as x, x+1, x+2 or x+3. Since it is undefined it could even be 0 or call abort().

edited Dec 05 '19 at 09:03

Eric Postpischil

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answered Dec 05 '19 at 05:19

Zan Lynx

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the *evaluation order of arguments is unspecified*. And the duplicate exists. – Antti Haapala -- Слава Україні Dec 05 '19 at 05:29
@AnttiHaapala Yes I linked the question to the order of parameter evaluation question. But it had already been made a duplicate of pre and post increment evaluation. At any rate I feel it is also nice to provide an answer if it is simple and quick. – Zan Lynx Dec 05 '19 at 05:36
no, you should use your "close as a duplicate" privileges as you can single-handedly do so. – Antti Haapala -- Слава Україні Dec 05 '19 at 05:37
@AnttiHaapala How would you propose that I close it again when it has already been closed as a duplicate? – Zan Lynx Dec 05 '19 at 05:40
@AnttiHaapala Never mind, I figured out I can edit the duplicates. That wasn't there in 2009, I don't think. – Zan Lynx Dec 05 '19 at 05:42
The canonical `++` QA has [this answer too](https://stackoverflow.com/a/34536741/918959) – Antti Haapala -- Слава Україні Dec 05 '19 at 05:44
I appreciate the responses. I had a hard time finding the answer on here. I did not know how to word it properly. – Tony Dec 05 '19 at 06:10
"order ... could be left to right, right to left, or randomized _or at the same time_ (that is: no order) or .... – chux - Reinstate Monica Dec 05 '19 at 07:21
Function arguments would not be evaluated by multiple threads. – Eric Postpischil Dec 05 '19 at 09:04
@EricPostpischil What would you call explicit handoffs to coprocessors? Or explicit instruction slots like Itanium? Or the weird things GPUs do? Handing complex function calls down to threads might be entirely reasonable. – Zan Lynx Dec 05 '19 at 18:20

score 0 · Answer 2 · answered Dec 05 '19 at 05:19

It is a undefined behaviour. It means anything can happen.

See GCC warning:

prog.c: In function 'main':
prog.c:15:30: warning: operation on 'x' may be undefined [-Wsequence-point]
   15 |     int y = addmeup(x++,x++,x++);
      |                             ~^~
prog.c:15:30: warning: operation on 'x' may be undefined [-Wsequence-point]

Which argument in a function is called first?

2 Answers2