Pointers in Arduino, not updating outside variable

Question

Not sure what I'm doing wrong because aValue and bValue aren't being updated.

int aValue;
int bValue;

void setup() {
    aValue = 0;
    bValue = 0;
}

void loop() {
    someFunc(&aValue, &bValue);
    // code for printing aValue and bValue
}

void someFunc(int *a, int *b) {
    *a++;
    *b++;
}

You are incrementing the pointers; not the values that they point to. Use `(*a)++;` or `*a +=1;` instead. — wildplasser, Apr 26 '15 at 16:13
I just saw your post wildlasser. Thank you too! Someone helped me earlier with that syntax :) — devwannabe, Apr 26 '15 at 21:11
Your post was really earlier than mine. I was just saying that I didn't see it. :) — devwannabe, Apr 27 '15 at 06:21

score 3 · Accepted Answer · edited May 23 '17 at 12:30

3

The problem is that pointers and post-increment does not do what you want.

If you write

void someFunc(int *a, int *b) {
    *a = *a+1;
    *b = *b+1;
}

it works

See ++ on a dereferenced pointer in C? for an explanation of why *a++ increments the pointer itself.

edited May 23 '17 at 12:30

Community

1
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answered Apr 26 '15 at 16:43

drRobertz

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I got it working too by using (*a)++; Thanks for the help too and I accepted it. – devwannabe Apr 26 '15 at 21:09

score 0 · Answer 2 · answered May 08 '15 at 17:12

The variables a and b in someFunc are copies and you are incrementing the copy. Precedence order for post increment is higher then the pointer de-reference so you are incrementing the copies of your pointers. The de-reference has no effect.

When in doubt use parentheses.

void someFunc(int *a, int *b) 
{
    (*a)++;
    (*b)++;
}

although some people say you shoud do

void someFunc(int *a, int *b) 
{
    ++(*a);
    ++(*b);
}

Since post increment technically returns a value while pre increment just increments. In this case most compilers would produce the same code. I have never looked at the AVR compiler.

Pointers in Arduino, not updating outside variable

2 Answers2