Strange behaviour on C array

Question

I found really strange behaviour when looping through NSString array. The thing is it happens only in iOS 32bit environment, but on 64bit works as expected.

here is the code I run:

static NSString * const HexColors[3] = {
    @"FFFFFF",
    @"FF0000",
    @"000000"};

static NSString * const ColorDescription[3] = {
    @"white",
    @"red",
    @"black"};

in implementation file I loop as following

- (void)loop {
    NSInteger i = 0;
    while (HexColors[i]) {
        NSLog(@"%@", HexColors[i]);
        i++;
    }
}

The result I get:

2014-04-25 09:57:45.374 loopApp[587:60b] FFFFFF
2014-04-25 09:57:45.375 loopApp[587:60b] FF0000
2014-04-25 09:57:45.376 loopApp[587:60b] 000000
2014-04-25 09:57:45.376 loopApp[587:60b] white
2014-04-25 09:57:45.377 loopApp[587:60b] red
2014-04-25 09:57:45.377 loopApp[587:60b] black

And then app throws EXC_BAD_ACCESS on NSLog line

I could use "for" loop but this is not the case

Any idea why it happen? Is it the clang issue?

Instead of that use i =3; while (i){ i--;NSLog(@"%@", HexColors[i]);} — Midhun MP
– Midhun MP, Commented Apr 25, 2014 at 8:07

paxdiablo · Accepted Answer · 2014-04-25 08:31:01Z

2

The line:

while (HexColors[i]) ...

will only stop at the end of the array if there's some NULL marker there, so you have a couple of options (at least).

First, you can put a NULL marker there, with:

static NSString * const HexColors[] = {
    @"FFFFFF",
    @"FF0000",
    @"000000",
    NULL};

Note also there the indeterminate array size [] which creates an array based on the data itself. That's often preferred when you supply all the data and don't want to change too much when you add items.

Second (without adding the NULL element), you can limit your loop using a better test:

for (i = 0; i < sizeof (HexColors) / sizeof (*HexColors); i++) ...

That expression sizeof (HexColors) / sizeof (*HexColors) gives you the number of array elements in HexColors.

As an aside, the reason you're seeing both arrays output is because of how they're laid out in memory. The ColorDescription array immediately follows HexColors so it's as if that's just one array according to your slightly awry loop.

Then, following ColorDescription is a pointer (or arbitrary value interpreted as a pointer) that causes the memory fault.

                 +---------+
HexColors        | pointer | --> "FFFFFF" (all nul-terminated
                 | pointer | --> "FF0000"  character arrays)
                 | pointer | --> "000000"
                 +---------+
ColorDescription | pointer | --> "white"  (ditto)
                 | pointer | --> "red"
                 | pointer | --> "black"
                 +---------+
ArbitraryMemory  | ??????? | --> ???????  (except here, which
                 +---------+               could be anything)

edited Apr 25, 2014 at 8:31

answered Apr 25, 2014 at 8:13

paxdiablo

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1 Comment

PiotrDomo Over a year ago

Thanks for help. Obviously I missed terminating the array with NULL value. This what happens when I focus too much on objective-c, then I forget principles of C

Parag Bafna · Accepted Answer · 2014-04-25 08:09:35Z

1

Change your while (HexColors[i]) condition.

 while (i < arrayCount)

to avoid index 3 beyond bounds.

answered Apr 25, 2014 at 8:09

Parag Bafna

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