Objective-C Implicit conversion loses integer precision (size_t to CC_Long)

Question

I have a function that's generating a sha256 encryption of a string,

Here's the function:

    -(NSString*)sha256HashFor:(NSString*)input
{
    const char* str = [input UTF8String];
    unsigned char result[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(str, strlen(str), result);

    NSMutableString *ret = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH*2];
    for(int i = 0; i<CC_SHA256_DIGEST_LENGTH; i++)
    {
        [ret appendFormat:@"%02x",result[i]];
    }
    return ret;
}

Now this line right here CC_SHA256(str, strlen(str), result); is what's producing this warning (the warning is for the strlen(str) variable).

Implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'CC_LONG' (aka 'unsigned int')

I'm guessing I just need to convert the strlen(str) to a CC_Long, but I have no idea how to do that.

What kind of compiler/toolchain and settings are you using? This seems like something overly pedantic. — , Jun 09 '13 at 20:31
By the way, why would this be tagged osx or automatic-ref-counting... exactly zero of either of these is in the question. — mah, Jun 09 '13 at 20:31
@mah Just like when iOS programming questions are tagged with `xcode` - *why?* Would it really matter if OP used Eclipse instead? Whatever... — , Jun 09 '13 at 20:32

score 56 · Accepted Answer · edited May 02 '14 at 12:26

56

Presumably that's not an error but a warning.
"I just need to convert the strlen(str) to a CC_Long, but I have no idea how to do that." - explicit type conversion (type casting): (CC_LONG)strlen(str), but I don't think you really need this.

edited May 02 '14 at 12:26

neowinston

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answered Jun 09 '13 at 20:30

1

it is a warning indeed... warning you that in theory the size_t could be bigger than the CC_Long ... add a cast to say: yes I know and it is okay – Daij-Djan Jun 09 '13 at 20:52
@Daij-Djan Yep, I find it either unlikely or potentially screwed up if a string is more than 4GB long (on OS X `int` is 4 bytes wide). – Jun 09 '13 at 20:55
It was just was just a warning, however my app is breaking on this line every time I run it. – garetmckinley Jun 09 '13 at 23:41
I actually fixed it, turned out even though it was breaking on that line, it was from a different reason. However, this answer still answered the original question. Got rid of that warning :D – garetmckinley Jun 10 '13 at 13:33
Thanks for a quick around solution. it worked for me. Thnk u very much – Abdul Yasin Apr 08 '14 at 11:57

score 3 · Answer 2 · answered Sep 06 '17 at 11:03

This code will not show any warning and works perfectly.

- (NSString*) sha256 {
    const char * pointer = [self UTF8String];
    unsigned char result[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(pointer, (CC_LONG)strlen(pointer), result);

    NSMutableString *ret = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH*2];
    for(int i = 0; i<CC_SHA256_DIGEST_LENGTH; i++)
    {
        [ret appendFormat:@"%02x",result[i]];
    }
    return ret;
}

score 1 · Answer 3 · edited May 23 '17 at 12:02

Speaking from the iOS perspective, the fact that CC_LONG (a.k.a. uint32_t) and size_t (unsigned long) are incompatible sizes can be a security/stability concern in some applications, especially when dealing with reusable library functions.

An MD5 hash is a 128 bit hash of a potentially unlimited length message, so there is a good reason for this warning. If you truncate a length greater than 2^32, you will come up with an incorrect hash.

Your code should logically decide on how large of a string it can support. In this case using CC_MD5, it would have to be 2^32 bytes.

Maximum length for MD5 input/output

Objective-C Implicit conversion loses integer precision (size_t to CC_Long)

3 Answers3

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