How do you convert an iPhone OSStatus code to something useful?

No. Not completely.

Some OSStatus are four-character-codes, so you can use (extracted from iPhone SDK's sample code "CAXException.h")

static char *FormatError(char *str, OSStatus error)
{
    // see if it appears to be a 4-char-code
    *(UInt32 *)(str + 1) = CFSwapInt32HostToBig(error);
    if (isprint(str[1]) && isprint(str[2]) && isprint(str[3]) && isprint(str[4])) {
        str[0] = str[5] = '\'';
        str[6] = '\0';
    } else {
        // no, format it as an integer
        sprintf(str, "%d", (int)error);
    }
    return str;
}

(See iOS/C: Convert "integer" into four character string for some more ways to convert fourcc into string, including Swift)

NSError's NSOSStatusErrorDomain is able to decode some OS errors. See @tomk's answer.

If you don't need to decode the number in program for the user, you may use the macerror script to manually find out the meaning, as mentioned in @lros's answer. The list of OSStatus supported can be found from its source code in /System/Library/Perl/Extras/5.18/Mac/Errors.pm.

There is also an online service http://osstatus.com/ collecting errors from all public frameworks. They are still not really complete e.g. the mapping to -12792 mentioned in the comment is missing. Probably it is a code from a private framework.

This is available on macOS and for iOS from 11.3 and above.

I know this is an old post, but I was reading the apple docs in a section related to keychains. They mention a method that is used to convert OSStatus errors into something readable.

SecCopyErrorMessageString

Returns a string explaining the meaning of a security result code.

SecCopyErrorMessageString (OSStatus status, void* reserved );

Useage:

NSString* ErrMsg = (__bridge_transfer NSString *) SecCopyErrorMessageString(theOSStatusError, NULL);

It worked for me with my keychain OSStatus errors. Does it work for you? You will need Security.Framework added to your project to use this method.

OSStatus is a signed integer value. You cannot convert or "cast" it to a string. You can convert it to a NSError like this:

NSError *error = [NSError errorWithDomain:NSOSStatusErrorDomain code:osStatus userInfo:nil];