iOS/C: Convert "integer" into four character string

The type you're talking about is a FourCharCode, defined in CFBase.h. It's equivalent to an OSType. The easiest way to convert between OSType and NSString is using NSFileTypeForHFSTypeCode() and NSHFSTypeCodeFromFileType(). These functions, unfortunately, aren't available on iOS.

For iOS and Cocoa-portable code, I like Joachim Bengtsson's FourCC2Str() from his NCCommon.h (plus a little casting cleanup for easier use):

#include <TargetConditionals.h>
#if TARGET_RT_BIG_ENDIAN
#   define FourCC2Str(fourcc) (const char[]){*((char*)&fourcc), *(((char*)&fourcc)+1), *(((char*)&fourcc)+2), *(((char*)&fourcc)+3),0}
#else
#   define FourCC2Str(fourcc) (const char[]){*(((char*)&fourcc)+3), *(((char*)&fourcc)+2), *(((char*)&fourcc)+1), *(((char*)&fourcc)+0),0}
#endif

FourCharCode code = 'APPL';
NSLog(@"%s", FourCC2Str(code));
NSLog(@"%@", @(FourCC2Str(code));

You could of course throw the @() into the macro for even easier use.

char str[5];
str[4] = '\0';
long *code = (long *)str;
*code = 1919902568;
printf("%s\n", str);

In Swift you would use this function:

func str4 (n: Int) -> String
{
    var s: String = ""
    var i: Int = n

    for var j: Int = 0; j < 4; ++j
    {
        s = String(UnicodeScalar(i & 255)) + s
        i = i / 256
    }

    return (s)
}

This func will do the same like above in a third of the time:

func str4 (n: Int) -> String
{
    var s: String = String (UnicodeScalar((n >> 24) & 255))
    s.append(UnicodeScalar((n >> 16) & 255))
    s.append(UnicodeScalar((n >> 8) & 255))
    s.append(UnicodeScalar(n & 255))
    return (s)
}

The reverse way will be:

func val4 (s: String) -> Int
{
    var n: Int = 0
    var r: String = ""
    if (countElements(s) > 4)
    {
        r = s.substringToIndex(advance(s.startIndex, 4))
    }
    else
    {
        r = s + "    "
        r = r.substringToIndex(advance(r.startIndex, 4))
    }
    for UniCodeChar in r.unicodeScalars
    {
        n = (n << 8) + (Int(UniCodeChar.value) & 255)
    }

    return (n)
}