A simple way to detect at runtime if we're running in UIKit legacy mode or the new "flat" variant. Written for our PDF iOS SDK (http://pspdfkit.com), where the precompiled binary needs to detect at runtime in what variant it's running. Want more stuff like that? Follow me on Twitter: http://twitter.com/steipete

UIKitLegacyDetector.m

// Taken from http://PSPDFKit.com. This snippet is under public domain.
#define UIKitVersionNumber_iOS_7_0 0xB57
BOOL PSPDFIsUIKitFlatMode(void) {
    static BOOL isUIKitFlatMode = NO;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // We get the modern UIKit if system is running >= iOS 7 and we were linked with >= SDK 7.
        if (kCFCoreFoundationVersionNumber >= kCFCoreFoundationVersionNumber_iOS_7_0) {
            isUIKitFlatMode = (NSVersionOfLinkTimeLibrary("UIKit") >> 16) >= UIKitVersionNumber_iOS_7_0;
        }
    });
    return isUIKitFlatMode;
}

Looks like this doesn't work anymore as of iOS 7.1b2. I've updated the code for a new way that works.

NSVersionOfLinkTimeLibrary seems to be the most stable solution. Are those version numbers defined somewhere, or should I just use them as magic constants?

Check the dyld(3) man page:

int32_t
NSVersionOfLinkTimeLibrary(const char* libraryName);

NSVersionOfLinkTimeLibrary() returns the current_version number that the main executable was linked
against at build time.  The libraryName parameter would be "bar" for /path/libbar.3.dylib and "Foo" for
/path/Foo.framework/Versions/A/Foo.  This function returns -1 if the main executable did not link
against the specified library.

They're just whatever number the team for that library uses.

In order to check which SDK version was used to build a binary, UIKit uses the _UIApplicationLinkedOnOrAfter function. It is implemented by comparing the major version of UIKit at link time (NSVersionOfLinkTimeLibrary("UIKit") >> 16) to a value in the __UIKitVersionNumbers table.

Here is this UIKit version numbers table, built by running otool -L on UIKit of all these iOS versions (yes I have a huge archive!):

Index	iOS Version	Hexadecimal	Decimal
0	2.0	0x0E5	229
1	2.1	0x2EB	747
2	2.2	0x2F0	752
3	2.2.1	0x2F1	753
4	3.0	0x333	819
5	3.1	0x3E8	1000
6	3.2	0x44C	1100
7	4.0	0x4B0	1200
8	4.1	0x514	1300
9	4.2.1	0x578	1400
10	4.2.6	0x582	1410
11	4.3	0x5DC	1500
12	5.0	0x640	1600
13	5.1	0x6A4	1700
14	6.0	0x944	2372
15	6.1	0x94C	2380
16	7.0	0xB57	2903
17	7.1	0xB77	2935

Weirdly enough, the __UIKitVersionNumbers contains 0x2EC instead of 0x2EB for iPhone OS 2.1. But since nobody cares at all about iPhone OS 2 this should not be a problem.

Awesome thread!

One tiny thing to add:

If kCFCoreFoundationVersionNumber_iOS_7_0 is undefined, use following snippet:

ifndef kCFCoreFoundationVersionNumber_iOS_7_0

define kCFCoreFoundationVersionNumber_iOS_7_0 847.2

endif

Instead of the defining the var I would use the & ... like

if (&kCFCoreFoundationVersionNumber_iOS_7_0 &&
    kCFCoreFoundationVersionNumber >= kCFCoreFoundationVersionNumber_iOS_7_0) 
{ ...

For anyone googling their way here, 8.0 is 0xCF6.

iOS 9.3: 0xDB8
iOS 10b4: 0xE0C

For iOS 12 (GM), it seems that casting to unsigned integer is now required:

(lldb) p (int32_t)NSVersionOfLinkTimeLibrary("UIKit") >> 16
(int32_t) $4 = -4536
(lldb) p (uint32_t)NSVersionOfLinkTimeLibrary("UIKit") >> 16
(uint32_t) $5 = 61000

-> UIKitVersionNumber_iOS_12_0 = 0xEE48

iOS 15.0 is 0x13CB

(lldb) p (uint32_t)NSVersionOfLinkTimeLibrary("UIKit") >> 16
(uint32_t) $2 = 5067

iOS 15.2 is 0x1455

(lldb) p (uint32_t)NSVersionOfLinkTimeLibrary("UIKit") >> 16
(uint32_t) $1 = 5205