Availability Checking Internal Implementation - Everett

Simplest Example
Reverse to See
Stage Summary
How Clang Handles
Summary Again

This article step by step explores @avaliable’s essence.

WWDC 2017: What’s New in LLVM Apple introduced a new API availability checking method, using @avaliable and similar syntax. Details see this document Marking API Availability in Objective-C

Among them @available() can be used in conditional statements, as below:

if (@available(iOS 11, *)) {
    // Use iOS 11 APIs.
} else {
    // Alternative code for earlier versions of iOS.
}

Believe this is also most familiar to everyone, then what is @avaliable?

Simplest Example

We create a new iOS project, then in AppDelegate call code below:

void test_available() {
    if (@available(iOS 11, *)) {
        // Use iOS 11 APIs.
        NSLog(@"ios11");
    } else {
        // Alternative code for earlier versions of iOS.
        NSLog(@"ios other");
    }
}

Example code see here

Reverse to See

After compilation (simulator is fine), use Hopper to open generated executable, find this test_available, as shown:

Look at decompiled pseudo code:

Can see called ___isOSVersionAtLeast this function, 0xb is iOS 11, estimate following two 0x0 are second and third version digits.

Then Hopper finds this function,

At a glance, guess roughly in dispatch_once got current system version, then assigned to _GlobalMajor, _GlobalMinor and _GlobalSubminor three global variables.

Then how does dispatch_once get system version? Hopper’s disassembled pseudo code seems can’t see what block executed. Switch to assembly code view, can see:

Here called _parseSystemVersionPList function, continue looking at this function:

See operation on this file /System/Library/CoreServices/SystemVersion.plist, and there’s a fopen. Then run Xcode, breakpoint at this fopen.

This file path:

/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/System/Library/CoreServices/SystemVersion.plist

We open to see,

Then understand, finally is accessing this file, get ProductVersion in it, and through sscanf parse out three _GlobalMajor, _GlobalMinor and _GlobalSubminor three values.

sscanf so old, seeing it, seems back to year just learned C language.

Stage Summary

From current analysis, @available(iOS 11, *) finally becomes following pseudo code:

_GlobalMajor
_GlobalMinor
_GlobalSubminor

void _parseSystemVersionPList() {
    char *path = ".../System/Library/CoreServices/SystemVersion.plist";
    fp = fopen(path)
    read from fp
    parse ProductVersion
    sscanf into _GlobalMajor,_GlobalMinor,_GlobalSubminor
}

BOOL ___isOSVersionAtLeast(major,minor,subminor) {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _parseSystemVersionPList();     
    });
    return compare major,minor,subminor with _GlobalMajor,_GlobalMinor,_GlobalSubminor
}