在iOS中测试弱链接符号无法正常工作

Question

I am running into a weird issue when trying to test for the existence of a symbol that is introduced in a newer version of the OS. I follow the Apple guidelines on using weak-linked symbols, ie

Check the availability of an external (extern) constant or a notification name by explicitly comparing its address—and not the symbol's bare name—to NULL or nil.

To reproduce the issue, I am using the latest iOS 6 SDK on the latest Xcode 4.5.2, using the default compiler (Apple LLVM compiler 4.1). I weak-linked the Social framework (which is only available on iOS 6+). And I run this code on iOS 5.1 (the deployment target is lower than 6):

NSLog(@"%p", &SLServiceTypeFacebook);
if (&SLServiceTypeFacebook)
  NSLog(@"Yes1");
if (&SLServiceTypeFacebook != NULL)
  NSLog(@"Yes2");

The output is:

0x0
Yes1
Yes2

In other words, we can verify at runtime that the expression &SLServiceTypeFacebook evaluates to the value 0. Yet, if statements that test on this expression treat it as if it is true.

---

Update: From this question , I found that this workaround works with no optimization, but not with optimization:

typeof(&SLServiceTypeFacebook) foo = &SLServiceTypeFacebook;
if (foo)
  NSLog(@"Yes3"); // does not get executed on -O0, but does on any optimization

---

Update: It appears that this problem does not exist with UIKit symbols. Running the following on iOS 4.3:

NSLog(@"%p", &UIKeyboardDidChangeFrameNotification);
if (&SLServiceTypeFacebook)
  NSLog(@"Yes1");
if (&SLServiceTypeFacebook != NULL)
  NSLog(@"Yes2");

The output is:

0x0

I hypothesize that the difference is that the UIKit symbol has a NS_AVAILABLE_IOS() macro next to it, so somehow the compiler handles it correctly. In the case of the Social framework symbol, it doesn't have a NS_AVAILABLE_IOS() macro since the entire Social framework itself is only available since iOS 6 (ie the symbol is available since the version of the framework, so I guess the don't need this macro?); but then the compiler does not handle the symbol correctly.

Answer 1

Are you sure you don't want to check that the SLRequest class exists instead of checking for this constant?

In any case, the issue is that the compiler is optimizing the test away (it interprets this as testing a constant expression which is true at compile time). You can circumvent this by reading this address into a local volatile variable. Or you could dynamically search for the symbol at runtime.

But I would consider just checking for the SLRequest class instead.

Here are at least these three options:

#include <dlfcn.h>

NSString* const * volatile check = &SLServiceTypeFacebook;
if (check != NULL)
    NSLog(@"SLServiceTypeFacebook is defined");

// Another approach would be to call dlsym() at runtime 
// to search for this symbol:
if (dlsym(RTLD_DEFAULT, "SLServiceTypeFacebook"))
    NSLog(@"SLServiceTypeFacebook found via dlsym");

// But if you really just wanted to know is if SLRequest
// is available, you should really just do this:
if ([SLRequest class])
    NSLog(@"SLRequest class is available");

Any of these should work as you were expecting in iOS5.1 versus iOS6.

Hope that helps.

Answer 2

只需检查NSClassFromString是否可以获取这些类。objC无论如何都是所有类：D