调用objc_msgSend方法在64位下崩溃解决方法
之前一直在非64位机器下测试一切正常的程序，在iPhone5s下无缘无故崩溃。崩溃的位置是调用objc_msgSend时出现。经过一番辛苦搜索终于发现苹果官网上有一段这样的描述：
Dispatch Objective-C Messages Using the Method Function’s Prototype
An exception to the casting rule described above is when you are calling the objc_msgSend function or any other similar functions in the Objective-C runtime that send messages. Although the prototype for the message functions has a variadic form, the method function that is called by the Objective-C runtime does not share the same prototype. The Objective-C runtime directly dispatches to the function that implements the method, so the calling conventions are mismatched, as described previously. Therefore you must cast the objc_msgSend function to a prototype that matches the method function being called.
Listing 2-14 shows the proper form for dispatching a message to an object using the low-level message functions. In this example, thedoSomething: method takes a single parameter and does not have a variadic form. It casts the objc_msgSend function using the prototype of the method function. Note that a method function always takes an id variable and a selector as its first two parameters. After the objc_msgSendfunction is cast to a function pointer, the call is dispatched through that same function pointer.
Listing 2-14 Using a cast to call the Objective-C message sending functions
- (int) doSomething:(int) x { ... }
- (void) doSomethingElse {
int (*action)(id, SEL, int) = (int (*)(id, SEL, int)) objc_msgSend;
action(self, @selector(doSomething:), 0);
}
貌似是说不能直接使用objc_msgSend的原型方法来匿名调用，否则会出现异常。结果尝试了上面的方法强制转换成一定的方法后，再次运行没有崩溃了，Luck!!
原文来自：https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaTouch64BitGuide/ConvertingYourAppto64-Bit/ConvertingYourAppto64-Bit.html
补充说明（2015-8-5）：
对于返回结构体的方法，如果使用objc_msgSend进行调用是会抛出异常EXC_BAD_ACCESS：
CGSize (*action)(id, SEL, int) = (CGSize (*)(id, SEL, int)) objc_msgSend; CGSize size = action(self, @selector(doSomething:), 0);
这样的做法是有可能会出现崩溃问题的（但是不一定每个程序都必然会出现），经过查看官方文档发现，objc_msgSend有一个扩展版本objc_msgSend_stret，是专门用于处理返回结构体的情况，因此把代码改为如下所示即可解决问题：
CGSize (*action)(id, SEL, int) = (CGSize (*)(id, SEL, int)) objc_msgSend_stret; CGSize size = action(self, @selector(doSomething:), 0);
注：此调整只有在armv7架构下需要进行调整，如果包含arm64则不需要。