非8字节对齐的内存访问会导致内存访问冲突在iPhone 3GS与iOS 4.0 [英] Non 8 Byte aligned memory access causes memory access violation on iPhone 3GS with iOS 4.0

问题描述

考虑下面的Objective-C ++的iPhone程序（TestMemAppDelegate.mm）。它崩溃与iPhone（3GS与iOS 4.0）的EXC_BAD_ACCESS。它在模拟器工作正常。这显然​​是一个内存对齐的东西，因为它适用于iPhone的罚款，如果在数据A结构上的8字节边界开始。

任何人能解释的原因是什么？它说的是与ARM架构？ ARM编译器？

  @implementation TestMemAppDelegate
typedef结构数据A
{
 浮X;
 unsigned char型ÿ;
};
 - （BOOL）应用：（*的UIApplication）的应用didFinishLaunchingWithOptions：（NSDictionary的*）launchOptions { 字符* MEM1 =（字符*）malloc的（4096）; 数据A * PTRA =（数据A *）（MEM1 + 1）; //这里我们却将对齐
 ptrA-＆GT; X = 10.0f;
 的printf（A：％.2f \\ n，ptrA-＆GT; X）; //这崩溃    //视图控制器的视图添加到窗口和显示。
    [窗口addSubview：viewController.view];
    [窗口makeKeyAndVisible]    返回YES;
}
@结束
 

解决方案

是的，它是一个定位的问题。一个浮点必须是4字节对齐。英特尔的x86处理器允许未对齐访问（但性能损失）。在ARM这是不允许的，产生你看到错误。

consider the following Objective-C++ iPhone Application (TestMemAppDelegate.mm). It crashes with an EXC_BAD_ACCESS on the iPhone (3GS with iOS 4.0). It works fine in the Simulator. It is clearly a memory alignment thing, because it works fine on the iPhone if the "DataA" struct starts on a 8 Byte border.

Can anyone explain the cause? Is it something with the ARM architecture? ARM compiler?

@implementation TestMemAppDelegate


typedef struct DataA
{
 float    x;
 unsigned char  y;
};


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

 char* mem1 = (char*)malloc(4096);

 DataA* ptrA = (DataA*)(mem1 + 1); // Here we shift the alignment
 ptrA->x = 10.0f;
 printf("A: %.2f\n", ptrA->x); // Here it crashes



    // Add the view controller's view to the window and display.
    [window addSubview:viewController.view];
    [window makeKeyAndVisible];

    return YES;
}


@end

解决方案

Yes it's an alignment issue. A float needs to be 4-byte aligned. An Intel x86 CPU allows mis-aligned access (but at a performance penalty). On ARM it is not allowed and generates the error you see.

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