NSLog（...）不正确的格式说明符影响其他变量？ [英] NSLog(...) improper format specifier affects other variables?

问题描述

我最近浪费了大约半个小时的时间，在NSLog中追踪这个奇怪的行为：

  NSString * text = @abc; 
 long long num = 123; 
 NSLog（@num =％lld，text =％@，num，text）; //（A）
 NSLog（@num =％d，text =％@，num，text）; （B）
  

行（A）打印预期的num = 123，text = abc ，但行（B）打印num = 123，text = （空）。

显然，打印 long long with ％d 是一个错误，但是有人可以解释为什么它会导致 text 打印为空？

解决方案

我认为比你使用x86处理器的最新苹果产品。考虑到这些假设你的堆栈看起来就像在这两种情况下：

 
 |堆栈|第一|秒| 
 + --------------------- + ------- + -------- + 
 | 123 | | ％d | 
 + --------------------- +％lld + -------- + 
 | 0 | | ％@ | 
 + --------------------- + ------- + -------- + 
 |指向文本|的指针％@ |被忽略| 
 + --------------------- + ------- + -------- + 
 

在第一种情况下，您将堆栈8个字节，然后4个字节。并且NSLog被指示从堆栈12个字节（％lld 8字节，％@ 4字节） ）。
$ b 在第二种情况下，指示NSLog首先占用4个字节（％d ）。由于你的变量是8个字节长，并且保存的数字非常小，所以它的高4字节将是0.然后，当NSLog将尝试打印文本时，它将从堆栈中取 nil 因为发送消息到 nil 在Obj-C中是有效的NSLog只会发送描述：


/ code>到 nil 可能没有任何东西，然后print（null）。



Objective-C只是C的补充，调用者清理整个这个烂摊子。


I recently wasted about half an hour tracking down this odd behavior in NSLog(...):

NSString *text = @"abc";
long long num = 123;
NSLog(@"num=%lld, text=%@",num,text); //(A)
NSLog(@"num=%d, text=%@",num,text); //(B)

Line (A) prints the expected "num=123, text=abc", but line (B) prints "num=123, text=(null)".

Obviously, printing a long long with %d is a mistake, but can someone explain why it would cause text to be printed as null?

解决方案

You just messed up memory alignment on your stack. I assume than you use newest Apple product with x86 processor. Taking these assumptions into account your stack looks like that in both situations:

   |      stack          | first | second |
   +---------------------+-------+--------+
   |        123          |       |  %d    |
   +---------------------+ %lld  +--------+
   |         0           |       |  %@    |
   +---------------------+-------+--------+
   |   pointer to text   | %@    |ignored |
   +---------------------+-------+--------+  

In first situation you put on stack 8 bytes and then 4 bytes. And than NSLog is instructed to take back from stack 12 bytes (8 bytes for %lld and 4 bytes for %@).

In second situation you instruct NSLog to first take 4 bytes (%d). Since your variable is 8 bytes long and holds really small number its upper 4 bytes will be 0. Then when NSLog will try to print text it will take nil from stack.

Since sending message to nil is valid in Obj-C NSLog will just send description: to nil get probably nothing and then print (null).

In the end since Objective-C is just C with additions, caller cleans up whole this mess.

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