我正在用gdb(Xcode内部)介绍一些简单的Objective-C代码,并注意到一些奇怪的东西。这里是相关的片段:
NSString * s = nil; int x =(s == nil);正如我所料, x 在这两行之后是 1 。奇怪的是,如果我在gdb中尝试类似的方法,它的工作方式不一样:
(gdb)print ret $ 1 =(NSString *)0x0 (gdb)print(int)(ret == nil) $ 2 = 0 (gdb)print nil $ 3 = {<文本变量,无调试信息>} 0x167d18< nil>似乎gdb除了objective-C使用的(0x0)之外还有一些零的定义。有人可以解释一下这里发生了什么吗?
当你的代码被编译时, nil 是一个预处理器常量,定义为 __ null (一个特殊的GCC变量,用作 NULL ), 0L 或 0 : < objc / objc.h> #ifndef nil #define nil __DARWIN_NULL / * id为无实例* / #endif < sys / _types.h> #ifdef __cplusplus #ifdef __GNUG__ #define __DARWIN_NULL __null #else / *! __GNUG__ * / #ifdef __LP64__ #define __DARWIN_NULL(0L) #else / *!__ LP64__ * / #define __DARWIN_NULL 0 #endif / * __LP64__ * / #endif / * __GNUG__ * / #else / *! __cplusplus * / #define __DARWIN_NULL((void *)0) #endif / * __cplusplus * /因此,gdb在运行时拾取的 nil 来自哪里?您可以从消息中得知gdb给出的 nil 是位于该地址的变量的名称:
(gdb)p nil $ 1 = {<文本变量,无调试信息>} 0x20c49ba5da6428< nil> (gdb)i addr nil 在没有调试编译的文件中,符号nil位于0x20c49ba5da6428处。其值不出所料,结果是 0 :
(gdb)p *(long *)nil $ 2 = 0 (gdb )x / xg nil 0x20c49ba5da6428< n> ;: 0x0000000000000000这个变量来自哪里从? GDB可以告诉我们:
(gdb)我共享nil 3基金会F - init YY / System / Library / Frameworks / Foundation.framework / Versions / C / Foundation at 0x20c49ba5bb2000(offset 0x20c49ba5bb2000)的确,当我们检查基金会中定义的符号,我们发现 nil :
$ nm -m /System/Library/Frameworks/Foundation.framework/Foundation | grep nil 00000000001f4428(__TEXT,__ const)external _nil
I was stepping through some simple Objective-C code with gdb (inside of Xcode) and noticed something strange. Here is the relevant snippet:
NSString *s = nil; int x = (s == nil);As I'd expect, the value of x after these two lines is 1. Strangely, if I try something similar in gdb, it doesn't work the same:
(gdb) print ret $1 = (NSString *) 0x0 (gdb) print (int)(ret==nil) $2 = 0 (gdb) print nil $3 = {<text variable, no debug info>} 0x167d18 <nil>It seems like gdb has some definition for nil other than what objective-C uses (0x0). Can someone explain what's going on here?
解决方案When your code is being compiled, nil is a preprocessor constant defined to be either __null (a special GCC variable that serves as NULL), 0L, or 0:
<objc/objc.h> #ifndef nil #define nil __DARWIN_NULL /* id of Nil instance */ #endif <sys/_types.h> #ifdef __cplusplus #ifdef __GNUG__ #define __DARWIN_NULL __null #else /* ! __GNUG__ */ #ifdef __LP64__ #define __DARWIN_NULL (0L) #else /* !__LP64__ */ #define __DARWIN_NULL 0 #endif /* __LP64__ */ #endif /* __GNUG__ */ #else /* ! __cplusplus */ #define __DARWIN_NULL ((void *)0) #endif /* __cplusplus */So, where does the nil that gdb picks up at runtime come from? You can tell from the message gdb gives that nil is the name of a variable located at that address:
(gdb) p nil $1 = {<text variable, no debug info>} 0x20c49ba5da6428 <nil> (gdb) i addr nil Symbol "nil" is at 0x20c49ba5da6428 in a file compiled without debugging.Its value, unsurprisingly, turns out to be 0:
(gdb) p *(long *)nil $2 = 0 (gdb) x/xg nil 0x20c49ba5da6428 <nil>: 0x0000000000000000Where does this variable come from? GDB can tell us:
(gdb) i shared nil 3 Foundation F - init Y Y /System/Library/Frameworks/Foundation.framework/Versions/C/Foundation at 0x20c49ba5bb2000 (offset 0x20c49ba5bb2000)Indeed, when we check the symbols defined in Foundation, we find nil:
$ nm -m /System/Library/Frameworks/Foundation.framework/Foundation | grep nil$ 00000000001f4428 (__TEXT,__const) external _nil
更多推荐
gdb中的nil没有被定义为0x0?
发布评论