Xcode调试技巧(一)

 XCode 内置GDB，咱们能够在命令行中使用 GDB 命令来调试咱们的程序。下面将介绍一些经常使用的命令以及调试技巧。

po 命令：为 print object 的缩写，显示对象的文本描述（显示从对象的 description 消息得到的字符串信息）。

好比：

上图中，我使用 po 命令显示一个 NSDictionary 的内容。注意在左侧咱们能够看到 dict 的一些信息：3 key/value pairs，显示该 dict 包含的数据量，而展开的信息显示 isa 层次体系（即class 和 metaclass结构关系）。咱们能够右击左侧的 dict，选中“Print Description of "dict"”，则能够在控制台输出 dict 的详细信息：

Printing description of dict:
<CFBasicHash 0x1001149e0 [0x7fff7e27ff40]>{type = immutable dict, count = 3,
entries =>
0 : <CFString 0x100002458 [0x7fff7e27ff40]>{contents = "first"} = <CFString 0x100002438 [0x7fff7e27ff40]>{contents = "one"}
1 : <CFString 0x100002498 [0x7fff7e27ff40]>{contents = "second"} = <CFString 0x100002478 [0x7fff7e27ff40]>{contents = "two"}
2 : <CFString 0x1000024d8 [0x7fff7e27ff40]>{contents = "third"} = <CFString 0x1000024b8 [0x7fff7e27ff40]>{contents = "three"}
}
(gdb)

Printing description of dict:
<CFBasicHash 0x1001149e0 [0x7fff7e27ff40]>{type = immutable dict, count = 3,
entries =>
	0 : <CFString 0x100002458 [0x7fff7e27ff40]>{contents = "first"} = <CFString 0x100002438 [0x7fff7e27ff40]>{contents = "one"}
	1 : <CFString 0x100002498 [0x7fff7e27ff40]>{contents = "second"} = <CFString 0x100002478 [0x7fff7e27ff40]>{contents = "two"}
	2 : <CFString 0x1000024d8 [0x7fff7e27ff40]>{contents = "third"} = <CFString 0x1000024b8 [0x7fff7e27ff40]>{contents = "three"}
}
(gdb) 

print 命令：有点相似于格式化输出，能够输出对象的不一样信息：

如：

(gdb) print (char *)[[dict description] cStringUsingEncoding:4]
$1 = 0x1001159c0 "{\n first = one;\n second = two;\n third = three;\n}"
(gdb) print (int)[dict retainCount]
$2 = 1
(gdb)

(gdb) print (char *)[[dict description] cStringUsingEncoding:4]
$1 = 0x1001159c0 "{\n    first = one;\n    second = two;\n    third = three;\n}"
(gdb) print (int)[dict retainCount]
$2 = 1
(gdb) 

 

注：4是 NSUTF8StringEncoding 的值。

info 命令：咱们能够查看内存地址所在信息

好比 "info symbol 内存地址" 能够获取内存地址所在的 symbol 相关信息：

(gdb) info symbol 0x00000001000017f7
main + 343 in section LC_SEGMENT.__TEXT.__text of /Users/LuoZhaohui/Library/Developer/Xcode/DerivedData/RunTimeSystem-anzdlhiwvlbizpfureuvenvmatnp/Build/Products/Debug/RunTimeSystem

(gdb) info symbol 0x00000001000017f7
main + 343 in section LC_SEGMENT.__TEXT.__text of /Users/LuoZhaohui/Library/Developer/Xcode/DerivedData/RunTimeSystem-anzdlhiwvlbizpfureuvenvmatnp/Build/Products/Debug/RunTimeSystem

 

好比 "info line *内存地址" 能够获取内存地址所在的代码行相关信息：

 

(gdb) info line *0x00000001000017f7
Line 62 of "/Users/LuoZhaohui/Documents/Study/RunTimeSystem/RunTimeSystem/main.m" starts at address 0x1000017f7 <main+343> and ends at 0x10000180a <main+362>.

(gdb) info line *0x00000001000017f7
Line 62 of "/Users/LuoZhaohui/Documents/Study/RunTimeSystem/RunTimeSystem/main.m" starts at address 0x1000017f7 <main+343> and ends at 0x10000180a <main+362>.

 

show 命令：显示 GDB 相关的信息。如：show version 显示GDB版本信息

(gdb) show version
GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Mon Aug 8 20:32:45 UTC 2011)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin".

(gdb) show version
GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Mon Aug  8 20:32:45 UTC 2011)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin".

help 命令：若是忘记某条命令的语法了，能够使用 help 命令名 来获取帮助信息。如：help info 显示 info 命令的用法。

(gdb) help info
Generic command for showing things about the program being debugged.

List of info subcommands:

info address -- Describe where symbol SYM is stored
info all-registers -- List of all registers and their contents
info args -- Argument variables of current stack frame
info auxv -- Display the inferior's auxiliary vector
info breakpoints -- Status of user-settable breakpoints
info catch -- Exceptions that can be caught in the current stack frame
info checkpoints -- Help
info classes -- All Objective-C classes
......

Type "help info" followed by info subcommand name for full documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)

(gdb) help info
Generic command for showing things about the program being debugged.

List of info subcommands:

info address -- Describe where symbol SYM is stored
info all-registers -- List of all registers and their contents
info args -- Argument variables of current stack frame
info auxv -- Display the inferior's auxiliary vector
info breakpoints -- Status of user-settable breakpoints
info catch -- Exceptions that can be caught in the current stack frame
info checkpoints -- Help
info classes -- All Objective-C classes
......

Type "help info" followed by info subcommand name for full documentation.
Command name abbreviations are allowed if unambiguous.
(gdb) 

在系统抛出异常处设置断点

有时候咱们的程序不知道跑到哪一个地方就 crash 了，而 crash 又很难重现。保守的作法是在系统抛出异常以前设置断点，具体来讲是在 objc_exception_throw处设置断点。设置步骤为：首先在 XCode 按 CMD + 6，进入断点管理窗口；而后点击右下方的 +，增长新的 Symbolic Breakpoint，在 Symbol 一栏输入：objc_exception_throw，而后点击 done，完成。 这样在 Debug 模式下，若是程序即将抛出异常，就能在抛出异常处中断了。好比在前面的代码中，我让 [firstObjctcrashTest]; 抛出异常。在 objc_exception_throw 处设置断点以后，程序就能在该代码处中断了，咱们从而知道代码在什么地方出问题了。

 

转自:http://blog.csdn.net/kesalin/article/details/7222153