上图中，我使用 po 命令显示一个 NSDictionary 的内容。注意在左侧我们可以看到 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）。我们可以右击左侧的 dict，选中“Print Description of "dict"”，则可以在控制台输出 dict 的详细信息： [cpp] view plaincopyprint? Printing description of dict: {type = immutable dict, count = 3, entries => 0 : {contents = "first"} = {contents = "one"} 1 : {contents = "second"} = {contents = "two"} 2 : {contents = "third"} = {contents = "three"} } (gdb) print 命令：有点类似于格式化输出，可以输出对象的不同信息： 如： [cpp] view plaincopyprint? (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：我们可以查看内存地址所在信息 比如 "info symbol 内存地址" 可以获取内存地址所在的 symbol 相关信息： [cpp] view plaincopyprint? (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 比如 "info line *内存地址" 可以获取内存地址所在的代码行相关信息： [cpp] view plaincopyprint? (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 [cpp] view plaincopyprint? (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：如果忘记某条命令的语法了，可以使用 help 命令名 来获取帮助信息。如：help info 显示 info 命令的用法。 [cpp] view plaincopyprint? (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 有时候我们的程序不知道跑到哪个地方就 crash 了，而 crash 又很难重现。保守的做法是在系统抛出异常之前设置断点，具体来说是在 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；然后点击右下方的 +，增加新的 Symbolic Breakpoint，在 Symbol 一栏输入：objc_exception_throw，然后点击 done，完成。 这样在 Debug 模式下，如果程序即将抛出异常，就能在抛出异常处中断了。比如在前面的代码中，我让 [firstObjctcrashTest]; 抛出异常。在 objc_exception_throw 处设置断点之后，程序就能在该代码处中断了，我们从而知道代码在什么地方出问题了。
