When the program is stopped, the first thing you need to do is to see where the program is parked. When your program calls a function, the address of the function, the function arguments, the local variables within the function are pressed into the stack. You can use the GDB command to view the information in the current stack.
Here are some GDB commands to view the function call stack information:
BACKTRACE/BT: Prints all the information for the current function call stack. Such as:
(gdb) bt#0 func (n=250) at tst.c:6#1 0x08048524 in main (argc=1, argv=0xbffff674) at tst.c:30#2 0x400409ed in __libc_start_main () from /lib/libc.so.6The function's call stack information can be seen from the:__libc_start_main–> main () –> func ()
BackTrace /BT : N is a positive integer representing only the stack information at the top n-tier of the stack.
If you want to view a layer of information, you need to switch the current stack, generally speaking, when the program stops, the topmost stack is the current stack, if you want to see the stack below the details of the layer, the first thing to do is to switch the current stack.
The above command will print out the information of the stack layer that is moved to. If you don't want it to be a message. You can use these three commands:
To view the information for the current stack, you can use the following GDB command:
Info frame/info f: This command prints out more detailed information about the current stack, except that most are memory addresses at runtime. For example: function address, call function address, called function address, the current function is written by what program language, function parameter address and value, local variable address and so on. Such as:
(gdb) info fStack level 0, frame at 0x7fffffffbb70: rip = 0x400642 in main (test.c:11); saved rip 0x2aaaaabe4304 source language c. Arglist at 0x7fffffffbb60, args: Locals at 0x7fffffffbb60, Previous frame‘s sp is 0x7fffffffbb70 Saved registers: rbp at 0x7fffffffbb60, rip at 0x7fffffffbb68Info args: Prints out the parameter names of the current function and their values.
First, display the source code
GDB 可以打印出所调试程序的源代码，当然，在程序编译时一定要加上-g的参数，把源程序信息编译到执行文件中。不然就看不到源程序了。当程序停下来以后，GDB会报告程序停在了那个文件的第几行上。你可以用list命令来打印程序的源代码。还是来看一看查看源代码的GDB命令吧。list <linenum> 显示程序第linenum行的周围的源程序。list <function> 显示函数名为function的函数的源程序。list 显示当前行后面的源程序。list - 显示当前行前面的源程序。
In general, the current line is printed on the top 5 rows and the next 5 lines, if the display function is 2 rows below 8 lines, the default is 10 rows, of course, you can also customize the display of the range, using the following command can be set to display the source program number of lines.
set listsize <count> 设置一次显示源代码的行数。show listsize 查看当前listsize的设置。
The list command also has the following usage:
list <first>, <last> 显示从first行到last行之间的源代码。list , <last> 显示从当前行到last行之间的源代码。list + 往后显示源代码。
In general, after the list can be followed by the following parameters:
<linenum> 行号。<+offset> 当前行号的正偏移量。<-offset> 当前行号的负偏移量。<filename:linenum> 哪个文件的哪一行。<function> 函数名。<filename:function> 哪个文件中的哪个函数。<*address> 程序运行时的语句在内存中的地址。
Second, search source code
Not only that, GDB also provides the command for source code search:
forward-search <regexp>search <regexp> 向前面搜索。reverse-search <regexp> 全部搜索。
Among them, is the regular expression, but also the main string of the matching pattern, about the regular expression, I do not speak here, but also ask you to view the relevant information.
Third, specify the path of the source file
At some point, only the name of the source file is included in the Execute program that was compiled with-G, and there is no path name. GDB provides a command that lets you specify the path to the source file so that gdb can search.
directory <dirname ... >dir <dirname ... > 加一个源文件路径到当前路径的前面。如果你要指定多个路径，UNIX下你可以使用“:”，Windows下你可以使用“;”。directory 清除所有的自定义的源文件搜索路径信息。show directories 显示定义了的源文件搜索路径。
Iv. Source-code memory
You can use the Info command to see the address of the source code in memory. Info line can be followed by "row number", "Function name", "File name: line number", "File name: function name", this command will print out the specified source code at runtime memory address, such as:
(gdb) info line tst.c:func Line 5 of "tst.c" starts at address 0x8048456 <func+6> and ends at 0x804845d <func+13>.
The
also has a command (disassemble) that allows you to view the machine code of the current execution of the source program, which dumps the current in-memory instructions. The following example shows the assembly code to view the Func function.
(GDB) disassemble func Dump of assembler code for function func:0x8048450 <func>: Push%EBP 0x 8048451 <func+1>: mov%esp,%ebp 0x8048453 <func+3>: Sub $0x18,%esp 0x8048456 <func+6> ;: Movl $0X0,0XFFFFFFFC (%EBP) 0x804845d <func+13>: Movl $0x1,0xfffffff8 (%EBP) 0x8048464 <func+20 : mov 0xfffffff8 (%EBP),%eax 0x8048467 <func+23>: CMP 0x8 (%EBP),%eax 0x804846a <func+26>: Jle 0x8048470 <func+32> 0x804846c <func+28>: jmp 0x8048480 <func+48> 0x804846e <fu NC+30>: mov%esi,%esi 0x8048470 <func+32>: mov 0xfffffff8 (%EBP),%eax 0x8048473 <func+35>: Add%EAX,0XFFFFFFFC (%EBP) 0x8048476 <func+38>: Incl 0xfffffff8 (%EBP) 0x8048479 <func+41>: JMP 0x8048464 <func+20> 0x804847b <func+43>: Nop 0x804847c <func+44>: Lea 0x0 (%esi,1) ,%esi 0x8048480 <fUNC+48>: mov 0xfffffffc (%EBP),%edx 0x8048483 <func+51>: mov%edx,%eax 0x8048485 <func+53> : jmp 0x8048487 <func+55> 0x8048487 <func+55>: mov%ebp,%esp 0x8048489 <func+57>: P Op%ebp 0x804848a <func+58>: ret End of assembler dump.在你调试程序时，当程序被停住时，你可以使用print命令（简写命令为p），或是同义命令inspect来查看当前程序的运行数据。print命令的格式是：print <expr>print /<f> <expr> <expr>是表达式，是你所调试的程序的语言的表达式（GDB可以调试多种编程语言），<f>是输出的格式，比如，如果要把表达式按16进制的格式输出，那么就是/x。
I. Expressions
print和许多GDB的命令一样，可以接受一个表达式，GDB会根据当前的程序运行的数据来计算这个表达式，既然是表达式，那么就可以是当前程序运行中的const常量、变量、函数等内容。可惜的是GDB不能使用你在程序中所定义的宏。表达式的语法应该是当前所调试的语言的语法，由于C/C++是一种大众型的语言，所以，本文中的例子都是关于C/C++的。（而关于用GDB调试其它语言的章节，我将在后面介绍）在表达式中，有几种GDB所支持的操作符，它们可以用在任何一种语言中。@ 是一个和数组有关的操作符，在后面会有更详细的说明。:: 指定一个在文件或是一个函数中的变量。{<type>} <addr> 表示一个指向内存地址<addr>的类型为type的一个对象。
Second, the program variable
在GDB中，你可以随时查看以下三种变量的值： 1、全局变量（所有文件可见的） 2、静态全局变量（当前文件可见的） 3、局部变量（当前Scope可见的）如果你的局部变量和全局变量发生冲突（也就是重名），一般情况下是局部变量会隐藏全局变量，也就是说，如果一个全局变量和一个函数中的局部变量同名时，如果当前停止点在函数中，用print显示出的变量的值会是函数中的局部变量的值。如果此时你想查看全局变量的值时，你可以使用“::”操作符： file::variablefunction::variable可以通过这种形式指定你所想查看的变量，是哪个文件中的或是哪个函数中的。例如，查看文件f2.c中的全局变量x的值：gdb) p ‘f2.c‘::x当然，“::”操作符会和C++中的发生冲突，GDB能自动识别“::” 是否C++的操作符，所以你不必担心在调试C++程序时会出现异常。另外，需要注意的是，如果你的程序编译时开启了优化选项，那么在用GDB调试被优化过的程序时，可能会发生某些变量不能访问，或是取值错误码的情况。这个是很正常的，因为优化程序会删改你的程序，整理你程序的语句顺序，剔除一些无意义的变量等，所以在GDB调试这种程序时，运行时的指令和你所编写指令就有不一样，也就会出现你所想象不到的结果。对付这种情况时，需要在编译程序时关闭编译优化。一般来说，几乎所有的编译器都支持编译优化的开关，例如，GNU的C/C++编译器GCC，你可以使用“-gstabs”选项来解决这个问题。关于编译器的参数，还请查看编译器的使用说明文档。
Three, array
有时候，你需要查看一段连续的内存空间的值。比如数组的一段，或是动态分配的数据的大小。你可以使用GDB的“@”操作符，“@”的左边是第一个内存的地址的值，“@”的右边则你你想查看内存的长度。例如，你的程序中有这样的语句： int *array = (int *) malloc (len * sizeof (int));于是，在GDB调试过程中，你可以以如下命令显示出这个动态数组的取值： p *[email protected]@的左边是数组的首地址的值，也就是变量array所指向的内容，右边则是数据的长度，其保存在变量len中，其输出结果，大约是下面这个样子的： (gdb) p *[email protected] $1 = {2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40}如果是静态数组的话，可以直接用print数组名，就可以显示数组中所有数据的内容了。
Iv. output format
一般来说，GDB会根据变量的类型输出变量的值。但你也可以自定义GDB的输出的格式。例如，你想输出一个整数的十六进制，或是二进制来查看这个整型变量的中的位的情况。要做到这样，你可以使用GDB的数据显示格式：x 按十六进制格式显示变量。d 按十进制格式显示变量。u 按十六进制格式显示无符号整型。o 按八进制格式显示变量。t 按二进制格式显示变量。a 按十六进制格式显示变量。c 按字符格式显示变量。f 按浮点数格式显示变量。 (gdb) p i $21 = 101 (gdb) p/a i $22 = 0x65 (gdb) p/c i $23 = 101 ‘e‘ (gdb) p/f i $24 = 1.41531145e-43 (gdb) p/x i $25 = 0x65 (gdb) p/t i $26 = 1100101
V. View memory
你可以使用examine命令（简写是x）来查看内存地址中的值。x命令的语法如下所示：x/<n/f/u> <addr>n、f、u是可选的参数。n 是一个正整数，表示显示内存的长度，也就是说从当前地址向后显示几个地址的内容。f 表示显示的格式，参见上面。如果地址所指的是字符串，那么格式可以是s，如果地十是指令地址，那么格式可以是i。u 表示从当前地址往后请求的字节数，如果不指定的话，GDB默认是4个bytes。u参数可以用下面的字符来代替，b表示单字节，h表示双字节，w表示四字节，g表示八字节。当我们指定了字节长度后，GDB会从指内存定的内存地址开始，读写指定字节，并把其当作一个值取出来。<addr>表示一个内存地址。n/f/u三个参数可以一起使用。例如：命令：x/3uh 0x54320 表示，从内存地址0x54320读取内容，h表示以双字节为一个单位，3表示三个单位，u表示按十六进制显示。
VI, automatic display
You can set up some automatically displayed variables that are automatically displayed when the program stops, or when you step through the tracks. The associated GDB command is display.
Display <expr>display/<fmt> <expr>display/<fmt> <addr>expr is an expression that FMT represents the format of the display, Addr represents a memory address, and when you set one or more expressions with display, GDB automatically displays the values of the expressions you set as long as your program is stopped. The format I and S are also supported by display, a very useful command is: display/i $pc $pc is the environment variable of GDB, indicating the address of the instruction,/I means the output format is the machine instruction code, namely the assembly. So when the program stops, there will be the source code and machine instructions corresponding to the situation, this is a very interesting feature. Here are some of the GDB commands associated with display: Undisplay <dnums...>delete display <dnums...> Delete Auto Show, dnums means the set of automatic explicit numbering. If you want to delete several, the number can be separated by a space, if you want to delete a range of numbers, you can use a minus sign (for example: 2-5) Disable display <dnums...>enable display <dnums...> Disable and Enalbe do not remove the settings that are automatically displayed, but just let them fail and resume. Info display to view the information displayed automatically on the display settings. GDB will print out a form to report to you, of course, how many auto-display settings are set in the debug, including, set number, expression, whether enable. Vii. setting display Options gdb has more options on display, here I just cite most of the common options. Set Print AddressSet print address on opens the output, and when the program displays the function information, GDB shows the function's parameter address. The system defaults to open, such as: (GDB) F #0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>") at input.c:530 530 if (lquote! = def_lquote) Set print address off function parameter address display, such as: (GDB) Set print addr off (gdb) F #0 set_quOTEs (lq= "<<", rq= ">>") at input.c:530 530 if (lquote! = Def_lquote) Show print address view current display selection Whether the item is open. The Set Print Arrayset print array on opens the arrays display, when the array is displayed, each element takes up a row, and each element is separated by commas if it is not opened. This option is off by default. The two commands associated with it are as follows, and I will not say more. Set Print array offshow print arrayset print elements <number-of-elements> This option is primarily an array, and if your array is too large, you can specify a <nu Mber-of-elements> to specify the maximum length of the data display, and when this length is reached, GDB does not display down. If set to 0, no limit is expressed. Show Print elements view option information for print elements. Set Print Null-stop <on/off> If this option is turned on, when the string is displayed, the Terminator is stopped. This option is off by default. Set print pretty on if you turn on printf pretty this option, it will be pretty when GDB displays the struct body. For example: $ = {next = 0x0, flags = {sweet = 1, sour = 1}, ME at = 0x54 "Pork"}set print pretty off printf pretty this option, GDB displays the struct as follows: $ = {next = 0x0, flags = {SWE ET = 1, sour = 1}, meat = 0x54 "Pork"}show print pretty See how GDB shows the structure. Set Print sevenbit-strings <on/off> sets character display, whether by "/nnn" formatDisplay, if open, the string or character data is displayed as/nnn, such as "/065". Show print sevenbit-strings See if the character display switch is turned on. Set Print Union <on/off> Sets whether the union data is explicitly inside when the struct is displayed. For example, the following data structures are available: typedef enum {Tree, Bug} species; typedef enum {big_tree, Acorn, seedling} tree_forms; typedef enum {Caterpillar, Cocoon, Butterfly} bug_forms; struct thing {species it; Union {tree_forms Tree; Bug_forms Bug; } form; }; struct thing foo = {Tree, {Acorn}}; When this switch is opened, the p foo command is displayed as follows: $ = {it = tree, form = {tree = Acorn, bug = Cocoon}} When the switch is closed, the p foo command is displayed as follows: $ = {it = Tree, form = {...}} Show Print Union View how union data is displayed set Print object <on/off> in C + +, if an object pointer points to its derived class, if this option is turned on, GDB automatically displays the output according to the rules called by the virtual method, if it is turned off This option, GDB does not care about the virtual function table. This option is off by default. The show Print object view settings for object options. Set Print static-members <on/off> This option indicates whether static data members are displayed when the content in a C + + object is displayed. The default is on. Show print static-members view static data member option settings. Set Print Vtbl <on/off> When this option is turned on, GDB will display the virtual function table in a more structured format。 It is turned off by default. Show print VTBL view options for virtual function display format.Viii. Historical records
当你用GDB的print查看程序运行时的数据时，你每一个print都会被GDB记录下来。GDB会以$1, $2, $3 .....这样的方式为你每一个print命令编上号。于是，你可以使用这个编号访问以前的表达式，如$1。这个功能所带来的好处是，如果你先前输入了一个比较长的表达式，如果你还想查看这个表达式的值，你可以使用历史记录来访问，省去了重复输入。
Nine, GDB environment variables
你可以在GDB的调试环境中定义自己的变量，用来保存一些调试程序中的运行数据。要定义一个GDB的变量很简单只需。使用GDB的set命令。GDB的环境变量和UNIX一样，也是以$起头。如：set $foo = *object_ptr使用环境变量时，GDB会在你第一次使用时创建这个变量，而在以后的使用中，则直接对其賦值。环境变量没有类型，你可以给环境变量定义任一的类型。包括结构体和数组。show convenience 该命令查看当前所设置的所有的环境变量。这是一个比较强大的功能，环境变量和程序变量的交互使用，将使得程序调试更为灵活便捷。例如： set $i = 0 print bar[$i++]->contents于是，当你就不必，print bar[0]->contents, print bar[1]->contents地输入命令了。输入这样的命令后，只用敲回车，重复执行上一条语句，环境变量会自动累加，从而完成逐个输出的功能。
Ten, view register
Change the execution of a program要查看寄存器的值，很简单，可以使用如下命令：info registers 查看寄存器的情况。（除了浮点寄存器）info all-registers 查看所有寄存器的情况。（包括浮点寄存器）info registers <regname ...> 查看所指定的寄存器的情况。寄存器中放置了程序运行时的数据，比如程序当前运行的指令地址（ip），程序的当前堆栈地址（sp）等等。你同样可以使用print命令来访问寄存器的情况，只需要在寄存器名字前加一个$符号就可以了。如：p $eip。
一旦使用GDB挂上被调试程序，当程序运行起来后，你可以根据自己的调试思路来动态地在GDB中更改当前被调试程序的运行线路或是其变量的值，这个强大的功能能够让你更好的调试你的程序，比如，你可以在程序的一次运行中走遍程序的所有分支。
First, modify the value of the variable
修改被调试程序运行时的变量值，在GDB中很容易实现，使用GDB的print命令即可完成。如： (gdb) print x=4x=4这个表达式是C/C++的语法，意为把变量x的值修改为4，如果你当前调试的语言是Pascal，那么你可以使用Pascal的语法：x:=4。在某些时候，很有可能你的变量和GDB中的参数冲突，如： (gdb) whatis width type = double (gdb) p width $4 = 13 (gdb) set width=47 Invalid syntax in expression.因为，set width是GDB的命令，所以，出现了“Invalid syntax in expression”的设置错误，此时，你可以使用set var命令来告诉GDB，width不是你GDB的参数，而是程序的变量名，如： (gdb) set var width=47另外，还可能有些情况，GDB并不报告这种错误，所以保险起见，在你改变程序变量取值时，最好都使用set var格式的GDB命令。
Second, jump execution
一般来说，被调试程序会按照程序代码的运行顺序依次执行。GDB提供了乱序执行的功能，也就是说，GDB可以修改程序的执行顺序，可以让程序执行随意跳跃。这个功能可以由GDB的jump命令来完：jump <linespec>指定下一条语句的运行点。<linespce>可以是文件的行号，可以是file:line格式，可以是+num这种偏移量格式。表式着下一条运行语句从哪里开始。jump <address>这里的<address>是代码行的内存地址。注意，jump命令不会改变当前的程序栈中的内容，所以，当你从一个函数跳到另一个函数时，当函数运行完返回时进行弹栈操作时必然会发生错误，可能结果还是非常奇怪的，甚至于产生程序Core Dump。所以最好是同一个函数中进行跳转。熟悉汇编的人都知道，程序运行时，有一个寄存器用于保存当前代码所在的内存地址。所以，jump命令也就是改变了这个寄存器中的值。于是，你可以使用“set $pc”来更改跳转执行的地址。如：set $pc = 0x485
Iii. generation of Signal volume
使用singal命令，可以产生一个信号量给被调试的程序。如：中断信号Ctrl+C。这非常方便于程序的调试，可以在程序运行的任意位置设置断点，并在该断点用GDB产生一个信号量，这种精确地在某处产生信号非常有利程序的调试。语法是：signal <singal>，UNIX的系统信号量通常从1到15。所以<singal>取值也在这个范围。single命令和shell的kill命令不同，系统的kill命令发信号给被调试程序时，是由GDB截获的，而single命令所发出一信号则是直接发给被调试程序的。
Iv. Mandatory function return
如果你的调试断点在某个函数中，并还有语句没有执行完。你可以使用return命令强制函数忽略还没有执行的语句并返回。returnreturn <expression>使用return命令取消当前函数的执行，并立即返回，如果指定了<expression>，那么该表达式的值会被认作函数的返回值。
V. Force call function
Using GDB in different languagescall <expr>表达式中可以一是函数，以此达到强制调用函数的目的。并显示函数的返回值，如果函数返回值是void，那么就不显示。另一个相似的命令也可以完成这一功能——print，print后面可以跟表达式，所以也可以用他来调用函数，print和call的不同是，如果函数返回void，call则不显示，print则显示函数返回值，并把该值存入历史数据中。
GDB supports the following languages: C, C + +, Fortran, PASCAL, Java, Chill, assembly, and Modula-2. In general, GDB will determine the debug language of course according to the program you are debugging, such as: find the filename suffix ". C", GDB will consider it a C program. The filename suffix is ". C,. CC,. CP,. cpp,. cxx,. C + +, GDB will be considered a C + + program. The suffix is ". F,. F ", GDB will consider it to be a FORTRAN program, and the suffix is". S, ". S "will be considered to be assembly language.
In other words, GDB will set its own locale based on the language of the program you are debugging, and let GDB's commands change as the locale changes. For example, when some GDB commands need to use expressions or variables, the syntax of these expressions or variables is changed entirely according to the current locale. For example, the syntax for pointers in C/s + + is *p, while in Modula-2 it is p^. And, if your current program is compiled by several different languages, GDB can automatically switch the locale to different languages during debugging. This function, which changes with the language environment, is a thoughtful design for developers.
Here are a few commands related to the GDB locale:
show language 查看当前的语言环境。如果GDB不能识为你所调试的编程语言，那么，C语言被认为是默认的环境。info frame 查看当前函数的程序语言。info source 查看当前文件的程序语言。
If GDB does not detect the current program language, you can also manually set the current program language. You can do this using the Set Language command.
Postscript当set language命令后什么也不跟的话，你可以查看GDB所支持的语言种类： (gdb) set language The currently understood settings are: local or auto Automatic setting based on source file c Use the C language c++ Use the C++ language asm Use the Asm language chill Use the Chill language fortran Use the Fortran language java Use the Java language modula-2 Use the Modula-2 language pascal Use the Pascal language scheme Use the Scheme language于是你可以在set language后跟上被列出来的程序语言名，来设置当前的语言环境。
GDB is a powerful command-line debugging tool. You know the power of the command line is that it can form an execution sequence and form a script. The software under UNIX is full of command line, which gives the program development a great convenience, the advantage of the command line software is that they can be very easy to integrate together, using a few simple tools of the command, you can make a very powerful function.
So the software under UNIX is more organically integrated than the software under Windows, each exerting its own merits and combining it into a more powerful function. and the graphics software under Windows is basically the respective battalion, cannot call each other, very unfavorable to the mutual integration of various software. Here is not what to compare with Windows, the so-called "inch, ruler is short", graphical tools are not as good as the command line place. (see this sentence, I hope you never think I was "despise the graphical interface", and I contradicting)
I am writing this article based on the version of 5.1.1 GdB, so there may be some features that have been modified or more powerful. And, I write very hasty, write relatively simple, and, I have seen a lot of typos (I use Wubi, so you do not understand the typo), so, I am here on the mistakes of my words are extremely apologetic.
The GDB features listed in the article, I just listed some of the use of GDB's command and usage, in fact, I only talk about the features of the GDB only about 60% of all the features, detailed documentation, or please see GDB's help and manuals, perhaps, over time, if I am free, Let me write another article about GDB's advanced use.
I personally like GDB's automatic debugging function, this function is really very powerful, imagine, I write a script under UNIX, let the script automatically compile my program, be automatically debugged, and report the results, debugging success, automatic checkin Source library. A command, compile with debugging with checkin, how cool ah. GDB is only the current support for automated debugging is not very mature, can only achieve semi-automation, sincerely expect the automation of GDB debugging function of maturity.
If you are interested in GDB or any other technical issues, you are welcome to discuss the exchange with me. I am currently mainly in the UNIX under the development of product software, so, UNIX software development is familiar with, of course, not only technology, software engineering implementation, software design, system analysis, project management I also have a little experience. Welcome everyone to contact me, (QQ is: 753640,MSN is: [email protected])
Linux Advanced Programming--04.gdb Debug Program (View data)
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