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How-to-debug-a-C or C++ program with GDB command-line
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SPccman Translating
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How to debug a C/C++ program with GDB command-line debugger
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================================================================================
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What is the worst part of coding without a debugger? Compiling on your knees praying that nothing will crash? Running the executable with a blood offering? Or just having to write printf("test") at every line hoping to find where the problem is coming from? As you probably know, there are not many advantages to coding without a debugger. But the good side is that debugging on Linux is easy. While most people use the debugger included in their favorite IDE, Linux is famous for its powerful command line C/C++ debugger: GDB. However, like most command line utilities, GDB requires a bit of training to master fully. In this tutorial, I will give you a quick rundown of GDB debugger.
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### Installation of GDB ###
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GDB is available in most distributions' repositories.
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For Debian or Ubuntu:
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$ sudo apt-get install gdb
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For Arch Linux:
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$ sudo pacman -S gdb
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For Fedora, CentOS or RHEL:
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$ sudo yum install gdb
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If you cannot find it anywhere else, it is always possible to download it from the [official page][1].
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### Code Sample ###
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When you are learning GDB, it is always better to have a piece of code to try things. Here is a quick sample that I coded to show the best features of GDB. Feel free to copy paste it to try the examples. That's the best way to learn.
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#include <stdio.h>
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#include <stdlib.h>
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int main(int argc, char **argv)
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{
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int i;
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int a=0, b=0, c=0;
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double d;
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for (i=0; i<100; i++)
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{
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a++;
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if (i>97)
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d = i / 2.0;
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b++;
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}
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return 0;
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}
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### Usage of GDB ###
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First and foremost, you will need to compile your program with the flag "-g" (for debug) to run it via GDB. From there the syntax to start debugging is:
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$ gdb -tui [executable's name]
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The "-tui” option will show your code in a nice interactive terminal window (so-called "text user interface") that you can navigate in with the arrow keys, while typing in the GDB shell below.
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We can now start playing around placing breakpoints anywhere in the source code with debugger. Here you have the options to set a breakpoint at a line number of the current source file:
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break [line number]
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or at a line number of a specific source file:
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break [file name]:[line number]
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or at a particular function:
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break [function name]
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And even better, you can set conditional breakpoints:
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break [line number] if [condition]
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For example, in our code sample, I can set:
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break 11 if i > 97
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which will have an effect of stopping me at "a++;" after 97 iterations of the for loop. As you have guessed, this is very handy when you do not want to step through the loop 97 times on your own.
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Last but not least, you can place a "watchpoint" which will pause the program if a variable is modified:
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watch [variable]
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Here, I can set one like:
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watch d
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which will stop the program as soon as variable d is set to a new value (i.e. when i > 97 is true).
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Once our breakpoints are set, we can run the program with the "run" command, or simply:
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r [command line arguments if your program takes some]
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as most words can be abbreviated in just a letter with gdb.
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And without surprises, we are stopped at line 11. From there, we can do interesting things. The command:
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bt
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for backtrack will tell us how we got to that point.
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info locals
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will display all the local variables and their current values (as you can see I didn't set my d variable to anything so its value is currently garbage).
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Of course:
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p [variable]
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will show the value of a particular variable. But even better:
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ptype [variable]
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shows the type of a local variable. So here we can confirm that d is double type.
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And since we are playing with fire, might as well do it all the way:
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set var [variable] = [new value]
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will override the value of the variable. Be careful though as you can't create a new variable or change its type. But here we can do:
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set var a = 0
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And just like any good debugger, we can "step" with:
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step
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to run the next line and potentially step into a function. Or just:
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next
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to just go straight to the line below, ignoring any function call.
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And to finish testing, you can delete a breakpoint with:
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delete [line number]
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Keep running the program from the current breakpoint with:
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continue
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and exit GDB with:
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quit
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To conclude, with GDB, no more praying to compile, no more blood offerings to run, no more printf("test"). Of course this post is not exhaustive and GDB's capabilities run beyond this, so I really encourage you to learn more about it on your own (or in a future post maybe?). What I am the most interested now is to integrate GDB nicely in Vim. In the meantime, here is a very [big memo][2] of all the GDB commands for future reference.
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What do you think of GDB? Would you consider its advantages over a graphical debugger or an IDE's? And what about integrating into Vim? Let us know in the comments.
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/gdb-command-line-debugger.html
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作者：[Adrien Brochard][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://xmodulo.com/author/adrien
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[1]:https://www.gnu.org/software/gdb/
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[2]:http://users.ece.utexas.edu/~adnan/gdb-refcard.pdf
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@ -0,0 +1,166 @@
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使用GDB命令行调试器调试C/C++程序
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============================================================
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没有调试器的情况下编写程序时最糟糕的状况是什么？编译时，跪着祈祷不要出错？用生命在运行可执行程序（blood offering不知道怎么翻译好...）？或者在每一行代码间添加printf("test")语句来定位错误点？如你所知，编写程序时不使用调试器的话是不利的。幸好，linux下调试还是很方便的。大多数人使用的IDE都集成了调试器，但linxu著名的调试器是命令行形式的C/C++调试器GDB。然而，与其他命令行工具一致，DGB需要一定的练习才能完全掌握。这里，我会告诉你GDB的基本情况及使用方法。
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###安装GDB###
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大多数的发行版仓库中都有GDB
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Debian 或 Ubuntu
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$ sudo apt-get install gdb
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Arch Linux
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$ sudo pacman -S gdb
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Fedora,CentOS 或 RHEL：
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$sudo yum install gdb
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如果在仓库中找不到的话，可以从官网中下载[official page][1]
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###示例代码###
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当学习GDB时，最好有一份代码，动手试验。下列代码是我编写的简单例子，它可以很好的体现GDB的特性。将它拷贝下来并且进行实验。这是最好的方法。
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#include <stdio.h>
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#include <stdlib.h>
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int main(int argc, char **argv)
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{
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int i;
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int a=0, b=0, c=0;
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double d;
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for (i=0; i<100; i++)
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{
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a++;
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if (i>97)
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d = i / 2.0;
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b++;
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}
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return 0;
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}
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###GDB的使用###
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首先最重要的，你需要使用编译器的 “-g“选项来编译程序，这样可执行程序才能通过GDB来运行。通过下列语句开始调试：
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$ gdb -tui [executable's name]
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使用”-tui“选项可以将代码显示在一个窗口内（被称为”文本接口”），在这个窗口内可以使用光标来操控，同时在下面输入GDB shell命令。
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现在我们可以在程序的任何地方设置断点。你可以通过下列命令来为当前源文件的某一行设置断点。
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break [line number]
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或者为一个特定的函数设置断点：
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break [function name]
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甚至可以设置条件断点
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break [line number] if [condition]
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例如，在我们的示例代码中，可以设置如下：
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break 11 if i > 97
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这样，程序循环97次之后停留在“a++”语句上。这样是非常方便的，避免了我们需要手动循环97次。
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最后但也是很重要的是，我们可以设置一个“观察断点”，当这个被观察的变量发生变化时，程序会被停止。
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watch [variable]
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可以设置如下：
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watch d
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当d的值发生变化时程序会停止运行（例如，当i>97为真时）。
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当设置后断点后，使用"run"命令开始运行程序，或按如下所示：
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r [程序的输入参数（如果有的话）]
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gdb中，大多数的单词都可以简写为一个字母。
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不出意外，程序会停留在11行。这里，我们可以做些有趣的事情。下列命令：
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bt
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回溯功能可以让我们知道程序如何到达这条语句的。
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info locals
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这条语句会显示所有的局部变量以及它们的值（你可以看到，我没有为ｄ设置初始值，所以它现在的值是任意值）。
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当然
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p [variable]
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这可以显示特定变量的值，但是还有更好的：
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ptype [variable]
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可以显示变量的类型。所以这里可以确定d是double型。
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既然已经到这一步了，我么不妨这么做：
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set var [variable] = [new value]
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这样会覆盖变量的值。不过需要注意，你不能创建一个新的变量或改变变量的类型。我们可以这样做：
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set var a = 0
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如其他优秀的调试器一样，我们可以单步调试：
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step
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使用如上命令，运行到下一条语句，也可以进入到一个函数里面。或者使用：
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next
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这可以直接下一条语句，并且不进入子函数内部。
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结束测试后，删除断点：
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delete [line number]
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从当前断点继续运行程序：
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continue
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退出GDB：
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quit
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总结，有了GDB，编译时不用祈祷上帝了，运行时不用血祭（？）了，再也不用printf(“test“）了。当然，这里所讲的并不完整，而且GDB的功能远不止这些。所以我强烈建议你自己更加深入的学习它。我现在感兴趣的是将GDB整合到Vim中。同时，这里有一个[备忘录][2]记录了GDB所有的命令行，以供查阅。
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你对GDB有什么看法？你会将它与图形调试器对比吗，它有什么优势呢？对于将GDB集成到Vim有什么看法呢？将你的想法写到评论里。
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/gdb-command-line-debugger.html
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作者：[Adrien Brochard][a]
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译者：[SPccman](https://github.com/SPccman)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://xmodulo.com/author/adrien
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[1]:https://www.gnu.org/software/gdb/
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[2]:http://users.ece.utexas.edu/~adnan/gdb-refcard.pdf
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