A GDB Tutorial with Examples--转
A GDB Tutorial with ExamplesBy Manasij Mukherjee A good debugger is one of the most important tools in a programmer's toolkit. On a UNIX or Linux system,GDB (the GNU debugger) is a powerful and popular debugging tool; it lets you do whatever you like with your program running under GDB. Should you read this? You should... if you can relate to two or more of the following:
A crash course on compiling with gcc (or g++)?is the de facto compiler in Linux or any other *nix system. It also has Windows ports but on Windows,you'll probably find the??'easier'. Suppose you have a file called main.cpp containing your c++ code. You should compile it with the following command:
|
If you use the next command,the line (and the function,provided there aren't breakpoints in it) gets executed and the control advances to the next line,readinput(),where you can perhaps examine 'value' to get an idea of how display() worked.
But if you use the step command,you get to follow what display() does directly,and the control advances to the first line of display(),wherever it is.
Examining your Variables
When you want to find the misbehaving portion of your program,it often helps to examine local variables to see if anything unexpected has occurred. To examine a variable,just use
Note: You can also modify variables' values by
=
You can modify variables to see if an issue is resolved if the variable has another value or to force the program to follow a particular path to see if the reason for a bug was due to a variable having the wrong value.
Setting Watchpoints
Setting watchpoints is like asking the debugger to provide you with a running commentary of any changes that happen to the variables. Whenever a change occurs,the program pauses and provides you with the details of the change.
The command to set a simple watchpoint (a write watchpoint,i.e you are notified when the value is written) is
Here's some example output when GDB pauses due to a change in :
Old value = 0
New value = 1
0x08048754 at main.cpp:31
31 variable=isalpha(ch)
Note: You can only set watchpoints for a variable when it is in scope. So,to watch something within another function or a inner block,first set a breakpoint inside that scope and then when the program pauses there,set the watchpoint.
Quit
To stop your program,when it is paused,use kill and to quit GDB itself,use quit.
An Example Debugging Session
The given code computes the factorial of a number erroneously. The goal of the debugging session is to pinpoint the reason of the error.
>n;
Into the Debugger
Now follow the commands and the outputs carefully,especially the watchpoints. What I'm doing is basically:
- Setting a breakpoint just in the line of the function call
- Stepping into the function from that line
- Setting watchpoints for both the result of the calculation and the input number as it changes.
- Finally,analyzing the results from the watchpoints to find problematic behaviour
Notice that result starts from 0 and is initialized to 1.
43. (gdb)
Notice that I didn't put in a command,I just hit
47. Old value = 3 48. New value = 2
Notice that n gets is immediately decremented from 3 to 2.
51. (gdb) 52. Continuing. 53. Hardware watchpoint 3: result 54. 55. Old value = 1 56. New value = 2
Now result becomes 2 (by multiplying result's earlier value with n's value). We've found the first bug! result is supposed to be evaluated by multiplying 3 * 2 * 1 but here the multiplication starts from 2. To correct it,we have to change the loop a bit,but before that,lets see if the rest of the calculation is correct.
59. (gdb) 60. Continuing. 61. Hardware watchpoint 2: n 62. 63. Old value = 2 64. New value = 1
n gets decremented from 2 to 1. Result doesn't change since n is 1.
67. (gdb) 68. Continuing. 69. Hardware watchpoint 2: n 70. 71. Old value = 1 72. New value = 0
n gets decremented from 1 to 0.
75. (gdb) 76. Continuing. 77. Hardware watchpoint 3: result 78. 79. Old value = 2 80. New value = 0
Now result becomes 0 (by multiplying result's earlier value with n's value,0). Another bug! How can result hold the value of the factorial when it is multiplied by 0? The loop must be stopped before n reaches 0.
83. (gdb) 84. Continuing. 85. Hardware watchpoint 2: n 86. 87. Old value = 0 88. New value = -1 89. 0x08048654 in factorial (n=-1) at main.cpp:20 90. 20 while(n--) 91. (gdb) 92. Continuing.
Now n becomes -1 and the loop isn't permitted to run anymore because n-- returns 0,and the function returns result's current value 0. Let's see what happens when the function exits.
This is what happens to a watchpoint when the variable goes out of scope.
101. (gdb) print val 102. $1 = 1293357044print val shows a garbage value because gdb points to a line before it is executed,not after.
103. (gdb) next 104. 12 cout<105. (gdb) continue 106. Continuing. 107. 0[Inferior 1 (process 2499) exited normally] 108. (gdb) quit Here's what the fix should look like:
0)
GDB in Conclusion
You have now seen enough to try GBD out on your own. Some important topics have not been touched upon here for the sake of simplicity,such as dealing with??or using tools like??to find memory leaks.
Remember that GDB comes built in with an excellent help system. Just type help in the (gdb) prompt and you will be presented with options of what you could need help with. For details about a specific command,use the syntax
Another important point to note is the use of shortcuts (like 'q' for 'quit'). GDB lets you use shortcuts for commands when it is not ambigious.
After learning about GDB,you do not have to panic the next time your program goes crazy. You have an excellent weapon in your arsenal now.
(编辑:李大同)
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