CISC2200: Data Structure, Intro. to gdb

Please use the following command to copy two sample codes:

cp  ~zhang/public_html/cs2200/LabClass/gdbLabCodes/*.cpp . 

Sometimes it's very difficult to debug memory related problems. For cases like this, you can use Gnu debugger gdb to trace the execution of your program. Essentially, running your program in the debugger allows you to view the values and addresses of the variables, contents of stack, and so on. So it's also a good tool for teaching purposes.

1. Compile a program with degbug information by using -g option:

   g++ -g fibonacci.cpp -o fib
   

2. Trace the execution of the given sample program

   Run the following command to load the program using gdb

   [zhang@storm labclass2]$ gdb fib 
   GNU gdb (GDB) Fedora 7.9.1-19.fc22
   Copyright (C) 2015 Free Software Foundation, Inc.
   License GPLv3+: GNU GPL version 3 or later 
   This is free software: you are free to change and redistribute it.
   There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
   and "show warranty" for details.
   This GDB was configured as "x86_64-redhat-linux-gnu".
   Type "show configuration" for configuration details.
   For bug reporting instructions, please see:
   .
   Find the GDB manual and other documentation resources online at:
   .
   For help, type "help".
   Type "apropos word" to search for commands related to "word"...
   Reading symbols from fib...done.
   (gdb) 
   (gdb) break main
   Breakpoint 1 at 0x4008a5: file fibonacci.cpp, line 28.
   (gdb) run
   Starting program: /u/erdos/xzhang/OSFall15/labclass2/fib 
   Missing separate debuginfos, use: dnf debuginfo-install glibc-2.21-8.fc22.x86_64
   
   Breakpoint 1, main () at fibonacci.cpp:28
   28         int localVarMain=10;
   Missing separate debuginfos, use: dnf debuginfo-install libgcc-5.1.1-4.fc22.x86_64 libstdc++-5.1.1-4.fc22.x86_64
   (gdb) where
   #0  main () at fibonacci.cpp:28
   (gdb) print &main
   $1 = (int (*)(void)) 0x40089d 
   (gdb) print &localVarMain
   $2 = (int *) 0x7fffffffe2bc
   (gdb) break Fibo
   Breakpoint 2 at 0x400852: file fibonacci.cpp, line 12.
   (gdb) cont
   Continuing.
   
   Breakpoint 2, Fibo (n=10) at fibonacci.cpp:12
   12         int localVar=n;
   (gdb) where
   #0  Fibo (n=10) at fibonacci.cpp:12
   #1  0x00000000004008b6 in main () at fibonacci.cpp:30
   (gdb) 
   
   

3. A list of gdb commands
   • help: to see a list of categories of commands available;
   • help breakpoints: to see all commands to set breakpoints
   • break function_name: to make program stop at the entry to a function

     break search // pause the program whenever it calls search function
     

   • run: to execute the program in gdb's environment. If your program takes command line arguments, you specify the arguments (except the command name) after run.

     run 4
     

   • break filename.cpp:line#: to set a break point at the given line of a source file

     break Unsorted.cpp:25 //pause the program's execution whenever line 25 of Unsorted.cpp is reached 
     

   • cont: to continue the execution of a program (after it stops at a breakpoint), until the next breakpoints or the end of a program
   • step: execute the next statement (after the program stops at a breakpoint), this allows you to trace the program's exection one statement at a time
   • help stack: list commands related to the calling stack
     1. where/backtrace: to view the current calling stack content
     2. up and down: to go to the previous and next frame in the caling stack
     3. frame: to go to a certain stack frame
   • help data: list commands related to examine or modify data
     1. print: to print the values of a varaible or expression. For example, you can find out the address of variable n:

          print &n
          

     2. set: to assign a value to a variable

1. Compile code with -g option,

   In order to allow gdb to understand your program, the program needs to be compiled with debugging information in the exectuable (which gives a larger executable file).

   g++ -g samplecode.cpp -o prog
   

2. Start gdb command, passing your program as argument:

   gdb ./prog 
   

3. After gdb finish starting, type run command to run your program in gdb.
4. When your program crashes due to memory related issue, you can then use gdb commands such as where,up,down to see where in the code the program crashes.
5. You can also use command print to view the value of variables.