Monday, 29 September 2014

Gdb tutorial

  GDB (GNU Debugger)

Gdb is a debugger for C (and C++). It allows you to do things like run the program up to a certain point then stop and print out the values of certain variables at that point, or step through the program one line at a time and print out the values of each variable after executing each line. It uses a command line interface.

It allows you to inspect what the program is doing at a certain point during execution. Errors like segmentation faults may be easier to find with the help of gdb.

1.   Compilation:

For normal compilation without enabling gcc
#gcc <flags> <source file > -o <output file>
e.g. #gcc –Wall hello.c :q–o hello
For compilation with gcc
#gcc <flags> -g  <source file > -o <output file>
e.g. #gcc –Wall –g  hello.c –o hello
NOTE: Use g++ instead of gcc for c++ programs.

    2.  Start gdb

After this prompt will appear like this ->  (gdb)

    3.   load program:
# (gdb) file <executable_name>
e.g.  # (gdb) file hello
We can start gdb and run a program in one command
#gdb hello

   4.   Run program

# (gdb) run
This will run complete program in one go.
If program have any issue in running (like segmentation fault), then gdb will throw some useful information like line number and parameters to function that caused error.

If your program has some bugs / issues then you don’t want to run program in one go, instead you would like to step through program a bit at a time, until you receive upon error.


These can be used to stop a running program at a designated point. The command is “break”
Breakpoint using line number
# (gdb) break hello.c:13
This will set breakpoint at line 6 in hello.c , So when control of execution reach at line 6, it will stop here and wait for another  command.
Breakpoint using function name
 # (gdb) break my_function
Once you have set break points, you can run your program using “run” command. This time execution should stop at 1st break point (unless a fatal error occurs before reaching that point)
Conditional breakpoints:
Use to avoid unnecessary stepping in program
# (gdb) hello.c:13 if len >= MAX_LEN
Here, pausing of program will trigger only if condition is satisfied.
# (gdb) delete  <breakpoint-name>  Use to delete breakpoint.
                    # (gdb) info breakpoints show information about all declared breakpoints.

More useful commands:

#(gdb) continue    Use to proceed from one break point to next break point.
#(gdb) step  It simply execute next line of program and wait for further command
#(gdb) next  It work similar to step , but it doesn’t execute each line of subroutine instead  “next” treat it as one instruction
NOTE: Instead of typing “step” or “next” again and again, you can just press ENTER, gdb will repeat last command fired

Checking value of variables in program:

#(gdb) print  my_variable
 It prints value of variable named “my_varaible” at that moment
You can also print value of structure elements.
Using watch on variables
Breakpoints work on line number or function name, watch act on variable. It pause the program whenever value of watched variable is modified. Gdb keep watch on variable within variables’ scope  
# (gdb) watch var


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