Debugging Programs with gdb

CS 325

gdb is a utility for debugging and executing programs. In order to be able to debug a program written in C or C++, it has to be compiled with the -g option:
gcc -g -o filename filename.c...
g++ -g -o filename filename.cc...
To start off gdb, type gdb filename at the command line. A few messages are printed, and then you are left at the (gdb) prompt:

      % gdb filename
      <various messages>
      (gdb)

Some of the important and most often used commands at the gdb prompt:

• break sourceline
  break function

  Used to set a breakpoint at the sourceline or the function. In the case of the sourceline, execution is stopped before any code on the line is executed. In the case of the function, execution stops when the function is entered.

• run

  Start execution of the program. If breakpoints are set, execution stops when the sourceline or function is reached. Otherwise, the program runs to completion. gdb prints a message stating the status of the program on termination.

• c

  Continue execution from where it stopped.

• k

  Kill execution of the program begin run. Typically used to prepare to re-start the program from the beginning.

• step
  step [n]

  Execute the next or next n source line(s). This command steps into functions.

• next
  next [n]

  Same as step, but the command steps past functions, treating them as if they were single statements.

• Removing Breakpoints
  • delete

    Deletes all breakpoints

  • clear sourceline
    clear function

    Deletes any breakpoints set on the sourceline or at the entry of function.

• bt

  Print a backtrace of all the active functions on the stack. This is very useful in determining the order in which functions call each other.

• print expression

  Print the value of expression. The contents of variables in the program can be viewed through this command.

  • print i
    Print the value of variable i.
  • print *p
    Print the contents of memory pointed to by p, where p is a pointer variable.
  • print x.field
    Check the different members of a structure.
  • print x
    Check all the members of a structure, assuming x is a structure.
  • print y-$\rangle$field
    y is a pointer to a structure.
  • print array[i]
    Print the i'th element of array.
  • print array
    Print all the elements of array.

• list sourceline
  list sourcefile:sourceline
  list function
  list

  Print 10 lines centered at sourceline or starting from the beginning of function. By itself, print 10 more lines.

• help

  Display the set of commands available in gdb.

• quit

  Exit gdb.

The commands in the file .gdbinit are executed as gdb initializes. gdb executes (if present) the file in the home directory. Then, this process is repeated using the current working directory. For more information on gdb, run the info (see the man page) facility from the shell prompt, then use the m command to enter the gdb documentation. You may also look at the man pages and gdb's help system.

Tom Kelliher