Debugging C programs with GDB - (GNU DeBugger)

All programs to be debugged should be compiled with the -g switch.  E.g.
  gcc -g ex5.c    (for C++ use  g++ instead of gcc)
This includes a Global symbol table and allows the debugger
the use variable names instead of memory addresses only.
(Note that the -g option also works for javac but in this case you need
to use jdb instead of gdb.)

To start debugging type   gdb
At the (gdb) prompt you can type the following commands (and many others). 
Most commands can be abbreviated to the first letter

The new version of gdb seems to require

file a.out  -- to load the program (could have any name)

help     -- gives a summary of commands which is a bit overwhelming

list     -- list 10 lines about where you are
l        -- ditto
RETURN   -- list 10 more
list 20,40    -- list lines 20 up to 40
list myfun    -- list first 10 lines of myfun()

break main   -- inserts a breakpoint at beginning of main()
break 27     -- inserts a breakpoint at start of line 27
             -- useful to put a break at key while loops
run      -- runs program up to crash or end 
         -- input/output redirection and command line arguments are ok
run <ex5data  -- run with stdin connected to ex5data
run swim "8" < file -- command line arguments are allowed 
If your program crashes use list to see where you are and

bt       -- prints a back trace of all procedure calls to get to where you are

continue --  continues execution after a break
c        -- ditto

step     -- executes next statement and if it is a function
         -- it takes you to the first line of the function
s        -- guess what
next     -- executes whole line including any procedures - useful to execute
           entire procedures like printf() that you think are OK
n        -- ditto
print x  -- prints out the current value of x
p x      -- ditto
print *p -- prints value pointed at by pointer p

call printf("%d%f\n", x, y)  -- useful to print several things

Note you don't see partial line output until a newline is printed.
Once you have typed a long command like this you can use the back arrow
key to do it again. Also use arrow keys to correct typing mistakes  

quit     -- quit gdb