Quick GDB Information

Displaying stuff:

print stuff displays the value in (stuff) or evaluates something (such as print sizeof(foo)
print/x stuff or p/x stuff displays stuff in hexadecimal
print $eax displays the value of register %eax
x stuff displays the value pointed at by stuff
display stuff displays stuff after each command
undisplay stuff removes display number stuff
info registers displays the contents of all registers, including some you've never heard of, in both hexadecimal and decimal.

Any of these commands can have a format argument appended:

/d decimal
/u unsigned
/x hex
/t binary
/i instruction
/s string (displays ascii values until a NUL is encounte
/c char

display /i $eip is a useful command. It displays the value pointed at by $eip after each command and interprets it as an instruction. Basically it shows the next instruction to be run. Also, if a size and number are given, it will print that many of those size items after the given thing. So, for example, x/20w $esp displays 20 words at and after $esp. The available sizes are:

/b byte
/w /code> word



Breakpoints:


Setting and removing single breakpoints:

     break (some function)
    
 break (line number)
    
 break *(some memory address)
    
 delete (breakpoint number)
 


Removing all breakpoints:

     clear  [clears current break point]
    
 clear (some function) 
    
 clear (some line number) 
    
 clear *(some memory address)
 

 Running:


 

     run arg1 arg2 ... starts or restarts the
	program with the given arguments
    
 run       starts or restarts the program at full
	speed.  If restarting, uses the same arguments used last time.
    
 s or step  steps by one line of
	source code,
	going into function calls.  This only works after the program
	is running, so you usually need to set a breakpoint somewhere
	so that you can get to where you want to start stepping.
    
 n or next  steps by one line of
	source code,
	not going into function calls
    
 stepi steps by one instruction, going into
	function calls
    
 nexti steps by one instruction, not going into
	function calls
    
 c or continue  goes at full speed
	after a breakpoint
    
 kill	  end the running program
    
 enter  do the same command again.
    
 finish step out of the current function.
 

 Stack:


 

     bt or backtrace shows the current stack.
    
 frame N goes to the Nth stack frame.
    
 info locals prints all local variables.
    
 info args prints all of the arguments to the
	current function as they are now (as opposed to as they were
	at the top of the function).
    
 call function calls function.
	Arguments can be provided.  Note: this works
	by pushing arguments on the stack, resetting %eip
	to point the the function, and letting the program run.  In
	some circumstances, this can fail
    
 whatis something prints the type of
	something
  



Command Line:


 

     set args (stuff)  passes
	stuff as command line arguments to the program the next
	time run is used.
    
 file stuff sets stuff as the
	program to be run and debugged.
 

Lazy Typing:



     Enter (the key) at an empty command prompt repeats the
	last command.  This is especially handy for step
	and next commands
    
 Ambigious abreviations will resolve to the last command with that 
      abbreviation


Lazy Math:



     print/d 0xsome hex number  will convert
	it to decimal
    
 print/x some decimal number will convert
	it to hex
    
 print complicated expression will
	evaluate the expression and print the result in decimal or
	hex.  You can use standard C notation, variables of your
	program, and register names ($eax, etc.)


Useful Tricks:



     info b will tell you how many times a breakpoint
	has been hit.
    
 continue n (or c
	n) will continue past a breakpoint n times.
	For example, if the fourth call to a function is the one that
	fails, you can use "c 3" to skip the first three
	calls.
    
 You can combine the two above tricks to deal with a function
	that crashes after many calls.  Set a breakpoint in the
	function, run the program, and type "c 9999999".
	When it crashes, use info b to find out how many
	times the function was called.  Then rerun the program and use
	continue n-1 to get to the invocation
	that crashes.






© 2007, Geoff Kuenning


This page is maintained by  Geoff
Kuenning.