Gdb

• Commands
• Formats
• Convenience Variables
• Helpful Hints

Commands

M-x gdb

invoke gdb from emacs

C-c

interrupt, return control to gdb

q

quit gdb

run

start program from beginning

kill

cancel the running program

cont

continue execution (from breakpoint, etc.)

fin

finish this function and break on return

ret value

return immediately from this function with value

s

continue and break at next statement

n

continue and break at next statement in this function

break function

set breakpoint at entry to function function

clear function

clear any breakpoint set at entry to function function

M-x gdb-break

set breakpoint at source line containing emacs cursor

info break

display list of breakpoints

delete bnum

clear breakpoint specified by bnum

frame 0

display source, etc., for innermost stack frame

frame n

for n>0, display source, etc., for caller of frame n-1

bt

display backtrace of function calls.

p exp

evaluate exp in the context of current frame and print its value in the format implied by its type.

p/letter exp

evaluate exp in context of current frame and print its value in the format specified by letter.

x/letter exp

similar to p, but exp is treated as an address and the memory contents at that address are displayed.

Formats

a address

c character

d signed decimal

f floating point

i instruction (assembler syntax)

o octal

s string

u unsigned decimal

x hexadecimal

Convenience Variables

$

most recent value printed

$$

next to most recent value printed

$n

nth value printed

$r0, $r1, ..., $r15

registers r0, r1, ..., r15

$pc, $ap, ...

registers pc, ap, ...

$ps

process status longword

Helpful Hints

To debug a function, try setting a breakpoint at the function, and using p commands to call the function, e.g., p f(5,s) calls f with arguments 5 and s and, if a breakpoint has been set, breaks immediately on entering the function. You can then step through the function, examine variables, etc.

It is often useful to set a breakpoint at main before running the program.

After giving an x command, subsequent returns (i.e., the RETURN key) are intepreted as x commands with the same format and applied to the memory address following the one just examined.