Guide to Faster, Less Frustrating Debugging
Summary of GDB commands for x86-64 Systems
Command Effect
Starting:
gdb
gdb <file>
Running and stopping
quit Exit gdb
run Run program
run 1 2 3 Run program with command-line arguments 1 2 3
kill Stop the program
quit Exit gdb
Ctrl-d Exit gdb
Note: Ctrl-C does not exit from gdb, but halts the current
gdb command
Breakpoints
break sum Set breakpoint at the entry to function sum
break *0x80483c3 Set breakpoint at address 0x80483c3
delete 1 Delete breakpoint 1
disable 1 Disable the breakpoint 1
(gdb numbers each breakpoint you create)
enable 1 Enable breakpoint 1
delete Delete all breakpoints
clear sum Clear any breakpoints at the entry to function sum
Execution
stepi Execute one instruction
stepi 4 Execute four instructions
nexti Like stepi, but proceed
through function calls without stopping
step Execute one C statement
continue Resume execution until the next breakpoint
until 3 Continue executing until program hits breakpoint 3
finish Resume execution until current function returns
call sum(1, 2) Call sum(1,2) and print return value
Examining code
disas Disassemble current function
disas sum Disassemble function sum
disas 0x80483b7 Disassemble function around 0x80483b7
disas 0x80483b7 0x80483c7 Disassemble code within specified address range
print /x $rip Print program counter in hex
print /d $rip Print program counter in decimal
print /t $rip Print program counter in binary
Examining data
print /d $rax Print contents of %rax in decimal
print /x $rax Print contents of %rax in hex
print /t $rax Print contents of %rax in binary
print /d (int)$rax Print contents of %rax in decimal after
sign-extending lower 32-bits.
You need this to print 32-bit, negative
numbers stored in the lower 32 bits of
%rax. For example, if the lower 32-bits of
%rax store 0xffffffff, you will see
(gdb) print $rax
$1 = 4294967295
(gdb) print (int)$rax
$2 = -1
(gdb)
print 0x100 Print decimal representation of 0x100
print /x 555 Print hex representation of 555
print /x ($rsp+8) Print (contents of %rsp) + 8 in hex
print *(int *) 0xbffff890 Print integer at address 0xbffff890
print *(int *) ($rsp+8) Print integer at address %rsp + 8
print (char *) 0xbfff890 Examine a string stored at 0xbffff890
x/w 0xbffff890 Examine (4-byte) word starting at address
0xbffff890
x/w $rsp Examine (4-byte) word starting at address in $rsp
x/wd $rsp Examine (4-byte) word starting at address in $rsp.
Print in decimal
x/2w $rsp Examine two (4-byte) words starting at address
in $rsp
x/2wd $rsp Examine two (4-byte) words starting at address
in $rsp. Print in decimal
x/g $rsp Examine (8-byte) word starting at address in $rsp.
x/gd $rsp Examine (8-byte) word starting at address in $rsp.
Print in decimal
x/a $rsp Examine address in $rsp. Print as offset from
previous global symbol.
x/s 0xbffff890 Examine a string stored at 0xbffff890
x/20b sum Examine first 20 opcode bytes of function sum
x/10i sum Examine first 10 instructions of function sum
(Note: the format string for the `x' command has the general form
x/[NUM][SIZE][FORMAT] where
NUM = number of objects to display
SIZE = size of each object (b=byte, h=half-word, w=word,
g=giant (quad-word))
FORMAT = how to display each object (d=decimal, x=hex, o=octal, etc.)
If you don't specify SIZE or FORMAT, either a default value, or the last
value you specified in a previous `print' or `x' command is used.
)
Useful information
backtrace Print the current address and stack backtrace
where Print the current address and stack backtrace
info program Print current status of the program)
info functions Print functions in program
info stack Print backtrace of the stack)
info frame Print information about the current stack frame
info registers Print registers and their contents
info breakpoints Print status of user-settable breakpoints
display /FMT EXPR Print expression EXPR using format FMT
every time GDB stops
undisplay Turn off display mode
help Get information about gdb