gdb.md

Collect backtraces for multiple processes

ps -ef | grep PATTERN | tr -s ' ' | cut -d ' ' -f 2 | \
  xargs -I{} gdb -p {} --batch -ex=bt > backtraces

Restart

r # run

Print stack trace / stacktrace

where

Compile with debugging info

Use -g.

https://www.geeksforgeeks.org/gdb-command-in-linux-with-examples/

Set breakpoint

# at line number
b <file_name>:<line_number

b foo.c:123

# at function
b MyFunction

http://web.eecs.umich.edu/~sugih/pointers/gdbQS.html

Run multiple commands

Must define function.

(gdb) define fn
> finish
> next  # step over line (does not enter function calls)
> end

https://unix.stackexchange.com/questions/297982/how-to-step-into-step-over-and-step-out-with-gdb https://stackoverflow.com/questions/1262639/multiple-commands-in-gdb-separated-by-some-sort-of-delimiter

One operand multiplication

imul edx fmul edx

Implicitly multiplies with eax.

https://stackoverflow.com/questions/3818755/imul-assembly-instruction-one-operand

Examine word stored at address

x/g $rsp  # examine 8-byte word starting at address $rsp

x/wx 0xf7bd3040  # examine 4-byte word at address
p/x *0xf7bd3040

x/gx $rsp+0x28  # examine 8-byte word at $rsp+0x28, print as hex

x/48b $rsp  # print 48 bytes from $rsp onward

https://sourceware.org/gdb/current/onlinedocs/gdb/Memory.html

Print contents of array | print array

p/x array[index]

# array is array variable
# length is length of array
p *array@length

p/x *roots@6

https://sourceware.org/gdb/current/onlinedocs/gdb/Arrays.html

Print string at address

print (char*) $esi

Print next 10 instructions

x/10i $pc

https://stackoverflow.com/questions/1902901/show-current-assembly-instruction-in-gdb

Print first 10 instructions of function

x/10i my_function

https://stackoverflow.com/questions/10874298/gdb-disassemble-one-line

Step into function

s  # will step into function if line is function call

http://web.eecs.umich.edu/~sugih/pointers/gdbQS.html

Step out of function

fin

https://stackoverflow.com/questions/24712690/step-out-of-current-function-with-gdb

Check meaning of command

# help FOO
help n

Show breakpoints

i b

Print integer at address

print *(int *) ($rsp)

Print string / print char*

print (char*) $rbp

Print multiple variables

print {var1, var2, ...}

https://stackoverflow.com/questions/1882857/print-multiple-variables-with-one-command-in-gdb

Print multiple registers

print /x {$rbx, $rbp}

https://stackoverflow.com/questions/1882857/print-multiple-variables-with-one-command-in-gdb

FLAGS register

IF = interrupt flag

https://en.wikipedia.org/wiki/FLAGS_register

Print all registers and flags

i r

Setting breakpoints

b *0x0000000000401417  // set breakpoint at address 0x401417
b FOO  // set breakpoint at function FOO

Disassemble function

disas = disassemble current function disas FOO = diassemble function FOO