How to print register values in GDB? - gdb

Question

How do I print the value of %eax and %ebp?
(gdb) p $eax
$1 = void

Answer 1

info registers shows all the registers; info registers eax shows just the register eax. The command can be abbreviated as i r

Answer 2

If you're trying to print a specific register in GDB, you have to omit the % sign. For example,
info registers eip
If your executable is 64 bit, the registers start with r. Starting them with e is not valid.
info registers rip
Those can be abbreviated to:
i r rip

Answer 3

There is also:
info all-registers
Then you can get the register name you are interested in -- very useful for finding platform-specific registers (like NEON Q... on ARM).

Answer 4

If only want check it once, info registers show registers.
If only want watch one register, for example, display $esp continue display esp registers in gdb command line.
If want watch all registers, layout regs continue show registers, with TUI mode.

Answer 5

Gdb commands:
i r <register_name>: print a single register, e.g i r rax, i r eax
i r <register_name_1> <register_name_2> ...: print multiple registers, e.g i r rdi rsi,
i r: print all register except floating point & vector register (xmm, ymm, zmm).
i r a: print all register, include floating point & vector register (xmm, ymm, zmm).
i r f: print all FPU floating registers (st0-7 and a few other f*)
Other register groups besides a (all) and f (float) can be found with:
maint print reggroups
as documented at: https://sourceware.org/gdb/current/onlinedocs/gdb/Registers.html#Registers
Tips:
xmm0 ~ xmm15, are 128 bits, almost every modern machine has it, they are released in 1999.
ymm0 ~ ymm15, are 256 bits, new machine usually have it, they are released in 2011.
zmm0 ~ zmm31, are 512 bits, normal pc probably don't have it (as the year 2016), they are released in 2013, and mainly used in servers so far.
Only one serial of xmm / ymm / zmm will be shown, because they are the same registers in different mode. On my machine ymm is shown.

Answer 6

p $eax works as of GDB 7.7.1
Tested as of GDB 7.7.1, the command you've tried works:
set $eax = 0
p $eax
# $1 = 0
set $eax = 1
p $eax
# $2 = 1
This syntax can also be used to select between different union members e.g. for ARM floating point registers that can be either floating point or integers:
p $s0.f
p $s0.u
From the docs:
Any name preceded by ‘$’ can be used for a convenience variable, unless it is one of the predefined machine-specific register names.
and:
You can refer to machine register contents, in expressions, as variables with names starting with ‘$’. The names of registers are different for each machine; use info registers to see the names used on your machine.
But I haven't had much luck with control registers so far: OSDev 2012 http://f.osdev.org/viewtopic.php?f=1&t=25968 || 2005 feature request https://www.sourceware.org/ml/gdb/2005-03/msg00158.html || alt.lang.asm 2013 https://groups.google.com/forum/#!topic/alt.lang.asm/JC7YS3Wu31I
ARM floating point registers
See: https://reverseengineering.stackexchange.com/questions/8992/floating-point-registers-on-arm/20623#20623

Answer 7

Easiest for me is:
(gdb) x/x $eax
First x stands for examine and second x is hex. You can see other formats using:
(gdb) help x
You can easily print strings with x/s $eax or return addresses with x/a $ebp+4.