How to print register values in GDB?

Question

How do I print the value of %eax and %ebp?

(gdb) p $eax
$1 = void

Answer 1

Gdb commands:

• i r : print a single register, e.g i r rax, i r eax
• i r ...: 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.