How To Print Register Values In Gdb

This is going to be a small-scale demonstration or 'tip' to clarify registers and memory via gdb when debugging a program. These commands are pretty much useful when debugging a program. Information technology has its own utilize cases.

Examine registers:

$info registers is the command which tin exist used to see current register values at the moment from gdb prompt. Below control can exist used every bit a short cut to view registers:

(gdb) i r rax 0x1 ane rbx 0x7fff955a9df0 140735699131888 rcx 0xffffffffffffffff -1 rdx 0x7fff955a9e70 140735699132016 rsi 0x7fff955a9df0 140735699131888 rdi 0x16 22 rbp 0x7fff955a9e70 0x7fff955a9e70 rsp 0x7fff955a9dc0 0x7fff955a9dc0 r8 0x7fff955a9dd0 140735699131856 r9 0x1 1 r10 0x7fff955a9ef0 140735699132144 r11 0x293 659 r12 0x7fff955a9ef0 140735699132144 r13 0x0 0 r14 0x1 ane r15 0x0 0 rip 0x37e78da373 0x37e78da373 eflags 0x293 [ CF AF SF IF ] cs 0x33 51 ss 0x2b 43 ds 0x0 0 es 0x0 0 fs 0x0 0 gs 0x0 0

Register data can exist fetched individually . For ex: "Stack pointer" and "Instruction pointer" information can be fetched by:

                                  
                  (gdb) i r $sp
                  sp: 0x7fff955a9dc0
                  (gdb) i r $rip
                  rip            0x37e78da373    0x37e78da373
                  (gdb)

Examining retentivity :

This is pretty much useful when debugging a programme:

"x" is the command which can be used for the aforementioned purpose.. The general format of 'x' command every bit shown hither.

                                  
                  (gdb) help x                  Examine memory: x/FMT Address.                  
                  ADDRESS is an expression for the memory address to examine.
                    FMT is a repeat count followed by a format letter and a size letter of the alphabet.
                    Format messages are o(octal), ten(hex), d(decimal), u(unsigned decimal),
                    t(binary), f(bladder), a(address), i(teaching), c(char) and southward(string).
                    Size letters are b(byte), h(halfword), w(word), g(giant, 8 bytes).
                    The specified number of objects of the specified size are printed
                    according to the format.
                  Defaults for format and size letters are those previously used.
                    The default count is 1.  The default address is the post-obit the last matter printed
                    with this command or "impress".
                    (gdb)
                  In short :
                  Formats:
                    o – octal
                    d – decimal
                    x – hexadecimal
                    u – unsigned integer
                    south – string
                    t – binary
                  Units:
                    b – byte
                    h – one-half
                    west – give-and-take
                    g – double word
                  Instance utilise of 'x' command:
                  "3" words of memory 'in a higher place' stack pointer can be displayed by:                  
                                      Just why I used "higher up" here? 'Ans': It is homework/consignment for you 🙂
                  (gdb) x/3xw $sp
                    0x7fff955a9dc0:    0x00000000    0x00000000    0x0041ecb1
                    (gdb)
                  "two" motorcar instructions from 0x37e78da373/eip
                  (gdb) 10/2i 0x37e78da373
                    => 0x37e78da373 :    mov    (%rsp),%rdi
                    0x37e78da377 :    mov    %rax,%rdx
                    (gdb)
                  To brandish a string you can apply: ' I selected a random address', and so it may non give a man-readable example string as output.
                  (gdb) x/s 0x0041ecb1
                    0x41ecb1:     "A\211\307藟\001"
                    (gdb)