How To Print The Value Of A Register In Gdb

This is going to exist a modest demonstration or 'tip' to clarify registers and retentiveness via gdb when debugging a program. These commands are pretty much useful when debugging a programme. Information technology has its own use cases.

Examine registers:

$info registers is the command which can exist used to meet electric current annals values at the moment from gdb prompt. Below command can exist used as a short cut to view registers:

(gdb) i r rax 0x1 1 rbx 0x7fff955a9df0 140735699131888 rcx 0xffffffffffffffff -i 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 1 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 be fetched individually . For ex: "Stack pointer" and "Instruction pointer" information can exist fetched by:

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

Examining memory :

This is pretty much useful when debugging a plan:

"x" is the control which tin can be used for the aforementioned purpose.. The general format of '10' control every bit shown here.

                                  
                  (gdb) assistance ten                  Examine retention: x/FMT Address.                  
                  ADDRESS is an expression for the retention address to examine.
                    FMT is a echo count followed by a format letter and a size letter.
                    Format letters are o(octal), ten(hex), d(decimal), u(unsigned decimal),
                    t(binary), f(bladder), a(address), i(instruction), c(char) and s(cord).
                    Size letters are b(byte), h(halfword), west(give-and-take), k(giant, viii 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 one.  The default accost is the following the concluding affair printed
                    with this control or "print".
                    (gdb)
                  In short :
                  Formats:
                    o – octal
                    d – decimal
                    x – hexadecimal
                    u – unsigned integer
                    s – string
                    t – binary
                  Units:
                    b – byte
                    h – one-half
                    w – word
                    g – double word
                  Example use of 'x' command:
                  "3" words of memory 'above' stack pointer tin can exist displayed by:                  
                                      But why I used "above" here? 'Ans': Information technology is homework/assignment for you lot 🙂
                  (gdb) x/3xw $sp
                    0x7fff955a9dc0:    0x00000000    0x00000000    0x0041ecb1
                    (gdb)
                  "2" machine instructions from 0x37e78da373/eip
                  (gdb) x/2i 0x37e78da373
                    => 0x37e78da373 :    mov    (%rsp),%rdi
                    0x37e78da377 :    mov    %rax,%rdx
                    (gdb)
                  To brandish a string you tin use: ' I selected a random address', then it may non give a human-readable case string as output.
                  (gdb) x/southward 0x0041ecb1
                    0x41ecb1:     "A\211\307藟\001"
                    (gdb)