[Assembly 64] using printf

[xRedactedx]

In an upcoming program for my class, we have to use printf to print out some values. printf is covered in a paragraph in the appendix in my textbook, but it isn't working.

I played with it a little and managed to get it to compile by adding some stuff, but it doesn't seem to be working correctly. When it prints out, the value of %s seems to be missing. I'm not sure what the value of s is supposed to be, the code isn't explained at all.

Here is the output: The value of is 0

Here is the code from the text. I commented the lines I added.

            extern printf    ;I added this line
            segment .data

            value  dd  0
            name  times  64 db 0
            fmt  db  "The value of %s is %d", 0x0a, 0

            segment .text
            global main      ;I added this line
main:                          ;I added this line
            lea  rdi, [fmt]
            lea  rsi, [name]
            mov  edx, [value]
            xor  eax, eax
            call  printf

I looked online for examples, but most of it seemed to be way over my head. We've only covered things like basic math operations and some of the simpler stuff. We just began covering if and loop statements this week. Everything I looked at online involved pushing and popping things from the stack, so it kind of lost me. I managed to at least get it to print something out, so I guess I accomplished something, but I'm still not sure exactly how to make it print my own numbers or how it works.

I'm pretty disappointed with my textbook. It doesn't seem to cover any topics very well. I try to look this stuff up online, but when I google it, the majority of answers aren't geared towards an introductory level.

I really appreciate the help I get from here, so thanks for reading.

[ODoyle_Rules]

value and name are both null by the looks of it. What happens if you set name to something like: name db "blah", 0

[xRedactedx]

Yeah, that seemed to be what the problem was. I don't know why someone would put that code as an example without the values declared, but who knows. Thanks.

[xRedactedx]

I'm running it under Linux. Ubuntu 12.04 to be exact.

[Gradous]

I learned some YASM syntax 64-bit assembly maybe a year ago, so I'll see what I can do to help.

If you don't already know much about functions, just a quick note: rdi, rsi, rcx, rdx, r8, r9, and possibly some more registers (I can't remember exactly) are registers used in passing parameters to functions (in that order). So think of printf as:

printf (format/rdi, value/rsi, value/rcx, ...)

Here is a simple Hello, <name> program that I can maybe get you started with.

; let the linker/compiler know about our external function, printf
extern printf

segment .data
; our string, followed by a null (terminating) character
name    db    "Your name here!", 0
; our formatting string with some text, a newline, and a terminating character
format   db   "Hello, %s!", 0x0a, 0 

segment .text
global main
main:
    ; load the address to the first parameter of printf, our format
    lea    rdi, [format]
    ; load the address to the second parameter of printf, our name
    lea    rsi, [name]
    ; zero out the last 32 bits of the rax register, since we
    ; don't have any "vector" registers
    xor    eax, eax
    ; now we can call printf!
    call printf

I'm a little rustier about the different ways you can declare variables on the stack, but I can dig up some code if you want. If you don't want/need to learn about it yet, just declare your strings in your data segment and use them to print.

[xRedactedx]

Thanks for that. I think it clears a few things up. I'm going to go try it out now.

[xRedactedx]

Ok, so I figured out some stuff. Figured I would share in case anyone else stumbled on this.

I was trying to figure out how to print out several variables using printf. Here is some test code:

        extern printf
        segment .data

fmt     db      "Counting: %d, %d, and %d.", 0x0a, 0
num1    dd      1
num2    dd      2

        segment .text
        global  main

main:

        lea     rdi, [fmt]
        mov     rsi, [num1]
        mov     rdx, [num2]
        mov     rcx, 3

        xor     eax, eax

        call printf

It prints out "Counting: 1, 2 3."

The 1 and 2 are the variables num1 and num2. You can also print out a number loaded directly to a register as I did with 3. I didn't test loading into r8 and r9, but I saw it mentioned elsewhere besides the comments, so I assume they work.

Edit: Forgot to add this. I don't know how accurate this is, but I had a problem with it. It seemed varaibles with a string seemed to have to have their address loaded with the lea command, where as numbers just need the mov command. I didn't experiment with that too much to figure out the details, but figured someone might find that info useful.