X86-64 NASM calling extern c functions

Question

Im very new to assembly but know a bit of c. Im playing around with extern function calls like

extern _printf
str db \"Hello\", 0
push str
call _printf
         


        

Answer 1

Edit: I made a few mistakes here, see comments for details!

Here is a 32bit version (64bit below):

SECTION .data
    ; global functions in this file
    global main

    ; extern functions
    extern strcmp

hworld: ; our first string
    db "hello world", 0

hworld2: ; our second string
    db "hello world2", 0

SECTION .text

;=============================================================================
; The program entrypoint
;
main:

    ; int rv = strcmp("hello world", "hello world2");
    push    hworld2
    push    hworld
    call    strcmp

    ; _exit(rv);
    mov     ebx, eax
    mov     eax, 1
    int     0x80

Then you can compile it with:

nasm -f elf main.s -o main.o
cc -m32 main.o -o hello_world

And here is the 64bit version:

SECTION .data
    ; global functions in this file
    global main

    ; extern functions
    extern strcmp

hworld: ; our first string
    db "hello world", 0

hworld2: ; our second string
    db "hello world2", 0

SECTION .text

;=============================================================================
; The program entrypoint
;
main:

    ; int rv = strcmp("hello world", "hello world2");
    mov     rsi, hworld2
    mov     rdi, hworld
    call    strcmp

    ; _exit(rv);
    mov     rdi, rax
    mov     rax, 60
    syscall

Then you can compile the x64 version with:

nasm -f elf64 main.s -o main.o
cc main.o -o hello_world

And run it:

$ ./hello_world 
$ echo $?
206