How to make a simple Hello World using C conventions?

Question

Knowing a little of the conventions in C and the generalization of code that it can provide us, I tried to implement my "Hello World":

global main
extern printf

section .data
helloworld db 'Hello World',0

section .text
main:
    push helloworld
    call printf
    ret

Soon followed by nasm -fwin64 test.asm and gcc test.obj. The compilation is perfect. I don’t know much about Assembly (just a bit of higher-level languages), so I don’t see what’s wrong. What happens to my program? Why does this happen? How to fix this?

PS: Machine running Windows 8, 64-bit Intel i3 CPU, and Linker Mingw64. By simple, I mean "no Winapi conventions".

Answer 1

Every function must have its own stack frame, this is part of the function convention. So, how main is a function, you must start the stack frame with push ebp and mov ebp, esp and end with leave. The reason you have to do this with main it’s because she nay is the main function of your executable. Actually there will be another, provided by the compiler, which will call main. In Windows would be WinMain while on Linux is the _start. So the code needs to be like this:

global main
extern printf

section .data
helloworld db 'Hello World',0 

section .text
main:
    push    ebp
    mov     ebp, esp
    push    helloworld
    call    printf
    leave              ; Equivalente a "pop ebp"
    mov     eax, 0     ; Retorne 0
    ret

But you are also interested in using code in x64. This way it won’t work. The convention for passing arguments in numerical x86 (or pointers as is the case) is to add them to the stack. But in the case of x64 the convention changes to use the following registers for the first 6 parameters: rdi, rsi, rdx, rcx, r8 and r9. From the seventh onwards, you will need to stack as usual. So the code looks like this:

global main
extern printf

section .data
helloworld db 'Hello World',0 

section .text
main:
    push    rbp
    mov     rbp, rsp
    mov     rdi, helloworld
    call    printf
    leave
    mov     rax, 0     ; Retorne 0
    ret

It is worth noting, however, that different systems or different architectures can change these conventions. You will always need to choose exactly what your target system is and search its conventions.